Hail isn't common in the Bay Area. I took this for granted until today, when I got hailed on, unexpectedly. The wind kicked up, blowing icy rocks against my face and down the back of my shirt. "Why is this happening to me?" I thought. No, really- why does hail happen?
Hail needs two ingredients to form: water and wind. Water in the atmosphere condenses into droplets. When conditions are right (especially during thunderstorms) strong upward winds called updrafts push the droplets high into the atmosphere, where temperatures are low enough for them to freeze. The frozen drops grow heavy and fall towards the ground as small hailstones. If updraft conditions persist, the hailstones are pushed back up to where they started, where they're coated with another layer of freezing water. Again, the hail grows and becomes heavy enough to fall down through the atmosphere. The cycle of falling, updraft, refreezing continues: each time the winds bring the hailstones back up, they grow larger, accumulating layer upon layer of ice. Stronger updrafts are associated with larger hailstones. In storms with very strong winds hailstones can grow to the size of oranges. Wait, make that grapefruits, according to this -highly technical- hail sizing chart from NOAA:
HAIL SIZE (in.) ..... OBJECT ANALOG REPORTED
.50 Marble, moth ball
1.50 Walnut, ping pong
1.75 Golf ball
2.00 Hen egg
2.50 Tennis ball
3.00 Tea cup
Why doesn't the Bay Area get more hail? Probably because conditions need to be both wet and windy. Winter is the wet season here, but strong winds are usually linked to rising warm air, which is less common in the winter. We don't frequently get the combination of rising warm air, gusty winds, and wet weather, all at the same time.