Cold Fronts• When cold, continental polar air advances into a region that is occupied by warmer air, this condition results in a cold front.• As it happens with warm fronts, the area of the cold front (and warm front) nearest the ground moves more slowly due to friction as compared to the air that is aloft.• Thus, the cold front steepens as it moves.
Cold Fronts• On average, cold fronts are about twice as steep as warm fronts.• In addition, cold fronts advance at speeds of 20 to 35 miles per hour compared to 15 to 20 miles per hour with warm fronts.• These two differences, steepness and rate of movement account for the more violent nature of cold fronts.
Cold Fronts• The arrival of cold fronts is sometimes preceded by alto-cumulus clouds.• As the front advances, mainly from the west and northwest, towering clouds can often be seen in the distance.• Close to the front, a band of dark, ominous clouds foretells the ensuing weather.
Cold Fronts• The forceful lifting of warm, moist air along the cold front is often so rapid that the released latent heat increases the air’s buoyancy.• Heavy downpours and vigorous wind gusts that are associated with mature cumulonimbus clouds frequently result.
Cold Fronts• Because a cold front produces roughly the same amount of lifting as a warm front but over a shorter distance, the precipitation intensity is greater, but of shorter duration.• Marked temperature drop and windshift from the southwest to the northwest usually accompanies the front’s passing.
Cold Fronts• The sharp temperature contrast and sometimes violent weather along cold fronts are symbolized on a weather map by a blue line with blue triangular points extending into the warm air mass.
Cold Fronts• Most often, the weather behind a cold front is dominated by subsiding air within a continental polar air mass.• Thus, the drop in temperature is accompanied by clearing that begins soon after the front passes.• Although the effect is a fair amount of cooling of air aloft, the effect near ground level is relatively minor.
Cold Fronts• In winter, the long, cloudless nights that follow the passage of a cold front result in radiation cooling that reduces surfaces temperatures.• When a cold front moves over a relatively warm surface, radiation emitted from the Earth can heat the lower atmosphere enough to produce shallow convection.• This can result in low cumulus or stratocumulus clouds behind the front.