LAPSE RATE IN ATMOSPHERE

On the basis of vertical distribution of temperature, the atmosphere is classified into four characteristic regions, namely troposphere, stratosphere, mesosphere and thermosphere.

Environmental or Normal Lapse Rate: The lapse rate refers to the rate at which temperature decreases with an increase in altitude in the atmosphere. There are different lapse rates in different layers of the atmosphere:

1. Troposphere (Negative Lapse Rate):

 In the troposphere, which is the lowest layer(8-17KM) of the atmosphere where weather phenomena occur, the lapse rate is called the environmental lapse rate (ELR). On average, the ELR is about 0.64°C /100 m or 6.5°C per kilometer (or 3.5°F per 1,000 feet) of altitude gained. However, it can vary due to various factors such as atmospheric stability, moisture content, and geographical location. In the upper limits of the troposphere, the temperature decreases to about –56°C. 

The upper boundary of the troposphere is known as tropopause. The transition region between troposphere and stratosphere i.e., tropopause, is a region of isothermal distribution.  For the same variations of solar radiation, the height of tropopause is nearer to earth at the poles in winter, and farthest at the equator during summer.

2. Stratosphere (Positive Lapse Rate): 

Above troposphere lies the Stratosphere extends upto approximately 20-50 km, the stabilized air forms a stratified cover over the troposphere and forms the basis for its name stratosphere. In this part, there is a temperature inversion i.e. the temperature increases to about –2°C (from–56°C in tropopause) with increases with height, due to photochemical reactions and absorption of solar radiation by ozone. Stratosphere is characterised by its temperature (– 68° to –51°C) and absence of convective air currents. The upper layer of stratosphere is called stratopause. Convection is a process in which heat is transmitted by transporting groups of molecules from place to place e.g. in water and air.

3. Mesosphere (Negative Lapse Rate): 

The mesosphere layer extends upto about 50-85Km. The upper boundary of this layer is known as mesopause. In the mesosphere, the lapse rate is generally negative, meaning that the temperature decreases with altitude. Here the temperature decreases and falls to about 180° K (−93°C).

4. Thermosphere (Positive Lapse Rate): 

In the thermosphere is the outermost/top layer of the atmosphere lies above 85-100Km. At this altitude the air is extremely thin and the molecules acquire high speed (kinetic energy) and hence result in high temperatures. Therefore, here the temperature rises and may reach as high as 1200°C. The temperature increases with altitude due to absorption of solar radiation. Thus, lapse rate in the thermosphere is highly variable and can be positive or even superadiabatic (increasing faster than the adiabatic lapse rate).