10.3: Winds during thunderstorms

Gust Fronts

Recall that mature thunderstorms contain both updrafts of warm, buoyant moist air and downdrafts of chilled, dry, dense air. Downdrafts are the result of a combination of evaporative cooling, precipitation drag, and dry air entrainment into the cloud. With the onset of precipitation in a mature thunderstorm, there is often a downward rush of cold air. As this cold air hits the surface, it spreads out in all directions along the surface, as illustrated in Figure \(\PageIndex{1}\). This surface boundary between the cold, horizontally rushing air and the warmer air around it is called the gust front. Along the base of the cloud, there oftentimes exists a rotating section of cloud along the boundary between the warm inflow and cold downdraft called the roll cloud.

Figure \(\PageIndex{1}\): Schematic of a thunderstorm base showing a gust front formed by horizontally spreading downdrafts. The gust front separates the cold downdraft from the thunderstorm and the warm convective flow. A roll cloud may form along the gust front due to the strong wind shear. (Public Domain; What is a Gust Front via NOAA)

Along this boundary, winds change rapidly in both direction and speed. The gust front is the leading edge of the outflowing cold air at the surface, and resembles a mini cold front to surface observers. If the gust front of an advancing thunderstorm were to pass by you, you would experience a sharp drop in temperature accompanied by a shift in wind direction and an increase in wind speeds, which may occasionally exceed 55 knots. The air is extremely turbulent along the leading edge of the gust front. As the cold air pushes warm, moist air upward along the boundary, a shelf cloud (or arcus cloud) may form, as shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): A shelf, or arcus cloud, appearing as a wedge-shaped cloud formed on the edge of a thunderstorm gust front over a city. (CC BY-SA 3.0; John Kerstholt via Wikimedia Commons).

When the atmosphere is conditionally unstable, the gust front may produce additional multicell thunderstorms along its leading edge by forcing warm, moist air upward. Each of these additional thunderstorms will have its own gust front, which may merge with other gust fronts to form a large gust front known as an outflow boundary.

Downbursts and Microbursts

Figure \(\PageIndex{3}\): Cross-section of a microburst showing a rapid outflow caused by a strong downdraft of cold air from the cloud base. The outflow spreads horizontally away from the precipitation, producing a gust front or outflow front. Horizontal vortexes may form as the outflow front curves upward due to interaction with the rising warm air near the surface. (Public Domain; USFAA via Wikimedia Commons).

Under a severe thunderstorm, the downdraft may become narrow, intense, and localised. As it reaches the surface, it may spread radially along the ground in a windburst, analogous to water rushing from a faucet and striking the bottom of the sink. This sort of downdraft is called a downburst. A short downburst, with width of 4 km (approximately 2.5 miles) or less, is called a microburst, as illustrated in Figure \(\PageIndex{3}\). A microburst consists of a downdraft of rain (or virga) falling out of the cloud base, which flows outward when it reaches the ground. As the outflow extends horizontally, horizontal vortexes develop on the edges of the outflow, known as the outflow front. Despite their small spatial extent, strong microbursts can generate winds exceeding 146 knots, capable of downing trees and severely damaging weak structures and boats. Some damage caused by microbursts may have been incorrectly attributed to tornadoes. In addition, wind shear, turbulence, and gusts associated with microbursts have been responsible for several airplane crashes. Microbursts are often associated with severe thunderstorms but can also occur with ordinary cell thunderstorms and even with clouds that produce only scattered showers.