Atmospheric subsidence occurs when the normal upward flow of air in the atmosphere, known as atmospheric convection, is disturbed. To understand what happens during convection, imagine a parcel of air located immediately above the ground during sunrise. As solar energy warms the earth, heat is transferred to that parcel of air. Warm air is less dense than cold air, so the heated parcel has a tendency to rise upward, or convect, into the atmosphere. As the parcel rises, it expands, causing cooling. Cooling causes water vapor (gas) in the air to change state to a liquid; water droplets form, producing clouds. An air parcel will continue to rise upward until its density is equal to the surrounding atmosphere, that is, until it is stable.
Convection, which creates a large area of low pressure and converging winds at the earth's surface, known as a cyclone, is not always present. Subsidence, or sinking of air, may happen instead, forming an area of high pressure, an anticyclone. Large scale subsidence occurs when air several thousands of feet overhead is denser than the surrounding air. This denser air is produced when winds aloft converge or air aloft is very cold, or warm, but unusually dry. The dense air sinks due to the pull of gravity, compressing the air, creating high pressure at the surface and diverging winds just above the surface. Warming of the air as it subsides increases evaporation, causing clear skies. That is why high pressure systems are usually associated with fair weather.
The subsiding air may settle onto a cooler air layer, creating what is known as a temperature inversion, or subsidence inversion. In a temperature inversion, a warm air layer, several hundred or thousand feet above the surface, is trapped between cooler layers above and below. This inversion resists convection of surface air, since the surface air is stable, and causes air pollution to be trapped at the surface. Subsidence inversions commonly occur at high latitudes during the winter and over the eastern United States during the late summer months. During an inversion, an urban area's air pollution may become a serious health hazard.