Distribution of temperature across the latitudes over the surface of the earth is called its horizontal distribution.
On maps, the horizontal distribution of temperature is commonly shown by “Isotherms”, lines connecting points that have equal temperatures.
The distribution of temperature is uneven.
The factors responsible for the uneven distribution of temperature are as follows:
The angle of incidence goes on decreasing from equator towards poles.
Higher the angle of incidence, higher is the temperature.
Lower angle of incidence leads to the lowering of temperature.
It is because of this that higher temperatures are found in tropical regions and they generally decrease at a considerable rate towards the poles.
Temperature is below freezing point near the poles almost throughout the year.
 Land and Sea Contrast
Land and sea contrast affects temperature to a great extent.
Land gets heated more rapidly and to a greater degree than water during sunshine.
It also cools down more rapidly than water during night.
Hence, temperature is relatively higher on land during day time and it is higher in water during night.
In the same way there are seasonal contrasts in temperature.
During summer the air above land has higher temperature than the oceans.
But the air above oceans gets higher temperature than landmasses in winter.
A snow covered land as in polar areas warms very slowly because of the large amount of reflection of solar energy.
A vegetation covered land does not get excessively heated because a great amount of insolation is used in evaporating water from the plants.
 Relief and Altitude
Relief features such as mountains, plateaus and plains control the temperature by way of modifying its distribution.
Mountains act as barriers against the movement of winds.
The Himalayan ranges prevent cold winds of Central Asia from entering India, during winter.
As we move upwards from sea level, we experience gradual decrease in temperature.
Temperature decreases at an average rate of 60C per 1000 m. altitude. It is known as normal lapse rate.
The air at lower elevations is warmer than that of higher elevations because it is closest to the heated surface of the earth.
As a result mountains are cooler than the plains even during summers.
The rate of decrease of temperature with altitude varies with time of day, season and location.
 Ocean Currents
Ocean currents are of two types – warm and cold.
Warm currents make the coasts along which they flow warmer, while cold currents reduce the tempeeture of the coasts along which they flow.
Winds also affect temperature because they transport heat from one region to the other, about which you have already studied under advection.
 Vegetation Cover
Soil devoid of vegetation cover receives heat more rapidly than the soil under vegetation cover.
Because vegetation cover absorbs much of sun’s heat and then prevents quick radiation from the earth whereas the former radiates it more rapidly.
Hence the temperature variations in dense forested areas are lower than those in desert areas.
For example, annual range of temperature in equatorial regions is about 5°C while in hot deserts, it is as high as 38°C.
 Nature of the Soil
Colour, texture and structure of soils modify temperature to a great degree.
Black, yellow and clayey soils absorb more heat than sandy soils.
Heat radiates more rapidly from sandy soils than from black, yellow and clayey soils.
Hence temperature contrasts are relatively less in black soil areas than those of sandy soils.
 Slope and Aspect
Angle of the slope and its direction control the receipt of insolation.
The angle of incidence of sun’s rays is greater along a gentler slope and smaller along a steeper slope.
The rays in both the cases carry an equal amount of solar energy.
Greater concentration of solar energy per unit area along gentler slope raises the temperature while its lesser concentration along steeper slopes lowers the temperature.
For such reasons, the southern slopes of the Himalaya are warmer than the northern ones.