Damaging wind from thunderstorms is much more common than damage from tornadoes. In fact, many confuse damage produced by "straight-line" winds and often erroneously attribute it to tornadoes. Wind speeds can reach up to 100 mph (161 kph) with a damage path extending many miles.
Downdrafts are generated when rain-cooled, more dense air sinks inside a thunderstorm. Also some of the strong winds aloft are carried down with the downdraft by a process called "momentum transfer". As precipitation begins to fall, it drags some of the air with it. This "precipitation drag" initiates a downdraft. The downdraft is intensified by evaporative cooling as drier air from the edges of the storm mix with the moist air within the storm.
These processes lead to a rapid downward rush of air. As the air impacts the ground it is forced to spread out laterally causing the gusty winds associated with thunderstorms. Occasionally, thunderstorms will produce intense downdrafts that create damage as the wind spread out along the ground.
Downbursts can create hazardous conditions for pilots and these events have been responsible for several disasters.
- As aircraft descend (right) into the airport they follow an imagery line called the "glide slope" to the runway (solid light blue line).
- Upon entering the downburst, the plane encounters a "headwind", an increase in wind speed over the aircraft. The faster wind creates lift causing the plane to rise above the glide slope. To return the plane to the proper position, the pilot lowers the throttle to decrease the plane's speed thereby causing the plane to descend.
- As the plane flies to the other side of the downburst, the wind direction shifts and is now from behind the aircraft. This decreases the wind over the wing reducing lift. The plane sinks below the glide slope.
- However, the "tailwind" remains strong and even with the pilot applying full throttle trying to increase lift again, there is little, if any, room to recover from the rapid descent causing the plane to crash short of the runway.
Since the discovery of this effect in the early to mid 1980's, pilots are now trained to recognize this event and take appropriate actions to prevent accidents. Also, many airports are now equipped with equipment to detect downbursts and warn aircraft of their location.
Macrobursts and Microbursts
Downbursts are divided into two catagories; macrobursts and microbursts. A macroburst is more than 2½ miles (4 km) in diameter and can produce winds as high as 135 mph (215 kph). Microbursts are smaller and produces winds as high as 170 mph (270 kph).
In wet, humid environemts, macrobursts and microbursts will be accompanied by intense rainfall at the ground. If the storm forms in a relatively dry environment, however, the rain may evaporate before it reaches the ground and these downbursts will be without precipitation, known as dry microbursts.
Dry heatbursts are responsible for a rare weather event called "Heat Bursts". Heat bursts usually occur at night, are associated with decaying thunderstorms, and are marked by gusty, sometimes damaging, winds, a sharp increase in temperature and a sharp decrease in dewpoint.
While not fully understood, it is thought that the process of creating a dry microburst begins higher in the atmosphere for heat bursts. A pocket of cool air aloft forms during the evaporation process since heat energy is required. In heat bursts, all the precipitation has evaporated and this cooled air, being more dense than the surrounding environment, begins to sink due to gravity.
As the air sinks it compresses and with no more water to evaporate the result is the air rapidly warms. In fact, it can become quite hot and very dry. Temperatures generally rise 10 to 20 degrees in a few minutes and have been known to rise to over 120°F (49°C) and remain in place for several hours before returning to normal.
A derecho is a widespread and long lived windstorm that is associated with a band of rapidly moving showers or thunderstorms. The word "derecho" is of Spanish origin, and means straight ahead. A derecho is made up of a "family of downburst clusters" and by definition must be at least 240 miles in length.
Derechos are associated with a band of showers or thunderstorms that are often "curved" in shape. These bowed out storms are called "bow echoes". A derecho can be associated with a single bow echo or multiple bow echoes. The bow echoes may vary in scale and may die out and redevelop during the course of derecho evolution.
Winds in derecho can exceed 100 mph. For example, a derecho in northern Wisconsin on July 4, 1977 produced winds of 115 mph. The winds associated with derechos are not constant and may vary considerably along the derecho path.