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Old Wednesday, November 14, 2007
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Horizontal Factors

The horizontal distribution of temperature depends on insolation, seasonal changes, winds, currents and nature of the land. The mean temperatures are taken on average of observations over a definite period of time. These temperatures are further reduced to sea level for convenience.

1. Winds

There are two ways by which distribution of temperature is influenced by the winds:
a. Winds carry the temperature from one place to another;
b. Winds blow the surface layers of a body of water in the direction in which they themselves are travelling.
In the figure, the winds carry the warmer surface water to the windward shore, cold water comes to the top on the leeward shore and the isotherms are displaced as shown. (See handwritten notes)
In this way, the winds have an effect of raising the temperature of the shore towards which the wind is blowing and to lower the temperature of the opposite shore.
In the north of latitude 40 oN, the prevalent winds are from the SouthWest. They blow the surface waters of the ocean towards the east, warming the western coasts and cooling the eastern coasts. These winds are coming from the warmer sea (from down under). Consequently the temperature of the western coasts is raised.
The isotherms within the region of the southwesterly winds move towards the equator on the eastern shores and farther away from it on the western shores. (Left)

2. Seasonal Variations

During the revolution of the earth around the sun, the inclination of the axis remains the same. Therefore all its positions are parallel to each other. This is evident from the following two factors:
a. Temperature changes everywhere according to the seasons; and
b. each place has the hottest and the coldest period or month.
The inclination of the axis of the earth determines the number of hours during which a particular place would receive sunlight and also the angle of incidence of the sunrays.
Some general facts about the seasonal change of temperature are:
1. There is no seasonal change of temperature in the tropics. Except the tropics of Cancer and Capricorn, everywhere there are two maximum and two minimum temperatures.
2. In the temperate and frigid zones the seasonal change of temperature is greatest. In the summer season, the days are the longest and the sun is also at its greatest height above the horizon. On the other hand, the days are the shortest and the sun is at its lowest height above the horizon. Hence there is only one maximum and one minimum.
3. Water takes longer time to heat and cool. Certain currents also warm the coasts. As a result the places situated near the sea have less acute change of temperature. On the sea shores, the hottest and the coldest months are later than those in the interior of the continents (i.e., August and February).
4. The seasonal change of temperature declines with the increase in altitude.

Vertical Factors

Normally the temperature goes on declining as we proceed higher up from the sea level. It is because of two causes:
1. The lower layers get warmed as they are in contact with the earth. The lower layers also get hot due to compression from above.
2. At high altitudes, the water vapor and CO2 content of the air is reduced and it gets rarified (thinned) leading to low absorption.

How the air gets heated?
Sun is a primary source of heat but it is not the rays of the sun which directly heat the air. A considerable portion of heat and light is absorbed by atmosphere but the increase of temperature is small due to the great mass of air.
The earth emits infra-red rays after receiving the heat energy from the sun. the air less transparent to Infra Red Rays than the visible rays. Consequently the air absorbs a larger proportion of heat radiated out from the earth.
Another stronger reason for the air to be warmed by earth is that the air is warmed at once by actual contact with earth. This is the major reason for the warming of air from below. It is therefore natural that the temperature should decrease upwards.
This can be demonstrated by a simple experiment. A thermometer hung in open air shows a high temperature, but if it is swung round and round, still in the open air, still in the sun, its temperature falls. The reason is that it comes in contact with the relatively cooler air.
Following are the factors affecting the vertical distribution of temperature:

1. Altitude

On the top of a mountain, it is always colder than at the foot. The rate of fall of temperature with the increase of altitude is called the Vertical Gradient of Temperature, or simply the Lapse Rate. It has been found to vary a great deal but on the average it can be stated that on the mountain slopes, the average rate of change is about 1oF for each 300ft. of ascent.
The warm earth heats the air in contact with it. The heat spreads upward through conduction or convection. As earth is the principle source of heat, the temperature will decrease upwards. On clear nights, the earth rapidly looses heat by radiation and its surface becomes colder than air. This cold air spreads upwards and there is a partial reversal of the temperature gradient.
This methodology has been supported by the observations at the Eiffel Tower. (draw figure from notes)
The tower is 300 m (less than 1,000 ft.) high having observing stations at different heights. Both in December and July, the average temperature is higher at the bottom than at the top. The maximum is also higher at the bottom than at the top, but the minimum is lower. At an altitude of 1,000 ft. the temperature does not rise so high nor so low as on the ground; the range of temperature is low.
In both the months, the temperature at the foot of the tower is higher than at the top as long as the sun is up and for sometimes afterwards; but in the early morning, when has ground has cooled, the temperature gradient is reversed.
It is clear that the sun’s rays do not warm the air to any great extent, but that they warm the earth and the earth warms the air.
Clouds reflect a great deal of radiation falling upon, moreover, the presence of as cloud sheet markedly checks the fall of temperature after sunset.

2. Expansion and Compression

When the air is compressed, without any heat being added to it, its temperature is raised. On the contrary when the air is allowed to expand by reduction of pressure, without any addition or subtraction of heat, its temperature falls.
At the surface of the earth the pressure is greater than it is above. If the air is forced to rise, it moves into a region of lower pressure and therefore it expands and becomes cooler. Similarly any mass of air moving down will face a higher pressure leading to compression and increased temperature.
There are three principle ways in which the temperature of air may be forced to rise:
a. Heating by contact with earth;
b. By blowing against a mountain side; and
c. Descent of a heavier mass of air through cooling.
These vertical movements do not extend indefinitely upwards. Above a certain altitude, their effect is imperceptible and beyond that the temperature does not decrease.

3. Water Vapor

Any suspended drops of water or dust increase the temperature of air. The water vapors condense upon cooling leading to the liberation of Latent Heat, which arrests the rapid fall of temperature.
The Adiabatic Lapse Rate, or Gradient for Indifferent Equilibrium is less for saturated than for dry air; and with saturated air, it varies with the water content.

4. Form of Land

Mountains: If the air is still, the isothermal lines will be horizontal over a plane. The mountain will also be warmed by the sun and it will also heat the surrounding air.
AB represents a plane surface while BC is a mountain. During the day, the air in contact with the mountain gets heated and becomes hotter than the air at the same level above the plane. The isotherms therefore rise towards the mountains. The warm air rises.
During the night, the plane and the mountain grow cold and cool the air in contact with them. The isotherms bend down towards the mountain. The colder and heavier air on the mountain side sinks downward and the warmer air from outside flows towards the mountains.

Inversion of Temperature: Under special circumstances, there is an increase in temperature with an increase in elevation. This state of affairs of known as Temperature Inversion.
During calm cool nights (winter), the sinking cold air collects in hollows and valleys causing a decrease of temperature. In mountain areas, the valley floors are colder than the places at a higher level. It happens in winter and spring nights. The ideal conditions of temperature inversion are:
1. Absence of winds;
2. Clear skies;
3. Long winter nights;
4. Cold dry air; and
5. Snow covered earth.

Plateaux: A plateau differs from a mountain or a mountain chain in the fact that it is much broader. It has infact a much larger mass projecting into the atmosphere and it produces a great effect upon the temperature of the air.
AB represents the plane and CD the plateau rising above it. The plateau will be hotter in sun raise as the raise have to penetrate a smaller thickness of the atmosphere. The isothermal surface will bend upwards on the slope of BC.
With such a distribution, the air cannot be at rest, for the air at 60o above the plateau is on a level with air at 30o above the plane. In general, the air above the plateau is hotter, lighter and less dense than the air at the same level above the plane.
Accordingly cooler air from outside flows in, displacing the hotter and forcing it to rise. The cold air is continually being warmed by the hot plateau and the temperature difference is maintained. If the plateau, the temperature difference between the plane and the plateau would be far greater, leading to stronger winds. Moreover, the rising hot air soon finds itself in a much cooler environment and may continue to greater heights. This produces violent thunder storms. The South African Veldt gives a good example.

5. Effect of Winds

Mountain and Valley Winds: there will be a flow of air towards the mountain up its slope during the day and if the sky is clear, there will be a flow of cold air down the mountain side at night.
These winds are felt in the Alps and in the Himalayas. These local winds may sometimes be over powered by other winds.

The FOHN or Chinook Winds: This is a special type of wind which blows from the mountains to the planes. In Switzerland, it is called the FOHN, while in the rocky mountains it is called the Chinook. It has several other local names. The special characteristic of these winds is their dryness and their high temperature.
If the barometric pressure on one side of a mountain chain is higher than on the other, a wind will blow towards the chain and if the atmosphere is in a state of stable equilibrium, it will descend the other side.
The wind mostly contains water vapor which will be condensed and often precipitated. The decrease of temperature on leeward side is therefore less than if the air were dry.
On the other side, while descending, the pressure increases on the air through compression causing a rise of temperature. Sometimes the difference of temperature of both sides could be quite high or often doubled.
The FOHN is therefore a warm wind for the season and very dry. It rapidly melts the snow on the slopes and valleys.
The changes in the temperature of the atmosphere are limited to the troposphere (7 miles). In the region of stratosphere, the temperature is very low (-80o to –70oF), but it is almost constant and uniform.


The isothermal lines represent the surface distribution of temperature and run generally from East to West. At the junction of land and sea, there is a deviation in their uniformity because of the difference in the insolation of land and sea.
An isotherm is a line along which the temperature is everywhere the same. The distribution of air temperature over large areas can best be shown by a map composed of isotherms.
The isotherms near the Equator bend pole-wards over the land, equator-wards over the sea, towards the poles the bends are in the opposite direction. (see Gupta - 94)
At some intermediate latitude, land and sea will be at about the same temperature and the isotherms will be nearly straight.
The isotherm of 30o in the northern hemisphere from West to East bends pole-wards over the Atlantic and equator-wards over Euro-Asia, pole-wards over the Pacific and again equator-wards over North America.

Method for drawing

All available stations are located on a base map, temperature plotted, and the lines drawn to represent selected temperatures. The values plotted may be either by actual means or reduced to sea level by adding a correction for the altitude of the station, thus a map either of “actual isotherms” or “sea level isotherms” can be constructed.
Isotherms may represent single readings taken at the same time everywhere, or they may represent the averages of many years of records for a particular day or month of a year depending upon the purpose of the map. Usually, isotherms representing 5o or 10o differences are chosen but they can be drawn for any selected temperatures. The isotherms pass through the observing stations only when the station readings coincide with the value selected for an isotherm. Otherwise, it is necessary to draw isotherms by estimating their proper positions between stations.

Value of Isotherms

The value of isothermal map is that they make clearly visible important characteristics of the prevailing temperatures. Countries of high or low temperatures are clearly outlined. Zones of gradation are readily seen.

Zones of Temperature

a. Torrid Zone: It extends on both sides of equator between the Tropics of Cancer and Capricorn.

b. Temperate Zone: It lies in the northern hemisphere between the Tropic of Cancer and the Arctic circle and between the Tropic of Capricorn and the Antarctic circle in the southern hemisphere.

c. Frigid Zone: The arctic and the Antarctic circles form the frigid zones.
a. 68 oF isotherm separates Torrid from Temperate
b. 50 oF isotherm separates Temperate from Frigid

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