How hot can it be and still hail?

[Cudzoziemiec]

On Saturday I was going to the Foreigners' Registration Office, never an inspiring destination, when it started raining heavily. I'd seen that it was coming on to rain, so I had my cap on and it kept the rain out of my eyes. Nevertheless, the rain here tends to be very heavy when it falls, and of course the heaviest rain fell at the trickiest part of the journey, where I have to cross 2 lanes of heavy, chaotic traffic to make a right turn at the top of a rise.

And then it started hailing! Only for 10 seconds or so, but nevertheless, with temperatures in the high 20s this was unexpected. I know hail is formed by rain freezing as it falls, but how hot can it be and still fall as hail?

[rogerzilla]

IME hail is more common in summer.  It just depends how high it comes from.

[Cudzoziemiec]

So air temperature at ground level has no bearing on hail (at least until it lands)? It could even fall in, say, Death Valley?

What about altitude? It was about 1,000m asl when it landed on me, but that presumably doesn't make much difference in comparison to cloud height?

[Chris S]

Hail formation requires deep and powerful convection to drive the process of growing hailstones.

This convection can be fuelled by very unstable airstreams, such as those over the UK in winter from a West or North Westerly direction. This is what I would term "cold hail".

The most powerful and sustained convection comes in summer thunderstorms however. The amount of vertical windshear inside summer storms is awesome, which is why pilots literally go out of their way to avoid them. Also, in summer the atmosphere is deeper, so cloud tops are colder and more conducive to hail formation.

The hail can reach the ground, even on a hot day in summer, because the rain/hail plume falling from a thundercloud drags down very cold air from aloft which slows the melting process enough for the hail to be intact when it reaches the ground. The big powerful storms over the American Mid-west are good examples of this - temperatures into the 30s Celsius initially drive the storms, which are later often accompanied by large hail and much cooler temperatures.

[andygates]

Or in translation: It's only hot down here.  Weird stuff happens up in clouds.  As long as the hail is big enough to make it down before it melts, hail on a hot day works just fine.

[Attitudeless Badger]

you need the ground heat to help create instability to get the thunderstorms.  The warm damp air is thrust upwards in the clouds, creating the hail stones, which then drop faster than they can melt (non-meteorologists explanation !).

Down here in central France, we get 2 or 3 such storms per summer.  There is usual enough hail to turn the lawn white, and the hail is of sufficient size to puncture plastic guttering and 'sandblast' bits of varnish off the window sills.  Cars are also liable to damage, although it does give a wonderful hammerite type finish.  Only ever happens in summer after v warm (30 degrees +) humid days.

[Cudzoziemiec]

Thanks for the informative replies.