Troposphere

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Atmosphere diagram showing the mesosphere and other layers. The layers are not to scale: from Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius.

View of Earth's troposphere from an airplane.

The troposphere is the lowest 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 vapor 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 17 km) and smallest at the poles (about 7 km). The lower part, where friction on the Earth's surface influences with air flow, is the planetary boundary layer or peplosphere which is 2 km deep on average, depending on the landform, and which is separated from the rest of the troposphere by the capping inversion layer.

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.

Temperature and pressure against altitude from the NRLMSISE-00 standard atmosphere model

1 Pressure and temperature structure
2 Atmospheric circulation
3 External links

[edit] Pressure and temperature structure

[edit] 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 altitudes, the weight of the air above is less 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 is the standard gravity
ρ is density
h is height
p is pressure
R is gas constant
T is temperature

assuming a constant temperature, pressure decreases exponentially with height:

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

[edit] Temperature

Main article: Lapse rate

In the troposphere, the temperature decreases with height at an average rate of 6.5 °C for every 1 km (1000 meters) increase in height. This decrease in temperature is caused by adiabatic cooling—as air rises the atmospheric pressure falls 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.

[edit] Tropopause

Main article: 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.

An idealised view of three large circulation cells.

[edit] Atmospheric circulation

Main article: 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.