AccuWeather's Melissa Constanzer breaks down how dew forms.

Dew forms when moisture in the air condenses onto cool surfaces, particularly during calm, clear nights. As daytime temperatures rise, the sun heats the ground and surrounding air. However, after sunset, the absence of sunlight causes rapid cooling at the surface, especially when winds are light and skies are clear.

This cooling process is known as radiational cooling, which leads the surface temperature to drop. When the surface temperature falls to the dew point—the temperature at which air becomes saturated—moisture from the air condenses into tiny water droplets on grass, roofs, and other surfaces.

In the morning, as the sun rises and surfaces warm again, this dew evaporates. Occasionally, if conditions are right, the evaporating water can be visible as steam, particularly from surfaces like roofs that heat quickly.




How does dew form?

It's OK, it's OK, don't worry. My neighbor's house wasn't on fire. That was a spectacular May morning a few years ago where I captured do burning off as steam. It's the evaporative process that dries up the grass and so many other surfaces. Now many roofs can heat up quickly in the morning sun, so a lot of water is trying to evaporate quite quickly too. Thus we can actually see it as steam because there's so much water coming off of it. But how do we even get that evaporation coating that you see there? Of due to form in the first place, right? Because it had to come from that moisture. Well, there's a little bit into that process. First, you have to understand what happens as we go into the nighttime hours. We get what we call radiational cooling. So on a calm, clear night, this is when it happens. The to the greatest extent. If there's a little bit of a wind, we can have more mixing in the atmosphere, but ultimately on a calm, clear night. We watch that surface temperature as we go throughout the day. The sunshine comes out and of course you have the warming that takes place so we know those numbers will go up during the day. The sun is heating the ground and of course all the air then is also absorbing some of that warmth as well. So that's why we can get into the 70s, for example, for a spring afternoon. But then with those clear skies and really no wind at night, all that heat rises and it cools very quickly. The surface is no longer warming, therefore it has to cool down into the 40s. Knowing that, let's talk moisture, because if we have a dew point in the 40s, that's when we know that do will condense. Then you can see as we go through the nighttime hours that temperature drops right to about near that dew point. And then once we get it to that dew point or around it, we start to see the dew form, and that's how we get do.

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