Temperature

Temperature

• It is the measurement of available heat energy in a system.
• It is a measure of hotness and coldness of the body.

The atmosphere is not heated directly by insolation or the Sun's rays; rather it is heated from below by the warmed surface of the earth, i.e. from terrestrial radiation.

• The lower levels of the atmosphere are heated by conduction.
• The earth's surface is heated during day time after receiving solar radiation.
• The upper levels of the earth get heated by convection.

The air coming in contact with the warmer surface of the earth gets heated and expands in volume. The warmer air rose up and forms vertical circulation of air. This mechanism transports heat from the ground surface to the atmosphere, thus helps in heating up of the atmosphere.

Factors controlling the distribution of temperature:

• Latitude: In general, average temperature decreases form the equator towards the poles because the sun rays become more and more oblique poleward.

• Altitude: the temperature decreases with increasing height from the earth's surface at an average rate of 6.5 °C per 1000 m. The lower layer of air contains more vapour hence it absorbs more heat radiated from the earth's surface than the upper air layers.

• Mountain Ranges: In certain areas of the world the existence of high ranges of mountains acts as formidable barrier to the free circulation of air in the lower reaches of the atmosphere. The Himalayas, for example, prevents the monsoon conditions extending further north into the interior of Asia and prevents the extremely cold anticyclonic winter conditions in Central Asia from penetrating Indian subcontinent. 

• Distance from the Sea: The Sea heats up and cools down much more slowly than theland. The effects of this phenomenon are more noticeable in the temperate latitudes where the warming effect of the sea particularly affects coastal regions in winter. In general, the sea has a moderating effect on the temperatures of the coastal areas throughout the year. On the other hand, regions deep within the interior of land masses experience extreme temperatures. This phenomenon is known as continentality.

• Ocean Currents: Ocean currents influence the temperature of the coastal regions particularly where onshore winds carry the influence of warm currents towards coastal regions in winter. Cold currents have a cooling effect on the nearby coasts but have a lesser effect than warm currents due to the fact that they often flow below offshore winds.

• Clouds: The presence or absence of clouds in the atmosphere over different regions of the earth's surface has a significant bearing on the temperature of the regions. Clouds have the effect of reducing the amount both of insolation, which reaches the surface of the earth and of outgoing radiation from the earth's surface. As a result, tropical rain forests with dense cloud cover have very little range of temperature, where as, the hot deserts, which havecomparatively less cloud cover, have both high diurnal and annual temperature ranges.

• Prevailing Winds: Prevailing winds also affect the temperature conditions of the areas. The moderating effects of oceans are brought to the adjacent lands through winds. On the contrary, off shore wind stake the effects of warm or cold currents away from land.

• Local Weather: Local weather comprises different types of storms, cloudiness, precipitation and other weather conditions. In the equatorial regions, despite the vertical rays of the Sun, large amount of cloudiness obstructs the solar radiation from reaching the earth surface. It is due to the clear skythat near the Tropic of Cancer and the Tropic of Capricorn the amount of solar radiation incident on the earth exceeds that reaching the equatorial regions. Thus, in the subtropical high pressure belt the surface water temperature in the oceans is a little higher. Besides, the incidence of daily afternoon rains in the equatorial regions does not allow the temperatures to rise further, whereas the extremely dry weather and cloudless skies prove helpful in raising the temperatures in the subtropical regions. In the same way in regions of stormy weather the ocean water temperatures are relatively lower.

Concept of Inversion of Temperature

The temperature decreases with the altitudes in the troposphere at an average rate of 6.5 °C per 1000 m, this is known as normal lapse rate. But sometimes the temperature increases upward upto a few kilometers from the earth's surface. This is known as inversion of temperature i.e. presence of warm layer of air above the cold layer of air.

Types of inversion of temperature:

1. Ground surface inversion:

The most common condition for inversion of temperature is through the cooling of the air near the ground at night. Once the sun goes down, the ground loses heat very quickly, and this cools the air that is in contact with the ground. However, since air is a very poor conductor of heat, the air just above the surface remains warm. Conditions that favour the development of a strong surface inversion are calm winds, clear skies, and long nights. Calm winds prevent warmer air above the surface from mixing down to the ground, and clear skies increase the rate of cooling at the Earth's surface. Long nights allow for the cooling of the ground to continue over a longer period of time, resulting in a greater temperature decrease at the surface. Since the nights in the winter time are much longer than nights during the summer time, surface inversions are stronger and more common during the winter months. During the daylight hours, surface inversions normally weaken and disappear as the sun warms the Earth's surface.

2. Upper air inversion:

 The thermal upper air inversion is caused by the presence of ozone layer in the stratosphere. The ozone layer absorbs most of the ultraviolet rays radiated from the sun  thus the temperature of this layer becomes  higher than the other layers.

Consequences of Temperature Inversions

a) Fog is formed due to the presence of warm air above the cold air. The cold air cools the warm air from below thus forms the tiny droplets around dust particles and smokes during winter season that result in formation of fog.

b) The urban smog is formed by the intensification of fog by pollution. When smog gets mixed with sulphur dioxide it becomes poisonous and deadly for human beings.

c) Inversion of temperature leads to formation of frost. Frost is economically unfavourable weather phenomenon as it damages fruit orchids and crops.

d) The inversion of temperatures creates anticyclonic conditions thus inhibits rainfall and encourages dry conditions.

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