During last Friday night's hailstorm I ended up w/ a significant accumulation under my garage eaves. I noticed that it was still there when I got up Saturday morning. When it was STILL there @ 7PM Saturday I went out to investigate & it was still REALLY cold; is hail uber-cold when it is formed in the atmosphere & hits the ground? I would have thought it would have melted before sunrise Saturday.
Posted May 17, 2008
MIKE MOSS SAYS: Richard, That does sound like a long time for the hail to last. However, I'd guess that maybe the area around your garage is a bit of a terrain depression, or that there is perhaps a shallow drainage ditch that runs alongside the garage beneath the eaves. I may be wrong, but if this is the case then the hail may have collected there and developed a shallow "pool" of dense, cold air as the outside of the hail started to melt (this melting process absorbs a lot of heat from the surrounding air as well as from adjacent layers of ice inside the hailstones, which lowers the temperature of each and reduces the tendency toward further melting unless the air is disturbed and displaced by warmer air, for example by wind blowing across it. If the area around the garage was sheltered from daytime sunlight and largely sheltered from wind as well, this layer of dense cool air would act as a strong insulator for the hail within, allowing it to melt much more slowly than you might expect given the overall ambient temperature at the time (which ranged from mid 50s to near 80 through the day). I don't think the temperature at which the hail formed initially would have a great impact here, although there could be some influence on the melting rate from the particular arrangement of clear and/or rimed layers (those containing air bubbles) that made up the hailstones. In any case, I'm assuming that the hail that really lasted a long time was in fairly particular areas, and that there was a lot of other hail in your yard, on the road, on the deck, etc, that disappeared pretty quickly by comparison. If the initial temperature or particular morphology of the stones was the major issue involved, then all of the other hail should also have melted at a noticeably slow rate as well.
Interesting question! I've had to make a few assumptions in the course of answering above, so if there's anything that seems off track compared to what you observed, or the layout and exposure of the area under your garage eaves doesn't square with my presumption that there was a "trough" or "bowl" in the ground for cold air to settle/collect in, please feel free to write back with more details.