Troposphere and tropopause

The troposphere is the lowest and thickest layer of the atmosphere. In contact with Earth's surface, the troposphere is heated by solar illumination and conduction. The tropopause is the boundary layer between the troposphere and the stratosphere .

The thickness of the troposphere depends upon a number of atmospheric variables at latitude . The troposphere ranges from a thickness of approximately 5.5 mi (9 km) in the polar regions, to a thickness of approximately 10 mi (16 km) in equatorial regions.

Although the troposphere contains more than 70% of Earth's atmosphere by weight, it is much thinner than the stratosphere or ionosphere . Because density increases with increasing mass, the troposphere exhibits a high pressure and density gradient wherein density and pressure decrease with increasing altitude.

Weather phenomena (e.g., rain, snow, etc.) take place in the troposphere. Convective currents provide mixing of air masses with different temperatures. These currents pass through regions of the troposphere that differ widely in pressure. The troposphere atmospheric pressure gradient varies by approximately 90% from sea level pressures to tropopause atmospheric pressures.

Under normal atmospheric conditions, the standard lapse rate describes decreasing temperatures encountered with increased altitude within the troposphere. The standard temperature lapse rate means that temperature decreases with altitude at a fairly uniform rate. Because the atmosphere is warmed by conduction from Earth's surface, this lapse or reduction in temperature is normal with increasing distance from the conductive source. The tropopause is specifically defined as that upper boundary layer of the troposphere where the thermal lapse rate no longer exists and temperature exhibit stability prior to increasing within the stratosphere.

The bulk of tropospheric heating occurs via conduction of heat from the surface. Differing amounts of sunlight (differential levels of solar insolation ) result in differential temperatures at the interface between Earth's surface and the troposphere. Warmer surface temperatures and higher rates of conduction allow warm air to create low-pressure zones where air is uplifted. Because surrounding air must rush in to replace the uplifted air, these warmer areas become zones of convergence (inward rushing air). Cooler surface temperatures and lower rates of conduction result in cooler, denser, higher pressure areas that form zones of divergence in which air moves outwards from the high pressure area .

The thermal instability means that the troposphere is the turbulent thermal boundary layer of Earth's atmosphere. In contrast to the unstable and vertical currents encountered in the humid troposphere, the stratosphere exhibits a near laminar (horizontal or sheet-like) flow.

See also Air masses and fronts; Atmospheric chemistry; Atmospheric circulation; Atmospheric composition and structure; Atmospheric inversion layers; Atmospheric pollution; Atmospheric pressure; Weather forecasting methods; Wind chill; Wind shear; Wind