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Author Topic: Does rain start off as snow or hail and then melt on the way down?  (Read 11420 times)

paulat

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What is the temperature inside the average cloud? Are the water droplets that form clouds actually frozen or ice crystals? When it "rains", does the rain start off as snow or hail, and then turn into rain on the way down, if the air temperature is high enough?

Heliotrope

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Some of it does indeed start of as ice formed around dust particles which then get too heavy for the cloud to support and then melt on the way down to fall as rail.

As far as I'm aware the temperature inside a cloud depends upon it's altitude, the amount of updraught and the prevailing conditions locally at that time.
There are many other factors but I don't know that much about meteorology so I can't tell you them all.

paul.fr

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" Does rain start off as snow or hail and then melt on the way down?"

It most cases, yes. This is a process known as the Bergeron-Findeisen process. In most area's of the earth the clouds grow high enough to reach area's where the temperature is below the freezing point of water. Bergeron said that such clouds would contain both water droplets and ice crystals, and because water does not instantly freeze at 0 deg.C in may exist in a supercooled state.
The ice crystals and supercooled water coexist in the cloud with the water either evaporating or joining with the ice crystal to make a larger crystal, the more this happens the faster and heavier the crystal grows and the faster it ascends the cloud, eventually becoming snowflakes.
The falling ice crystals that are now snowflakes will melt and become rain if they encounter an air temperature on their way down that is = or + 4 deg. C

But not all rain starts off as snowflakes. In the tropical regions the clouds do not get as high, and as such are free from ice, they are "warm clouds". The rain here is produced by Langmuir's Chain Reaction, or more commonly the collission and coalescence process.
The water droplets collide and coalesce due to atmospheric turbulence and convection.

Drizzle on the other hand is different to rain, and should not be called rain. Drizzle comes from low stratus clouds and does not start off as snowflakes.
« Last Edit: 03/08/2008 23:14:38 by Paul. »

blakestyger

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In a similar vein, years ago when I used to go mountaineering, I was on the side of a mountain in the Torridons (Scotland) when it started to hail. What we saw from about 2000 ft was remarkable: the hailstones were falling past us and were then caught in an up-draught. They were carried up and fell again, this seemed to be repeated with a swirling, circular motion until they were heavy enough to fall to Earth. It was quite fascinating to see this 'weather in action'.

DoctorBeaver

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In a similar vein, years ago when I used to go mountaineering, I was on the side of a mountain in the Torridons (Scotland) when it started to hail. What we saw from about 2000 ft was remarkable: the hailstones were falling past us and were then caught in an up-draught. They were carried up and fell again, this seemed to be repeated with a swirling, circular motion until they were heavy enough to fall to Earth. It was quite fascinating to see this 'weather in action'.

That's freaky

paul.fr

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In a similar vein, years ago when I used to go mountaineering, I was on the side of a mountain in the Torridons (Scotland) when it started to hail. What we saw from about 2000 ft was remarkable: the hailstones were falling past us and were then caught in an up-draught. They were carried up and fell again, this seemed to be repeated with a swirling, circular motion until they were heavy enough to fall to Earth. It was quite fascinating to see this 'weather in action'.

Nice, the rest of us have to make do with walking in fog to get something similar to walking through stratus. Although this guy did go one better than all of us...

William Rankin:
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Lieutenant Colonel William Rankin is the only person ever to survive a fall from the top of a cumulonimbus thunderstorm cloud.[1] He was a US Air Force pilot, and a World War II and Korean War veteran. He was flying a jet fighter over a cumulonimbus cloud when his plane's engine stalled and he was forced to eject and parachute into the cloud.[1]


[edit] The fall
In the summer of 1959, Rankin was on a routine flight from South Weymouth Naval Air Station, Massachusetts to Beaufort, North Carolina. He was climbing over a thunderhead which peaked at 45,000 ft (13.7 km). Flying at 47,000 ft (14.3 km) and at mach 0.82, a loud bump and rumble was heard from the engine, the rpm fell to zero, and the fire warning light began flashing.[1] As he pulled the lever to deploy the auxiliary power, the lever unexpectedly broke off in his hands. Although the temperature outside was −50C and he was not wearing a pressure suit, his only choice was to eject. At 6:00 pm, he ejected from the plane.[1] He suffered immediate frostbite, and decompression caused his eyes, ears, nose, and mouth to start bleeding. His abdomen swelled as if he was pregnant. Pain seared across his body, but it was numbed by the cold. He managed to use his emergency oxygen supply.[1] Five minutes had passed since the beginning of his descent, but his parachute did not yet open. He was still in the upper regions of the thunderstorm, with near-zero visibility, when his parachute opened. After ten minutes, when he should already have landed, Rankin was being carried upward by updrafts and was getting hit by hailstones. The violent spinning and pounding caused him to vomit. Lightning appeared, resembling blue blades several feet thick, and the thunder was felt physically. The rain was posing a drowning hazard, and he had to hold his breath. One lightning strike lit up the parachute, making Rankin believe he had died.[1] After this, conditions began calming, and he descended in a forest. His watch read 6:40 pm. He went searching for help and was admitted into a hospital at Ahoskie, North Carolina.[1] He suffered from frostbite welts, bruises, and severe decompression.

http://en.wikipedia.org/wiki/William_Rankin

 

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