Insolation and Temperature: Horizontal Distribution of Temperature in January and July

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Horizontal Distribution of Temperature in January

  • In January, the sun shines vertically overhead near the Tropic of Capricorn. Hence it is summer in southern hemisphere and winter in northern hemisphere. High temperature is found over the landmasses mainly in the regions including north-west Argentina, east and central Africa, and, central Australia in the southern hemisphere. Isotherm of 30°C closes them. In the northern hemisphere landmass arc cooler than oceans.

  • During this time northeast Asia experiences lowest temperatures. As the air is warmer over oceans than over landmasses in the northern hemisphere, the Isotherms bend towards poles when they cross the oceans. In southern hemisphere, the position of the isotherms is just reverse. They bend towards poles when they cross the landmasses and towards equator when they cross oceans. Large expanse of water exists in southern hemisphere. Hence, isotherms are regular and widely spaced in the southern hemisphere.

Average Sea Level Temprature (January)

Average Sea Level Temprature (January)

Average Sea Level Temprature (January)

Horizontal Distribution of Temperature in July

During this period the sun shines vertically overhead near the Tropic of Cancer. Hence, high temperatures are found in the entire northern hemisphere. Isotherm of 30°C passes between 10° N and 40° N latitudes. The regions having this temperature include south western USA, the Sahara, Arabia, Iraq, Iran, Afghanistan, desert region of India, and China. However, lowest temperature of 0°C is also noticed in the Northern Hemisphere during summer in the central part of Greenland. During summer in the northern hemisphere, isotherms bend equatorward while crossing oceans and poleward while crossing landmasses. In southern Hemisphere the position of isotherms is just opposite.

Average Sea Level Temprature (July)

Average Sea Level Temprature (July)

Average Sea Level Temprature (July)

The difference between the average temperatures of warmest and the coldest months is known as annual range of temperature. Annual range of temperature is larger in the interior parts of the continents in middle and high latitudes of the northern hemisphere. Verkhoyansk in Siberia records 66°C, the highest annual range of temperature in the world. Its lowest average winter temperature is-50°C. Hence it is aptly called cold pole of the earth.

Vertical Distribution of Temperature

The permanent snow on high mountains, even in the tropics, indicates the decrease of temperature with altitude. There is a fairly regular decrease in temperature with an increase in altitude. The average rate of temperature decrease upward in the troposphere is about 6oC per km, extending to the tropopause. This vertical gradient of temperature is commonly referred to as the standard atmosphere or normal lapse rate, but is varies with height, season, latitude and other factors.

Inversion of Temperature

  • Long winter night, clear sky, dry air, and absence of winds leads to quick radiation of heat from the earth’s surface, as well as from the lower layers of the atmosphere. This results in cooling of the air near the earth’s surface. The upper layers which lose their heat not so quickly are comparatively warm. Hence the normal condition in which temperature decreases with increasing height, is reversed. The cooler air is nearer the earth and the warmer air is aloft. In other words, temperature increases with increasing height temporarily or locally. This phenomenon is termed as the inversion of temperature.

  • This phenomenon is especially observed in intermontane valleys. During winters the mountain slopes cool very rapidly due to the quick radiation of heat. The air resting above them also becomes cold and its density increases. Hence, it moves down the slopes and settles down in the valleys. This air pushes the comparatively warmer air of valleys upwards and leads to the phenomenon of inversion of temperature. Sometimes the temperature falls below freezing point in the valleys leading even to the occurrence of frost. In contrast, the higher slopes remain comparatively warmer. That is why the apple growers of Himachal Pradesh avoid the lower slopes of the mountains to escape winters frost.

In brief, it can be said that

  • Sun’s energy reaching the earth in short waves is called insolation. The amount of insolation depends upon angle of incidence, duration of the day, and transparency of the atmosphere. The processes involved in the heating and cooling of the atmosphere are radiation, conduction, convection, and advection. Terrestrial radiation is the amount of heat radiated back from the earth. There is a balance between the receipt of insolation and the terrestrial radiation on earth’ s surface. It is known as heat budget. Global warming is the worldwide increase of atmospheric temperature due to depletion of ozone layer and increase in carbon dioxide

  • Temperature measures the intensity of heat. Distribution of temperature varies both horizontally and vertically. The factors controlling its distribution are latitude, land and water contrast, winds, ocean currents, altitude, and aspect of slope.

  • Temperature also decreases with increasing altitude. The rate at which it decreases in normal conditions is known as normal lapse of temperature. It is 6oC per 1000m of height. The phenomenon of inversion of temperature also occurs when temperature increases with increase in height. It is generally local and temporary in character.

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