The thermal regime of a sphagnum peat bog

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The thermal regime of a sphagnum peat bog

Williams, G. P.

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S e r *

TH1

N21r2

no.

435 ,

c .

2 BLDG

THE

THERMAL REGIME

OF

A

SPHAGNUM

PEAT BOG

BY

G. _P.

_WILLIAMS

REPRINTED F R O M

PROCEEDINGS

T H l R D

INTERNATIONAL

P E A T CONGRESS HELD IN

QUEBEC, C A N A D A ,

1 8

-

23 _{AUGUST 1968}

P. 195

-

200

RESEARCH PAPER NO. 435

O F THE

831ViSION

O F

BLBl kDll

NG

RESEARCH

WATloNAL RESEARCH C D B N E ~ 1 0 C E N T S

OTTAWA

MAY 1970

N R C C 11342

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The Thermal Regime

of

a Sphagnum

Peat

Bog

G . P. WILLIAMS

CANADA

ABSTRACT

In this study air and ground temperature measurements taken over a 3-year period in a virgin peat bog are compared w i t h similar measurements taken at nearby mineral soil sites. The purpose of the study was t o assess the significance of temperature differences that can be expected due to terrain and soil thermal properties. The drainage of cold air into the peat bog was found to be the major factor causing minimum air temperatures to be lower over the bog than at nearby weather stations. The condition of the bog, i.e., whether it

was w e t or dry, thawing or snow-covered, appeared t o have some effect on minimum air temperatures but was not as important a factor as air drainage. Monthly and annual average peat soil temperatures were several degrees colder than corresponding mineral soil temperatures. The peat soil temperatures were colder because heat available from radiation and convection was used for evaporation rather than for warming the soil.

L'auteur Btablit dans la presente etude une comparaison entre les temperatures de I'air et du sol d'une fagne vierge, relevees au cours d'une periode de trois ans et celles de terrains voisins

a

sol mineral. Le but vise par cette Btude est d'evaluer I'importance des differences de temperatures attribuables aux caracteristiques thermiques du terrain et du sol. L'auteur a constate que I'appel d'air froid vers la fagne est la principale raison pour laquelle les temperatures minimales y sont plus basses qu'au-dessus des terrains a sol mineral avoisinants. Les conditions physiques de la fondriere 2 sphaignes, qu'elle soit humide ou skche, couverte de neige stable ou fondante, y ont quelques effets sur la temperature minimale de I'air, mais le facteur le plus important reste I'appel d'air froid. Les moyennes mensuelles et annuelles de la temperature du sol tourbeux sont plus basses de plusieurs degres que celles des sols mineraux avoisinants. La raison en est que la chaleur reCue par rayonnement ou convection est employee B Bvaporer I'eau plut6t qu'a rechauffer le sol.

Vast areas of Canada are covered by extensive peat deposits overlain by Sphagnum and other vegetation. Because there are n o permanent air or ground temperature records for these areas it is necessary to use temperature records from n~eteorological stations (usually located a t urban centres or airports) for studies of the relation between climatic factors and the freezing of bog land, permafrost distribution, and the growth of different types of vegetation in peat bogs. T h e purpose of this paper is to assess the significance of temperature differences that can be expected due to terrain and soil thermal properties. T h e study is based o n a comparison between air and ground temperature measurements taken over a 3-year period in a virgin peat bog with similar measurements obtained from nearby standard nieteorological stations.

G . P. Williams, Snow and Ice Section, Division of Building Research, National Research Council of Canada, Ottawa, Canada.

DESCRIPTION OF SITE AND MEASUREMENTS Ground and air temperature measurements analyzed in this report were made a t the Mer Bleue, a peat bog located about 6 to 8 niiles (10 to 12 k m ) southeast of the City of Ottawa. This peat bog, lying in a depression of a former river channel partly separated by three sand ridges, has an area of about 8 sq mi (20 sq kni). The principal surface vegetation is Sphagnuin, covering peat deposits varying from 5 to about 1 6 ft (1.5

-

5 ni) in depth. Other fornis of vegetation frequently associated with a peat bog are present, such as blueberry bushes, spruce and tamarack. T h e growing Sphagnunz and other vegetation is typical of bog vegetation occurring a t higher latitudes.

Figure 1 shows a plan of the bog area, with cross-section A'A' where ground and air temperature records were niain- tained. A t Station No. 1, air temperature has been measured continuously since September 1964, and ground temperature

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225

-

w * Y Y I ' d S a n d .U S c a l e 1. 25 in. = 1 m i l e S E C T I O N A ' A ' S c a l e H o r i z o n t a l 1" = 2 0 0 0 ' V e r t i c a l 1" = 4 0 '

Figure 1. Sketch of Mer Bleue peat bog.

since September 1965. Air temperatures were measured at the other stations for shorter time intervals to check the general air temperature pattern across the section A'A1.

Air temperature measurements at all stations were made using recording thermographs placed in Stevenson screens 4 f t above the bog surface. Maximum and minimunl mercury thermometers were also placed in the screens and used as standards t o check, each week, the accuracy of the recording thermograph.

Ground temperature was measured a t Station No. 1 using

mercury-in-steel thermometers connected to a 3-pen recording thermograph. F r o m September 1965 to November 1966 the thernlon~eters were placed at three depths below the surface:

0.5 c n ~ , 2.5

-

5 cnl, and 100 cnl. In November 1966 the thermo-

meters were repositioned at depths of 10, 3 0 and 100 c m and the calibration of the thernlonleters checked. T h e accuracy of ground temperature n~easurenlents was checked in situ after several months' operation by inserting thermocouple probes down through the layers of peat to the appropriate depths.

Daily maxinlunl and minimum air temperatures were ex-

tracted from the thern~ograph temperature record a t Station No.

1 and compared with corresponding maximum and minimum air temperatures recorded at the meteorological station at

Ottawa's International Airport, located at Uplands about 8 miles

(13 km) southwest of the Mer Bleue. Monthly and annual

average temperatures were calculated and compared.

Daily maximum and minimum ground temperatures measured at the various depths at the Mer Bleue were extracted and monthly averages calculated. These monthly averages were then

compared with corresponding monthly averages for soil temperatures measured at the Central Experimental F a r m (C.E.F.) located about 10 miles ( 1 6 k m ) west of the M e r Bleue.

COMPARISON OF AIR TEMPERATURE

T h e conlparison of annual average air temperature a t Uplands and Station No. 1 for the period of record is as shown in Table

I. T h e differences in annual average daily rnininzunz air tempera-

ture ranged from -5.0°F in 1964-65 to - 2 S ° F in 1965-66.

T h e differences between annual average daily maxi~num air

temperatures were usually less than one degree F and, during

the last two years, the average maxinlunl at Uplands was slightly lower than that at Mer Bleue. Annual average temperatures are

TABLE I

Comparison of Annual Average Air Temperature at Uplands and Mer Bleue

Annual Average

Temperature Difference

Mer

Period Bleue Uplands

1 September 1964 - 31 August 1965 37.9"F 41 . I "F -3.2"F

1 September 1965

-

31 August 1966 41 . 6 4 2 . 4 - 0 . 8

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thus lower over Mer Bleue than at Uplands, primarily because of lower niininium air temperatures.

T o affect annual average air temperature significantly the daily niinimum teniperature niust be lower quite frequently durlng the year. F o r example, from 1 September 1964 to 31 August 1965, the year with the largest annual temperature difference, dally niinimum air temperatures were colder by 8'F o r more on 100 days, compared with 58 days for the year 1 September 1965 to 31 August 1966, and to 79 days for the succeeding year.

T o show which weather conditions result in significant minimum air temperature differences, an analysis was made of available data for the 3-year period. First, the daily minimum alr teniperature differences between M e r Bleue and Uplands were d~vided into two groups (based o n weather records at

Uplands) :

1 ) sky conditions essentially clear;

2 ) sky cond~tions essentially cloudy.

The cases when the sky was essentially clear were subdivided into two classes:

a ) average wind speed during night-time, from 2000 h r t o 0600 hr, under 1 0 mph (4.5 m/sec);or

b ) greater than 1 0 m p h (4.5 ni/sec).

Histograms were constructed from these data showing the occurrence in per cent of daily mininluni air temperature differences for these different groups (Figure 2 ) .

T h e results show that larger minimum air temperature differences occur more frequently under clear skies than under cloudy conditions and when the average wind speed is under 1 0 mph (4.5 m/sec) than when it is over 1 0 mph (4.5 m / s e c ) . When night-time wind speed averages under 1 0 m p h (4.5 ni/sec) and the sky is essentially clear, the daily niinimum air teniperature will usually be lower over Mer Bleue than at Uplands.

There are exceptions to this, however. After several days of clear, calm weather, large n~ininiunl teniperature differences will persist for a day even after the sky becon~es partly cloudy or even overcast. Frequently when invers~on conditions are first developing and the sky is clear and wind speed light, air temperature differences are not great. T o explain all cases conipletely it would be necessary to take into consideration all factors, such

C L O U D Y n

.

2 1 0 ( n o . o f c a s e s ) C L E A R W i n d S p e e d > l O m p h 20

1

W i n d S p e e d < l O m p h n

-

1 3 8 0 e P C ,

.

9 h y ; - b C L A S S I N T E R V A L 3 ° F

Figure 2. Per cent occurrence of daily minimum air temperature difference ( M e r Bleue - Uplands) for different weather conditions 1964-67.

as wind direction, type of air masses and duration of clear sky conditions.

I n general, the weather conditions that cause large differences in niinimum temperatures are those associated with the drainage of cold air into low-lying areas, well described in the literature, e.g., Geiger ( 1 9 6 5 ) . T h e drainage of cold air into this low-lying bog (Figure 1 ) is, without doubt, a major factor in the large mininiuni air temperature differences observed.

EFFECT OF BOG SURFACE CONDITIONS ON AIR TEMPERATURE DIFFERENCES

T h e question arose whether the condition of the surface would affect minimum air teniperature recorded over the bog. It is generally assumed that surface layers of peat o r Splzagn~lin under similar atniospheric conditions will cool more rapidly than surrounding mineral soil because of lower thermal conductivity. The relatively smaller amount of heat stored in the moss or peat cannot be returned as readily to the air during night-time cooling. As a first step in the investigation of this question, monthly average differences in daily maximum a n d niinimum air temperature, the general surface condition of the bog and per cent of normal precipitation at Uplands have been plotted (Figure 3 ) . L T e m p . D l f l . ( M o n t h l y A v e r a g e - D a l l y M a x . ) . + A

-

M e r B l e u e - U p l a n d s

-

CI - T e m p . D i l l . ( M o n t h l y A v e r a g e

-

D a l l y M l n . 1,

-

0 - -

-

- c _{S u r f a c e C o n d i t i o n}

UFLllHG MELTING MELTING

0 m ' ~ o n t h l ~ P r e c l p l t a t l o n 1% o f N o r m a l 1 LL 0. I I I I I l I I I I I 1 I ~ I l l f ~ f 1 1IIIII I I I I I I I 1 I l S O N O J F Y P Y J J b S O H O J F M P L I J J P 1964-65 1965-66 1966-67

Figure 3. Monthly average air temperature differences ( M e t Bleue

-

Uplands) and general surface conditions and precipitation.

The bog was classified as "dry" when the level of the water table was several inches below the ground surface. It was classified as "wet" when the water table was close to the surface, resulting in the ponding of water in the irregular hollows, characteristic of the surface of the bog. When the air temperature was below freezing and the surface snow-covered, the surface was classified as "dry snow." T h e bog surface was classified as "nlelting" during the active snow-melt period and during the period when the bog was still thawing.

Figure 3 shows also that during dry snow periods temperature differences were much greater than during the melting period. During the melting period minimum air temperature differences were not pronounced even when conditions should result in low night-time air temperatures (clear sky, light winds, etc.). During the 3-year period daily niinimum air temperatures for March and April were colder by 8OF o r m o r e a t M e r Bleue

only 11 tinies compared with 5 2 times during the months of

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T h e next step in the analysis was to subdivide all n ~ i n i m u m air temperature differences that occurred under clear skies and wind speed below 1 0 mph (4.5 m/sec) into three classes: 1 ) dry snow-dry Sphagnunz (combined) ;

2 ) melting snow o r thawing S p h a g n u m ; 3 ) wet Splzagnutn.

Histograms were constructed from these data (Figure 4 ) . Large air temperature differences occur most frequently during periods of dry snow and dry S p h a g n ~ l n z . Comparing dry snow-dry Sphagtzlit7z with wet S p h a g n ~ l n z , the wet S p h a g n ~ ~ t n has fewer temperature differences greater than -13OF and a larger number of cases where minimum temperatures at Mer Bleue were higher than at Uplands. During the melting period the frequency of occurrence of large air temperature differences is considerably less than in the other periods, with n o cases exceeding -13OF.

Dry Snow Dry Sphagnum n

.

47 W

::"

fi

1

M e l t i n g Snow M e l t i n g Sphagnum n

-

4 1 0 r + p p c f : y n CLASS INTERVAL 3 O F

Figure 4. Per cent occurrence of daily minimum air temperature difference (Mer Bleue - Uplands) for different surface conditions when wind

speed

<

1 0 mph clear sky 1964-67. EFFECT OF TOPOGRAPHY ON AIR TEMPERATURE DIFFERENCES

F r o m the foregoing analysis it was concluded that the major factor causing low air temperature over Mer Bleue was air drainage and that the condition of the bog has a n effect but is of secondary importance.

T o show the effect of topography o n the air temperature pattern across Mer Bleue, several thermographs were installed across a section of the bog (A1*, Figure 1 ) during the fall of 1967. Station No. 3 was located o n a sand ridge about 2 0 ft ( 6 m ) above Station No. 1. Station Nos. 4 and 5 were located o n a cultivated slope (tilled loam) across the bog, about 1 0 and 5 f t ( 3 to 1.5 nl) higher in elevation than station No. 1.

Night-time air temperatures were much lower for several days at Mer Bleue in early September than at Uplands. During the first night or two, minimum air temperatures were 4 to S ° F higher o n the sand and loam ridge than over the bog. Towards the end of the period, however, night-time temperatures were almost as low over the higher mineral soil as over the low-lying peat bog. Figure 5 shows the night-time temperature recorded

3 0 1 2 " " 1 6 " " 20"" 24"" 4 " " So" 1 2 " " ( T i m e ) Sept 1 3 Sept 14, 1967 o...~

.----.

U p l a n d s

-

G r a s s M e r B l e u e

-

S p h a g n u m ( S t a t i o n s 1 & 2 0--0 S t a t i o n 3

-

G r a s s K n o l l S t a t i o n 4

-

S o u t h f a c i n g T i l l e d - L o a m Slope

Figure 5. Comparison of air temperature at different sites.

on 13 to 1 4 September for the stations at Mer Bleue and Uplands after several days of low night-time air temperatures.

Some of the studies on low night-time air temperature over peat bogs reported in the literature may have underestimated the importance of air drainage. F o r example, Rigg (1947) i n explaining the occurrence of low air temperatures in a Sphag- n u m bog o n the Pacific Coast states

".

. .

that air drainage is a factor but not a critical one." Yet he describes the bog where low air temperatures were recorded thus: "The swampy area around the bog is entirely surrounded by low, gently sloping hills" and "There is very little air drainage from the bog and the swampy area which surrounds it because trees and brush form a barrier a t the origin of the stream." His description of the site would indicate that air drainage could have been the major factor causing the low air temperatures recorded.

Longley and Louis-Byne (1967) in making some air temperature traverses across frost hollows in Central Alberta noted a consistent drop of about 3OF over a peatland area. They considered that the change in elevation was so slight that the cooler temperatures must have been due to the peat. Geiger (1965) notes that only a slight difference in elevation is needed for cold air to collect in low areas. Duffy and Fraser (1963), in a study of local frost occurrences in eastern Ontario woodlands, noted examples where only a slight inclination caused a frost hollow. Weather conditions and topography must be such that the draining of cold air occurs quite frequently in order to affect monthly and annual average air temperature values t o a con- siderable extent. From 1 September 1964 to 31 August 1965 minimum air temperatures were S ° F colder at Mer Bleue than at Uplands on 100 days of the year, yet annual average air temperature was only -3.2OF colder. Monthly average air temperatures were several degrees colder a t Mer Bleue than a t Uplands only during dry periods when weather conditions were favourable for cold air t o settle quite frequently and last f o r several days during a month.

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COMPARISON OF GROUND TEMPERATURES AT

MER BLEUE AND AT THE C.E.F., O'ITAWA

Daily maximum and minimum ground temperature measured at the various depths in Mer Bleue were extracted from the continuous record and monthly averages calculated. The monthly averages for two depths were plotted and compared with corresponding monthly averages for soil temperatures measured at the Central Experimental Farm (C.E.F.), Ottawa.

Soil temperature records at C.E.F. are part of a soil temperature program carried out at several stations across Canada by the Meteorological Branch, Department of Transport in co- operation with the Canada Department of Agriculture (Potter, 1962). Thermistors are used as the temperature sensing elements. The soil is an imperfectly drained silt-loam with a 2-in.-high grass cover. C.E.F. monthly and annual average soil temperatures were compared with corresponding average soil temperatures observed in mineral soil at another site in the Ottawa area (Gold, 1967). As the average agreed within 1 ° F for 1961 and 1962, the years when observations were available at the other site, it is assumed that values for monthly and average annual soil temperature at C.E.F. are representative for mineral soils in the Ottawa area.

Temperatures measured at 1

-

2.5 cm depth at Mer Bleue from 1 September 1965 to 1 November 1966 are compared with temperatures measured at 1 cm depth at the C.E.F. in Figure 6 ( a ) . From 1 November 1966 to 31 August 1967 the monthly average temperatures are compared for a depth of 10 cm at both stations. Monthly average temperatures at depth 100 cm are compared in Figure 6(b) for September 1965 to September 1967.

Monthly average temperatures near the surface at C.E.F. are always several degrees warmer than at Mer Bleue. The length of the period during which temperatures near the surface are below 32OF is several weeks shorter than at Mer Bleue.

The annual average temperature at 100 cm at Mer Bleue over the 2-year period is 41.5OF (5.2OC) compared with 45.8OF (7.7OC) at the C.E.F. The annual amplitude at the 100-cm depth is much less at Mer Bleue than at the C.E.F. There is also a pronounced "lag" to the peat soil reflecting its different thermal properties. This ground temperature record is too short to warrant detailed analysis, i.e., the calculation of

G r o u n d T e m p e r a t u r e N e a r S u r f a c e 1 - 2 . 5 c m s 1 0 c m s /"\ G r o u n d T e m p e r a t u r e a t 1 0 0 c m s -I

pFO*

0 3 0 A v e r a g e = 4 5 . 8

.

-.-

A v e r

'

_I age = 4 1 . 5

Figure 6. Comparison of monthly average ground temperature Mer Bleue - C.E.F. (Ottawa) 1965-67.

soil properties such as thermal diffusivity by Fourier analysis. Unusual snow cover depth and duration, or unusually wet or dry conditions for such a short period, could give a distorted comparison of soil "climate."

DISCUSSION OF GROUND TEMPERATURE

DIFFERENCES

The three major factors determining the annual pattern of soil temperature are:

1) depth and density of snow cover; 2 ) thermal properties of the soil;

3) heat exchange at the surface by radiation, convection, and evaporation.

By understanding the relationship between these factors one can understand why surface ground temperatures of a peat bog should be colder than those at mineral soil sites in the same climatic region.

Snow depth and density are major factors affecting heat exchange into or out -of the ground. Snow depth and density at Mer Bleue are about the same as those at mineral soil sites in the area that have the same exposure to sun and wind. There is a tendency for the irregular surface of the bog to catch snow and prevent drifting but this is a factor only during the early winter. Snow depth is thus not a major factor causing lower soil temperatures over this marsh.

The thermal properties that determine heat transfer in soil are volumetric specific heat (C,), latent heat of fusion, thermal conductivity ( K ) , the ratio K/C, defined as thermal diffusivity ( a ) , and the product C,V;, termed the conductive capacity. Table I1 lists a range of values for thermal properties of peat, soil, ice, and snow obtained from the somewhat limited information available in the literature (Geiger, 1965; Williams, 1966; Lettau, 1966). Low values for peat and soil are for low moisture contents; high values are for high moisture contents.

The high latent heat of fusion of peat is much higher than that of mineral soils because of its high moisture content. Con- sequently, for the same imposed surface heat exchange, peat soils should freeze at a slower rate and thaw at a slower rate than mineral soils.

The conductive capacity term, c , v ~ , sometimes called the thermal contact coefficient (Carson, 1961), has a critical role in determining the amount of heat that will be transferred into the earth's surface from the atmosphere. For example, the low conductive capacity of Sphagnum means that heat will be trans- ferred into and out of the surface at a slower rate than for mineral soils for the same imposed surface heat exchange. At Mer Bleue the growing Sphagnum is shallow (about 4

-

6 in.; 10

-

15 cm) and the effect of reduced heat transfer would not be as significant as in bogs where Sphagnum makes up a large percentage of the layer that is subject to seasonal freezing.

The third factor affecting soil temperatures that should be considered is heat exchange at the surface during the different seasons of the year. Considering first the active snow melt season, the snow cover should disappear from mineral and bog sites with the same exposure at about the same time, providing that snow depths and density and the energy available for snow melting and albedo are comparable.

In contrast with mineral soil sites, where the melt water will normally run off or be absorbed in the soil, most of the snow melt water remains on a low-lying bog and floods the surface for several weeks. In the mineral soil, surface heat available from radiation and convection warms the soil layers and, depending on the degree of saturation, causes evaporation of moisture from the soil. Over the flooded bog in contrast, most of the available surface heat is probably used for evaporation, and the warming-up process is slow compared with that of a

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TABLE I I

Approximate Value of Thermal Properties of Different Natural Materials

Thermal C,, volumetric K, thermal a = K/C, thermal C , d c conductive

Property heat capacity conductivity diff usivity capacity

x 1000 . x 1000 cal/(cu cm) (OC) Unfrozen Peat 0 . 6 - 1 .O Frozen Peat 0 . 3 - 0 . 6 Wet Clay 0 . 3 - 0 . 4 Wet Sand 0 . 2 - 0 . 6 Ice 0 . 5 Snow (density 0 . 2 gmlcc) 0 . 1 Sphagnum 0 . 2 estimated

millicals/(cm) (sec) ("C) sq cm/(sec)

0 . 7 - 1 . 5 1 - 1 . 3 2 - 5 8 - 9 2 - 5 6 - 1 6 2 - 6 4 - 1 0 5 - 7 11 - 1 5 0 . 2 - 0 . 3 2 - 4 - 0 . 7 cal/(sq cm)( "C) (sect ) 1 9 - 3 9 26

-

5 3 23 - 50 1 2 - 6 0 52 - 61 2 - 6 0 . 5 Reference (6) (6) (1 (1 (1) (1 ) (7)

mineral soil. This factor, combined with the effect of thermal Monthly and annual average surface temperatures are

properties in delaying thawing, explains why the thawing of several degrees colder a t Mer Bleue than over mineral soil in

a bog is prolonged, often several weeks longer than mineral soil. the same general area. A major reason causing the colder soil

A t M e r Bleue ice lenses were observed in late June in 1965. Ice temperatures is the use of energy for evaporation rather than

lenses have been observed in July in N e w Brunswick (Ganong, f o r warming the soil.

1897).

This slow warming of the bog might also explain why the

_{ACKNOWLEDGMENTS}

difference in minimum air temperatures between M e r Bleue and

Uplands was seldom greater than

soF

(4.50C) during the melt T h e assistance of several colleagues in taking observations and

period. The temperature of the saturated surface layers, resting for discussion is gratefully This paper is

on frozen ice lenses, would tend to remain close to 320F ( O O C ) a contribution from the Division of Building Research, National

until all the ice was melted. ~f the air was cooled at night so Research Council of Canada, and is published with the ap-

that the dew-point temperature went much below 32OF, there proval of the Director of the Division.

would be condensation. T h e heat released from condensation would tend to keep surface temperatures a t higher levels than otherwise would be the case. Condensation at other times of the year probably explains why differences in minimum air temperatures between M e r Bleue and Uplands never exceeded about l g ° F (10.5"C).

After the ice in the bog is completely melted, the level of the water table will remain high if there is above-average rainfall; otherwise it will gradually drop. Much of the energy available from solar radiation and other sources will be used in evaporation, the amount depending on the rate at which water can be supplied t o the evaporating surface and this depends o n the depth of the water table. Evaporation measurements made in 1964 a t M e r Bleue indicated that when the water table is 1 2 in. below the surface the rate of surface evaporation is about three-quarters of what it is when the water table is at the surface.

T h e foregoing discussion indicates that the soil temperature regime of bogs will be affected by changes in water table level. During dry years less heat will be used in evaporation and more will be available f o r warming the soil. This effect will be somewhat offset by the increase in thermal insulation of the surface layers due t o decreased moisture content.

CONCLUSIONS

T h e drainage of cold air into the low-lying M e r Bleue peat bog is the major factor causing minimum air temperatures to be lower over this bog than at nearby weather stations. Weather conditions must be such that the drainage of cold air occurs quite frequently throughout a month o r year in order t o affect significantly monthly or annual average air temperatures.

T h e condition of the bog, i.e., whether it is wet o r dry, thawing o r snow-covered, appears to have some effect o n mini- m u m air temperatures over the bog, but this is not as important a factor as air drainage. A s peat bogs are usually located in low- lying areas, some of the references in the literature may have underestimated the importance of air drainage in causing low air temperatures over peat bogs.

REFERENCES

Carson, J. E. 1961. Soil temperature and weather conditions. Argonne National Laboratory, ANL-6470, Meteorology.

Duffy, P. J. B., and J. W. Fraser. 1963. Local frost occurrences in eastern Ontario woodlands. Can. Dept. of Forestry, Pub. No. 1029. Ganong, W. F. 1897. Upon raised peat bogs in the province of New

Brunswick. Trans., Roy. Soc. Canada, Sect. IV, p. 131-163.

Geiger, R. 1965. The climate near the ground. Cambridge, Mass., Harvard University Press.

Gold, L. W. 1967. Influence of surface conditions on ground temperature. Can. J. of Earth Sciences, 4:199.

Lettau, B. 1966. The use of Sub-Arctic bogs as natural climatic indicators. Univ. of Wisc., Dept. of Met., Tech. Rep. No. 23. Longley, R. W., and M. Louis-Byne. 1967. Frost hollows in west

central Alberta. Canada Dept. of Transport, Meteorological Branch, Circ. 4532.

Potter, J. G. 1962. Soil temperature records at eight localities in Canada 1959-1960. Canada Dept. of Agriculture, Research Branch, in co-operation with Meteorological Branch, Dept. of Transport.

Rigg, C . B. 1947. Soil and air temperature in a Sphag~zurn bog of the Pacific coast of North America. Amer. J. Bot., 34:452-469. Williams, G. P. 1966. Soil freezing and thawing at the Muck Experi-

ment Station, Bradford, Ontario. School of Agricultural Engineer- ing, University of Guelph, Eng. Tech. Pub. No. 14.

DISCUSSION

Dr. J. A. Taylor (U.K.)

There is need to establish meteorological stations which enable a triplication of observations per type-site to be obtained. This is required because of the extensive variations in the readings obtained from any individual instrument o r for any individual station. I t is necessary t o differentiate quite clearly i n the siting of stations between a ) the effects of air drainage and b ) the effects of the organic site as compared with the mineral sites, regardless of altitude.