Troposphere

The Troposphere is the lowermost portion of Earth's atmosphere. It is the densest layer of the atmosphere and contains approximately 75% of the mass of the atmosphere and almost all the water vapour and aerosol.

The troposphere extends from the Earth's surface up to the tropopause where the stratosphere begins. The depth of the troposphere is greatest in the tropics (about 16km) and smallest at the poles (about 8km).

The word troposphere stems from the Greek "tropos" for "turning" or "mixing". The troposphere is the most turbulent part of the atmosphere and is the part of the atmosphere in which most weather phenomena are seen. Generally, jet aircraft fly just above the troposphere to avoid turbulence.

Pressure and temperature structure

Pressure

The pressure of the atmosphere is highest at the surface and decreases with height. This is because air at the surface is compressed by the weight of all the air above it. At higher levels the weight of the air above is smaller and so the air is compressed less and has a lower pressure. This change in pressure with height can be predicted with the hydrostatic equation:

\frac{dp}{dh} = -\rho g = - \frac {pg}{RT}

where:

g = the acceleration due to gravity

ρ = density

h = height

p = pressure

R = universal gas constant

T = temperature

assuming a constant temperature, pressure decreases exponentially with height:

p(h) = p(0) e^{ - \frac{gh}{RT}}

Temperature

In the troposphere the temperature decreases with height at an average rate of 6.4 °C for every 1 km increase in height. This decrease in temperature is caused by adiabatic cooling - as air rises the atmospheric pressure falls and so the air expands. In order to expand the air must do work on its surroundings and therefore its temperature decreases (due to Conservation of energy).

Temperatures decrease at middle latitudes from approx. +17°C at sea level to approx. -52°C at the beginning of the tropopause. At the poles, the troposphere is thinner and the temperature only decreases to -45 °C, while at the equator the temperature at the top of the troposphere can reach -75 °C.

Tropopause

The tropopause is the boundary region between the troposphere and the stratosphere.

Measuring the temperature change with height through the troposphere and the stratosphere identifies the location of the tropopause. In the troposphere, temperature decreases with altitude. In the stratosphere, however, the temperature increases with altitude. The region of the atmosphere where the lapse rate changes from positive (in the troposphere) to negative (in the stratosphere), is defined as the tropopause.

Atmospheric circulation

The basic structure of large scale circulation in the atmosphere remains fairly constant. There are three convection cells in each hemisphere: the Hadley cell, the Ferrel cell, and the Polar cell which guide the prevailing winds and transport heat from the equator to the poles.

External links

The vertical structure of the atmosphere
Composition of the Atmosphere, from the University of Tennessee Physics dept.
Chemical Reactions in the Atmosphere