why does it snow vs. hail?

Question

I am trying to understand what allows the weather to snow vs. hail vs. sleet, etc? I am assuming it's temperature based, but am not clear

Answer 1

Execellent question. Hail forms in clouds with much greater vertical motion than those in which snow forms. These clouds are called cumulonimbus. As the liquid water grows in the cloud it is lifted aloft and freezes. As it drops it begins to melt but soon the updrafts are strong enough to carry it upward again adding more liquid water to be frozen on its surface. This process may be repeated several times before the hail is either shot out of the top of the storm or falls out the bottom of the cloud.

In the clouds that form snow, there is little vertical motion and no melting and re-freezing. The snow can easily fall under the weight of gravity.

Answer 2

Hail and snow are both formed in the cloud when temperatures in the cloud are cold enough to freeze water. Hail is typically larger water droplets, and happens in more humid conditions, but it's still all freezing water (this is VASTLY simplified...look to your and my friend Wikipedia for more in-depth detail).

What differentiates between what comes down is the ambient temperature at ground level. It must be at freezing or lower (32oF, 0oC) for snow to reach the ground as snow--if it's warmer, it will convert to rain in the atmosphere and hit the ground as such.

Hail can fall when the ambient temperature is higher, though, because it is a larger mass of frozen water and can therefore hold its own temperature a little longer (what lasts longer, an ice cube, or a tiny bit of shaved ice?).

Answer 3

Hail is formed in the vertical circular motion of a thunderstorm. Rains fall and are then blown back skyward several thousand feet where it freezes. Due to the cohesive properties of water, when the frozen rain falls again it collects more rain, and on and on. Finally, when the hail is too heavy to be send back up, it falls to the earth. A meteorology professor of mine calculated that for golf ball size hail, the vertical winds would have to be about 200 mph. That's some kind of wind!

Answer 4

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RE:
why does it snow vs. hail?
I am trying to understand what allows the weather to snow vs. hail vs. sleet, etc? I am assuming it's temperature based, but am not clear

Answer 5

Most all precipitation begins as snow or ice crystals that form about 20000 feet above the surface. As this precip falls toward earth many different things can happen. With hail it melts on the way down (Summer/spring implies warmer air near the surface) and strong verticle winds move it back up to where it is able to refreeze and it again falls, partially melts, is carried aloft and refreezes - each time accumulating more and more water until it is to heavy for the updraft to carry it aloft, hence it falls to the gound as hail. If you were to cut throught it you would see the layers where the freezing and refreezing happened. Go to http://www.theweatherprediction.com/seve... For rain the snow melts as it passes through warmer air and the air remains above freezing through the remainder of the descent. Freezing rain occurs when objects on the ground are near or below freezing and as rain comes into contact with them the rain (if supercooled - see link) freezes. Sleet is snow that melts on the way down as it passes through air above 32 degrees and then freezes as it reenters air that is below 32. Snow, of course stays in air below 32 from formation until it hits the ground. As for it raining when the temp is under 32 (ask yourself who and where was this measured), it takes some time for the rain to freeze and obviously it didn't have enough time to freeze. Hail occurs in summer at a level in the atmosphere that is well below 32 and falls so fast it doestn't have time to melt - an ice cube sitting outside in the summer takes quite a while to melt completely. Also be advised that "official" temp measurements are taken at about 6-10 feet above the surface, which is why we see frozen dew at times that the official low temp is above freezing. Even in a stable airmass temperatures can vary by more than 20 degrees across a large metropolitan area. Try http://ww2010.atmos.uiuc.edu/(gh)/guides... and http://www.islandnet.com/~see/weather/el... Hope these help and thanks for asking