AIR TEMPERATURE

AIR TEMPERATURE Air temperatures depend upon various geographical factors including elevation, aspect, proximity to sea, direction of prevailing winds and patterns of insolation.

Land-water Differences The absorption and radiatio properties of land and water differ. Land gets heated u rapidly and intensely under the sun's rays whereas tl' water surfaces get slowly and moderately heated. Land ge cooled off faster than water when solar radiation is cut oj These differing qualities result in greater temperatuJ contrasts over land areas, i.e., the middle of the continent while over the water areas (near the coasts), thesecontras are moderate. However, the air temperature over tr ocean has two features: (i) As water bodies heat and COI slower than land, maximum and minimum temperaturl are reached a month later than on land. (ii) The year] range of temperature in water is less than that over tl' land.

Annual Temperature Patterns The air temperatw varies at different latitudes. Isotherms (lines. connectir places having the same air temperature) which run mOl or less parallel to the lines of latitudes in the east-west zonl reflect the general decrease of insolation from equator 1 poles. During the year, the isotherms follow the declinatio of the sun and change position north or south.

The patter of temperature changes also varies at different altitudes: ( In equatorial areas, annual temperature shows little sei sonal variation. as they receive constant amount of diurni insolation throughout the year. (ii) In mid and high-Iatituc areas, seasonal variation of annual temperature is muc more marked. (iii) In areas between the tropics and pole circle in each hemisphere, air temperatures and insolatio amounts have a marked seasonal pattern. This is due 1 the fact that the sun's path in these areas shifts throug a relatively large range of noon altitude and substanti, differences exist in the length of the days. (iv) In polar regions, there are large seasonal contrasts in air tempera­ture-very low in winter or during polar nights and extremely high during the summer. As the atmosphere is only indirectly heated by the insolation through the me­dium of ground, there exists a time-lag between the air temperatu.re pattern and the insolation pattern despite a strong correlation. In winter, in the northern hemisphere, the coldest air temperatures are experienced in January,
while the winter solstice is in December because the ground continues to lose heat even after insolation has begun to increase.

Daily Cycle of Temperature The daily pattern of temperature changes illustrates energy changes on a small time-scale. Air temperature falls if the ground is cooler than air owing to the fact that the atmosphere is largely heated from the earth's sur­face. On a calm day with little cloud, air tempera­tures fall to the lowest; as the ground becomes cold during the night, the air above it is cooled by conduction. As the ground temperature rises after sunrise due to insolation, air temperature also begins to rise with the lag of about an hour. Maximum insolation received is at midday, but the maximum air temperature is usually at about 1400 hours. After this time, temperature drops as the convection mixes cooler upper air with warm air near the ground. After sunset, the air remains warm for some time on still being heated by radiation from the ground before the temperature drops eventually.

Vertical Changes of Temperature Air tempera­ture also varies according to the altitude. At higher altitudes as air becomes less dense, it is unable to absorb heat, resulting in colder air temperature. The normal drop of temperature with height is known as normal lapse rate, which is 6.4°C per km on an average. But this can vary according to geographic position, season and time of the day. Temperature inversion is the situation where there is increase in temperature with height, before beginning to drop. into the normal lapse rate. In cases where the temperature remains the same with increase in altitude, the layer of atmosphere is called isothermal.