Sun is the ultimate source of heat. And the differential heat received from sun by different regions on earth is the ultimate reason behind all climatic features. So understanding the patterns of distribution of temperature in different seasons is important for understanding various climatic features like wind systems, pressure systems, precipitation etc.
Horizontal Distribution of Temperature
The distribution of temperature across latitude over the Earth’s surface is known as the horizontal distribution of temperatures. The horizontal distribution of temperature on Earth is shown by Isotherms. Isotherms are the line joining points that have an equal temperature. When the isotherm map is analyzed, it can be observed that the horizontal distribution of temperature is uneven.
Some of the factors affecting the temperature distribution are:
The angle of Incidence or the Inclination of the Sun’s Rays
Duration of sunshine
- Heat received depends on day or night; clear sky or overcast, summer or winter etc..
Transparency of Atmosphere
- Aerosols (smoke, sooth), dust, water vopour, clouds etc. effect transparency.
- If the wavelength (X) of the radiation is more than the radius of the obstructing particle (such as a gas), then scatteringof radiation takes place.
- If the wavelength is less than the obstructing particle (such as a dust particle), then total reflectiontakes place.
- Absorption of solar radiation takes place if the obstructing particles happen to be water vapour, ozone molecules, carbon dioxide molecules or clouds.
Most of the light received by earth is scattered light.
Differential heating of land and water
- Albedo of land is much greater than albedo of oceans and water bodies. Especially snow covered areas reflect up to 70%-90% of insolation.
- Average penetration of sunlight is more in water – up to 20 metres, than in land – where it is up to 1 metre only. Therefore, land cools or becomes hot more rapidly compared to oceans. In oceans, continuous convection cycle helps in heat exchange between layers keeping diurnal and annual temperature ranges low. (more while studying salinity and temperature distribution of oceans)
- The specific heat of water is 2.5 times higher than landmass, therefore water takes longer to get heated up and to cool down.
Prevailing Winds
- Winds transfer heat from one latitude to another. They also help in exchange of heat between land and water bodies.
- The oceanic winds have the capacity to take the moderating influence of the sea to coastal areas – reflected in cool summers and mild winters. This effect is pronounced only on the windward side (the side facing the ocean).
- The leeward side or the interiors do not get the moderating effect of the sea, and therefore experience extremes of temperature.
Aspects of Slope
- The direction of the slope and its angle control the amount of solar radiation received locally. Slopes more exposed to the sun receive more solar radiation than those away from the sun’s direct rays.
- Slopes that receive direct Sun’s rays are dry due to loss of moisture through excess evaporation. These slopes remain barren if irrigational facilities are absent. But slopes with good irrigational facilities are good for agriculture due to abundant sunlight available. They are occupied by dense human settlements.
- Slopes that are devoid of direct sunlight are usually well forested.
Ocean currents
- Ocean currents influence the temperature of adjacent land areas considerably. (more while studying ocean currents).
Altitude
- With increase in height, pressure falls, the effect of greenhouse gases decreases and hence temperature decreases (applicable only to troposphere).
- The normal lapse rate is roughly 1⁰ C for every 165 metres of ascent.
Earth’s Distance from the Sun
- During its revolution around the sun, the earth is farthest from the sun (152 million km on 4th July). This position of the earth is called
- On 3rd January, the earth is the nearest to the sun (147 million km).This position is called
- Therefore, the annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July.
- However, the effect of this variation in the solar output is masked by other factors like the distribution of land and sea and the atmospheric circulation.
- Hence, this variation in the solar output does not have great effect on daily weather changes on the surface of the earth.
Vertical Distribution of Temperature
- The temperature in the troposphere decreases with increase in altitudes but the rate of decrease in temperature changes according to seasons.
- The decrease of temperatures is known as vertical temperature gradient or normal lapse rate which is 1000 times more than the horizontal lapse rate.
- The decrease of temperature upward in the atmosphere proves the fact that the atmosphere gets heat from the Earth surface through the process of conduction, radiation, and convection.
- Hence, it is obvious that as the distance from the Earth’s surface (the source of direct heat energy to the atmosphere) increases (i.e as the altitude increases), the air temperature decreases.
Global Distribution of Temperature
- The global distribution of temperature can be effectively understood by considering the temperature distribution for the month of January and July.
- The distribution of temperature is usually shown on the map using the isotherms.
- The isotherms are line joining places of equal temperature.
- Generally, the effects of latitude is well shown on the map as isotherms are generally parallel to the latitudes.
- The deviation from this trend is more generally observed in January rather than in July, especially in the northern hemisphere.
- The land surface is much larger in the northern hemisphere than the southern hemisphere. Hence, the effects of land masses and ocean currents are well observed.
Temperature Distribution in January
- In January, there is winter in the Northern hemisphere and summers in the southern hemisphere.
- The western margins of continents in January are much higher than the Eastern counterparts as the westerlies can carry high temperatures into the landmasses.
- The temperature gradient is much closer to the Eastern margins of continents. The isotherms observe more steady behavior in the southern hemisphere.
Temperature Distribution in July
- During July, it is winter in the Southern hemisphere and summers in the Northern hemisphere. The isotherm behavior is the opposite of what it was in January.
- The isotherms are generally parallel to the latitudes in July.
- The equatorial oceans record warmer temperatures more than 27 degrees celsius.
- More than 30 degrees celsius is noticed over the land in the subtropical continent region of Asia, along the 30 ° N latitude.
