This is my first actual question on this board, so I hope I am following the correct protocol.
The question is this. I live in Toronto and drive into work every day on the lakeshore route, which has a good view of lake Ontario. This winter has been very, very cold, with many days below -20 C or even colder. On some clear, still mornings, when it is very cold, I have noticed that the lake is literally covered with what looks like plumes of steam or fog rising from the water - sometimes, right out to the horizon. To me, this looks very odd indeed, as I associate steam with hot, not cold, water.
I am sure this has a logical explaination that any meteorologist would know right away - can anyone explain this cold-weather steam?
The water is indeed warmer than the air. Which is what causes the water vapor. As for the dynamics someone will be along soon and explain them I’m sure.
Bodies of water hold heat much better than the air does. On a chilly morning, the air may have gotten quite cold overnight, but the water will still retain much of the warmth from the previous day, and so can be several degrees warmer than the air over it. Water vapor evaporating from the surface will condense quickly in the cold air, forming a fog over the lake.
Naturally the water must be warmer than the air - there is no ice, so it must be warmer than 0 degrees C.
Is steam then a function of the extreme difference in temperature between the water and the air - rather than, as I had previously thought, a function of boiling/really hot water? If so, how much temperature difference is required to produce steam?
The misconception here is that you can see steam. You can’t. Steam is water vapor at the local boiling point of water or higher (212 F or 100 C at sea level). You can’t see water vapor until it drops below that temperature and begins to condense into tiny droplets suspended in the air. What you are seeing over the lake is not steam (which is a gas), but microscopic droplets of liquid water in the air. Hope this clears things up.
Actually water does not change temperature as fast as air does. In warmer climates the water will be cooler than the air. Land also changes temperature faster than water, thus causing winds in the morning and evening near the shore.
I think kniz’s point is that, not only do bodies of water hold heat better than air, bodies of water are also harder to heat than air. The effect is reversed, in other words.
Whether the stuff coming off the surface be steam or fog, or some yet third definition, I have yet to hear a reason why it forms over the lake when the air is really, really cold.
I must say, the effect is really striking. Is there a weather person in the house who can explain why it happens?
Okay, maybe I am just having a hard time loosing my “common sense” or rather mistaken opinion that evaporation is caused by high heat, rather than temperature difference … I now understand that it is the temperature difference between the water, which is relatively warmer, which causes the evaporation.
You see, not being a scientist, I associate steam or fog with the stuff that comes off of a hot cup of tea … I see now that the answer is, that as far as the water in the lake is concerned it is as much “hotter” than the air as the water in the teacup is.
So my next question is - how much difference must this be, in order for evaporation to form visible fog? When I look out my car window, can I tell approximately what temperature the air is by observing how foggy the lake is (assuming that the water temp is just above zero C and there is no wind?). I realize that the more fog, the colder, but at what temperature does this phenominon begin to occur?
)