Downbursts
A "downburst" is a strong downdraft that originates within the lower part of a cumulus cloud or thunderstorm and descends to the ground.
Winds from downbursts can exceed 100 mph and can cause "straight-line" wind damage equivalent to weak F0 & F1 tornadoes.
Small and intense downbursts (diameters less than 4 km) are called "microbursts".
Downbursts form by two primary mechanisms: 1) evaporation of rain drops or ice crystals; 2) the downward drag force of the falling precip.
Enviromental characteristics favorable to downbursts: 1) a large environmental lapse rate below the cloud; 2) dry air below the cloud base; 3) an increase of the air's actual moisture content near the surface; 4) below freezing temperatures in much of the cloud.
Evaporating ice crystals above the cloud base may contribute to the initiation of a downburst, due to sublimating ice crystals requiring the latent heat of fusion (melting), in addition to, the latent heat of vaporization; more effective cooling than by evaporating rain drops.
A descending downburst hits the ground, forms a "stagnation cone", spreads out as a "vortex ring", sometimes creating "runaway vortex rolls."
A wet microburst is accomanied by measureable precip (>= 0.01"); most common in the Midwest, East and Southern U.S.; are associated with moist, unstable sounding structure.
A dry microburst has no rain reaching the ground ... mostly virga ... most common in the western U.S. And Great Plains; are associated with the "Inverted V" sounding structure (e.g. dry up to about 550 mb; moist above; sub-freezing air from 600 mb upward, allowing ice crystals to fall into the dry air below the cloud base).
Dry microbusts can be especially dangerous to aircraft, since they can only been seen if there is blowing dust on dry ground.
Microbursts are especially dangerous to aircraft that are landing or taking off and have been responsible for many "aircraft mishaps"; aircraft flying into a microbust experiences a surge of wind and a rapidly changing wind direction ... leading to potential loss of lift and control.
Terminal Doppler Weather Radar and Low Level Wind-Shear Alert Systems (LLWAS) have greatly reduced "aircraft mishaps" in recent years.