Why did `steam` appear over the Chicago River at freezing temperatures?

Arabia Weather - In the past few days, polar air has brought subzero temperatures in many parts of the United States, including Chicago and Lake Michigan. People there noticed a very strange phenomenon where “steam” began to form over the Chicago River and lake, which prompted many people to ask: “What is happening?”

Firstly, it should be said that this is not steam, steam is water vapor that is released when water is heated. What you see is actually water vapor, but we must be precise about its composition. The phenomenon you see is closer to seeing a person breathing on a cold day. You certainly would not describe yourself as evaporating.

Smoke on the surface of the water

This phenomenon was previously called sea smoke, but it more closely resembles fog than anything else. This phenomenon is caused by the temperature difference between cold air and warmer water. At the border between the two, a thin layer of air forms, in which there is some water vapor. Warm air rises, water vapor condenses, and when it suddenly comes into contact with cooler air, it condenses into thin, weak fogs across rivers, seas and lakes.

The best conditions occur when there is a large temperature difference between air and water, as is the case in Chicago right now. This is easiest when there is little wind, so that condensation can occur without being immediately discharged by the wind.

Here's a more detailed science lesson: Dew

Why do you need those conditions to create sea smoke? This has to do with the dew drop. The amount of water vapor needed to saturate a given volume of air depends on temperature. The lower the temperature, the easier it is to reach the dew point. After that, the water vapor begins to condense. At high altitudes, we call this condensation clouds. At ground level, we call it fog.

There is a relationship between fog and sea smoke, but fog forms in the opposite direction – warm air over cold water. A similar phenomenon to sea smoke is the evaporation of snow when it is under direct sunlight.

Read also:

Why does steam come out of a person's mouth in winter?

What is dew, how is it formed, and what are its benefits?

Source: iflscience