The lowermost part of the atmosphere in which we live, in which most clouds form and which is the theatre for weather as we know it, is termed as the troposphere. It contains about 75 per cent of the total gaseous mass of the atmosphere and practically all the moisture and dust particles.

The term troposphere was first suggested by Teisserence de Bort. Troposphere literally means the ‘region of mixing’. It has been derived from the Greek word ‘tropos’ meaning ‘mixing’ or ‘turbulence’.

The average height of this lowermost layer of the atmosphere is placed at about 14 kilometers above sea level. However, its height varies from place to place and from season to season.

Under normal conditions, the height of the troposphere at the poles is about 8 kilometers, while at the equator it is about 16 kilometers. Thus, there are marked variations in the height of this layer as between different latitudes.

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Troposphere is marked by turbulence and eddies. It is also called the convective region, for all the convective activities cease at the upper limit of the troposphere.

Various types of clouds, thunderstorms as well as the cyclones and anticyclones occur in this sphere because of the concentration of almost all the water vapour and aerosols in it.

Another important characteristic of the troposphere is that the wind velocities increase with height and attain the maximum at the top.

The most important feature of the troposphere is that there is a decrease of temperature with increasing elevation at a mean lapse rate of about 6.5° Celsius per kilometer (or 3.6° F/1000 ft.).

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However, there is an abrupt change in the lapse rate at an altitude of about 14 kilometers. The level of change is the tropopause, which also marks the upper boundary of the troposphere.

The lowest part of the troposphere is further divided into three sub-layers: the friction layer, the surface boundary layer and the laminar layer. The height of the friction layer is about 1 km from the surface of the earth.

In this layer the surface topography controls the wind speed and its direction. The surface boundary layer extends up to a few meters from the earth’s surface, whereas the laminar layer comprises only a few millimeters.

In this sub-layer the air is held almost stationary around all solid and liquid surfaces by molecular forces. It provides thermal insulation.

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At the top of the troposphere there is a shallow layer separating it from the next thermal layer of the atmosphere, i.e. the stratosphere. This shallow layer is known as the tropopause.

The word tropopause has also been taken from the Greek word which literally means, ‘where the mixing stops’. It was Sir Napier Shaw who for the first time used this word.

Because of warn temperatures and highly developed thermal mixing the tropopause has its greatest height near the equator, i.e. about 18 kilometers.

It is interesting to note that the lowest temperatures in the entire troposphere are found directly over the equator and not at the poles.

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In the middle-latitudes the jet streams may go beyond the limits of the troposphere causing variations in the height of the tropopause. On an average, the tropopause shows a gradual slope from equator to poles, but especially in the middle latitudes the height of the tropopause changes with the weather situation.

Tropopause is fairly well defined in the tropics, but less so in the Polar Regions. The tropopause is the upper limit of most clouds and storms. In fact, the tropopause marks the general upper limit of the transfer of atmospheric properties by large scale vertical turbulence and mixing.

The tropopause is usually characterized by a sharp temperature inversion. Above this narrow transition zone, there is a slight increase in temperature with increasing elevation.

It may be noted that the increase in temperature does not start just at the tropopause. However, the rate of increase is much lower at the tropopause.

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In the middle and high latitudes the height of the tropopause varies according to seasons. For example, at latitudes 45° N and S the average height of the tropopause in January is about 12.5 kilometers, while in July it becomes 15 kilometers.