Finally, the speed at which a hailstone hits is not the same as its falling speed. For one thing, there may be a horizontal component – side winds can increase the impact speed of a hailstone compared to if it had hit in freefall. The most damaging hail events are downbursts, driven by powerful downdraughts – where air rapidly descends from storms and spread outwards when they hit the ground, producing very high wind speeds. Downbursts are typically only a few kilometres or miles across and last a matter of minutes, but can feature vertical windspeeds of 70-80m/s (156-179mph) with correspondingly destructive hail.
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