A downburst is a strong descending air current within a thunderstorm that, upon reaching the surface, spreads out horizontally in all directions producing destructive winds. It is distinguished from the microburst by its larger scale: a downburst's damage area exceeds 4 km in diameter (microbursts are less than 4 km). Winds can reach 150–200 km/h, comparable to a weak tornado, but with a divergent (radial) damage pattern rather than a convergent (rotational) one. Downbursts are classified as dry and wet. Dry downbursts occur when precipitation evaporates before reaching the ground (virga): evaporative cooling generates a dense air column that plunges to the surface with violent wind but no visible rainfall. Wet downbursts accompany heavy precipitation and are easier to detect by radar. Downbursts cause significant damage: trees felled in radial patterns, roof damage, overturned vehicles, and downed power towers. A particular hazard is aviation: downbursts during takeoff or landing have caused serious accidents, which is why modern airports are equipped with low-level wind shear detection systems.