A downburst is a sudden burst of cold air emerging from the base of a
thunderstorm. During most of a thunderstorm's lifetime, air is being drawn up into the storm, often explosively, bringing warm moist air into the heart of the storm. The interior of the thunderstorm cools, and
rain and
hail are often also embedded in the updraft. After the thunderstorm has built up enough cold, wet air, a downburst sometimes forms. Cold air is much denser than warm air, and in the case of downbursts, it is also often filled with rain and hail. The cold air blasts downward until it hits the ground, where it spreads out laterally.
Downbursts are one of the main causes of strong winds in a thunderstorm... if you experience cold, strong wind coming from a nearby thunderstorm, it is probably either a downburst or a gust front. (downbursts may become gust fronts as they move away from the storm). Downbursts are also associated with torrential rain or hail. Although downbursts aren't usually as visually dramatic as tornados, they can cause as much damage as a small tornado. The sharp downward winds can also cause planes near the ground to crash.
Although downbursts are sometimes invisible, sometimes they can be seen approaching. Often, a downburst will be associated with lowerings in the cloud, which do not show rotation. (if the lowered cloud is rotating it is probably a funnel cloud). Downbursts are also reflected on the ground as clouds of debris. Downbursts can contain hurricane-strength winds - if a severe downburst seems to be approaching, seek shelter away from windows.
Downbursts are usually associated with solitary thunderstorms (as opposed to squall lines). Usually, a downburst indicates that the updraft of a thunderstorm has weakened and the storm is nearing the end of its lifetime. However, mesocyclones often sustain such a structure that they can contain both mature updrafts and downdrafts for hours. In dry areas, downbursts may not be associated with rain. In the desert, dry thunderstorms can kick up large dust storms.
Compact, severe downbursts are sometimes called microbursts.