Why Does A Cold Glass Of Water 'Sweat' On A Hot Day?

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By By Steve Pool

SEATTLE - Say it's a hot day, and to beat the heat, you go inside and grab a nice glass of lemonade. But a few minutes later, the glass is coated with water drops. Why?

The first steps to understand are that the air always has tiny droplets of water vapor in it, and that warmer air can hold more water than colder air.

In our example, the lemonade will cool the glass it's in, which in turn will cool the air immediately around the glass.

As that air near the glass cools, it can't hold as much water vapor, so the water changes from its water-vapor gas state to its liquid form, which you see as water drops clinging to the glass. That process is called "condensation."

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