The controls of temperature include those factors that bring about spatial variations in temperature

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The controls of temperature include those factors that bring about spatial variations in temperature.

The following are some of the more important factors which determine the temperature of a particular place on the earth’s surface: latitude, altitude, distribution of land and water, ocean currents, prevailing winds, cloudiness, mountain barriers, nature of the surface, relief and convection and turbulence.

Latitude:

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Intensity of insolation depends on the latitude. Since insolation is a prime factor in determining the temperatures of the air, it is but natural the temperatures would vary with latitude. Thus, the temperature of the atmosphere at a particular place near the earth’s surface is a function of the insolation received at that location.

The total insolation is more intense in the low than in the high latitudes. As we move away from the equator pole-ward, the mean annual temperatures show lower values.

The most important causes for variation in the amount of solar energy reaching the earth are the seasonal changes in the angle at which the sun’s rays strike the earth’s surface and in the varying lengths of the day.

The total amount of the radiant solar energy received at any point on the earth is largely controlled by the duration and intensity of insolation.

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These two factors, in turn, are controlled by the rotation and the revolution of the earth as well as the inclination of its axis.

There is no doubt that of all the factors, latitude is the most important. This is proved by the general east-west trend of the isotherms drawn on the maps showing the mean annual temperature distribution on the earth.

However, the irregularities of the isotherms do suggest the influence of other factors on the distribution of temperature on the surface of the earth.

Altitude:

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We know that temperature decreases with increasing altitudes from the earth’s surface. This vertical decrease in temperature of the atmosphere is limited to the troposphere only in which the normal lapse rate is about 6.5°C per kilometer.

Since the direct source of atmospheric heat lies at the surface of the earth, the higher we ascend in the atmosphere, the lower the temperatures are.

The second factor which causes the lowering of temperature with elevation is the fact that the lower layers of air at or near the surface are denser and contain more water vapour, more water particles and more dust particles than the upper air.

Since these substances absorb a large amount of terrestrial radiation, the lower part of the atmosphere has higher temperatures than the less dense and cleaner upper part. The perma­nent snow caps on high mountains, even in the low latitudes, indicate the decrease of temperature with altitude.

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This shows the effect of altitude on temperature. Mt. Kenya (5199 meters high), even though located at the equator, is cold enough to have a permanent snow cover in its upper parts.

Distribution of land and water:

Land and water surfaces react differently to the incoming solar radiation. That is why different surfaces such as land, water or ice heat up and cool down differently.

For example, land surface is heated more quickly and to a greater extent than the water surface when subjected to an equal amount of insolation, and it also cools more rapidly. Because of the great contrasts between land and water surfaces, their capacity for heating the atmosphere varies.

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This arises from the fact that whereas water tends to store the heat it receives, land quickly returns it to the atmosphere. Variations in air temperatures, therefore, are much greater over land than over water.

It is the differential heating of land and water that accounts for the distinct types of marine and continental types of climates found on the surface of the earth.

The factors which account for the differential heating of land and water are discussed below;

(i) Since water is essentially transparent, it allows some heat and light energy to penetrate to a depth of several meters. Land surfaces, on the contrary, are opaque, so that there is a greater concentration of insolation in a thin layer of soil and rock with the consequent greater heating.

Because the solar energy on a square meter of ground is distributed over such a small mass of material, it naturally causes a greater temperature increase than would be caused by the same amount of insolation falling on the water surface.

(ii) The specific heat of water is much higher than that of land. In other words, more heat is required to raise the temperature of one gram of water by 1°C than the same amount of land.

For water the specific heat is 1 which is among the highest of all such values. Specific heat is almost three times greater for water than it is for land.

Thus, a given quantity of incoming solar radiation would increase land temperatures more than the temperature of an equal mass of water. Similarly, for a temperature decrease of 1° C a mass of water gives off much more heat than does the same mass of land.

(iii) Water being fluid is highly mobile. Heat in it is redistributed mainly through turbulence. Ocean waves, tides and currents aided by convection distribute the absorbed solar energy to greater depths.

Besides, the cool and heavy water of the surface at night sinks and is replaced by warm water from below. Hence, a large mass of water must be cooled before the temperatures of the surface layers are reduced.

That is why sea temperatures change relatively little from summer to winter. In other words, the sea surfaces show little seasonal variations. Thus, the temperatures of the water surfaces tend to fluctuate more slowly than those of the land surface.

Land surfaces, on the other hand, are static. Heat in solid earth is distributed by the slow process of conduction. The vertical distribution of heat in land is such a slow-going process that diurnal temperature changes are very small below a depth of 10 centimeters.

On land, therefore, only a thin layer is heated to much higher temperatures during the summer. During the winter the same shallow layer cools rapidly, producing large diurnal and annual temperature ranges on land surfaces.

(iv) Evaporation from water bodies is greater than that from land surfaces. The evaporation process consumes a large amount of solar energy in the conversion of liquid water to water vapour without any increase in the temperature.

Evaporation process is more effective in moderating the temperature in all the large bodies of water during summer. However, it is only a minor factor. But on the land surface evaporation takes place only from wet ground and that too during the period of sunshine.

It is, however, interesting to note that in the wet tropics, evaporation from the continents is greater than from the oceans.

Thus, according to Trewartha, large-scale evaporation and its retarding effects on excessive heating and cooling are not restricted to ocean surfaces alone.

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