Outflow, in meteorology, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather satellite imagery or a fine line on weather radar imagery. For observers on the ground, a thunderstorm outflow boundary often approaches in otherwise clear skies as a low, thick cloud that brings with it a gust front.
The outflow boundary indicated by the presence of this shelf cloud preceded a derecho in Minnesota
Outflow boundary that preceded a strong thunderstorm in Oklahoma
Structure of a tropical cyclone. The upper level outflow is depicted by cirrus clouds in the upper part of the schematic
A sandstorm (Haboob) approaching Al Asad, Iraq, just before nightfall on April 27, 2005.
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.
Thunderstorm with lead gust front near Brookhaven, New Mexico, United States, North America. The gust front is marked by a shelf cloud.
Illustration of a microburst. The wind regime in a microburst is opposite to that of a tornado.
This shelf cloud preceded a derecho in Minnesota
Satellite image of an undular bore