troposphere n : the lowest atmospheric layer; from 4 to 11 miles high (depending on latitude)

English

Etymology

From troposphère.

Noun

1. The lower levels of the atmosphere extending from the earth's surface up to the tropopause. It is characterized by convective air movements and a large vertical temperature change.

• ρ stands for density
• z stands for height
• p stands for pressure
• R stands for the gas constant
• T stands for temperature in kelvins
• m stands for the molar mass

Since temperature in principle also depends on altitude, one needs a second equation to determine the pressure as a function of height, as discussed in the next section.

Temperature

The temperature of the troposphere generally decreases with altitude. The rate at which the temperature decreases, -dT/dz, is called the lapse rate. The reason for this decrease is as follows. When a parcel of air rises, it expands, because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work; but generally it does not gain heat in exchange from its environment, because its thermal conductivity is low (such a process is called adiabatic). Since the parcel does work and gains no heat, it loses energy, and so its temperature decreases. (The reverse, of course, will be true for a sinking parcel of air.)

p(z)T(z)^=constant

where \gamma is the heat capacity ratio (\gamma=7/5, for air). Combining with the equation for the pressure, one arrives at the dry adiabatic lapse rate,

\frac=- \frac \frac=-9.8^\mathrm/\mathrm

If the air contains water vapor, then cooling of the air can cause the water to condense, and the behavior is no longer that of an ideal gas. If the air is at the saturated vapor pressure, then the rate at which temperature drops with height is called the saturated adiabatic lapse rate. More generally, the actual rate at which the temperature drops with altitude is called the environmental lapse rate. In the troposphere, the average environmental lapse rate is a drop of about 6.5 °C for every 1 km (1000 meters) increase in height.

The environmental lapse rate (the actual rate at which temperature drops with height, dT/dz) is not usually equal to the adiabatic lapse rate (or correspondingly, dS/dz \ne 0). If the upper air is warmer than predicted by the adiabatic lapse rate (dS/dz > 0), then when a parcel of air rises and expands, it will arrive at the new height at a lower temperature than its surroundings. In this case, the air parcel is denser than its surroundings, so it sinks back to its original height, and the air is stable against being lifted. If, on the contrary, the upper air is cooler than predicted by the adiabatic lapse rate, then when the air parcel rises to its new height it will have a higher temperature and a lower density than its surroundings, and will continue to accelerate upward.

Temperatures decrease at middle latitudes from an average of 15°C at sea level to about -55°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 remains constant for a while and then 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. Thus, the tropopause is an inversion layer, and there is little mixing between the two layers of the atmosphere.

See also

• Table of Global Climate System Components
• Transition altitude
• Tropospheric scatter

References

External links

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

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