 A macroburst, normally associated with squall lines, is a strong downburst larger than 4 kilometres (2.5 mi). A wet microburst consists of precipitation and an atmosphere saturated in the low-levels. A dry microburst emanates from high-based thunderstorms with virga falling from their base. All types are formed by precipitation-cooled air rushing to the surface. Downbursts can occur over large areas. In the extreme case, a derecho can cover a huge area more than 200 miles (320 km) wide and over 1,000 miles (1,600 km) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds, but the generative process is somewhat different from that of most downbursts.
