The climate of cities

T H E C L I M A T E

a thesis by larz torquil anderson

submitted to the department of city planning

M I,T

author . department . institute ...

562 Newbury Street Boston 15, Mass. 15 January, 1948

Professor Frederick J. Adams Department of City Planning

Massachusetts Institute of Technology Cambridge, Mass.

Dear Professor Adams;

Presented here is my thesis entitled THE GLIMATE OF CI TIES,

submitted as a requisite for the degree of Master in City Planning.

Very truly yours, A

F 0 R EW 0 R D

The general purpose of any thesis is, or should be,

to conduct an orderly and logical investigation into some

subject beyond the realm of general knowledge. The results

of the investigation cannot be foretold; if they could, the

thesis subject would not fit the specification that it be

*original research. Therefore, no thesis should be judged

by the value of the new material uncovered. As in mining gold, one cannot hit pay dirt every time; yet mining and

thesis writing go on; both are gambles.

This thesis was also a gamble. From the surface

indi-cations there seemed a likelyhood that valuable findings

could be uncovered by investigation. Planners, in their

literature, have indicated a desire to know more of how

climate affects their cities, and what they can do about

it. Climatologists, in their literature, have been

think-ing of how they can apply climatology to the design of new cities; some of them have been told of City Planning. In

fact, it was a portion of a speech to a graduating class

of Meteorologists from University of Chicago that indicated

the horizons toward which City Planners and Climatologists

should work. So that the reader can also share in the hope

"A new profession has slowly begun to form during the last ten years or so. It is going to have a tremendous

influence after the war. It is the profession of City Planners.

Since it is new, and it has not yet defined its contents or

boundaries, nor has it clearly formulated its prerequisites of

knowledge, it is recruited from many fields. Public

adminis-trators, writers, sociologists, traffic experts, architects,

and medical men make up the bulk of the membership. Meteorology

has been little heard from as yet.

"I should like to see a meteorologist offer a city plan

which would create a city of calm in a windy location. While

complete achievement is impossible, it is equally certain that

tremendous improvement is perfectly feasible. I should like

to see the meteorologist take a hand in determining the use of

heat generating or light reflecting facades in city building. I should like to see the arbitrary rules about building forms

and arrangements, designed to avoid shadow, give room to intel-ligent and deliberate use of light and shadow by the

microclima-tologist and architects working in conjunction, as painters use

their colors on canvas. We know that the microclimate of cities

is considerably different from the microclimate of surrounding nature. And by this very knowledge it is proven to us that a limited but quite considerable amount of outdoor

air-condition-ing, if you will, is perfectly feasible by intelligent applica-tion of meteorological principles to city planning."*

In doing research on this thesis the author came in

contact with climatologists and meteorologists. Without

exception they were all kind, courteous, and helpful, and

showed what seems to be a profession-wide eagerness to

apply their sciences in whatever way they can to

improv-ing the life of man.

Especial thanks for cooperation are due to Dr. Charles

F. Brooks of Harvard, and to the staff of the Blue Hill

Observatory in Milton, Massachusetts. It should be noted,

however, that no person other than the author should be

I N T R 0 D U C T IO N

The city is a special area of Climatology, and has a climate unlike that of surrounding nature. The climate

of the city, however, is dependent upon all the forces

of nature that are found elsewhere as well. Therefore,

in order to understand the workings of the climate of the

city, we must understand the workings of climate in general.

This is no easy task. By referring to Figure One, the

reader will see that there is much material to go through

before arriving at the consideration of the city. All the

items listed in Figure One are related to the climate of

cities, as indicated on the Organization Chart of the

Influences on Climate.

Part One of this study will be a brief discussion of

the major influences of all these factors. Part Two will

be a discussion of their effects within the city, and what

can be done about it.

In the glossary will be found definitions of technical

terms. The glossary is in the rear of the book; just

follow-ing the glossary will be found a set of maps, published by

the U. S. Weather Bureau, which give the climates to be found

in various parts of this country. They are included for

refer-ence to matters in the text, and also for general interest to

the readers who wish to see how the climate of their region

I'TCLI2 1

I N F L U E N C E S O N C L I M A T E

MAIN CAUSE

9 1

SOLAR RADIATION

INTENSITY AND SPECTRAL COMPOSITION VARIATIONS ON SUN

DISTANCE SUN-EARTH ECCENTRICITY OF CRBIT

REVOLUTION AROUND SUN INCLIN&TION OF EARTH'S AXIS LATITUDS

EARTH'S AKIAL ROTATION

a -INTENSITY OF RADIATION AT BOUNDARY OF ATMCSPHERE

SEASOXAL ai

V AR I.ATION

DIURNAL

VAhIATION

STATIC PROPERTIES OF ATMOSPHERE

REFLECTION ABSORBTION SCATTERING

HIGHER ATMOSPHERE

OZONE NITROGEN PENTOXIDE

L4WER ATMOSPHERE

WATER VAPOR CARBON DIGKIDb SUSPENSIONS

CLOUDS-ATMOSPHERIC BACK-RADIATION

DYNAMIC PROPERTIES OF ATMOSPHERE

GENERAL CIRCULATION SEMI-PERM*NENT HIGHS AND LOWS

PLANETARY WINDS AIR MASSES FRONTS

CCNDENSATION

RADIATION PROPERTIES OF SURFACE ABSORBTION RKFLECTION RADIATION LOSS

ALBEDO OF VARIOUS SURFACES

EVAPORATION

DIFFERENTIATIOV OF SUhFaE

ICEFIELDS CONTINENTS OCAANS

CURRENTS

LAND COVER LAND FORMS HUMAN ACTIVITY

-DESERT CONCAVE QUVATION

-PLAIN -CONVEK

-FOREST LAKES - ORIES

-SIAMP I C ASTRONOMICAL pOsITION ATMOSPHERE I EARTH'S SURFACE CLIMATS MICRO ;LIK&is ? .JLIN RESUsI

C H A P T E R I.

THE SOLAR SYSTEM:

In our consideration of climate on the earth, we find

immediately that by far the most important single element

in climate is the sun. It is the sun that furnishes the

earth with enormous amounts of energy, used for heating

the surface of the earth, for heating and circulating the

atmosphere, and for giving us light. All other planets,

even the moon, are of negligible influence on climate.

THE SUN:

As for the body of the sun itself, it has a diameter

of 864,000 miles (as compared with the earth's 8000) and

has a mass 332,000 times as great as the earth. The

densi-ty of the sun is about one-fourth that of our planet. The

item of greatest interest to us, however, is that the sun

radiates energy, of which the earth receives less than a

millionth part. As far as is known, the surface of the sun

is a mass of flaming gases, with a temperature of

approxi-mately 6,0000 Absolute. The sun keeps supplying this energy

at what is thought to be a constant rate, except for slight

variations from time to time which may possibly be caused

by sunspots. Sun spots are still a thing of mystery to us; we have been observing them and recording them since 1749,

but so far we are not even positive that there is any

cor-relation between their frequency and the variation of the

Footnote 1 - see Landsberg in PHYSICAL CLIMATOLOGY, page 86.

For complete bibliography, see section at the

-page

two-energy received by the earth, but there is good reason to suspect that this correl.tion does exist.

SPECTRAL COMPOSITION:

The sun, with a temperature of about 6,0000 A, sends out radiant energy to space. This energy is distributed

in various wavelengths, with the peak amount of energy being sent out with a wavelength of 0.5 microns. Ninety-nine

per cent of the energy is within the limits of Ol5Aand

4.OM. Roughly one-half of this radiation lies within the visible spectrum (between 0.38 and 0.7 7 _{q ); the remainder}

lies in the infra-red and the ultra-violet ranges, invisible to man, but very real nevertheless. All of this is shown

graphically in figure 2.

OUR RELATIONSHIP WITH THE SUN:

It is not within the scope of this paper to explain just how the sun is related to the rest of the universe, or where

it came from. So far as we are concerned, it always has been, and always will be, to borrow an often hackneyed

phrase. At any rate, we do know that the earth moves in an elliptical orbit about the sun, making one revolution every year, with a mean diameter of about 93,000,000 miles (see figure 3). On January the first we are 91,000,000 miles from the sun; but on July the first we are 94,500,000 miles away. This change of distance is of relatively minor im-portance compared with other changes, as we shall see. It will be noticed in illustration 3 that the earth's axis is

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three-not vertical. The fact of the matter is that the axis of

the earth is inclined 230 30' away from the vertical. It

is this tilt that causes our seasons. On June 21 of every

year, the earth is tilted so that the sun's rays are di-rectly above latitude 230 30' in the Northern Hemisphere.

As the year passes, and the earth moves on around the sun,

by September 23 the sun shines directly on the equator.

By December, the sun is perpendicular to 230 30' south lati-tude; in March it is again over the equator. Figure 4

il-lustrates the effects of this change of this sun-earth

re-lationship. It will be noted that on June 21 the sun shines

mainly on the northern hemisphere, with less sunshine in

the southern half of the world. Notice that the land below

670S never gets any sun at all on June the 21, no matter

what time of day it is, and that land north of 670N has sun

24 hours a day.

It is getting ahead of our story here, but the reader may have noticed that the sun's rays hit the surface of the earth at different angles in different places. For instance,

on September 23 the sun hits the land of the equator

per-pendicularly, but just grazes the surface at the North and

South Poles. If we look at these sections of the surface of

the earta, as shown in figure 4, we see that the INTENSITY

of the sun's rays is far greater where the plane is at right angles to the rays of the sun. Therefore, the sun

means more in terms of effectiveness to people living near

FIGURE 4

THE EARTH AID TIE. SUJ'S RAYS

4~~1

-page four-EFFECT OF ATMOSPHERE:

Because there is a layer of atmosphere about the earth, and atmosphere absorbs the sun's rays and diminishes their effects, this geographical distribution of the effectiveness is accentuated. Again, looking at figure 4, we can see that the sun has to penetrate a much thicker layer of atmosphere at the poles than it does at the equator. If the thickness of the atmosphere is given a relative thickness of 1 when the sun is at 900 to the surface, the thickness of the at-mosphere is 5.60 times as great when the sun's rays are at

an angle of 100 to the surface. The following table shows how this effect varies with the difference of the sun's

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five-TABLE I

RELATIVE THICKNESS OF ATMOSPHERIC LAYERS FOR VARIOUS ANGLES OF SOLAR ALTITUDES

Altitude of Sun Thickness of Atmosphere

90 80 70 60 50 40 301. 25 20 15 10 8 6 4 Source: Landsberg in PHYSICAL CLIMATOLOGY, page 89 1.00 1.02 1.06 1.15 1.30 1.55 2.00 2.36 2.90 3.82 5.60 6.88 8.90 12.44

-page si

x-RESULT: SEASONAL VARIATIONS:

These foregoing factors; the inclination of the earth's

axis, and the motion of the earth around the sun, as

accen-tuated by the effects of the layer of atmosphere on the

surface of the earth, all go to make seasonal changes in

ou climates, for these factors influence the amount of sun

that rieabhes the earth; it is this amount of sun that is the

greatest factor in determining our climates.

ROTATION OF THE EARTI:

As the earth moves in its yearly tour around the sun,

it also turns around its own axis once a day. This has the

effect of exposing one side of the earth to the sun at

one time of day, and, since the earth moves at a constant

speed, moves this one-time sunlit area around to the shaded

side of the earth, where there is no sun, and it is therefore

night. This is the cause of what is known as DIURNAL, or

daily, changes. When one side ofihe earth is exposed to

the sun, it gains heat energy from the incoming radiation.

When the earth has turned so that this one spot has moved

into the shaded area, it has a chance to radiate away some

of this heat, and cool off. For this reason, we experience

the earth getting warmer under us as we move into the sun

and we notice the earth cooling at night. These diur-nal

changes are of great importance to human comfort. It is the

pattern of these diurnal changes plus the seasonal changes

-page seven-INSOLATION PATTERNS:

In figure 5 we see how much sun the surface of the earth would get if there were no atmosphere. Notice that land at the equator would get from 800 to 1000 cal/cm2_{/day if there}

were no air, and that land at the equator would get sun up to the intensity of 1100 cal/cm2/day. Now look at figure 6. Here is what happens whenaatmosphere is on the surface of the earth. The range of sun at the equator is from 480 to 550 cal/cm2_{/day, while the polar regions get from 0 to 400}

cal/cm2/day. This is much closer to the actual picture, although the computations used in figure 6 neglected the effect of cloudiness

frem the ote eufc garth,

By comparing figures 5 and 6 we can see the drastic effect that the atmosphere has on the amount of sunlight that reaches the earth. The effect is far greater at the polar regions than it is in the equatorial belt. This is what we would expect now that we are familiar with Table I. SKY RADIATI ON:

Fortunately for those that live in areas where there is relatively little sunshine, there is such a thing as s

radiation. This is the radiation of heat by the atmosphere. It so happens that the atmosphere, as we will be told in the next chapter, absorbs heat, and like all bodies containing heat, radiates it away; some of it towards space and some

of it towards the earth. Figure 7 shows just how important this sky radiation is in comparison to the direct solar

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eight-radiation. Note that in some parts of the world, such as that between the latitudes of 600 to 900 in September, that the sky radiation supplies more heat than does tie direct sun.

Even for those dCus that do not live in the extreme

latitudes, sky radiation is of importance. It is due to the radiation from the atmosphere (or tsky') that the tempera-ture of the surface of the earth does not drop to a very low level as soon as the sun goes down. The atmosphere stores up heat in the daytime and radiates it toward the earth (and to space) at night. We will go further into

this complicated subject later when we audit the heat balance of the earth.

-page nine-THE TRUTH ABOUT RADIATION:

Every body does it. Yes, every body that contains heat radiates energy, and so far, we have not been able to find

a body that contains no heat.

Here, however, is a point that may be new to some people; the energy radiated by bodies is different if the tempera-ture of the bodies is different. We can quote Wien's law of radiation here - "The maximal value of wavelength is inversely proportional to the absolute temperature of the body."

That is to say, that the higher the absolute temperature of the body, the shorter the wavelength of the energy it emits. With this in mind, let us think about the radiant heat that we run into in our daily lives. The sun has a temperature of about 60000 Abs. We saw in figure 2 that the peak, or the greatest amount of energy enitted by the sun, comes at a wavelength of about 0.5p, with practically all of the

energy given off between the wavelengths of 0.15 and 4.OA. The earth absorbs the energy from the sun. We know that the earth would get much warmer if this energy were just incoming and the earth kept it all. However, the earth is not getting appreciably warmer from year to year, so we realize that the earth must get rid of the heat energy of

the sun that it absorbs. It does this by radiating heat

out to space again. This goes on day and night, and is known as terrestial radiation.

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ten-TERRESTIAL RADIATION:

Terrestial radiation behaves in the same way that we have said all radiation behaves. Since we know that the

earth, which does the radiating, has a relatively low tem-perature (compared with that of the sun), we can expect a relatively long wavelength for the peak of energy emission.

This is exactly what we do find. In figure 8 we see the

amount of energy given off by bodies at temperatures of 60000A

and 3000A, which is about the temperature of the sun and

the earth respectively. (In figure 9 these curves are plotted on semi-log paper, and so become clearer)

Note that these energy curves are for the qualitative dis-tribution of energy; not the quantitative. Actually, the sun gives off very intense radiation, and so would have a very high peak, while the earth, emitting less intensely, would have a lower peak on the graph. Figure 10 shows the effect on intensity and peak wavelength of various temperatures

near that of the earth. It can be seen that the warmer bodies give off more radiation and at shorter wavelengths. This

is very fortunate for us; It means that in wintertime, when the earth is cool enough anyway, that the heat lost by the earth is relatively little compared to that lost in the summer. We see this when we note that the heat lost in

summer, when the temperature of the surface is 400, would be proportional to the area under the curve for 4000 in

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eleven-about 000, the heat lost is much less; it is proportional to the area under the 000 curve. It is because of this automatic control that the earth's tenperature is as even

as it is. Without this law in effect (the Stefan-Boltzmann law) we would be plagued with intolerable changes in tem-perature.

C H A P T E R II.

THE ATMOSPHERE:

With the advent of the air age, the term 'stratosphere'

is heard quite often; we all know it to be the word used

to denote the upper air. Not quite so well known, however,

is when and where the stratosphere starts, and where it stops.

Unhappily, scientists don't know very much about this either.

It has been very difficult to gather data on the upper air,

because no one and nothing has been up there. Even in the

lower air, we do not have all the date we can use. The

atmosphere is so large in extent that it is a tremendous

task to gather and analyse samples from all levels and from

all parts of the world. We do know the following things

about the atmosphere, however.

By sending up small balloons with recording instruments attached to them, scientists have found that the atmosphere

is.divided into two parts, namely the troposphere, or lower

air, and the stratosphere, or upper air. The point of

separation between them is called the tropopause.

The main difference between the upper and the lower air

is that in the lower air the temperature decreases at a

constant rate with an increase in altitude. Above the

tro-popause, however, the air is nearly all the same tenperature

(about -80 C at the equator and about -40 C at the north pole). The tropopause is not a constant thing. It varies during the

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thirteen-seasons (is highest during the summer, and lowest in the winter), and it varies with latitude (it is about 8 or 9 km in the polar regions, and 11 km near the equator).

COMPOSITION OF THE LOWER AIR:

Near the surface of the earth the composition of the air (when dry) is as in Table II.

TABLE II.

COMPOSITION OF THE LOWER AIR

Element Vol. in Dry Air

Nitrogen 78.03 Oxygen 20.99 Argon 0.937 Carbon dioxide 0.03 Hydrogen 0.01 Source: Neon 0.0012 LANDSBERG Krypton 0.0010 p. 77. Helium 0.0004 Xenon 0.0001

The amount of these elements may vary from time to time. For instance, in closed rooms, the oxygen content may drop to 19.75%. The most variable component is carbon dioxide, which is usually about 0.029% in fresh country air, but is often as high as 0.038% in cities. Indoors, it may get as high as 0.100%, and, if there is a leaky furnace, up to

3.00%, in which case there is danger of asphyxiation.

Water vapor forms another very important part of the air. The anount of water that can be held by the air is proportional to the tenperature of the air (the warmer the

air, the greater the possible moisture content). In volume percentage, in the cold polar regions, the air contains only

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fourteen-about 0.5% moisture; in the moderate latitudes the content ranges from 0.5 to 1.5%; vhile in the equatorial belt the moisture may be as great as 3%. We all know from our daily

lives ("It isn't the heat; it's the humidity") how important water vapor in the air is to our comfort. It is of even

greater importance in its effect on the climate by control-ling the amount of heat that reaches us and leaves us by radiation. This will be discussed in the section on Heat Balance.

One more component of the atmosphere that has a great deal to do with urban climates is the amount of dust in the air. This is so important that a separate chapter will be devoted to it later on.

THE UPPER ATMOSPHERE:

When discussing the composition of the upper air, it is necessary to repeat tae warning that very little is known

about this subject. Sounding balloons have gone no higher than 35 km (22 miles), and it is only a few that have gone as far as 25 km (15 miles). The upper air is still one of the mysteries to be explored by man. Slowly we are gathering data on it, but in only a few sections of the world (North America and Europe). Perhaps with the advent of the rocket,

and radar tracing, we will be able to go further in our investigations.

We know that the stratosphere is of great importance in the movement of air-masses, and therefore in our weather, but data is lacking on just how the upper air does move. We

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fifteen-also know that the world air circulation is keyed up with the movement of both upper and lower air strata, but, again, we do not know enough of how the air moves to even tell the basic mechanism of the world air circulation.*

COMPOSITION OF THE UPPER AIR:

The following table on the distribution of gases in the air from Humphreys' PHYSICS OF THE AIR, was compiled from computed material (see Humphreys, pages 62-72) derived from equations based upon some very definite hypotheses. The table should not be used to illustrate any known facts about the atmosphere, and should not be quoted or reproduced

further without reference to the hypotheses on which the equations were based. The values in the table have been supported by observational data up to the height of 30 km, but no further.

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sixteen-TABLE III.

PERCENTAGE DISTRIBUTION OF GASES IN THE ATMOSPHERE

Height

in km. Argon

Water Nitrogen Vapor

Carbon

Oxy- Diox-

Hydro-gen ide gen_ Helium

Total pressure in mm. 20.69 0.03 0.01 20.95 0.03 0.01 20.99 0.03 0.01 19.66 0.02 0.02 18.10 0.01 0.04 12.61 . 0.67 7.69 ... 10.68 1.85 .. 64.70 0.11 ... 95.58 ... ... 98.74 ... - 99*15 0.02 0.23 1010 1.31 1.07 0.84 760 405 168 89.66 40.99 1.84 000935 0.0123 0.0067 0.0052 0.0040

Source: Humphreys, PHYSICS OF THE AIR, page 70 from computations, not observations

THE SIGNIFICANCE OF THE COMPOSITION OF THE AIR:

We cannot be certain of the full significance of the composition of the air until we have all the facts; until we know what the air does contain, and the true effects that the contents have. As previously stated, we lack this data; we can only make conjectures on the subject. Clima-tologists, meteorologists, and other scientists think that the heat balance of the earth is closely tied up with the effects of the atmosphere on radiation.

INCOMING SOLAR RADIATION:

In the stratosphere, it is thought that then is a relatively high percentage of ozone, and that this ozone absorbs about 5% of the incoming solar energy with vir-tually complete absorption below the wavelength 0.29pu.

0 5 11 15 20 40 60 80 100 120 140 0093 0.94 0.94 0.77 0.59 0022 0.03 .. 0 ... ... ... 0 77.14 77.89 78.02 79.52 81024 86.42 81.22 32.18 2.95 0.019 0.01 1.20 0.18 0.01 0.01 0.02 0.06 0.15 0.17 0.05 ... 0 ...

-page

seventeen-This absorption may explain the high tEnperatures of the

outer air.*

With the exception of ozone, the stnificant absorption

of solar radiation by the gaseous components of the

atmos-phere can be entirely attributed to water vapor. Oxygen

has several absorption bands, but these are so narrow that

they represent a very minute loss of solar energy. The

re-maining gases either do not absorb at all, or absorb

negli-gible quantities.**

The solid particles in the atmosphere do much to absorb

and reflect the incoming solar radiation, but are or such a variable nature that it is impossible to give any set rule

about their behavior.

The total incoming solar radiation may be diminished,

depleted, or reflected by any or all of the following

methods: (1) absorption by ozone in the upper atmosphere;

(2) scattering by dry air; (3) absorption scattering, and

diffuse reflection by suspended solid particles; (4) ab-sorption and scattering by water vapcr.

ATMOSPHERIC EFFECTS ON TERRESTIAL RADIATION:

Here it is that the effects of the atmosphere are

es-pecially noticeable, for, although the sun's energy can get

to the surface of the earth, it sometimes has a hard time

getting away. Terrestial radiation, being of a long

wave-*See Berry, Bollay, and Beers, page 294.

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eighteen-length (between 4gand 120s,) has different characteristics than does the shortwave energy of the sun. The water-vapor in the atmosphere does not permit all longwave radiation to pass thru it. Therefore, outgoing longwave radiation is

trapped, and kept near the earth. This is known as the 'greenhouse effectt, (in greenhouses the sun energy can pass thru the glass, but the longwave terrestial energy cannot pass back to space again; therefore the heat is trapped inside the greenhouse. The atmosphere vaks in the same way). Water vapor is the chief heat trap to terrestial radiation, but carbon dioxide has a very important and

noticeable effect aswell; it has a strong absorption band centered at 14.7m, which extends from 12kto l6.3k. Ozone has weaker bands centered at 7mand 10m, The absaption

spectrum for water vapor is shown in figure 11.

In summary form., the effects of atmospheric absorption are as follows:

Below 4.0 m effectively complete transparency 4.0 to 5.5A incomplete absorption

5.5 to 7.Om effectively complete absorption 7.0 to 8.5m incomplete absorption

8.5 to ll.0A complete transparency 11.0 to 14.0g incomplte absorption

14.0.qupwards effectively complete absorption

FG9GGW- from BERRY, BOLLAY, and BEERS, page 298 and from HAURWITZ, page 96

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nineteen-CLOUDS:

Clouds come in many sizes, shapes and forms, but they are essentially all similar in structure; clouds occur when

water molecules glomerate onto 'condensation nucleii'

(with-out these condensation nucleii, the water vapor has to be

supersaturated to 420% before condensation occurs, if anyone

is interested)*

These condensation nucleii are readily to be had in

nature; there are many dt particles in the air, and there

are many salt particles in the-a-ir, from sea surf whose moisture has evaporated and left the salt solid in the air.

The role that clouds play in the climate of the world

is a very important one. Clouds reflect heat from the sun

and from the earth, they radiate heat themselves, and they transport energy from one part of the world to another. This last point, the transportation of energy by clouds, is so complex and so little is known about it, either

quanti-tatively or qualiquanti-tatively, that it will be passed by here;

we know that it is one of the key factors in the world

energy distribution, a subject that is still lacking an

adequate explanation.**

The role of clouds in the local area we know more about. Clouds reflect 60 to 70% of the energy that hits their

sur-face, either from above or below. This means that in cloudy

*BERRY, BOLLAY, BEERS page 254, citing experimental data.

-page

twenty-areas the sun's energy from above cannot pass thru clouds

to warm the land during the day, nor can the terrestial

ra-diation escape to space in the day or night; therefore,

the ground temperature is relatively even. This effect is

very definitely born out in observed date. For an example,

.IGURE 12

TEMPRATURE

6 10 14 18 22

TIME OF D.Y

DIURNAL TfAIPERAiTURE VARIATIONS ON BRIGEAI idD OLOUDY DIYS

source: Landsberg 39.2 28.4 24.8 -2 -4 -6

CHA PTER III.

WORLD HEAT BALANCE:

By this time, the reader should know that (4) The only significant source of heat to the earth is the sun (it is true that the heat on the interior of the earth does have an effect. If it were not for a very hot interior, the surface of the earth would cool 0.10 C. lower than its present temperature); (b) The energy from the sun comes in

short wavelengths, while the energy leaving the earth is long wavelength; (c) Moisture in the air traps long wave-length energy; (d) The earth gives off energy to space day and night. With these facts in mind, we can make a summary of the distribution of heat as it affects the world. Figure 13 shall be our main reference in this short study.*

If we assume that 100 units of heat leave the sun and head towards the earth, this is what happens to them:

27 units pass thru the atmosphere and warm the earth's surface.

16 units are diffused by the sky, but end up

warming the earth's surface with diffuse, or sky, radiation.

9 units are diffused by the sky, but end up by returning to space.

18 units are absorbed by the atmosphere.

33 units are reflected by clouds and the atmosphere back to space.

*It should be kept in mind that the values given are merely estimates, due to the lack of reliable data, once again. The figures given by various authors range considerably in magnitude. The numerical values given here are from

Baur, F., and Phillips, H., in GERL. BEITR. GEOPHYS. vol 42, p 160, 1934, and vol 45, p. 82, 1935 as quoted in HAURWITZ and AUSTIN, page 14.

F IGURE 13

-Page

twenty-two-It will be noted that of the energy in the atmosphere,

9 of the 25 units are reflected back to space, while 16

pass to the earth. This difference is due to the fact

that the larger dust particles in the atmosphere scatter

more radiation in the direction away from the sun than in

the direction of the sun.*

So, from the sun, we have 42 units being reflected back to space, 18 units being absorbed by the atmosphere, and 43 units absorbed by the earth. All of this has to be

ac-counted for in heat losses, or else the receptor of this

energy will gain in heat and tEnperature.

This is what happens to the long-wave radiation leaving

the earth and the atmosphere:

Of the heat leaving the earth;

8 units pass thru the atmosphere to space 16 units are caught by the atmosphere.

The analysis of the heat in the atmosphere is more complex, because of the turbulent transfer of heat within the atmosphere, and because of condensation and evaporation.

23 units are assumed to be given up by the

earth to the atmosphere in condensation

4 units are assumed to be transferred from the atmosphere to the earth in the process

of turbulent transfer

50 units are lost by the atmosphere by radiation to space.

-page

twenty-three-It should be noted that this summary of the heat bal-ance applies primarily to the northern hemisphere and that while the basic principles are the same for the southern hemisphere, the numerical values may be quite different.

GLOBAL DISTRIBUTION:

We must return for a moment to figures 5 and 6, showing

the amount of heat that reaches the outside of the atmosphere,

and the amount of heat that might reach the surface of the

earth. It can be seen that the amount of heat that reaches

the equatorial region is markedly greater than that which

reaches the polar regions. If there were no such thing as

heat transportation and transfer, the equatorial regions

would be very hot indeed, and the polar regions would be

even colder than they are today. Just how great an effect

heat transportation has is shown in figure 14, which is

drawn from calculations by Simpson,* and has not been

thor-oughly substantiated with collected data.

Figure 14 shows that the latitudes up to 350 North

receive more heat energy than they give off, and that the

latitudes north of 350 lose more heat thru radiation than

they receive from the sun and the sky.

In order to create a net balance between incoming and

outgoing radiation, there must be a meridianal transport of

heat from lower to higher latitudes. The energy may be

transported by atmospheric or ocean currents as sensible heat, or as latent heat of evaporation. It does not follow that

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twenty-four-the atmospheric winds are entirely responsible for twenty-four-the heat

transport, but it is quite certain that any theory of

gen-eral circulation must take into account a considerable

poleward transport of heat by winds.*

WORLD AIR CIRCULATION:

There are no generally accepted theories on the general

circulation of the air. The only data that we have gathered

has been for a region close to the surface of the ground,

and we know very little of what happens in the stratosphere.

From analysing the surface air currents, tho, we have

been able to reconstruct a general pattern that seems to work

in design, but we are still at a loss to give any values

to the amount of heat transferred.

Figure 15, then, gives a very generalized picture of

how the atmosphere circulates about the earth. The sectional

view of the air at the edge of the sphere has been greatly

exaggerated in the vertical scale, for the height of the

division between the stratosphere and the troposphere, being

only 11 to 15 miles high, would be insignificant if drawn

in its true relative size. The general pattern is that

warmed air rises from the equatorial regions and moves

poleward. Because of the revolution of the earth, the air

moving towards the poles takes on an easterly component as

FIGURE 15

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-page

twenty-five-it moves (known as the Coriolis effect). Cold air at the

poles at the same times moves toward the equator, and takes

on a westerly component of motion. When these two air

currents meet, they set up a third motion in the temperate

zone, the effect of which is to produce a surface stream

of air moving easterly and poleward. All these surface winds

have been named by the sailors of the early days, as have

the regions on the surface where two of the circulation

systems intermix to produce fluky or light winds. These

names are given in figure 15. It can be seen why sailing

ships used to go around the world instead of just across the

ocean and straight back again. With the trades always

blowing westerly, it was easy to coast along before them,

once the ships passed through the horse latitudes, and

drifted through the doldrums best they could. To go around

the horn, it was necessary to battle the roaring forties

C H A P T E R IV.

INSOLATION:

It has been emphasised that the radiant energy from

the sun is the only significant source of heat to the earth. It is therefore fitting that we take a few minutes out to discuss the mechanics of insolation (or, incoming solar radiation) and the patterns that it sets up. We have re-viewed what solar energy is, its wavelengths, and so forth, and have given diagrams (figures 5 and 6) to show how

in-tensely it reaches the earth, in its global pattern. Now, therefore, let us go into the subject in greater detail.

ALBEDO:

Not all the radiations that strike a surface are

ab-sorbed into that surface; some of the radiations are reflected. The ratio of the reflected light to the incident light is

called the albedo. A surface that reflects much of the energy that impinges upon it therefore has a high albedo; conversely, a surface that absorbs most of the energy hitting its surface has a low albedo. Following is a table of

albedos of often-found surfaces:

twenty-six--page

twenty-seven-TABLE IV.

ALBEDOES OF VARIOUS SURFACES

Material Albedo

Pastures 6%

Conifer forests 7%

Leafed forests 9%

Lakes and rivers _{7 to 9%}

City areas 10%

Rock _{12 to 15%}

Sand _{13 to 18%}

Clouds _{60 to 70%}

Old snow and ice _{50 to 60%}

Fresh snow _{80 to 90%}

Water - depends upon altitude of sun

Altitude of sun - 50 100 200 300 400 500-900 Albedo of water - 40% 25% 12% 6% 4% 3%

Source off table - D. Brunt, page 101

Berry, Bollay, and Beers, page 933, 296 Haurwitz, page 93

Landsberg, page 90 in PHYSICAL CLIMATOLOGY

SUNSHINE:

As mentioned several times earlier, the intensity of the sun is not always the same (see pages 2. an d

3

)

because of the annual change in the sun-earth relationship. Also another important factor is the atmosphere that the rays of the sun must penetrate before reaching the .earth.

-page

twenty-eight-A more specific example will be found in figure 16,

which illustrates graphically the effect of the sun having to

pierce the atmosphere at various angles. Curve 1 shows the

amount of energy that reaches the boundary of the atmosphere,

while curves 2, 3, 4 and 5 illustrate the depletion of energy

as the rays have to pass through more and more atmosphere.

The moral to the story is that one gains a great deal more

heat if one is able to have the sun shine through relatively

little atmosphere, and also that, therefore, the sun is

really far more effective when it is in its zenith (as at

noon-time) than when low in the sky as it is early in the

morning or late in the afternoon.

The daily variation in the intensity of insolation is

shown in figure 17 which also demonstrates the effect of

other factors. This figure shows the variation of solar

intensity over the months of the year at LaJolla, California,

( Lat. 320 521 N., Long. 1170 15 W. ), The area beneath

the curves here indicate the average daily insolation for

each month. The area beneath the December curve is less

than half that under the May curve, which indicates that

the heat received from the sun in December is less than

half that received in May.

These figures include the sky radiation as well as the direct solar radiation, and therefore show the total heat input to the area. Other cities would have similar sets

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-page

twenty-nine-as setwenty-nine-asonal fog, or winter smoke, both of which would

decrease the amount of incoming heat at certain times of

the year.

The curves fcr Washington, D. C. are shown in figure

18. Note the sinusoidal shape of the family of curves,

which is entirely to be expected from knowledge of the

movement of the sun. Note also that in Washington there is

a far more atmospheric absorption of radiation in the summer

than in the winter months.

INSOLATION AND TFMPERATURE:

We notice that the time of maximum insolation (figure

17) is at noon, as would be expected. If we look back at

figure 12, however, we find that the highest temperature of

the day is at 14 or 15 hrs. (2 or 3 p.m.) Now, what is the

reason for this lag between incoming heat and temperature?

The answer is that the heat of the forenoon sun is used to

re-warm the surface of the earth that has cooled during the

night, while the heat of the afternoon sun can be used to

heat the surface of the ground to an above-average temperature.

By conduction, the air is warmed by the surface of the earth,

and a high afternoon temperature results. Note that the

atmosphere is warmed almost entirely by conduction from the

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C H A P T E R V.

MOISTURE IN THE AIR:

In the lower atmosphere (below 15,000 feet altitude)

the air contains an appreciable amount of water vapor. It is hard for some people to realize, but this water vapor is

present as a Z, and is not in the form of little droplets

of water visible to the eye. It is only when the water vapor is condensed that it loses its quality as a gas and behaves as a suspended liquid. When water vapor is not condensed,

it behaves as a true gas, just as does oxygen or helium.

Water vapor does not just condense out of the air by itself; in order to condense, it must have what is known as

a 'condensation nucleus', and it must have some body to

receive the latent heat of condensation that the water

releases on liquifying. (This is the reverse process of

evaporation. In evaporation, heat must be supplied to the

water in order to vaporize it. Here, the water releases

the same amount of heat it took in when evaporating)

HUMIDITY:

The amount of water vapor held by the air is called

'humidity'. This can be measured either in terms of the

mass of water vapor per unit volume (as grains per cubic

foot), a measurement known as 'absolute humidity', or else

the humidity may be expressed in terms of the ratio of the

actual amount of water held to the total possible amount

thirty--page

thirty-one-the air could hold if it were filled to capacity. This measurement is known as relative humidity. Humidity is im-portant to us because on it depends, to a great extent, human

comford. Cooling power is directly related to the humidity of the air, for the human body is cooled by the evaporation of water (perspiration) from the skin. If the air around the body is saturated, and can hold no more moisture, evap-oration cannot take place, and therefore the body cannot be cooled.

THE COMFORT ZONE:

The American Society of Heating and Ventilating Engineers has gone to great effort to delineate the physical

condi-tions that are normally called 'comfortable'.*

These conditions are shown in chart f orm in f igare 19. G. Schmidt, as quoted in Landsberg's PHYSICAL CLIMATOLOGY,4H:-has set up a scale of comfort reactions of man in terms of cooling power, in high and low humidities. See table five..

TABLE V.

HUMAN REACTION IN TERMS OF COOLING POWER

Climatic Comfort Zone Cooling power in m.cal/cm2/sec. LOW HUMIDITY HIGH HUMIDITY

extremely cold > 55 90 very cold 40-55 75-90 cold 30-40 58-75 cool 22-30 47-58 mild 15-22 38-47 warm 10-15 30-38 sultry <10 30

*See ASHVE Guide, page 53

FIGTR 19 4 . .. 4 4 .. - .. -- - 7 -- 7 -- - --- e -1 --W - 41 A -s +Y -- - T-V -M-5- -4* -T -4-4 T-t * :44 - t --T --- --- 4 4 - - -

----page

thirty-two-It is not clear to this author why Schmidt feels that

if a body loses 28.m.cal/cm2sec it will have the- readtion ,df

feeling sultry on a day with high humidity, and cool on a day with low humidity; the heat loss is the same in both

cases, so the cooling sensation should be the same.

Although defining comfort zones in terms of cooling

power is certainly the correct approach to the analysis of

human comfort, it should be remembered that subjective

reactions vary considerably, and that what will seem

com-fortable to an old lady and her lap-dog might seem

intoler-able to a high school youth. Because of the differences

of reaction due to difference of age, activity, nutrition,

and health, the best delineations so far are but mere

generalizations.

In addition to the physical reactions are the

psychol-ogical reactions that are necessary to have a state of

comfort. Other than temperature and humidity, etc., it is

necessary to have a 'comfortable amount', or lack of,

noise, color, movement, odor to the air, and so on. A

small room will be uncomfortable to a- person who suffers

from claustrophobia, no matter what the other factors are.

Therefore, it is clear that physical factors alone do not

and can not clearly set forth and define what comfort is.

To return to the specific subject of humidity once

again, attention is called here to the supplementary

-page

thirty-three-Figures 9-S, 10-S, and 11-S show the humidities found in

the United States. In figure 11-S, of Mean Relative Humidity

for the whole year taken at local noon, it is very noticeable

that the areas near large bodies of water have high humidities

(note the Pacific, Gulf, and Atlantic coasts, as well as the

Great Lake region). The driest areas are those between the

Sierra Nevada Mountains and the Western side of the Rocky

Mountains* The degree of comfort is not solely dependent upon the relative humidity, as has been pointed out

pre-viously; other charts in this supplemental series should

be examined in order to truly judge the climate.

For example, high humidities are bearable unless ac-companied by high temperatures. In the western coastal re-gions of high hunidity, however, there tends to be a

moder-ate temperature that does not get 'uncomfortably' warm

(see figure 6-S).

RAIN:

Rain is atmospheric moisture that has condensed on

dust particles in the air and fallen towards the ground.

Rains range from heavy mists to cloudbursts. The size of a

raindrop in a Scotch fog (or 'mizzlet, or light drizzle) is

about half a millimeter in demeter. In a medium rain, the

diameter of the drops -is about one millimeter, while in a

cloudburst, it may be as large as a centimeter. The rate

rain--page

thirty-four-drop falls at about 1000 feet per minute; fog particles will sometimes stay suspended in the air without appearing to fall at all.

The classification of rain by the size of drop also

indicates the intensity of the rainfall, for the larger drops fall faster, and a heavier rain is experiences.

The standard classification of rains is as in Table 12.

TABLE VI.

CLASSIFICATION OF PRECIPITATION

Name Diameter of Drop Rate of fall Lpm

Dry Fog 0.002 cm 0.15 Mist 0.02 100 Drizzle 0.04 500 Light Rain 0.09 700 Medium Rain 0.2 1000 Heavy Rain 0.3 to 0.4 1500 Cloudburst 0.6 to 1.0 2000

Besides the intensity of rainfall, and the amount, vaat happens to the water when it reaches the ground is also of climatological importance. The effect to the land is entirely different if the water sinks into the soil than if it runs off into a lake or sewer and away. If the water stays in the ground, the humidity of the area will likely be higher, as it is easier for water to get into the air. Also the ground will have a lower and more even temperature, as incoming heat will be used to evaporate the moisture, rather than heat the soil to higher temperatures.

-page

thirty-five-Rain is one of the most important factors in world

and local heat balance. As yet, however, we do not know the

exact proportion of its role, inasmuch as we know relatively

little about the general heat balance of the atmosphere.

The distribution of rainfall in the United States is

given in figure 1-S. Here is shown a large arid (desert)

area in red, and a larger semi-arid area in white. The

yellow and grey areas are humid, and the blue is very humid. In general, the yellow and grey areas are the most suitable for raising crops, and are therefore the most heavily settled

by farmers. Note that this map, like all others, cannot be

read by itself and be significant. Needed also is a knowledge

of topography, history, and of other climatic elements,

such as seasons and temperatures. One other key to the

relationship of rainfall to climate is figure 2-S, which

gives the number of rainy days per year. With this, we see

that there is a striking difference in the type of rain

found in the eastern half of this country and those in the

west.

FOG:

Fog is a special case of condensed atmospheric moisture.

We are especially interested in it because it is common in

cities, and it makes a great deal of difference to their

climate.

Fog, like rain, is condensed moisture, but, unlike rain, fog does not fall towards the ground at a noticeable speed; the particles are so small that they have a very slow sinking

-page

thirty-six-Why do fogs stay on for days and days then? The answer is that the fog particles do drop out to the ground, or are

blown away, but that new particles are formed to take their

place. This is noticeable in some city fogs; the billows

will be sooty and dirty at the end of the day, but overnight

the fog blanket is once again a fresh, pure white. And,

contrary to common opinion, there is no adhesive or cohesive

property of fog; fog is just like any other air, except that

it is a little heavier. Fog does not 'stick' in one place;

it is constantly settling out and being blown away, but, as

mentioned above, it is being constantly formed anew. As

long as the conditions that caused the original fonnation

of fog continue, fog will continue to be formed; not until

these conditions have changed will the fog disappear, or

'liftt.

Fog is self perpetuating quite often. What happens is

that fog brings on conditions that encourage further formation

of fog; it is sometimes a vicious circle, and can last for a

long time. The most often found circumstance is the

forma-tion of an 'inversion' in teaperature, where the air

tem-perature increases with an increase height above the ground,

instead of decreasing as is usually the case. This puts a

lid on vertical circulation; it means that lower air, when

it rises loses its temperature as it goes higher, but it finds

itself in warm air instead of air at its own temperature.

-page

thirty-seven-this cold air sinks to a lower level, bringing to an end its rise and its vertical circulation.

When fogs are over cold ground, and are caused by air

in contact with cold ground, these fogs will be

self-perpetu-ating. The fog will blanket the cold eaibth and prevent its warming with sun energy; this will cause an inversion and will

prevent vertical circulation; without vertical circulation,

the fog will not clear, and therefore the sun will be unable

to reach and warm the earth. The way that the fog will clear

will be for a wind to blow in some air that will not

con-dense out moisture when cooled to the temperature of the

ground.

There are four different basic types of fog, all of

which are formed by air cooling to a temperature at which

the moisture condenses and gloms onto small solid particles

in the air. These four types are:

(a) Radiation fog, formed by the cooling of the earth

by radiation, and then the cooling of the air

in contact with the earth.

(b) Mixture fog, formed by the mixing of cold air

and warm air of high relative humidity; the

resulting mixture having an excessive amount of

moisture for its temperature which must be

con-densed out.

(c) Advection fog, generally formed by warm air

-page

thirty-eight-mist by the movement of cold air over warm water surfaces.

(d) Upper air fog, found only in mountains, formed by the dynamic cooling of rising currents of air

to a point where they must release moisture (this is the way that cumulus clouds are formed).

SNOW:

Snow is atmospheric moisture ddich has frozen into crystal form and fallen to the earth. Snow is the second

most frequently found fonn of precipitation (rain being

the first), and is very important in cities, mainly because of its nuisance; snow-blocked roads must be cleared, and so on. Snow also provides the source for winter sports, which many enjoy; it is not an unmitigated curse.

The climatic effect of snow is an interesting one. As a ground cover, snow reflects virtually all the smlight that strikes the surface (it has an albedo of from 80 to 90%). However, it radiates away heat the same way that any black body of the same temperature would; therefore the mocturnal heat losses from snow-covered ground are often high, while the daytime heat gains are small. This has the effect of prolonging cold weather once the ground is covered with snow.

Even the snow itself on the surface of h~e earth is a large and important store of tcoldnesst that must be

-page

thirty-nine-done away with before the land will warm up. Because of the latent heat of fusion of ice (80 calories per gram of ice, or 144 BTU per pound), the heat used to melt snow, without raising its temperature, could be used to raise the temperature of bare ground many degrees.

Let us take a square foot of earth with a snow covering of six inches. Assuming a pretty soggy snow that is one part ice tofvez parts of air, in six inches of snow there is one inch of ice. This one inch of ice on the square foot of earth weighs as much as one twelfth of a cubic foot of ice, or 1/12 x 57 lb., or 4.75 lbs. To melt this 4.75 lbs. of ice requires 144 BTU per pound, or 686 BTU. However, since the snow reflects 85% of all the energy that strikes its

surface, far more than 686 BTU are needed to strike the sur-face of the snow; in fact, the amount will be 686/(1.00-.85), or 686/.15, or 4570 BTU.

If this amount of energy, 4570 BTU, were used to warm the bare soil the rise in temperature would be very notice-able. We can calculate just what the effect would be.

Assuming that the bare earth has an albedo of 10%, 4570 x (1.00-0.1), or 4570 x .9, or 4110 BTU would be absorbed by the earth (compare with the 686 that were absorbed when

the surface was covered with snow).

Now, assuming that the 'earth isffrozen, and that: 0O It, contained 20% water when frozen, the heat must thaw