It may be the end of April, but there were several reports of hail in the capital region today. It left many of you shaking your head thinking “Isn’t it almost May?” I know! I felt the same way.
While hail is ice, and we associate ice with wintery conditions, I thought I’d share how hail actually forms – and it doesn’t have to be cold at the ground to have little chunks of ice fall from the sky!
Start by looking at the diagram. It’s from the NOAA website. I really like how it shows the cycle I’m about to describe.
Let’s say we have a supercooled water droplet in a cloud. If it is to become rain, it will fall straight to the ground, melting along the way as it falls from the cold cloud through warmer air which we find closer to the surface.
But what if the droplet starts to fall, begins to melt, then gets caught in an updraft? If it is pushed back into the cloud where temperatures are below freezing, it will refreeze. The weight of the droplet may cause it to fall again, or it could get caught in a cold downdraft. Either way, as the droplet falls it begins to thaw as it moves into warmer air closer to the ground.
If the half-frozen droplet is again pushed back into the very cold cloud by another updraft, it will refreeze again. Every time the droplet goes through this freeze-thaw cycle, it adds another layer of ice. That’s what makes it larger in size.
Finally, if our frozen water droplet is too heavy to be supported by the updrafts, it falls to the ground as hail.
There you go! The good news: I’m not expecting hail tomorrow. Maybe some early morning frost, but no hail :)
