What Happens to Air Temp & Dewpoint Temp in a Descending Air Mass?

Descending Air Masses

• Descending air masses also are known in meteorology as areas that produce anticyclones. As sinking air reaches the surface, it spreads out in all directions, producing a surface divergence. This outward flow of air is affected by the Earth's spin, known as the Coriolis effect, creating a clockwise rotation in the Northern Hemisphere. At the same time, upper-level air is drawn into the area of high pressure to replace the descending air, producing a convergence aloft. The result is stable, clear skies, with light and variable winds. These conditions tend to be persistent, as high-pressure areas move slowly and are resistant to the influences of other weather systems.

Effects of High Pressure on Dewpoints

• The warmer a parcel of air is the more moisture it can hold. Conversely, the colder a parcel of air is the less moisture it can hold. The dewpoint represents the temperature to which a parcel of air must cool in order to reach saturation. At this temperature, the relative humidity would reach 100 percent and condensation (water droplets or dew) would form. In the troposphere, temperature and altitude are inversely related, meaning that as you rise in altitude, the temperature drops. In a high-pressure system, the descending air mass is warmed, causing a greater difference from the dewpoint temperature. Secondly, as the air mass warms, it dries out. This lowers the dewpoint temperature, as the parcel of air would have to cool even more for the air, now drier, to reach its saturation point.

Effects of High Pressure on Temperatures - Summer

• During summer months in the Northern Hemisphere, high-pressure systems cause temperatures to rise. This is the result of radiant heating. As the descending air warms and drops in water vapor content, the air's ability to transmit radiant heat increases. This means the sun is able to heat the Earth's surface more efficiently, thereby raising temperatures. This process is amplified by the fact that high-pressure systems produce stable atmospheric conditions. The dry air limits cloud formation and the downward circulation limits vertical development. All of this means that high-pressure systems are generally accompanied by clear skies, allowing more sunlight to more efficiently heat the surface -- driving up temperatures.

Effects of High Pressure on Temperatures - Winter

• During winter months, high-pressure systems in the Northern Hemisphere result in just the opposite from summertime -- lower temperatures. The reason for the change in temperature is the same, though the resulting direction of the change is the reverse. The moisture content of the descending air is still reduced, allowing the air to transmit radiant energy more efficiently. In this case, however, solar heating is limited by the sun's wintertime position. Instead, the clear skies and light winds allow ground heat to escape very quickly, radiating heat in the opposite direction - out into space. This in turn causes temperatures to drop, especially at night.