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The lowest part of the atmosphere is the troposphere, a layer where the temperature generally decreases with height. This layer contains most of Earth’s clouds and is where the weather mainly occurs.
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The lower levels of the troposphere are usually strongly influenced by the Earth’s surface. This sublayer, known as the planetary boundary layer, is the region of the atmosphere in which the surface affects temperature, humidity and wind speed through turbulent mass transfer. As a result of surface friction, winds in the planetary boundary layer are usually weaker than above and tend to blow towards areas of low pressure. For this reason, the planetary boundary layer was also called an Ekman layer, for the Swedish oceanographer Vagn Walfrid Ekman, a pioneer in the study of the behavior of wind-driven ocean currents.
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Under clear, sunny skies over land, the planetary boundary layer tends to be relatively deep as a result of the heating of the ground by the sun and the resulting generation of convective turbulence. In summer, the planetary boundary layer can reach heights of 1 to 1.5 km (0.6 to 1 mile) above the land surface—for example, in the humid eastern United States—and up to 5 km (3 miles) in the southwestern desert. Under these conditions, as unsaturated air rises and expands, the temperature decreases at the dry adiabatic lapse rate (9.8 °C per kilometer, or about 23 °F per kilometer) through most of the boundary layer. Near the heated surface of the Earth, the air temperature decreases superadiabatically (with a decay rate greater than the dry adiabatic lapse rate). In contrast, during clear, quiet nights, turbulence tends to stop, and radiation cooling (net loss of heat) from the surface results in an air temperature that increases with height above the surface.
When the rate of temperature decrease with height exceeds the adiabatic lapse rate for a region of the atmosphere, turbulence is generated. This is due to the convective overturning of the air as the warmer air in the lower level rises and mixes with the cooler air. In this situation, because the environmental lapse rate is greater than the adiabatic lapse rate, an ascending parcel of air remains warmer than the surrounding air, even as the parcel both cools and expands. Evidence of this circumstance is produced in the form of bubbles, or eddies, of warmer air. The larger bubbles often have enough buoyancy energy to penetrate the top of the boundary layer. The subsequent rapid air displacement brings the air from the height into the boundary layer, thereby deepening the layer. Under these conditions of atmospheric instability, the air aloft cools at the ambient lapse rate faster than the rising air cools at the adiabatic lapse rate. The air above the boundary layer replaces the rising air and undergoes compressional warming as it descends. As a result, this entrained air heats the boundary layer.
The ability of convective bubbles to break through the top of the boundary layer depends on the environmental lapse rate. The upward movement of penetrative bubbles will decrease rapidly if the pack rapidly becomes cooler than the ambient environment surrounding it. In this situation, the piece of air will become less buoyant with additional lift. The height that the boundary layer reaches on a sunny day is therefore strongly influenced by the intensity of surface heating and the environmental flow just above the boundary layer. The faster a rising turbulent bubble cools across the boundary layer relative to its surroundings, the less chance there is of subsequent turbulent bubbles penetrating far beyond the boundary layer. The top of the diurnal boundary layer is referred to as a mixed layer inversion.
On clear, calm nights, radiative cooling leads to an increase in temperature with altitude. In this situation, known as a night inversion, turbulence is suppressed by the strong thermal stratification. Thermally stable conditions occur when warmer air overlies cooler, denser air. Over flat terrain, an almost laminar wind flow (a pattern where winds from an upper layer slide easily past the winds from a lower layer) can occur. The depth of the radiatively cooled air layer depends on several factors, such as the moisture content of the air, the soil and vegetation characteristics and the terrain configuration. In a desert environment, for example, the nocturnal inversion tends to be found at higher altitudes than in a more humid environment. The inversion in wetter environments occurs at a lower altitude because longer-wave radiation emitted by the surface is absorbed by many available water molecules and returned to the surface. As a result, the lower levels of the troposphere are prevented from cooling rapidly. If the air is moist and sufficient surface cooling has occurred, water vapor will condense into what is called “radiation fog”.
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In windy conditions, the mechanical production of turbulence becomes important. Turbulent eddies produced by wind shear tend to be smaller in size than the turbulence bubbles produced by the rapid convection of buoyant air. Within a few tens of meters of the surface in windy conditions, wind speed increases dramatically with height. If the wind is strong enough, the turbulence generated by wind shear can overwhelm the resistance of stratified, thermally stable air.
In general, there is rather little turbulence above the boundary layer in the troposphere. Nevertheless, there are two notable exceptions. First, turbulence is produced in jet streams where large velocity shears exist both within and adjacent to cumuliform clouds. In these places, a buoyant turbulence arises as a result of the release of latent heat. Second, pockets of buoyant turbulence can be found at and just above cloud tops. In these places, the radiative cooling of the clouds destabilizes the air pockets and makes them more lively. Clear-air turbulence (CAT) is often reported when aircraft fly near one of these regions of turbulence generation.
The top of the troposphere, called the tropopause, corresponds to the level at which the pattern of decreasing temperature with height stops. It is replaced by a layer that is essentially isothermal (with the same temperature). In the tropics and subtropics, the tropopause is high, often reaching about 18 km (11 miles), as a result of vigorous vertical mixing of the lower atmosphere by thunderstorms. In polar regions, where such deep atmospheric turbulence is much less common, the tropopause is often as low as 8 km (5 miles). Temperatures at the tropopause range from as low as −80 °C (−112 °F) in the tropics to −50 °C (−58 °F) in polar regions.
The region above the planetary boundary layer is commonly known as the free atmosphere. The wind at this volume is not directly retarded by surface friction. Clouds occur most often in this part of the troposphere, although fog and clouds that rise or develop over elevated terrain often occur at lower levels.
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There are two basic types of clouds: cumuliform and stratiform. Both cloud types develop when clear air rises, cooling adiabatically as it expands until either water begins to condense or deposition occurs. Water undergoes a state change from gas to liquid under these conditions because colder air can hold less water vapor than warmer air. For example, air at 20 °C (68 °F) can contain almost four times as much water vapor as at 0 °C (32 °F) before saturation takes place and water vapor condenses into liquid droplets.
Cirrus fibratus are high clouds that are almost straight or irregularly curved. They appear as fine white filaments and are generally distinct from each other.
Stratiform clouds form when saturated air is mechanically forced upward and remains colder than the surrounding clear air at the same altitude. In the lower troposphere such clouds are called stratus. Advection fog is a stratus cloud with a base at the Earth’s surface. In the middle troposphere, stratiform clouds are known as altostratus. In the upper troposphere are the terms
Be used. The cirrus cloud type refers to thin, often wispy, cirrostratus clouds. Stratiform clouds that both extend through a large part of the troposphere and precipitate are called nimbostratus.
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Cumuliform clouds form when saturated air is turbulent. Such clouds, with their bubbly turreted shapes, exhibit the small-scale up-and-down behavior of air in the turbulent planetary boundary layer. Often such clouds are seen with bases at or near the top of the boundary layer as turbulent eddies generated near the Earth’s surface reach high enough to cause condensation.
Cumuliform clouds form in the free atmosphere when a parcel of air, at saturation, is warmer than the surrounding atmosphere. Because this air parcel is warmer than its surroundings, it will accelerate upwards, creating the saturated turbulent bubble characteristic of a cumuliform cloud. Cumuliform clouds that do not reach higher than the lower troposphere are known as cumulus humulus if they are randomly distributed and as stratocumulus if they are organized in lines. Cumulus congestus clouds extend into the middle troposphere, while deep, precipitating cumuliform clouds that extend through the troposphere are called
