thumb|upright=1.5|A picture of Earth's troposphere with its different [[cloud types of low to high altitudes casting shadows. Sunlight is reflected off the ocean, after it was filtered into a reddish light by passing through much of the troposphere at sunset. The above lying stratosphere can be seen at the horizon as a band of its characteristic glow of blue scattered sunlight.]]

thumb|right|upright=1.35|Atmospheric circulation: the three-cell model of the [[Atmospheric circulation|circulation of the planetary atmosphere of the Earth, of which the troposphere is the lowest layer.]]

The troposphere is the lowest layer of the atmosphere of Earth. Pronounced , the name comes . It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary surface of the Earth, the average height of the troposphere is in the tropics; in the middle latitudes; and in the high latitudes of the polar regions in winter; thus the average height of the troposphere is .

The term troposphere derives from the Greek words tropos (rotating) and sphaira (sphere) indicating that rotational turbulence mixes the layers of air and so determines the structure and the phenomena of the troposphere. The rotational friction of the troposphere against the planetary surface affects the flow of the air, and so forms the planetary boundary layer (PBL) that varies in height from hundreds of meters up to . The measures of the PBL vary according to the latitude, the landform, and the time of day when the meteorological measurement is realized. Atop the troposphere is the tropopause, which is the functional atmospheric border that demarcates the troposphere from the stratosphere. As such, because the tropopause is an inversion layer in which air-temperature increases with altitude, the temperature of the tropopause remains constant.

Temperature

The planetary surface of the Earth heats the troposphere by means of latent heat, thermal radiation, and sensible heat.

The gas layers of the troposphere are less dense at the geographic poles and denser at the equator, where the average height of the tropical troposphere is 13 km, approximately 7.0 km greater than the 6.0 km average height of the polar troposphere at the geographic poles; therefore, surplus heating and vertical expansion of the troposphere occur in the tropical latitudes. At the middle latitudes, tropospheric temperatures decrease from an average temperature of at sea level to approximately at the tropopause. At the equator, the tropospheric temperatures decrease from an average temperature of at sea level to approximately at the tropopause. At the geographical poles, the Arctic and the Antarctic regions, the tropospheric temperature decreases from an average temperature of at sea level to approximately at the tropopause.

Altitude

thumb|upright=1.5|A picture of Earth's atmosphere as viewed from an [[airplane, traveling over the Arctic.]]

The temperature of the troposphere decreases with increased altitude, and the rate of decrease in air temperature is measured with the environmental lapse rate (<math>-dT/dz</math>), which is the numeric difference between the temperature of the planetary surface and the temperature of the tropopause divided by the altitude. Functionally, the ELR equation presumes that the planetary atmosphere is static and that there is no mixing of the layers of air by either vertical atmospheric convection or winds that could create turbulence.

The difference in temperature derives from the planetary surface absorbing most of the energy from the sun, which then radiates outwards and heats the troposphere (the first layer of the atmosphere of Earth) while the radiation of surface heat to the upper atmosphere results in the cooling of that layer of the atmosphere. The ELR equation also assumes that the atmosphere is static, but heated air becomes buoyant, expands, and rises. The dry adiabatic lapse rate (DALR) accounts for the effect of the expansion of dry air as it rises in the atmosphere, and the wet adiabatic lapse rate (WALR) includes the effect of the condensation-rate of water vapor upon the environmental lapse rate.

{| class="wikitable" style="border-spacing: 5px; margin:auto;"

|+ Environmental Lapse Rate (ELR)

|-

! scope="col" style="width:150px;"| Altitude Region

! scope="col" style="width:100px;"| Lapse rate

! scope="col" style="width:100px;"| Lapse Rate

|-

! scope="col" style="width:150px;"| (m)

! scope="col" style="width:100px;"| (°C / km)

! scope="col" style="width:100px;"| (°F / 1000 ft)

|- align="center";

| &nbsp;&nbsp; &nbsp; 0.0 &nbsp; – 11,000 || &nbsp; 6.50 || &nbsp; 3.57

|- align="center";

| 11,000 – 20,000 || &nbsp; 0.0&nbsp; || &nbsp; 0.0&nbsp; &nbsp;

|- align="center";

| 20,000 – 32,000 || −1.0 || −0.55

|- align="center";

| 32,000 – 47,000 || −2.8 || −1.54

|- align="center";

| 47,000 – 51,000 || &nbsp; 0.0 &nbsp; || &nbsp; 0.0 &nbsp; &nbsp;

|- align="center";

| 51,000 – 71,000 || &nbsp; 2.80 || &nbsp; 1.54

|- align="center";

| 71,000 – 85,000 || &nbsp; 2.00 || &nbsp; 1.09

|-

|}

Compression and expansion

A parcel of air rises and expands because of the lower atmospheric pressure at high altitudes. The expansion of the air parcel pushes outwards against the surrounding air, and transfers energy (as work) from the parcel of air to the atmosphere. Transferring energy to a parcel of air by way of heat is a slow and inefficient exchange of energy with the environment, which is an adiabatic process (no energy transfer by way of heat). As the rising parcel of air loses energy while it acts upon the surrounding atmosphere, no heat energy is transferred from the atmosphere to the air parcel to compensate for the heat loss. The parcel of air loses energy as it reaches greater altitude, which is manifested as a decrease in the temperature of the air mass. Analogously, the reverse process occurs within a cold parcel of air that is being compressed and is sinking to the planetary surface.

Environment

The environmental lapse rate (<math>dT/dz</math>), at which temperature decreases with altitude, usually is unequal to the adiabatic lapse rate (<math>dS/dz \ne 0</math>). If the upper air is warmer than predicted by the adiabatic lapse rate (<math>dS/dz > 0</math>), then a rising and expanding parcel of air will arrive at the new altitude at a lower temperature than the surrounding air. In which case, the air parcel is denser than the surrounding air, and so falls back to its original altitude as an air mass that is stable against being lifted. If the upper air is cooler than predicted by the adiabatic lapse rate, then, when the air parcel rises to a new altitude, the air mass will have a higher temperature and a lower density than the surrounding air and will continue to accelerate and rise. The coldest layer of the atmosphere, where the temperature lapse rate changes from a positive rate (in the troposphere) to a negative rate (in the stratosphere) locates and identifies the tropopause as an inversion layer in which limited mixing of air layers occurs between the troposphere and the stratosphere.

Zonal flow

A zonal flow regime is the meteorological term meaning that the general flow pattern is west to east along the Earth's latitude lines, with weak shortwaves embedded in the flow. The use of the word "zone" refers to the flow being along the Earth's latitudinal "zones". This pattern can buckle and thus become a meridional flow.

Meridional flow

When the zonal flow buckles, the atmosphere can flow in a more longitudinal (or meridional) direction, and thus the term "meridional flow" arises. Meridional flow patterns feature strong, amplified troughs of low pressure and ridges of high pressure, with more north–south flow in the general pattern than west-to-east flow.

Solar System

right|thumb|Cross-section diagram of Titan's atmosphere

Within the Solar System, other planetary bodies with a substantial atmosphere have a troposphere. These include Venus, Mars, and the Saturnian moon Titan. Jupiter does not have a solid surface, and the lowest atmospheric layer, the troposphere, smoothly transitions into the planet's fluid interior.

The troposphere of Venus is the densest part of the atmosphere, starting at the surface and extendind upwards to 65&nbsp;km. The winds are slow near the surface, but at the top of the troposphere the temperature and pressure reaches Earth-like levels and clouds pick up speed to 100&nbsp;m/s (360&nbsp;km/h). The large amount of CO<sub>2</sub> in the atmosphere together with water vapour and sulfur dioxide create a strong greenhouse effect, trapping solar energy and raising the surface temperature to around 740&nbsp;K (467&nbsp;°C). The thick troposphere makes the difference in temperature between the day and night side small, even though the slow retrograde rotation of the planet causes a single solar day to last 116.5 Earth days. On the night side of Venus clouds can still be found at 80&nbsp;km (50&nbsp;mi) above the surface.

The troposphere of Mars contains most of the planet's weather phenomena, including convection and dust storms. Its dynamics are heavily driven by the daytime surface heating and the amount of suspended dust. Mars has a higher scale height of 11.1&nbsp;km than Earth because of its weaker gravity. The theoretical dry adiabatic lapse rate of Mars is 4.3&nbsp;°C km<sup>−1</sup>, but the measured average lapse rate is about 2.5&nbsp;°C km<sup>−1</sup> because the suspended dust particles absorb solar radiation and heat the air. The planetary boundary layer can extend to over 10&nbsp;km thick during the daytime. The near-surface diurnal temperature range is huge (60&nbsp;°C The temperature above 15&nbsp;km is controlled by radiative processes instead of convection.

Titan is the only planetary satellite with a substantial atmosphere, and it is the only atmosphere besides Earth's composed primarily of nitrogen. Titan's lower surface gravity creates a more extended atmosphere than Earth, with scale heights of . The troposphere of Titan is well defined, extending to a tropopause at an altitude of around 40 km, where the temperature is . Methane condenses out of Titan's atmosphere at high altitudes, with its abundance increasing below the tropopause, leveling off at a value of 4.9% between and the surface. Methane rain, haze rainout, and varying cloud layers are found in the troposphere.

See also

  • Jet stream
  • Trade winds

References

  • Chemical Reactions in the Atmosphere