ATMOSPHERE

The troposphere ends when there is a variation in temperature with height. This area, which is the top of the troposphere and bottom of the stratosphere, is known as the tropopause.

The inversion of temperature beyond the tropopause prevents further

convection of air, thereby confining most of the weather phenomenon in the troposphere.

Stratosphere

Stratopshere is the second layer of the Earth's atmosphere. It starts at the top of the tropopause and is 50 km above the sea level. The term stratosphere is derived from the Greek word strata which means arrangement in horizontal layers. Approximately 99% if the atmosphere is located up to this height which includes almost all of the gases.

Weather balloons and jet aircrafts fly in this region as the air present in this layer is very thin. This increases the fuel efficiency of the aircrafts.

The stratosphere also contains the ozone layer, at a height of about 30 km. The density or thickness of ozone layer above the surface of the Earth is measured in Dobson Units (DU). The ozone layer is thinnest near the equator (-260 DU) and its thickness increases towards the poles, though there are seasonal fluctuations. Ozone - which is a mainly formed photochemical smog - is considered to be dangerous for health if reached the ground level or near the surface of the Earth. However, in the stratosphere, the high energy UV radiation is absorbed by the ozone layer and heats up the stratosphere. This also prevents skin cancer and damage to vegetation.

The layer which is top of the stratosphere and bottom of the mesosphere is known as the stratopause. Here, the temperature becomes constant with altitude until it again starts increasing in the mesosphere.

Mesosphere

It is the middle layer of the Earth's atmosphere. The name derives from the Greek word misos meaning middle. This layer starts at 50 km above the sea level and extends to about 85 km. The air in this layer is very thin and molecules are at long distance from each other. Not much is known about this layer because weather balloons and jet aircrafts do not fly so high and it is too low from an orbit of satellites and space shuttles. Some scientific studies have been made using sounding rockets that are not required to go into orbit. It is found that meteors generally burn up in the mesosphere. Thus, this layer protects the surface of the Earth from being pockmarked with craters.

The temperature of this layer also decreases with increase in altitude. The temperature of upper layer of mesosphere is even less than -100°C. At these low temperatures, freezing of water vapour will take place and polar-mesospheric clouds are formed. They are known to be the highest water-based aerosols and ends at mesopause. It marks the minimum temperature of mesosphere (making it the coldest part of the Earth also) and is the bottom of the next layer - the thermosphere.

Thermosphere

It is the fourth and the widest among the layers of the atmosphere. It is at 90 km or starts from the top of mesopause to between 500 and 1000 km, varying with solar activity. When more high energy radiations are emitted by the Sun, the thermosphere expands under this heat and thus, its height at the top varies. The air is so thin in this layer, that it is generally considered to be part of the outer space. The Kármán Line at about 100 km which is called as the boundary of the atmosphere also lies in this layer. In this layer, we can also find many satellites and space shuttle orbits.

Temperature inversion takes place in the thermosphere and temperature increases as one goes up. The temperature can range from 500°C to 2000°C or higher, depending upon the position and activity of the Sun. However, the air feels cold because the molecules are so far apart that a molecule may travel up to 1km before collision with another molecule.