Yesterday, a southerly breeze swept warm, humid air into Central Missouri, typical conditions for late May. Only the hazy cloudtops to the southwest and a distant rumble of thunder warned of a coming change. Suddenly, about 4 PM, a strong, cold wind swept through Columbia, shaking the trees and dropping the temperature from 80 to 70 in five minutes. The wind died down as quickly as it came, the cluster of storms passed by to our south and, within an hour, the heat and humidity were back.
We had experienced outflow winds from a strong thunderstorm. As they boil up, these storms draw warm, moist air from the surrounding countryside and inject it into the cold, upper layers of the atmosphere. Condensing on the way up, water droplets soon become ice pellets and, depending upon the size and strength of the storm, may progress to large hail. Eventually, this cold air, with its cargo of frozen moisture, plunges toward the ground, producing heavy rain and variable degrees of hail. Striking the ground, this downburst sends out a wave of cold, fast-moving air in all directions.
Yielding a brief respite from the heat and often causing damage to trees and structures, such downdrafts may also give birth to other storms. As this cold, dense air undercuts and lifts the warm, humid air, clouds and showers form above the outflow boundary; some will develop into thunderstorms.
