In general, temperature drops with altitude. The rate of decrease, known as the Environmental Lapse Rate (ELR), depends on atmospheric conditions and the composition of air (water vapour, carbon dioxide, dust). On average, the atmosphere cools by 6°C every 1000 m.
A parcel of warm air which is forced upwards by the normal process of convection cools as it gains altitude. If the rate of cooling of that parcel is lower than the prevailing ELR, the air rises even higher. Such a phenomenon is common in summer and helps air pollution from metropolitan areas to rise and dissipate. The air either gets diluted or is blown away by winds of the upper atmosphere.
In the winter months, temperatures can sometimes rise (rather than decrease) with altitude. This phenomenon is known as “temperature inversion”. Among the many causes of inversion is “back radiation” combined with the strong cooling of the ground during winter nights. The heat of the day is radiated back into the atmosphere during the early part of the night, creating a layer of warm air over the surface. Subsequent parcels of air that rise are not as warm as this layer and hence tend to sink back. Such a layer of air, in essence, acts like a lid and puts an end to all vertical motion. Instead of rising and dispersing, pollutants produced by urban centers tend to hug the ground and become highly concentrated.
Bowl-shaped valleys like Kathmandu are specially affected by inversion because low-level winds which might otherwise drive away the polluted air are prevented from entering by the wall of surrounding hills. While the flow of Valley air is thus sealed from all sides, there is no letup in the production of polluting agents such as smoke and dust.
The early morning fog so characteristic of Kathmandu Valley is the result of inversion in its most natural form. The fog lifts and rapidly disappears as the morning sun’s radiation warms the cold layer of air at the Valley floor. The presence of pollutants in the air adds a new but undesirable dimension to the process of inversion.
Basnyet is a Senior Meteorologist in HMG’s Department of Hydrology and Meteorology.