Atmospheric Structure: a brief overview Essay

6.2.1 Atmospheric Structure 

The atmosphere comprises of four distinct layers, namely

1. Thermosphere
2. Stratosphere
3. Mesosphere
4. Troposphere

Each layer is briefly described here under

1. Thermosphere

It is the hottest and most distant (from earth) layer of the atmosphere beginning approximately at an altitude of 80 km. The temperature reaches about 1000 oC due to absorption of intense solar radiation by the limited amount of oxygen remaining. This layer is characterized by increase in temperature with altitude as shown in figure no 6.1.

2. Mesosphere

The coldest temperature, 110 oC, occur at the upper boundary of this layer. It extends from 50 km to 80 km in altitude and is characterized by the decrease in the temperature with increasing altitude. It accounts only for 0.1 % of the mass of the atmosphere as a whole. Average summer and winter temperatures recorded are  110 oC and  60oC respectively.

3.  Stratosphere

It extends from approximately 10 12 km to 50 km above the Earths surface and is characterized by very high concentration of atmospheric ozone. It contains nearly 90 % of the atmospheric ozone, a form of oxygen that absorbs high energy solar radiation. Due to this the temperature increases with altitude beyond 25 km. Up to 25 km altitude, the temperature remains fairly constant. Any substance entering this layer remains there for a long time due to very poor vertical mixing of air masses and absence of precipitation. The protective ozone layer exists in this layer.

4. Troposphere

This is the layer where we live and breath. It extends up to approximately 16 km in the tropics and 8 km in higher latitudes. Nearly 99% of the atmospheric water vapor is contained within this layer. The temperature and water vapor both decrease rapidly with increasing altitude. It accounts for more than 80 % of the total mass of the atmosphere. Practically all of the water vapor, gases, dust, clouds etc. are contained in this layer. This is the layer where most weather occurs. It is characterized by vertical and horizontal movements of air  resulting into formation of clouds and wind respectively. Air masses are mixed vertically due to which the emitted pollutants can reach the top within a few days. Any substance entering this layer may under go chemical changes and or be washed back to Earths surface due to precipitation.

Figure 6.1. Four layers of atmosphere

Table 6.1 Characteristics of troposphere and stratosphere

Troposphere Stratosphere

1.  Extent  Ground level to 16 km (i) Extent  16 to 65 km
2.  Temperature normally decreases with altitude, up to 700F (590C) (2.) Temperature increases with altitude, up to +320 F (00C)
3.  Much vertical mixing (iii) Little vertical mixing, slow exchange of gases with troposphere, via diffusion.
4.  Substances entering may be washed back to Earth (iv) Substances entering remain therefore long time unless attacked by sunlight or other chemicals
5.  All weather and climate take place here. (6.) Contains the protective ozone layer