Temperature Anomaly or Thermal Anomaly

Under normal circumstances, the temperature decreases from equator towards the poles and each latitude has its own temperature. But other factors such as altitude, distribution of land and water, prevailing winds, ocean currents, etc. also affect the temperature of a place. As a result, there is a difference between the mean temperature of any place and the mean temperature of its parallel which is known as temperature anomaly or thermal anomaly.

Due to more land area in the Northern Hemisphere and more water area in the Southern Hemisphere, the largest anomalies are found in the Northern Hemisphere while the smallest anomalies occur in the Southern Hemisphere. When the temperature of a place is less than the expected temperature of latitude, it is said to be negative anomaly. Contrary to this, if the temperature of a place is more than the expected temperature of its latitude, it is said to be positive anomaly.

For the year as a whole the anomalies are negative over the continents from about 40° latitude towards the poles and positive towards the equator. On the ocean, the anomalies are positive poleward from about 40° latitude and negative towards the equator.

Horizontal Distribution of Temperature

Horizontal distribution of temperature is shown by isotherms. An isotherm (isos : equal, therms : temperature) is an imaginary line joining places having equal temperatures, reduce to sea-level to eliminate the effects of altitude. Isotherms run almost parallel to latitudes because same amount of insolation is received and same temperature is experienced by almost all points located on a particular latitude.

However, the global pattern of isotheris is modid due to the inuence of land and sea. Since land is heated and cooled more quickly than sea, there is dierential heating of land and water. Consequently at any latitude the temperature over land mass is higher in summer and lower in winter than the temperature over the sea. Isotherms, therefore, bend slightly while crossing from landmasses to oceans and vice versa.

In January, it is winter in the northern hemisphere and summer in the southern hemisphere. As the air over the ocean is warmer than that over the landmasses in the northern hemisphere, the isotherms bend equatorward while crossing the landmasses and poleward while crossing the oceans. In the southern hemisphere, the conditions during this season are just the reverse. In July it is summer in the northern hemisphere and winter in the southern hemisphere. Land areas are hotter than the water areas in the northern hemisphere.

Consequently in the northern hemisphere the isotherms bend equatorward while crossing the oceans and poleward while crossing the land masses. In the southern hemisphere, it is vice versa. There is more water area in the southern hemisphere resulting in uniform temperature distribution. So there are less bends in the isotherms and their east-west trend is more clear in the southern hemisphere as compared to northern hemisphere. Distance between isotherms indicates the rate of change of temperature. Close spacing of isotherms indicates a rapid change in temperature and wide spacing means slow change.

Range of Temperature

The dierence between maximum and minimum temperature is known as range of temperature. It is of the following two types. viz., (i) Diurnal Range of Temperature and (ii) Annual Range of Temperature.

Diurnal or Daily Range of Temperature

The difference between the maximum temperature during day and the minimum temperature at night is called diurnal or daily range of temperature. For example, if the maximum temperature of a place during day time is 30°C and the minimum temperature during night is 20°C, then the diurnal range of that place will be 30–20°C = 10°C

Diurnal range of temperature is suciently high at the equator and gradually decreases towards the poles. Diurnal range of temperature is minimum near the sea coast due to moderating eect of the sea. It gradually increases as we move away from the sea coast towards continental interiors. Cloud cover reduces the range of temperature because it obstructs the incoming solar radiation during the day and outgoing terrestrial radiation at night. Desert areas have higher diurnal range of temperature because sand absorbs and radiates heat quicker than other surfaces.

Annual Range of Temperature

The difference between the average temperature of the hottest month and the coldest month of the year is known as annual range of temperature. If the temperature of the hottest month at a place is 40°C and that of the coldest month is 25°C, then the annual range of temperature at that place will be 40°–25° =15°C. Annual range of temperature is minimum near the equator and increases towards the poles. This is due to the fact that seasonal changes are minimum at the equator and maximum at the poles.

Singapore, located at 1°10’N latitude, has maximum and minimum temperature of 28°C and 27°C and its annual range of temperature is 1.0°C only. On the other extreme end of the scale is Verkhoyansk in Russia located at 67°50’N. It has mean temperature of –50°C in January and mean temperature of 15.5°C in July. Thus the annual range of temperature in Verkhoyansk is 65.5°C which is the highest recorded anywhere in the world.

Vertical Distribution of Temperature

Within the troposphere, temperature decreases with height at the rate of 1°C per 165 metres (or 6.4°C per km). This is known as normal lapse rate. This is due to the fact that atmosphere is mainly heated from below by the terrestrial radiation. As a result of this, the atmospheric layer lying immediately above the earth receives maximum heat and becomes the warmest layer. But as we go higher up, the successive layers receive lesser amount of heat and have lower temperature. The average normal temperature at the surface of the earth is about 20°C which is reduced between –55°C to –60°C at the upper limit of the troposphere (i.e. on reaching tropopause).

Above the troposphere, the temperature remains more or less constant at –55°C to –60°C upto 20 km height. Between 20 to 50 km height there is a layer of ozone gas. This gas is capable of absorbing ultraviolet rays coming from the sun and the temperature starts rising here. By the time we reach upto 50 km height the temperature rises to 0°C. After this, there is mesosphere layer in which the temperature starts falling again. It reaches –80°C at 80 km altitude. Beyond 80 km height, the temperature again starts rising. The temperature conditions in the atmosphere upto a height of about 160 km. It is estimated that the temperature rises to 1000°C at about 400 km height. s layer of the atmosphere is called ‘Thermosphere’ due to high temperature prevai ling there.

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