A squall is a sudden, powerful surge of wind that dramatically alters local weather conditions, often signaling the rapid approach of a storm system. This phenomenon is characterized by its abrupt onset and short duration, creating a significant hazard for aviation, marine, and ground activities. Squalls typically accompany active weather, such as thunderstorms or heavy precipitation, bringing a swift and sharp change to the atmosphere.
Defining the Phenomenon
A weather event must meet specific quantitative criteria to be officially classified as a squall by meteorological organizations. The definition centers on a rapid increase in wind speed that is sustained for a minimum period. According to the National Oceanic and Atmospheric Administration, a squall is a strong wind with a sudden onset in which the speed increases by at least 16 knots (18.4 mph) and is then sustained at 22 knots (25.3 mph) or more for at least one minute.
This precise definition separates a squall from a simple wind gust, which is a momentary spike in speed lasting only a few seconds. While gusts are brief, a squall involves winds that remain high for a prolonged period, typically several minutes, before returning to the long-term mean wind value.
The Mechanics of Squall Formation
Squalls are fundamentally a product of atmospheric instability, primarily driven by strong convective activity within thunderstorms. The process begins high in the storm cloud where precipitation cools the air through evaporation, making it colder and denser than the surrounding air. This heavy, cold air begins to sink rapidly toward the ground in a powerful column known as a downdraft.
When the downdraft hits the surface, the air is forced to spread out horizontally at high speed, creating the leading edge of the cold air known as the gust front. This fast-moving surge of cold air is what produces the sudden, strong winds of the squall. The arrival of a squall is often preceded by a visual indicator, such as a low-hanging shelf cloud or a rolling roll cloud, which forms as the gust front lifts warm, moist air ahead of it.
Classifications
Squalls are often categorized by the weather they accompany, with the most recognized being the line squall and the snow squall. A line squall is a large, organized system consisting of a long line of thunderstorms, sometimes stretching for hundreds of miles, typically forming ahead of a cold front. A snow squall is a brief, intense burst of heavy snowfall and gusty winds that dramatically reduces visibility.
Immediate Effects
The immediate effects of any squall are significant due to their suddenness and intensity. The rapid wind shift and increase in speed can be strong enough to cause widespread damage, including downing trees and power lines. A sharp drop in temperature frequently occurs with the passage of the cold, dense air mass. In a snow squall, the combination of heavy snow and high winds creates whiteout conditions, which can reduce visibility to near zero and pose a danger to travelers.