Cloud city! What is this weather phenomenon?

October 4, 2007 2:50 PM

Coulds are passing by outside my window. What's going on?

Is anyone else in Toronto? I'm in the core and I'm looking at something I've never seen before. It’s basically like everything's been reversed. The sky is completely clear, but clouds are passing through the city from ground level to about the height of a 40 story building. Above that, it’s nothing but blue sky. It’s not smoke. It might be some kind of fog, but it’s moving very quickly and it’s darker in colour than fog. It clouds my view, but not uniformly. It’s hard to describe, but it’s what you would expect to see if your city was torn out of the ground and flying amongst the clouds.

Anybody have any insight on this meteorological phenomenon?

posted by pantheON to science & nature (15 comments total)

I don't know exactly what the conditions are like in Toronto, but this is not uncommon elsewhere. For example, if you start climbing a mountain on a cloudy day, you'll sometimes pass through the clouds on your ascent and then emerge into sun above. Another common example of this (at least around here) is when it is very cloudy in the valley bottoms, but clear on the surrounding hills. Clouds generally tend to be found together at a particular elevation level. The clouds are probably below your window today, though it could be other things (fog blown off the lake perhaps).
posted by ssg at 3:05 PM on October 4, 2007

Thanks for your comments ssg. But this has never happened in Toronto before, at least to my recollection and that of my co-workers. Also, Southern Ontario is almost completely flat... no hills or valleys. Its actually gotten darker and thicker. The colour of moderately heavy rain clouds. I'm pretty sure they are cloud formations, but again, they are far lower than we have ever seen them before.
posted by pantheON at 3:18 PM on October 4, 2007

I'm a meteorologist, but not YOUR meteorologist (or whatever the lawyers say). The lake temperatures at Lake Ontario are about 16C and your air temperature in the city is about 15-16C. Also the air mass is very humid, as dewpoints are about 15C (which is a prerequisite for very low clouds). I am not exactly sure what is going on just picking through Internet charts but it appears you're getting development of low-level stratus due to cold advection off the lakes in a humid air mass. The air coming off the lakes is probably lifting the air over Toronto a little bit, and since it's nearly saturated, it doesn't have to rise much to form some cloud material.
posted by rolypolyman at 3:24 PM on October 4, 2007 [5 favorites]

Clarification: the rising motion of the air is due to colder temperatures coming in off the lake, which acts a lot like a cold front. Air likes to conserve its potential temperature, so it rises a little bit to higher altitudes as the cool air comes in, and rising air leads to increasing humidity, and if the air mass is very humid, saturation (and cloud) occurs.
posted by rolypolyman at 3:28 PM on October 4, 2007

Sounds like some sort of haar, or sea fog. Presumably the wind's coming off the lake? This happens a lot on the east coast of Scotland but I've no idea if it's the same sort of thing or not in Toronto.
posted by ComfySofa at 3:31 PM on October 4, 2007

When I read your question, "Coulds are passing by outside my window. What's going on?" I was going to suggest to you that the world is full of possibilities.
posted by found missing at 3:44 PM on October 4, 2007 [9 favorites]

We get a thing called an "inversion layer" (and by "we", I mean San Francisco Bay Area folk), whereby the fog will be kept quite close to the ground, but the tops of buildings or hills will be above it. I would link to some cool photos I took of that, but I am among the last people to have a flickr account, so no photos from me. It only happens a few times a year, but the effects are spectacular.
posted by rtha at 3:48 PM on October 4, 2007

After I read this I had to go up and look out the third floor window. Ya, that's freaky.
This camera doesn't quite capture it, but you get an idea.
(You can't see downtown through the clouds.)
posted by chococat at 4:06 PM on October 4, 2007

Whats the difference between clouds and a fog? Isnt this just a city fog?
posted by damn dirty ape at 4:25 PM on October 4, 2007

I disagree with you a little, rolypolyman. I think the fog is due to warm, humid (but not saturated) air coming off the lake and then cooling down when it is over the land (probably by radiative cooling) to the point where it is saturated; at which point droplets condense out of it and a fog forms-- similar to a lake-effect snowstorm.

It's interesting pantheON has never seen it before. By extrapolation from the fact that arctic ice is less this year than ever before in recorded history, I guess the lake is much warmer on this particular Oct. 4 than it has been on average for this date in the past.
posted by jamjam at 5:01 PM on October 4, 2007

jamjam: The problem is that the air coming off the lake is not warm; it is 16C and has travelled over the cool lake waters for many hours. There is really no other explanation for the cool temperatures in Toronto this past afternoon, as areas surrounding the lake are in the 20-25C range and Toronto was 16C. I considered the possibility of a "lake effect" cloud pattern coming onshore but that is a "frigid air mass moving onto warm lake" scenario.

As far as Lake Ontario being warm for this time of year, the SST anomaly charts do show it is running about a +1-2C anomaly, but it won't have much to do with this particular scenario.
posted by rolypolyman at 5:57 PM on October 4, 2007

Thank you for your very interesting response, rolypolyman. If the air coming off the lake has been traveling over the water for many hours, as you indicate, is it possible it was warmer before it started traveling over the lake, but has cooled by evaporating water from the lake, and so could be carrying enough water vapor so that a fog would form in it if cooled down just a bit more?

In that case, it seems conceivable to me that the part of Toronto it was flowing over had already been cooled down so much by air coming off the lake earlier in the day (the question was posted at ~6PM(?) local time, and wouldn't air coming of the lake earlier in the day tend to be cooler than that later in the day?) that Toronto cooled that later, more nearly water-saturated air coming off the lake enough for a fog to form. That is, in short, Toronto was cooled off sharply by wind coming off the lake all day long. By the end of the day Toronto was so cold that it cooled the somewhat warmer moisture-rich air coming off the lake at that time enough to cause a fog to form.
posted by jamjam at 6:41 PM on October 4, 2007

I just came across this picture - City Bird - on Daily Dose of Imagery.

It shows Toronto clouds from below and gives you an idea of how close they were to ground level.
posted by Nugget at 2:46 AM on October 5, 2007

We get this sort of effect in Juneau when we have had a fog in overnight followed by a clear day. The fog is burned off by the sun everywhere except over the very cold water in the channel. The channel has this really neat ribbon of low-laying cloud over it for most of the morning.
posted by Foam Pants at 1:45 PM on October 5, 2007

Low-lying clouds don't appear to be new to Toronto.
posted by zennie at 8:02 PM on October 5, 2007