Where is that rain coming from?

October 21, 2011 1:34 PM

Several people I know here in Chicago, including myself, have witnessed very small isolated rain showers. They are usually no more than 6 feet in diameter, clearly not coming from a building, no clouds in the sky. One of saw this happen in a wide open intersection, one in an open quad area and one on a sidewalk. These showers tend to last anywhere from 40 seconds to 2 minutes and seem to be coming from absolutely nowhere. They are clearly unusual enough that people stop, stare, comment and try to locate it's source. Is this a naturally occurring phenomenon? Is there a name for it? Where is that rain coming from?
posted by smich to Science & Nature (12 answers total)

I'd guess water devils (as a parallel construction to dust devils)-- little whirlwinds that pick up water from the lake.
posted by jamjam at 1:54 PM on October 21, 2011

I think they're called cloudbursts but that might refer to something else.
posted by Nattie at 2:05 PM on October 21, 2011 [1 favorite]

In one of Jerzy Kosinkski's books (probably Cockpit) he describes high-rise pranking while living in Manhattan -- he ran a hose from the kitchen sink out to his balcony, and (with the assistant operating the faucet) he learned how to pin-point bursts of water that would fall on specific parts of the street, many floors below. After several weeks of soaking various targets he went down and talked with a cop patrolling that beat who told a story much like yours, about very small rain showers from cloudless skies.

Any tall buildings around, possibly with residents out on the balcony, laughing?
posted by Rash at 2:12 PM on October 21, 2011 [2 favorites]

clearly not coming from a building

I would hazard to guess that they are still coming from a building. Tall buildings and skyscrapers do very strange localized things with wind. It might be rare, but to me, I could see a very localized wind phenomenon which can pick up water from a building, and deposit it, "water devil" like, a block or two away. It needn't even be a strong wind, so you and others are probably not making the connection.

While not the same, we all know about the John Hancock building in Boston.

Cities create artificial microclimates and are funny like that.
posted by xetere at 2:14 PM on October 21, 2011

I notice this all the time in the Loop. And every single time, with some searching, I'm able to look up and find the window washers who are working on a nearby building. (Sometimes they're on the other side...the wind definitely picks up the water and whips it around corners.)
posted by phunniemee at 2:17 PM on October 21, 2011 [1 favorite]

Hanging baskets.
posted by biffa at 2:33 PM on October 21, 2011

Rooftop AC units often spray their condensation up, and it can land dozens of yards from the source. I notice it all the time on Chicago Ave. by the Moody Bible Institute.
posted by hydrophonic at 2:35 PM on October 21, 2011

I've noticed the loop/window washer (maybe even just water dripping from a high rise air conditioner) phenomenon. But, I've also noticed a few times actual pockets of rain happen out of nowhere while being no where near tall buildings (while standing on the blue line platform at Cicero--which is right in the middle of an expressway, for instance, or one time recently in the parking lot of a Jewel in Wicker Park). Not that I think this is unique to Chicago, per se, but theories that this might be related to 'water devils' from the lake makes sense.
posted by marimeko at 2:48 PM on October 21, 2011

Meteorologist here. We would really need a clearer idea of the time of day, clouds, temperature, wind, and preferably the actual date/time of such an event so the weather regime can be considered. First thought is that buildings do indeed have complex flow patterns around them, and this can produce all scales of eddies and vortices. If the relative humidity is high enough or spray from the lake is entrained, or a weak shower is passing overhead, it would be pretty trivial to focus the precip into brief spots of drizzly rain sort of like you describe. if it's bright and sunny, or dry, then yeah it would almost certainly be water from something on the building, focusing also if there's wind.
posted by crapmatic at 5:09 PM on October 21, 2011 [5 favorites]

They are cloudbursts. I see the same thing living right on Lake Erie. They are not water devils, AKA waterspouts, because if they were you would know it. Waterspouts are basically tornadoes that form over a large body of water, which means that it's not going to be a bright sunny day when one hits.

Also, waterspouts/devils very rarely come ashore. It takes a certain atmospheric condition to make a tornado, and the air pressure/temperature that makes one over open water is not going to make one on land, and vice versa. I hope crapmatic can come back and explain exactly why not, but all I can say is that ain't the culprit.

Cloudbursts, on the other hand, are common here in the Great Lakes. I bet it happens most in Chicago when the air cools abruptly (i.e. late spring or early-ish fall) but the lake in question is still warmer than the air. (Lake Erie is still like 64 degrees, and it's about 50 degrees here in air temperature). So, I think (and please do gently correct me if I'm wrong about thermodynamics) that the water evaporates at a higher rate than the water in the air condenses, meaning there's some excess. Assuming no severe weather system present, the lake water takes the form of a lovely, puffy cloud that drifts over Chicago and rains for a minute or two.

Damn, I hope crapmatic comes back and either lambastes me for speculating or congratulates me for figuring it all out...
posted by deep thought sunstar at 5:44 PM on October 21, 2011 [1 favorite]

Yeah, that's just normal weather. Or at least normal for the particular weather pattern going on at that time. If the atmosphere is unstable, a weird updraft can stimulate some rain in a very small area.
posted by gjc at 8:38 PM on October 21, 2011

you should write a letter to geoffrey baer about this, he will know.
posted by c.larence at 2:12 AM on October 23, 2011