Why does it hail, even in summer?
Because hail is made high inside thunderstorms, not near the ground. Strong updrafts carry drops far above the freezing level, where they freeze and grow layer by layer until they're too heavy for the updraft to hold up. Summer has the strongest storms, so it gets the most hail.
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How a hailstone grows
Even on a 90°F day, the air a few miles up is well below freezing. A thunderstorm's updraft lifts water drops into that cold air, where many of them stay liquid even below 32°F (supercooled). When they hit a small ice pellet they freeze onto it, building a coat of ice. The stone keeps growing as long as the updraft keeps it in that zone, and cutting one in half often shows rings of clear and cloudy ice from its trip through wetter and drier parts of the storm.
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Why some storms make big hail
The bigger the stone, the stronger the updraft needed to hold it up. Golf ball–sized hail needs an updraft blowing upward at highway speed. Those updrafts come from very unstable air: warm, humid air near the ground with cold air above. When the updraft can no longer support a stone, or it drifts out of the updraft, it falls. Hail an inch across or larger is the threshold the National Weather Service uses for a severe thunderstorm.
Where it hails most
In the US, the most frequent large hail falls on the high plains where Colorado, Nebraska and Wyoming meet, nicknamed Hail Alley. Higher ground means the freezing level is closer to the surface, so stones have less distance to melt on the way down.
Questions people ask
- What's the difference between hail and sleet?
- Sleet is a raindrop that freezes as it falls through a cold layer near the ground in winter. Hail is grown inside a thunderstorm's updraft and can fall in any season, most often in spring and summer.
- Why does hail fall in a narrow strip?
- Hail falls from the storm's updraft region, which is only a few miles across, so the swath on the ground can be narrow and sharply defined.