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Issue

For the past few days, north Indian states are experiencing unusually cold days although the night temperatures are normal.

 

Background

Meteorology department has attributed this to cloud cover in the region. The western disturbance has brought precipitation resulting in cloudy conditions.

 

Details

• During day, clouds obstruct heat from the sun to reach ground, reflecting some into space. This lowers the temperature.

• At night they act as blankets and help retain some of the heat absorbed by the ground. An overcast weather acts as greenhouse warming.

 

Temperature inversion

• Temperature inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the thermal lapse rate.

• Normally, air temperature decreases with an increase in altitude. During an inversion, warmer air is held above cooler air; the normal temperature profile with altitude is inverted. 

• An inversion traps air pollution, such as smog, close to the ground. An inversion can also suppress convection by acting as a "cap".

• If this cap is broken for any of several reasons, convection of any moisture present can then erupt into violent thunderstorms. Temperature inversion can notoriously result in freezing rain in cold climates.

 

Favourable conditions

• Long winter nights: Loss of heat by terrestrial radiation from the ground surface during night maybe more than the amount of incoming solar radiation.

• Cloudless and clear sky: Loss of heat through terrestrial radiation is more rapid without cloud cover.

• Dry air near the ground surface: It reduces the absorption of the radiated heat from the Earth’s surface.

• Slow movement of air: It prevents mixing of heat in the lower layers of the atmosphere.

• Snow covered ground surface: It causes maximum loss of heat through reflection of incoming solar radiation.

 

Types of inversions

• Ground inversion develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by radiation.

• Turbulence inversion often forms when quiet air overlies turbulent air. Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer.

• Subsidence inversion develops when a widespread layer of air descends. The layer is compressed and heated by the resulting increase in atmospheric pressure, and, as a result, the lapse rate of temperature is reduced.

•  Frontal inversion occurs when a cold air mass undercuts a warm air mass and lifts it above. The front between the two air masses then has warm air above and cold air below.