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Good Question: Why Are The Windows Foggy?

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MINNEAPOLIS (WCCO) — With the hot, humid Minnesota air this week, many people are waking up to steamy, foggy windows. The glasses and sunglasses are steaming up when we walk out of the air conditioning, too. So what’s the science behind the foggy windows?

“It’s just like a pop can – when you take a pop can out of the refrigerator,” said Brent Woods, a guy who talks a lot about windows with St. Louis Park-based Wellington Windows.

It’s fairly simple science: the air is full of tiny water vapor droplets, and that vapor condenses into liquid droplets.

“When one side [of the window] is cooler, humidity is attracted to the cool side of the surface,” Woods said.

Even though it looks like the fog and the dew is inside your house, the truth is that during the summer the fog ends up on the outside. The cool air-conditioning inside the house gets to the outer pane of the window glass.

“The inside of your house is a giant refrigerator, because this [ glass] is the thinnest surface, its the most susceptible point for humidity to grab on,” Woods said.

When a meteorologist says the dew point is 79 degrees, if the outside of your window is 79 or cooler, water vapor is going to condense. The window will start to fog up.

“Even the best windows are gonna have humidity. Even commercial buildings with one-and-a-half–inch thick glass, they’re gonna have humidity on them,” Woods said.

Wellington sells windows that reflect the heat of the sun back out of the house, which can slow down the process. But when the dew point is extremely high, it’s impossible to completely prevent the condensation process from taking place, he said.

The same process of condensation happens on sunglasses and camera lenses, after they’ve been cooled in the AC.

Generally the fog shows up during the overnight hours, because the sun’s rays are no longer keeping the glass warm.

“As soon as the sun comes on, it warms up the surface. As soon as that surface warms up, you’re equalizing the temperature differential,” Woods said.

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