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 (cold front, dry line, another outflow boundary, etc.). 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.
Outflow boundaries create low-level wind shear which can be hazardous during aircraft takeoffs and landings. If a thunderstorm runs into an outflow boundary, the low-level wind shear from the boundary can cause thunderstorms to exhibit rotation at the base of the storm, at times causing tornadic activity. Strong versions of these features known as downbursts can be generated in environments of vertical wind shear and mid-level dry air. Microbursts have a diameter of influence less than 4 kilometres (2.5 mi), while macrobursts occur over a diameter greater than 4 kilometres (2.5 mi). Wet microbursts occur in atmospheres where the low levels are saturated, while dry microbursts occur in drier atmospheres from high-based thunderstorms. When an outflow boundary moves into a more stable low level environment, such as into a region of cooler air or over regions of cooler water temperatures out at sea, it can lead to the development of an undular bore.
