ET MICROCLIMATOLOGIE CLIMATOLOGIE

CLIMATOLOGY

AND MICROCLIMATOLOGY

Proceedings

of the Canberra Symposium

CLIMATOLOGIE

ET MICROCLIMATOLOGIE

Actes

du Colloque de Canberra

U N E S C O

ARID ZONE RESEARCH — XI

CLIMATOLOGY AND MICROCLIMATOLOGY PROCEEDINGS OF THE CANBERRA SYMPOSIUM

RECHERCHES SUR LA ZONE ARIDE — XI CLIMATOLOGIE ET MICROCLIMATOLOGIE

ACTES DU COLLOQUE DE CANBERRA

Titles in this series/Dans cette collection :

I. R e v i e w s of Research o n Arid Z o n e Hydrology.

I. C o m p t e rendu des recherches relatives à l'hydrologie d e la zone aride.

II. Proceedings of the A n k a r a S y m p o s i u m o n Arid Z o n e Hydrology.

II. Actes d u colloque d ' A n k a r a sur l'hydrologie de la zone aride.

I U . Directory of Institutions engaged in Arid Z o n e Research (en anglais seulement).

I V . Utilization of Saline W a t e r : R e v i e w s of Research.

IV. Utilisation des eaux salines : compte rendu de recherches.

V . Plant Ecology : Proceedings of the Montpellier Symposium/Écologie végétale : Actes jdu colloque de Montpellier.

V I . Plant Ecology : Reviews of Research/Ecologie végétale : compte rendu de recherches.

VII. W i n d a n d Solar E n e r g y : Proceedings of the N e w Delhi S y m p o s i u m / É n e r g i e solaire et éolienne : Actes d u colloque de N e w Delhi/Energía solar y eólica : Actas del coloquio celebrado e n N u e v a Delhi.

VIII. H u m a n a n d A n i m a l Ecology : Reviews of Research/Écologie h u m a i n e et animale : compte rendu de recherches.

IX. Guide Book to Research Data for Arid Zone Development.

IX. Guide des travaux de recherches sur la mise en valeur des régions arides.

X . Climatology : Reviews of Research.

X . Climatologie : c o m p t e rendu de recherches.

X I . Climatology and Microclimatology : Proceedings of the Canberra Symposium/Climatologie et microclimatologie : Actes du colloque de Canberra.

As from 1955 the Reviews of Research are published with a yellow cover, the Proceedings of the Symposia with a grey cover.

A partir de 1955, les comptes rendus de recherches sont publiés sous couverture jaune, les Actes des colloques sous couverture grise.

Published in 1958 by the United Nations Educational, Scientific and Cultural Organization

19, avenue Kléber, Paris-16^e Printed by L. P.-F. Léonard Danel - Loos

Publié en 1958

pour l'Organisation des Nations Unies pour l'éducation, la science et la culture

19, avenue Kléber, Paris-16e

Imprimé par L. P.-F. Léonard Danel - Laos

© Unesco 1958 Printed in France

NS.57.III.14.AF

C O N T E N T S

T A B L E D E S M A T I È R E S

Foreword 11 Avant-Propos 12 List of participants/Liste des participants 13

Introduction to arid zone climatology, b y C . W . Thornthwaite . 15 Introduction à la climatologie de la zone aride, par C . W . Thornthwaite 23

First session: Evaporation and the water balance Première séance: Evaporation et bilan hydrique

Turbulent transfer in the lower atmosphere, b y W . C . Swinbank 35 Le transfert par turbulence dans la basse atmosphère, résumé 37 Comparison between m o m e n t u m and water vapour transfer, by E . L . Deacon and W . C . Swinbank . . . 38

Comparaison des transferts de mouvement et de vapeur d'eau, résumé 40 The automatic, direct, measurement of natural evaporation, b y R . J. Taylor 42

L a mesure automatique directe de l'évaporation naturelle, résumé 44

A lysimeter installation at Aspendale, by I. C . McHroy 45

Installation lysimétrique à Aspendale, résumé 47 Climatic indices in relation to the water balance, b y J. A . Prescott 48

Les indices climatiques et le bilan hydrique, résumé 51 Evaporation pan coefficients in Australia, b y C . E . H o u n a m 52 Les coefficients d'évaporation en bac utilisés en Australie, résumé 60 Reduction of evaporation of stored water, b y W. W. Mansfield 61 Réduction de l'évaporation des eaux emmagasinées, résumé 63 The influence of salinity upon the rate of natural evaporation, b y C . W . Bonython 65

Effet de la salinité sur le taux d'évaporation naturelle, résumé 70

Water storage in semi-arid soils, by J. J. Burgos and M . Tschapek 72

Emmagasinage de l'eau dans les sols semi-arides, résumé 91

S u m m a r y of discussion 93 Compte rendu des débats 94

Second session: Radiation and the thermal balance Deuxième séance: Rayonnement et bilan thermique

The Australian radiation network, b y F . H . W . Albrecht 99 Le réseau australien d'observation du rayonnement, résumé 105 Sensible heat transfer from ground to air, by C . H . B . Priestley 106

Transfert de la chaleur sensible du sol à l'air, résumé 108 The thermal behaviour of soils, by D . A . de Vries 109 Le comportement thermique des sols, résumé 112 Note on the heat exchange between soil and air under the influence of an initial temperature difference, by

D . A . de Vries 114 Note sur l'échange de chaleur entre le sol et l'air sous l'effet d'une différence initiale de température,

résumé 115 Evaporation from soil, b y J. R . Philip 117

L'évaporation à la surface du sol, résumé 122 The heat balance of soil beneath crops, by J. L . Monteith 123

Le bilan thermique des sols portant des cultures, résumé 127 Phenomena controlling the thermal balance at the ground surface, by L . A . R a m d a s 129

Phénomènes régissant le bilan thermique à la surface du sol, résumé 132

S u m m a r y of discussion 134 Compte rendu des débats 136

Third session: Inter-relationships of climatic elements and flora Troisième séance: Relations entre les facteurs climatiques et la flore

Afrique du Nord et Australie méditerranéenne, par L . Emberger 141

North Africa and Mediterranean Australia, s u m m a r y 146 The nature of climatological problems encountered by the Land Research Section, C S I R O , by C . S. Christian. 148

Nature des problèmes climatologiques rencontrés par la Section de recherches sur les terres de l'Organisation

de recherches scientifiques et industrielles du Commonwealth, résumé 151 Micro-environment (soil) of a natural plant community, by R . L . Specht 152

Micro-milieu (sol) d'un groupement végétal naturel, résumé 155 S o m e observations on vegetation changes and water relationships in arid areas, by C . S. Christian and

R . 0 . Slatyer 156 Quelques observations sur les modifications de la végétation et les relations hydriques dans les régions

arides, résumé 158 Availability of water to plants, by R . O . Slatyer 159

Les disponibilités en eau de la végétation, résumé 163 The experimental control of water availability, by G . L . Wilson 165

Régulation expérimentale des disponibilités en eau, résumé 167

Transpiration from plants with a limited water supply, by R . L . Closs 168 La transpiration des plantes disposant d'une quantité d'eau limitée, résumé 171

Natural phenomena and microclimate, by R . M . Moore 172

Phénomènes naturels et microclimat, résumé 174 The influence of temperature on the growth of pasture plants, b y K . J. Mitchell 175

Effet de la température sur la croissance des plantes fourragères, résumé 177

Light intensity and plant growth, b y J. N . Black 178 Intensité lumineuse et croissance végétale, résumé 180 Light and pasture growth, b y J. L . Davidson and J. R . Philip 181

L a lumière et la croissance des plantes fourragères, résumé 186 A n example of climatic control of land utilization, by B . Mason 188 U n exemple d'exploitation rationnelle des terres en fonction des données climatiques, résumé . . . 194

Heatwaves and their significance in Queensland's primary industries, b y P . J. Skerman 195 Les vagues de chaleur et leur importance pour les industries primaires du Queensland, résumé . . . 197

S u m m a r y of discussion 199 Compte rendu des débats 201

Fourth session: Inter-relationships of climatic elements and fauna Quatrième séance: Relations entre les facteurs climatiques et la faune

The measurement of moisture in the environments of animals that can Uve in dry places, by H . G . Andre-

wartha 205 L a mesure de l'humidité dans les milieux où vivent des animaux qui supportent la sécheresse,

résumé 209 Meteorological aspects of desert locust control, b y E . B . Kraus 211

Aspects météorologiques de la lutte anti-acridienne, résumé 216 Animais and arid conditions, b y K n u t Schmidt-Nielsen 217

L'animal et l'aridité, résumé 221 S u m m a r y of discussion 222 Compte rendu des débats 223

Fifth session: Microclimate of man and domestic animals

Cinquième séance: Microclimat de Vhomme et des animaux domestiques

Experimental approaches to the functions of tropical livestock, by W . V . Macfarlane 227 Étude expérimentale des fonctions organiques du bétail des régions tropicales, résumé 233

The performance of sheep in semi-arid Queensland, by G . R . Moule 235 Le comportement du mouton dans le Queensland semi-aride, résumé 241 Factors affecting heat tolerance of cattle, by H . G . Turner , 243

Facteurs dont dépend la tolérance thermique du bétail, résumé 245 Facilities for studies in climate physiology of sheep and cattle at the Sheep Biology Laboratory, Prospect,

N . S . W . b y I. W . McDonald 247

Les moyens matériels d'étude de la physiologie climatique des ovins et des bovins, au laboratoire de biologie ovine de Prospect, résumé . . . 250

S u m m a r y of discussion 251 Compte rendu des débats 252

Sixth session: Modification of microclimate Sixième séance: Modification du microclimat

The modification of microclimate b y vegetation in open country and in hilly country, by Rudolf Geiger . 255 Modification du microclimat par la végétation en pays plat et en pays accidenté, résumé . . . . 257

Trees and microclimate, b y N . Hall, V . S. Russell and C . D . Hamilton 259

Les arbres et le microclimat, résumé 263 Frost prevention b y wind machines, b y D . E . Angus 265

L a lutte contre la gelée à l'aide de ventilateurs, résumé 268 The artificial stimulation of rainfall and its application to the arid regions, by E . G . B o w e n . . . . 269

L a pluie provoquée et ses applications dans les régions arides, résumé 271 Observations and interpretation of nocturnal density currents, by F . A . Brooks and H . B . Schultz. . . 272

Observations et interprétation de courants nocturnes de densité, résumé 277

S u m m a r y of discussion 278 Compte rendu des débats 280

Seventh session: Salting and chemistry of rainwater Septième séance: Salage et chimie des sols

The chemistry of rainwater with particular reference to conditions in South Eastern Australia, b y

J. T . Hutton 285 L a chimie de l'eau de pluie notamment dans les conditions particulières au sud-est de l'Australie,

résumé 289 S u m m a r y of discussion 291

Compte rendu des débats 292

Eighth session: Climatological observational requirements in arid zones

Huitième séance: Les besoins de Vobservation climatologique dans les régions arides

Aspects of soil moisture measurement with reference to arid soils, b y J. W . Holmes 295 Aspects de la mesure de l'humidité édaphique dans le cas de sols arides, résumé 299

Measurements of dew, b y D . E . A n g u s 301

Mesures de la rosée, résumé 303 Meteorological measurements in arid zones, by H . N . Brann 304

Les mesures météorologiques dans les régions arides, résumé 306 A n e w absolute method of hygrometry, by R . G . Wylie 307 U n e nouvelle méthode absolue d'hygrométrie, résumé 308

L a mesure de la rosée, par H . Masson 309

Measurement of d e w , s u m m a r y 314

O n some of climatological 'problems and microclimatological studies of arid and semi-arid regions in

U . S . S . R . , b y B . L . Dzerdzeevskii 315 Etudes climatologiques et microclimatologiques des régions arides et semi-arides de P U . R . S . S . , résumé . 324

Periodic variations in water balance in an arid region—a preliminary study of 100 years' rainfall at Karachi,

by S. N . Naqvi 326 Variations périodiques de l'équilibre hydrique d'une région aride, étude préliminaire d'un siècle de pluviosité

à Karachi, résumé 345 S u m m a r y of discussion 346 Compte rendu des débats 347 Authors mentioned / Auteurs cités 349

Subject index 352

Index des matières 354

F O R E W O R D

T H E Arid Zone Programme of Unesco was started in 1951 with the creation of an Advisory Committee on Arid Zone Research. Its object is to promote and stimulate research in the various scientific disciplines which have a bearing upon problems of the arid zones.

The ultimate aim is not only increase of knowledge but improvement of the living conditions of mankind and in particular of the peoples living in desert or semi- desert regions. The emphasis, however, is on fundamental research and the main function of the programme is that of co-ordinating and integrating the widespread efforts being made on specific subjects. Many highly specialized studies have contributed to the existing knowledge and in some fields the very mass of available data presents a serious problem for the research worker from a country where little work has as yet been done

on the subject. Unesco provides one solution by the publication of reviews of research devoted each year to an important scientific discipline, e.g. hydrology, plant ecology, human and animal ecology or the utilization of salt water.

The Advisory Committee on Arid Zone Research, as an international scientific body, which guides the Unesco programme for the arid zone, has the primary task of disseminating knowledge, indicating new lines of research or pointing out which particular discipline should receive attention. The composition of the committee assures the closest contact between the various disciplines so that results obtained in one branch of study can be assessed for their general significance and implications. Arid zone research has many facets and one of the tasks of the programme is the stimulation of effort in branches of research which are neglected because of their apparent lack of importance or

because no immediate economic interest is implied. This stimulation is achieved through the organization of symposia, award of fellowships, preparation and publi- cation of reviews of research, exchange of information between scientists and an information programme designed both to interest persons at college and university level and to inform the general public.

Symposia and other scientific meetings organized by Unesco have brought together research workers from all over the world who are specialized in many different fields.

The meetings have a central topic which is discussed in the light of the various disciplines involved.

At the invitation of the Australian Government a symposium on arid zone climatology, with special reference to microclimatology, was held in Canberra in October 1956, organized jointly by the Commonwealth Scientific and Industrial Research Organization and Unesco. This volume contains the papers presented at the symposium by the Australian participants as well as original contri- butions from scientists Unesco had invited to the meeting.

It is a companion volume to the reviews of research on arid zone climatology published in the same series. These also were presented at the symposium, and the two volumes thus form a whole.

It was decided that, instead of a full length publication of the discussion, the chairmen of the various sessions should be asked to provide reports which would summarize the discussion and underline the conclusions. These will be found after the last paper in each section.

As in the case of the proceedings of other symposia

jointly organized by Unesco and a Member State, the

papers are given in their original language, either English

or French, with a summary in the other language.

A V A N T - P R O P O S

L

Le Comité consultatif de recherches sur la zone aride, organisme scientifique international chargé de guider l'Unesco dans l'exécution du programme de la zone aride, a pour rôle essentiel de diffuser des connaissances, d'indi- quer des directions nouvelles à la recherche, et de signaler les domaines qui devraient retenir l'attention. La compo- sition même du comité garantit une parfaite coordination des différentes disciplines, et permet d'apprécier, d'un point de vue général, la portée et les incidences des résultats obtenus dans chaque branche. Les recherches sur la zone aride présentent de multiples aspects, et l'un ¿les buts du programme est précisément d'attirer les chercheurs vers ceux qui sont négligés, soit que l'importance n'en ait pas

été perçue, soit qu'ils ne présentent pas un intérêt écono- mique immédiat. Pour y parvenir, l'Unesco recourt à divers moyens: organisation de colloques, octroi de bourses, établissement et publication de comptes rendus de recherches, échanges d'informations entre hommes de science et mise en œuvre d'un programme d'information conçu à la fois pour les milieux universitaires et pour le grand public.

Les réunions scientifiques organisées par l'Unesco, et notamment les colloques, ont permis à des chercheurs de tous les pays du monde et de toutes les spécialités de se rencontrer. Chaque réunion s'organise autour d'un thème central, qui est traité du point de vue des différentes disci- plines intéressées.

Sur l'invitation du gouvernement australien, V Unesco et la Commonwealth Scientific and Industrial Research Orga- nization ont organisé conjointement, sur la climatologie de la zone aride, et plus particulièrement sa microclimato- logie, un colloque qui a eu lieu à Canberra en octobre 1956.

Le présent volume contient les communications des parti- cipants australiens, ainsi que des contributions originales d'hommes de science que l'Unesco avait invités à cette réunion. Il est destiné à accompagner le compte rendu de recherches sur la climatologie de la zone aride, qui a paru dans la même collection; celui-ci a d'ailleurs été présenté au colloque, si bien que les deux ouvrages forment un tout.

Il a été décidé de ne pas publier de compte rendu in extenso des débats, mais de prier les présidents de chaque séance de résumer la discussion et d'en mettre en lumière les conclusions dans des rapports qu'on trouvera à la fin de chaque section.

Pour les Actes des autres colloques organisés conjoin- tement par l'Unesco et un État membre, les communi- cations sont reproduites dans la langue originale (français ou anglais) et accompagnées d'un résumé dans l'autre langue.

L I S T O F P A R T I C I P A N T S L I S T E D E S P A R T I C I P A N T S

E X P E R T S P R E S E N T I N G PAPERS / E X P E R T S A Y A N T P R É S E N T É D E S C O M M U N I C A T I O N S

Dr. F. H . W . A L B R E C H T , Department of Meteorology, Uni- versity of Melbourne, Australia.

Professor H . G . A N D R E W A R T H A , University of Adelaide, Zoology Department, Adelaide, Australia.

D . E . A N G U S , CSIRO,1 Division of Meteorological Physics, Aspendale, Victoria, Australia.

J. N . B L A C K , Waite Agricultural Research Institute, Glen Osmond, South Australia.

Professor F . S. B O D E N H E I M E R , Académie internationale d'histoire des sciences, Jérusalem, Israël.

C. W . B O N Y T H O N , ICI Alkali (Aust.) Pty. Ltd., Osborne, South Australia.

Dr. E . G . B O W E N , CSIRO, Chief, Division of Radiophysics, Chippendale, N e w South Wales, Australia.

H . N . B R A N N , Department of Interior, Bureau of Meteorology, Brisbane, Australia.

Dr. F. A . B R O O K S , Department of Agricultural Engineering, University of California, U . S . A .

J. B U R G O S , Servicio Meteorológico Nacional, Buenos Aires, Argentina.

C. S. CHRISTIAN, CSIRO, Land Research and Regional Survey Section, Canberra, Australia.

J. L . D A V I D S O N , CSIRO, Regional Pastoral Laboratory, Deniliquin, Australia.

E . L . D E A C O N , CSIRO, Division of Meteorological Physics, Aspendale, Victoria, Australia.

Dr. D . A . D E V R I E S , CSIRO, Regional Pastoral Laboratory, Deniliquin, Australia.

Dr. A . J. D R U M M O N D , Chief Research Physicist, The Eppley Laboratory, Inc., Newport, Rhode Island, U . S . A . Professor B . L . DZERDZEEVSKII, Head of Division of Clima-

tology and Hydrology, Institute of Geography, Academy of Sciences, Moscow, U.S.S.R.

Professor L . E M B E R G E R , Institut de botanique de l'Université de Montpellier, Montpellier, France.

Dr. E . E R I K S S O N , University of Stockholm, Institute of Meteorology, Stockholm, Sweden.

Dr. E . M . F O U H N I E R D ' A L B E , Instituto de Ciencias Aplicadas, Ciudad Universitaria, Mexico.

Professor R . G E I G E R , Ludwig Maximilians Universitat, Munich, Germany.

N . H A L L , Division of Forest Resources, Forestry and Timber Bureau, Canberra, Australia.

J. W . H O L M E S , CSIRO, Division of Soils, Adelaide, Australia.

C. E . H O U N A M , Department of Interior, Bureau of Meteo- rology, Melbourne, Australia.

J. T . H U T T O N , CSIRO, Division of Soils, Adelaide, Australia.

Dr. E . B . K R A U S , Snowy Mountains Hydro-Electric Authority, Australia.

Dr. I. W . M C D O N A L D , CSIRO, Officer-in-Charge, Sheep Biology Laboratory, Animal Health and Production, Prospect, Australia.

Professor W . V . M A C F A R L A N E , University of Queensland, Physiology Department, Brisbane, Australia.

I. C. M C I L R O Y , CSIRO, Division of Meteorological Physics, Aspendale, Victoria, Australia.

B . M A S O N , Department of Interior, Bureau of Meteorology, Melbourne, Australia.

Dr. H . M A S S O N , Institut des Hautes Études, Dakar, A . O . F . Dr. K . J. M I T C H E L L , DSIR,² N e w Zealand Grasslands Division,

Fitzherbert West, Palmerston North, N e w Zealand.

Dr. J. L . M O N T E I T H , Physics Department, Rothamsted Expérimental Station, Harpenden, Herts, United Kingdom.

R . M . M O O R E , CSIRO, Assistant-Chief, Division of Plant Industry, Canberra, Australia.

G . R . M O U L E , Queensland Department of Agriculture and Stock, Australia.

S. N . N A Q V I , Director, Meteorological Service, Government of Pakistan, Karachi, Pakistan.

Dr. J. R . PHILIP, CSIRO, Division of Plant Industry, Canberra, Australia.

Professor J. A . P R E S C O T T , University of Adelaide, Adelaide, Australia.

Dr. C. H . B . PRIESTLEY, CSIRO, Chief, Division of Meteo- rological Physics, Aspendale, Victoria, Australia.

Dr. L . A . R A M D A S , Meteorological Office, Poona, India.

Dr. K . S C H M I D T - N I E L S E N , Duke University, Durham, North Carolina, U . S . A .

R . O . S L A T Y E R , CSIRO, Land Research and Regional Survey Section, Canberra, Australia.

W . C. S W I N B A N K , CSIRO, Division of Meteorological Physics, Aspendale, Victoria, Australia.

1. C o m m o n w e a l t h Scientific and Industrial Research Organization.

2. D e p a r t m e n t of Scientific and Industrial Research.

R . J. T A Y L O R , CSIRO, Division of Meteorological Physics, Aspendale, Victoria, Australia.

Professor A . V E R N E T , École d'Agriculture, Tunis, Tunisie.

E X P E R T S N O T PRESENTING P A P E R S / E X P E R T S N ' A Y A N T PAS P R É S E N T É D E C O M M U N I C A T I O N

T. A R T H U R , Wimmera Forest, Nursery Vale, Victoria, Australia.

Dr. L . A . T . B A L L A R D , CSIRO, Division of Plant Industry, Canberra, Australia.

A . T . B R U N T , Department of Interior, Bureau of Meteorology, Brisbane, Australia.

B . E . B U T L E R , CSIRO, Division of Soils, Adelaide, Australia.

Professor R . L . C R O C K E R , University of Sydney, Botany Department, Sydney, Australia.

R . S. D I C K , University of Queensland, Geography Department, Brisbane, Australia.

A . J. D Y E R , CSIRO, Division of Meteorological Physics, Aspendale, Victoria, Australia.

J. F R I T H , CSIRO, Land Research and Regional Surveys Section, Canberra, Australia.

D . J. G U Y A T T , N e w South Wales Department of Agriculture, Trangie, Australia.

M . J A C O B S , Forestry and Timber Bureau, Canberra, Australia.

A . F. J E N K I N S O N , Waite Agricultural Research Institute, Glen Osmond, South Australia.

R . W . JESSUP, CSIRO, Division of Plant Industry, Canberra, Australia.

Dr. J. A . K E A S T , Australian Museum, Sydney, Curator, Birds and Reptiles, Australia.

Dr. J. L . K E Y , CSIRO, Division of Entomology, Canberra, Australia.

Professor G . L E E P E R , University of Melbourne, Agriculture Department, Melbourne, Australia.

J. H . E . M A C K A Y , CSIRO, Division of Plant Industry, Canberra, Australia.

M . M I L L S , CSIRO, Land Research and Regional Surveys Section, Canberra, Australia.

B . J. O ' B R I E N , DSIR, N e w Zealand Dominion Physical Laboratory, N e w Zealand.

F. P E N M A N , CSIRO, Officer-in-Charge, Irrigation Research Station, Merbein, Victoria, Australia.

G . H . R A W T O N , University of Adelaide, Geography Depart- ment, Adelaide, Australia.

Dr. G . S. SCURFIELD, CSIRO, Division of Plant Industry, Canberra, Australia.

H . S U I J D E N D O R P , Western Australian Department of Agri- culture, Perth, Australia.

G . K . T R E L O A R , Forest Commission of Victoria, Mildura, Victoria, Australia.

A . K . T U R N E R , University of Melbourne, Department of Agricultural Engineering, Melbourne, Australia.

J. T U R N E R , CSIRO, Land Research and Regional Surveys Section, Canberra, Australia.

A . D . T W E E D I E , N e w South Wales University of Technology, Geography Department, Sydney, Australia.

R . D . W H A L L E Y , N e w South Wales Department of Agri- culture, Trangie, Australia.

C. J. W I E S N E R , N e w South Wales University of Technology, Sydney, Australia.

G . L . W I L S O N , University of Queensland, Botany Depart- ment, Brisbane, Australia.

REPRESENTATIVES OF T H E U N I T E D NATIONS A N D SPECIALIZED A G E N C I E S / R E P R É S E N T A N T S D E L ' O R - GANISATION D E S NATIONS UNIES E T D E S INSTI- TUTIONS SPÉCIALISÉES

WMO/OMM

H . T . A S H T O N , Department of Interior, Bureau of Meteo- rology, Brisbane, Australia.

A . GIBBS, Department of Interior, Bureau of Meteorology, Brisbane, Australia.

WHO/OMS

Dr. R . N . CLARKE, W H O , Geneva, Switzerland.

FAO

Dr. H . C. F O R S T E R , CSIRO, Assistant Executive Officer, Melbourne, Australia.

M E M B E R S OF T H E A D V I S O R Y C O M M I T T E E O N ARID ZONE R E S E A R C H / M E M B R E S D U COMITÉ CONSULTATIF D E R E C H E R C H E S SUR LA Z O N E ARIDE

Dr. G . A U B E R T , directeur, Division des sols, Office de la recherche scientifique et technique d'outre-mer, France.

Professor G . V . B O G O M O L O V , Geographical Geological Insti- tute, Academy of Sciences, Moscow, U . S . S . R .

Dr. B . T . D I C K S O N , Canberra, Australia.

Dr. M . H . G A N J I , Director, Meteorological Service of Iran, Teheran.

Dr. H . G R E E N E , Rothamsted Experimental Station, Harpenden, Herts, United Kingdom.

S. N . N A Q V I , Director, Meteorological Service, Government of Pakistan, Karachi, Pakistan.

Professor R . E . G . PICHI-SERMOLLI, Botanical Institute, University of Florence, Florence, Italy.

Professor H . O ' R E I L L Y - S T E R N B E R G , Rio de Janeiro, Brazil.

Professor M . S. T H A C K E R , Director, General Council of Scientific and Industrial Research, N e w Delhi, India.

Dr. C. W . T H O R N T H W A I T E , The Laboratory of Climatology, Drexel Institute, Centerton, Elmer, N.J., U . S . A .

O T H E R R E P R E S E NT ATI VES/AUTRES R E P R É S E N T A N T S Sir IAN CLUNIES R O S S , Chairman, CSIRO, Melbourne, Victoria,

Australia.

G . B . G R E S F O R D , Secretary, CSIRO (Industrial and Physical Sciences), Melbourne, Victoria, Australia.

Professor E . S. HILLS, University of Melbourne, Geology Department, Australia. Representing the International Council of Scientific Unions.

Professor C. W . N . S E X T O N , International Association for Hydraulic Research, Union of International Engineering Organizations, Australia.

Dr. F. W . G . W H I T E , CSIRO, Chief Executive Officer, Melbourne, Australia.

U N E S C O SECRETARIAT/SECRETARIAT D E L ' U N E S C O J. S W A R B R I C K , Department of Natural Sciences.

W . M O L L E R , Department of Natural Sciences.

INTRODUCTION

TO ARID ZONE CLIMATOLOGY

by

C. W . T H O R N T H W A I T E , The Laboratory of Climatology,

Centerton, N.J., U.S.A.

HISTORICAL B A C K G R O U N D

At least five centuries before the beginning of the Christian era, scholars in ancient Greece, when travelling in the Mediterranean region, noted the unchanging character of the country from west to east in contrast to the striking differences from north to south. As they travelled southward up the Nile in Egypt they found the country becoming increasingly hot and dry, while northward they experienced colder and wetter conditions. They reasoned on the basis of these observations that the earth must "slope u p " to the south and that the southern regions must be closer to the sun and thus, hotter. Similarly, the "slope d o w n "

toward the north placed those regions farther from the source of heat and made them cold and humid.

The word climate originally meant simply slope and in English is perhaps closely related to the word incline. In Creek experience, significant regional diffe- rences in weather, vegetation, and people occurred only from north to south. These differences were related to the curvature of the earth's surface, confirming the then current notion that the earth was a sphere, and were thought to be due to the slope or inclination of the sun's rays on the earth. F r o m this, there developed a threefold division of the earth into torrid, temperate, and frigid zones. Since the frigid and torrid zones appeared to be too rigorous for Ufe of any sort, it was argued that the inhabited world must be confined largely to the temperate zone. This explains in part w h y the world of the ancient Greeks was thought to consist simply of a fringe of land surrounding the Mediterranean Sea, hairpin shaped, and almost entirely within the temperate zone.

It was reasoned that there must be another tempe- rate zone south of the equator, and that it must contain another habitable world. However, this southern continent, conjured out of the mind of an ancient Greek philosopher and called Australis, was considered to be utterly inaccessible, being cut off by the impas-

sable equatorial zone. T o achieve symmetry, two additional habitable worlds were placed in the temperate zones opposite the original two, so that the earth was pictured as a sphere whose surface was covered with a great world ocean out of which arose four island continents. All four worlds were visualized as horseshoe or hairpin shaped, consisting of land masses bordering central seas which connected westward with the outer ocean.

M u c h later van Diemen, Tasman, Torres, d'Entre- casteaux, and others proved that Australis actually existed and could be reached although only with great difficulty. The Greek philosophers and geographers and the Dutch explorers would be incredulous of the facts of present-day travel. I myself do not find it easy to believe the time-table: I leave Sydney, Australia, at 1100 hours on Tuesday, and I a m in San Francisco, U . S . A . , at 0630 hours on Wednesday. O f course, the International Date Line is crossed on that journey.

At least two centuries before Christ the five original climatic zones of the Greeks were subdivided into a number of belts or climates which were defined by the length of the longest day and delimited by parallels of latitude which were determined by the height of the noonday sun on the longest day. Eventually, 24 climatic belts were delimited; the longest day within each belt varying by half an hour between 12 hours at the equator and 24 hours at each polar circle.

For more than 2,000 years the term climate was used in this sense. After A . D . 1450, when the period of rapid geographical exploration began, increased acquaintance with actual conditions in other parts of the world showed that the climates are not simple latitude belts.

They are instead usually highly irregular areas exhi- biting contrasts in moisture supply and wind movement as well as solar heat, which reflect the general circulation of the atmosphere and which, in turn, are strongly affected b y the distribution, orientation and configu- ration of the great land masses and water bodies over the earth's surface.

Climatology and microclimatology / Climatologie et microclimatologie This n e w direction in climatology was given consi-

derable impetus with the development of instruments and the collection of data. For the first time, regional differences in temperature, precipitation, wind and pressure could be given quantitative expression. But the thermometer and anemometer revealed large local differences in temperature and wind, both horizontally and vertically. For climatological purposes, it was thought that these local variations should be avoided in some w a y , or at least generalized. Accordingly, early investigators devoted m u c h effort to developing stan- dards of instrumentation and exposure.

A t the same time the basic geographic quality of cUmatology was gradually lost from view. T o m a n y people at the present time, the content and scope of climatology is only this—the measuring, recording and averaging of standard météorologie elements. I need not remind you that cUmatology w h e n circumscribed in this w a y is sterile and unrewarding.

The fault lies at least in part with the climatologists w h o are responsible for the collection and compilation of the basic data. They have, too often, been satisfied to accept the old conventional approach and to continue the old techniques without reference to relevant devel- opments in related fields. Only over the past few years has it been recognized that climatology must change and awareness of this necessity is a slow growth.

A standardized system of climatologie observations, although the only kind that can be used for the general purpose of establishing and recording the climatic conditions of an area as large as Australia or the United States, cannot be expected to provide answers to the m a n y climatologie questions that, for instance, arise in relation to the production of different kinds of crops and the other varied agricultural activities in so large an area. The standardized observations of synoptic meteo- rology or of general cUmatology seek to avoid the local influences of vegetation and soil as completely as pos- sible, whereas what is required for agricultural and biological purposes is observation near the ground in the zone where the plants actually Uve.

THE H E A T A N D MOISTURE BALANCE

Modern research in physical cUmatology shows us that cUmates owe their individual characteristics to the nature of the exchange of m o m e n t u m , heat and moisture between the earth's surface and the atmosphere. T h e climate at a place represents the existing balance between incoming and outgoing fluxes of heat and moisture.

In this sense, of course, the words "incoming" and

"outgoing" must be interpreted both horizontaUy and vertically. T h e horizontal component in the total flux is what w e call "advection", which belongs to the province of dynamic climatology. In general, advection is slow b y comparison with the vertical fluxes; the latter depend on radiative and turbulent exchanges parallel to

very strong gradients of temperature and humidity, and are of a higher order of magnitude at most times than advection. The physical cUmatologist therefore concen- trates on determining the vertical heat and moisture balances for a given locality.

T o obtain the heat balance it is necessary to determine rates and amounts of solar radiation, and to evaluate reflectivity and emissivity of various types of surfaces such as bare soils, vegetation-covered ground, water bodies and, in some regions, snow and ice. It is necessary to measure soil temperature at different depths and to determine the thermal properties of the soil material.

Finally, it is necessary to determine the distribution of temperature with height in the lowest several feet of the atmosphere.

To evaluate the water balance involves knowledge of the amount and distribution of precipitation, and of the variation in the amount of interception of water by plants, infiltration into the ground, and run-off from the various types of land surfaces and vegetation covers. In addition, the rate and amount of moisture transfer from the soil and vegetation cover to the atmosphere must be measured. T o complete the moisture balance it is necessary to k n o w the variation with depth of the soil moisture, the rate of moisture transfer through various soil materials, and the distribution of moisture with height in the atmosphere just above the ground surface.

Thus the proper field for study in climatology is not Umited to the atmosphere but must include the land surface as well. A n y region is a composite of innumerable local climates: the cUmate of the ravine, of the south- facing slope, of the hül top, of the m e a d o w , of the corn field, of the woods, and of the bare rocky ledge. Both the heat and moisture exchange vary from the ravine to the hiU top and to the rocky ledge, because of variation in the physical characteristics, position, exposure, and aspect of these diverse surfaces. T h e colour, apparent density, heat capacity, moisture content, and permea- biUty of the soil; the characteristics of the vegetation cover; the albedo and roughness of the surface—these are all factors that influence the heat and moisture exchange and are thus important cUmatic factors.

THE IMPORTANT ROLE OF MICROCLIMATOLOGY The cUmates of areas of very Umited extent are called microcUmates. They are clearly the ones that concern the farmer, the agronomist and the biologist. The stan- dardized cUmatologic stations are neither situated nor equipped to measure temperature or humidity at the places and at the times that are critical for plants. The standard observations do not, as has been supposed, give an average of climatic conditions over a consider- able area. They are in themselves microcUmatic obser- vations, and m a y differ appreciably from representative conditions, depending on the nature of the microclimate in which they are taken.

The pattern of cUmates over the earth has been pretty

Introduction to arid zone climatology well understood for a long time. Good maps of the

standard climatic elements have been prepared and considerable progress has been m a d e in the classification of climates. It is certainly worth while to refine and revise the climatic maps and to improve the classifi- cations. T h e most important present task, however, is probably in the field of microclimatology. For the biologist it m a y be more important to k n o w the pattern of climatic distribution between the ground and the tree tops or the pattern over a field or a farm than it is to k n o w the world pattern.

MICROCLIMATIC INFLUENCES ON THE HEAT A N D MOISTURE BALANCES

Microclimatology is concerned with the heat exchange near the ground and with the various influences upon it. The source of heat is the sun. A t the top of the atmosphere the normal incidence of incoming solar radia- tion amounts to about two gramme-calories per square centimetre each minute. If the earth had no atmosphere and were nothing more than a hollow ball m a d e of thin sheet copper one millimetre thick, floating in empty space, the incoming radiation from the sun would raise the temperature of the copper about 20° C . per minute, which would bring it to melting point in less than one hour. However, the earth is itself a radiation source whose rate varies with temperature. Thus, the tempera- ture of the surface of our copper shell would rise only to a point where the outgoing black-body radiation equalled the incoming radiation from the sun. If w e ignore the radiation flux from front to back within the hollow shell, this equilibrium would be reached at 122° C . If our imaginary earth had the astronomic motions of the real earth, and experienced night and day, the outgoing radiation would continue both day and night following the diurnal course of temperature, while the incoming radiation would be received only during the day. Under these conditions the surface temperature at the equator would range from a m a x i m u m of 122° C . just after noon to a m i n i m u m of —187° C . in the early hours of the morning.

If the copper sheets of this imaginary world were chrome plated, 65 per cent of the incoming radiation would be reflected back to the sky and the available energy would be reduced to 0.7 calories per square centimetre per minute. Under those conditions the temperature at the equator would range from a day- time m a x i m u m of 31° C . to a night-time m i n i m u m of - 1 9 4 ° C .

If the thickness of the copper shell of our imaginary earth were a hundredfold greater, it would take 100 times as m a n y calories to raise the temperature a given amount, and the solar radiation intake of two calories per square centimetre per minute would raise the temperature only one-hundredth as fast. Day-time equilibrium would still be reached at 122° C . but the nocturnal m i n i m u m would be only 34° C . If this thicker

copper sphere were chrome plated the day-time m a x i m u m would be 31° C . and the night-time m i n i m u m would be —23° C .

E v e n on this imaginary earth devoid of an atmosphere a great variety of conditions could exist as a result of nothing more than a simple variation in the mass and plating of the copper shell. W h e n w e consider the actual earth consisting of ocean and land surrounded with an atmosphere containing water vapour the number of variables increases greatly and w e have what indeed w e have observed: extreme climatic diversity, but a diver- sity in which a systematic climatic pattern is distinctly discernible.

A s the solar radiation enters the earth's atmosphere it is depleted in a number of ways. About one-third is reflected back to the sky from the tops of the clouds.

Smaller amounts are absorbed and diffusely scattered so that less than six-tenths of the sun's radiation reaches the earth's surface. There are only small differences in the available heat reaching the surface over wide geo- graphic areas. T h e differences which do exist are the result of differences in the absorption and scattering of insolation in the air because of the local presence of clouds, or other concentrations of water vapour, and the turbidity of the atmosphere. N o w , however, the surface of the earth exerts its influence and leads to the forma- tion of the innumerable microclimates which can exist even within a small area.

O f the energy that reaches the bare ground or a vegetation-covered surface some is reflected immediate ly back to the sky while the rest, less than 50 per cent of the original total of insolation from the sun, goes to heat the surface layers of soil or water, to heat the air in contact with the surface, and to be used in the evaporation of water.

Surfaces differ in their ability to reflect incoming radiation. T h e albedo, or reflectivity, w h e n expressed as a percentage of the reflected radiation to that arriving at the surface varies from 80 to 85 per cent for a fresh snow surface to 8 to 10 per cent for an ocean surface, although the albedo of a water surface depends strongly on the sun's angle. Forest vegetation loses 5 to 18 per cent of the incoming radiation b y reflection, while grass, shrubs and cultivated green crops lose 15 to 30 per cent.

The albedo of bare dry sand is 18 per cent, that of wet sand is 9 per cent. Thus, as soon as the ground or vegetation surface enters into the heat balance, consi- derable differences arise in available energy between adjacent areas.

Since different surfaces absorb varying amounts of heat, markedly different thermal regimes will exist both in the surface layers and in the air over the surface. A t one extreme is water, which retains almost all the heat radiated to it; because of its ability to absorb and hold heat, there is almost no daily range of temperature in the air over a water surface. A t the other extreme, a mulch or leaf litter, having a very low coefficient of thermal conductivity and thus good insulation proper-

Climatology and microclimatology / Climatologie et microclimatologie ties, will allow little of the energy to get into the soil,

and so most of it goes to heat the surface of the mulch itself and the overlying air b y conduction-convection.

Such a surface can get very hot in the day-time and cold at night resulting in a large diurnal range of air temperature.

The density, colour and?aspect of a surface all affect its thermal regime. Ploughing a soil or handling it in such a w a y as to increase its air content lowers the ability of the soil to absorb heat to any depth. This, in turn, increases the amount of heat going to the air layers near the ground and increases the amplitude of the daily range of air temperature over it. Dark-coloured surfaces absorb more heat than lighter ones. Aspect, or slope and exposure of the site, is important in determining the amount of heat absorbed or reflected at the surface. In the United States, south- and west-facing slopes receive more energy than north- or east-facing slopes, and hence they are generally warmer and drier than the latter. In Australia, it should be just the opposite.

Moisture affects in several ways the balance between the heat which enters the soil and that which goes to heat the air. Although a moist soil will absorb more radiation than the same soil when dry, water is a better conductor of heat than air, so that more of the heat reaching the surface will be used to heat the deeper layers of the soil and in evaporation and less will be utilized in heating the air. The day-time temperature of the surface of a moist soil will be less than that of a dry one; these differences will be reflected in the temperature of the air above. Of course, where the soil contains no moisture, there is no evaporation or transpiration and so none of the energy is utilized for evaporation. There is then an increased amount of energy going into heating the soil and air. However, when there is no deficiency of moisture in the soil, between 80 and 90 per cent of the energy reaching the surface will be used to evaporate water and only a small amount, the remaining 10 or 20 per cent, will go to heat the soil and the air. As the moisture content of the soil varies from fully moistened to completely dry, the percentage of radiation used in evaporation varies proportionally.

The type of vegetation growing on the surface is of secondary importance in determining the combined evaporation and transpiration from a moist soil. In this case the évapotranspiration is primarily determined by the energy available from the sun. Except for the differences in albedo, which will result in variations in the total amount of heat available for evaporation, the évapotranspiration from moist soil is independent of the type of vegetation. As the soil dries, however, different species of vegetation are able to utilize varying amounts of stored soil moisture. The actual évapotranspiration from a surface will, therefore, be influenced by the type of surface and the form of vegetation on it.

TOPOCLIMATOLOGY A N D MICROCLIMATOLOGY

Rudolf Geiger, w h o is truly the father of microclimato- logy, has discussed the rather large number of terms that have been employed in the designation of the space aspect of climate. Ordinarily, the terms macro-, meso-, and microclimate designate different scales of area with microclimate referring to the climate of the very small space. The geographers use four terms that relate to different sizes of area: cosmography is the description of the cosmos or the universe, geography refers to a description of the whole earth, chorography to a region such as the Murray-Darling catchment, and topography refers to the description of a place. The old topographies, popular in England two centuries ago, were detailed descriptions of places of very limited area—fields or villages. Gilbert White's Natural History of Selbourne is a classic topography. Thus, by analogy, the climate of a very small space might be called topoclimate and its study, topoclimatology.

Topoclimatology can only m e a n the study of the climate of a place. But the logical connotation of the term can be derived from the content of climatology itself. If climates owe their individual characteristics to the nature of the exchange of m o m e n t u m , heat and moisture between the earth's surface and the atmosphere, then climatology must study the earth's surface as well as the atmosphere.

MAPPING THE FACTORS OF THE HEAT A N D MOISTURE BALANCE

Here, then, is where the study of climate takes on a new aspect. Climatology has been an armchair study.

The task of the climatologist presumably has been to salvage and analyse meteorological observations as meteorologists discard them. But the complete study of climatology involves field work. It involves detailed study of certain aspects of the land. It is essential in topoclimatology to m a k e surveys and to m a p the aspects of the soil that affect the heat and moisture balance. These studies involve methods that have been developed by geographers and soil surveyors. Still, the pedologist has never concerned himself with the mapping of the heat capacity or heat conductivity of the soil.

N o geographer has ever attempted to m a p the qualities of vegetation that determine the surface roughness and thus the vertical wind profile or that determine the évapotranspiration and thus the soil moisture.

Some geographers have experimented with the mapping of elements of slope and with what they call surface configuration. They have never m a d e such maps, however, to assist in the interpretation of the heat and moisture balance. Unless a climatologist has had basic training in geomorphology and in soil science,

Introduction to arid ton» climatology he will not be qualified to work in this phase of climato-

logy. Conversely, ordinary geomorphologists and soil surveyors will not have the understanding of the objec- tives of climatology to permit t h e m to m a k e such studies.

I wish to emphasize the necessity of geographic mapping of the characteristics of the surface that influence the exchange of heat, moisture and m o m e n t u m . Hare and his associates at M c G U l University in Canada have recently m a d e a detailed m a p of albedo of Labrador, on a scale of 1 : 5,000,000. N o other comparable detailed m a p of aerodynamic roughness has been m a d e . M a p s showing the distribution of soil moisture in detail are similarly lacking. Those elements which are of impor- tance in heat and moisture exchange vary from season to season and in some instances from day to day. B u t to understand the heat and moisture exchange, it is necessary to produce m a p s showing the averages of these factors. S o m e years ago I published small-scale m a p s of the United States showing the concentration of moisture in the soil at the end of each of the 12 months. These were highly generalized m a p s . T o achieve the programme that is outlined here, it will be necessary to m a k e such m a p s on a large scale. M a p s of soil moisture would show the eifect of soil texture, slope, and topographic situation on the water content. Such m a p s of soil moisture would aid immeasurably in an understanding of the water balance of a small area. T h e ultimate objective of climatology m a y be to m a k e m a p s of the heat budget and the moisture budget of the earth on a topoclimatological scale. This would involve making very detailed large-scale m a p s . Generalization from such local studies would greatly enhance our understanding of climates of large regions.

O n e of the tasks of the climatologist is to develop skill in generalizing for a large area the observations of a point. This requires more than the simple plotting of data on a m a p and the drawing of smooth isolines the proper distances from the plotted points. T h e reason for this is clear. T h e standard climatic observations at a point do not give an average of climatic conditions over a large area. W e must therefore develop means which will permit us to extrapolate from the standard instru- ment shelters, which must always be located at points convenient to the observer, to other nearby areas and in this manner determine average climatic conditions over a considerable area. In this, topoclimatology is indispensable.

T H E HEAT A N D MOISTURE B A L A N C E IN ARID ZONE CLIMATOLOGY

W h a t has the foregoing to do with arid zone climato- logy? It is clear that the objectives and methods of climatology are the same in the arid regions as elsewhere.

Arid climates are distinctive, however, because of the distinctive characteristics of the heat and moisture balance in the regions of deficient rainfall. Arid areas

generally have clear skies and as a result are charac- terized b y a high amount of solar radiation. While there is very little reflection or absorption of short-wave radiation b y clouds, there is some reduction in radiation reaching the earth's surface because of dust and haze.

Nevertheless, the depletion of incoming radiation is lowest in the desert. Bernard's comparison of the annual insolation for various regions of the globe shows the tremendous influence of the variable cloud cover on incoming radiation in various tropical regions (Table 1).

T A B L E 1. Comparison of annual insolation for various regions

Station

Y u m a , Arizona Heluan, Egypt K a n o , Nigeria K a d u u a , Nigeria Y a n g a m b e , Belgian Congo

Vegetation

desert desert steppe savanna forest

Period

hourB

3 900 3 670 3 002 2 774 1861

M a x i m u m possible insolation

%

Bernard has estimated that about 75 per cent of the solar energy received at the surface in the Belgian Congo is used in évapotranspiration, leaving 25 per cent for heating the soil and the air. In a study in the Great Plains region of the United States, m y associates at the Laboratory of Climatology found that about 80 to 85 per cent of the incoming radiation was used in evapo- ration and transpiration w h e n the soil w a s at field capacity and that the percentage so used steadily dropped as the soil moisture was depleted. In the arid areas there is little moisture available for evaporation or transpiration from the sparse desert plants. Thus, the large amount of radiation which is received at the surface of the earth cannot be used in any large amounts for evaporation but must go into heating the air and the soil.

It follows from these remarks that in dry areas a high

albedo reduces the drain on soil water, and is accord-

ingly an ecologically desirable thing. Prolonged selec-

tion of drought-tolerant species in the hot arid and

semi-arid climates has resulted in a sparse vegetation

cover whose capacity to resist water loss is well marked

in the literature. It is rarely pointed out, however, that

such vegetation is nearly always light in colour, and

appears pale grey in the full sun. In other words, it has

a high albedo, a fact that m a y well be its most important

moisture-conserving property. T h e brilliant light of the

desert and semi-desert, so trying for the traveller, is

largely due to this high albedo. Remarkably enough, a

very similar response to drought is visible in the forested

sub-arctic, an area studied in detail b y Hare and his

colleagues. Here, the rainfall and melted snow are

usually adequate for tree growth, but the low tempera-

ture of the soil water in July, the season of peak cambial

activity in those latitudes, allows only the top few

inches of the soil to yield water to the trees. Hence,

Climatology and microolimatology / Climatologie et microclimatologie intense competition exists for water, poverty in the midst of plenty, the condition that Schimper called

"physiological drought". The trees have an albedo below 0.15, but they stand far apart, the intervening ground being covered b y a continuous carpet of Cladonia, the reindeer lichen, whose pale greenish grey surface has an albedo as high as that of dirty snow, which ranges between 30 and 70 per cent. Thus in two completely unrelated areas w e have an association of drought with high albedo of the plant cover.

With a greater percentage of the incoming radiation going into direct heating of the soil surface an increase in temperature of the adjacent layers of the air and soil should be expected. Furthermore, the absence of mois- ture in desert soils favours temperature extremes at the surface. Both heat capacity and heat conductivity of a soil vary directly with its moisture content. For example, the wetting of a dry sandy soil results in a twofold increase of its heat capacity and a tenfold increase in its heat conductivity.

A classic series of observations of the gradients of temperature in air and soil at Tucson, U . S . A . , on 21 June 1915, reported a variation of soil temperature at a depth of 0.4 centimetre from 17.0° C . at 0530 hours to a m a x i m u m of 71.5° C . at 1300 hours. The m a x i m u m temperature was 29° C . above the air temperature in a standard screen nearby, and 21.1° C . higher than at a depth of two centimetres in the soil. Similar observations are probably to be had from every desert in the world and certainly it is not necessary to go back 41 years for illustrative material.

The United States A r m y Quartermaster Corps has recently m a d e a number of microclimatic observations at a test site at Y u m a , U . S . A . T h e temperature in degrees Fahrenheit, in the air and at one inch below the surface in the soil, observed at four-hourly intervals for the week 14-20 August 1952, is summarized for three different locations in Table 2 .

T A B L E 2. Air and soil temperatures (°F.) at Y u m a , U . S . A . 0030 hrs 0430 hrs 0830 hrs 1230 hrs 1630 hrs 2030 hrs

Ta Ta T o Tg Ta Ta Ta Ta Ta Ts T a Ts

Sand dune 91 100 87 95 88 95 96 107 99 114 95 108 Swamp border 86 90 83 85 90 106 99 131 100 129 92 98 Swamp 86 86 83 86 88 85 100 90 100 90 92 87

These figures should detain us briefly, for they illus- trate the immense importance of terrain differences in determining microclimates. T h e sand dune has a low conductivity, a near-zero water content, and a high albedo. T h e s w a m p w e can suppose to have a m u c h lower albedo, and is presumably wet. Along the s w a m p border, albedo is probably still low, but the soil is evidently dry. Over the sand, the diurnal variation of air temperature is from 87° to 99° F . , whereas the soil w a r m s up from 95° to 114° F . ; the soil at one inch is thus warmer throughout the day than the air above.

But the fluctuation of air temperature is almost wholly due to heating or cooling from the radiating soil surface;

hence w e can infer high thermal gradients in the top inch of this almost non-conducting m e d i u m . Over the s w a m p and s w a m p border, however, air temperature has a higher diurnal range—from 83° to 100° F . in both cases. In this case the dramatic thing is the contrast within the soil at one inch, where at noon the s w a m p border attains 131° F . , although the wet s w a m p , with its high specific heat, remains at 90° F . Also the surface temperature of the s w a m p is probably lower than in the s w a m p border despite the fact that the air above it is warmed as m u c h as over the s w a m p border, and more than over the sand dune. Here is proof, then, of the immense local differences that can exist within a single macroclimate.

APPLICATION OF T H E HEAT A N D MOISTURE BALANCE IN OTHER FIELDS

I have indicated that the principal problems in climato- logy relate to the heat and moisture balance and involve the measurement of the exchange of heat and moisture between the earth's surface and the atmosphere. Clima- tologists, of course, do not have a monopoly of the problems of heat and moisture exchange. Chemical engineers are concerned with both in developing their industrial processes. Mechanical engineers have to deal with these problems also. Architectural engineers must consider both heat and moisture exchange in architec- tural design; some of their most fruitful recent results have been in "climate control", using physical principles in making house interiors cooler and drier in summer and warmer and moister in winter. Actually, this vast field of technology is the concern of technical climato- logy. In studies of the interrelationships of climate and soils, plants, animals and people and their clothing, houses, and materials, w e are still dealing with problems of the heat and moisture balance between the subject and its surroundings.

Recent studies of the relationship between climate and plants show clearly the important role played by those météorologie factors which comprise the heat and water balance. T h e plant is well designed to dissipate heat, the leaves being like the fins of a radiator. S o m e of the excess heat is conducted into the adjacent air and carried away in turbulent eddies. In this w a y the air is heated. But some of the excess heat energy is utilized in transpiration, to change water from a liquid into a vapour. Most of the heat of evaporation must come from the plant. The greater the intensity of the sunshine, the greater will be the tendency to overheat, and the larger will be the transpiration of a plant exposed to it, if water is available. Transpiration is a heat regulator, preventing temperature excesses in both plant and air.

It is clear that plants are thus affected b y the factors of the heat and water balance such as the intensity _of radiation, the albedo, the heat capacity and conducti-