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Jens
2012-Dec-06, 12:51 AM
This morning as I was walking to the station, it was raining, and yet the sky above me was completely clear. I looked it up, and there is a phenomenon called serein, but some people doubt it is a real phenomenon. In fact, though there weren't any clouds above me, I could see some clouds on the horizon in different directions, so I guess the rain might have come from one of the clouds and been blown over somehow.

Which leads to a question. I once looked up how long raindrops take to hit the ground, and I remember someone giving a figure of even up to an hour. But thinking about it, that seems doubtful. Rainclouds are usually at about 2,000 meters, I think, and if raindrops fall at about 20 km/h, that means they could cover the 2 kilometers in about six minutes. Is that pretty realistic?

Gillianren
2012-Dec-06, 01:00 AM
I doubt you'd find anyone around here who doubts it's a real phenomenon! But I would imagine winds higher in the atmosphere could affect how long it takes to fall.

Swift
2012-Dec-06, 01:51 AM
Which leads to a question. I once looked up how long raindrops take to hit the ground, and I remember someone giving a figure of even up to an hour. But thinking about it, that seems doubtful. Rainclouds are usually at about 2,000 meters, I think, and if raindrops fall at about 20 km/h, that means they could cover the 2 kilometers in about six minutes. Is that pretty realistic?
My understanding is that raindrops don't necessarily head straight down. If they get caught in updrafts, they can actually circulate around for a while.

Van Rijn
2012-Dec-06, 02:26 AM
I'm not sure if this is directly relevant, but I thought it was interesting:

http://blogs.scientificamerican.com/observations/2012/11/30/mysterious-atmospheric-river/


Northern California is experiencing the first days of what weather forecasters are warning will be a long series of torrential rainstorms that could cause serious flooding across the northern one-third of the state. The relentless storms are being driven by a feature in the atmosphere you have probably never heard of: an atmospheric river.
[snip]
An atmospheric river is a narrow conveyor belt of vapor about a mile high that extends thousands of miles from out at sea and can carry as much water as 15 Mississippi Rivers. It strikes as a series of storms that arrive for days or weeks on end. Each storm can dump inches of rain or feet of snow. For more details, see this feature story that Scientific American has just published, written by two experts on these storms.

Maybe cloudless rain is from more of an atmospheric creek?

Ara Pacis
2012-Dec-06, 06:24 AM
Depending on humidity, updrafts and horizontal winds aloft, I'm not too surprised it can travel miles. Or maybe it was a an-made shower.

Ivan Viehoff
2012-Dec-06, 10:11 AM
This morning as I was walking to the station, it was raining, and yet the sky above me was completely clear.
In Britain, especially in spring, it is a common occurrence that the sun comes out and it starts raining shortly afterwards, usually only briefly. Spring is a windy time of year with all sorts of strong updrafts associated with squally showers, hail, etc that occur at this time of year. It seems a reasonable explanation that some rain fell from a cloud, got blown around a lot, and then a patch made it to the ground a long way away.

adapa
2012-Dec-06, 06:33 PM
This morning as I was walking to the station, it was raining, and yet the sky above me was completely clear. I looked it up, and there is a phenomenon called serein, but some people doubt it is a real phenomenon. In fact, though there weren't any clouds above me, I could see some clouds on the horizon in different directions, so I guess the rain might have come from one of the clouds and been blown over somehow.

Which leads to a question. I once looked up how long raindrops take to hit the ground, and I remember someone giving a figure of even up to an hour. But thinking about it, that seems doubtful. Rainclouds are usually at about 2,000 meters, I think, and if raindrops fall at about 20 km/h, that means they could cover the 2 kilometers in about six minutes. Is that pretty realistic?
If the clouds on the horizon are cumulonimbus, then your observation makes perfect sense and here is why:
A powerful cumulonimbus has stong enough updrafts to throw hail out the top and up to 20 miles downwind. If the hail stones are small, then they can be carried even farther away and are more likely to melt into rain as they fall into warmer air.

Swift
2012-Dec-06, 07:15 PM
A related topic (at least to me) is a study from several years ago about how raindrops break-up as they fall and produce the variety of drops sizes that are seen.

ScienceMag.org (http://news.sciencemag.org/sciencenow/2009/07/20-02.html)


Here's a question for a rainy day: How do clouds create such a wide variety of raindrop sizes? The answer, according to stunning new high-speed movies, is much simpler than physicists thought.

The idea has been that raindrops grow as they gently bump into each other and coalesce. Meanwhile, more forceful collisions break other drops apart into a scattering of smaller droplets. All this action would explain the wide distribution of shapes and sizes. But trying to unravel how the drops crash and break up led to a tough set of equations.

The new movies, however, show a much more straightforward process. Researchers snapped 1000 pictures a second of an isolated water drop as it fell through an ascending air stream. The drop first flattens into a pancake shape, which then balloons like a parachute. The bottommost rim of this chute has a thick, irregularly corrugated rim. Pressure from the air drag eventually breaks the chute apart into numerous smaller droplets--their wide range of sizes is due to the wide range of sizes of the bumps in the rim.

cjameshuff
2012-Dec-06, 07:33 PM
My understanding is that raindrops don't necessarily head straight down. If they get caught in updrafts, they can actually circulate around for a while.

They certainly don't suddenly switch from droplets suspended in the clouds to raindrops that fall at 20 km/h. There's a distribution of sizes and terminal velocities, and updrafts certainly can make them circulate for a significant amount of time...think of how hailstones form.

Snow can get blown around even more easily, and fall as rain when it moves into a warmer area or into sunlight and melts into droplets with a much higher terminal velocity.