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Why does my drinking water look milky or cloudy sometimes?

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Welcome to CoreFacts, where we're always short on time and big on science. I'm Jessica Robertson. Today's question is about water.

Why does my drinking water look milky or cloudy sometimes?

Once in a while you get a glass of water that looks cloudy or milky, but after a few seconds, it miraculously clears up! The cloudiness is due to tiny air bubbles in the water. Like any bubbles, the air rises to the top of the water and goes into the air, clearing up the water. The water in the pipes coming into your house might be under a bit of pressure. Air bubbles are dissolved in pressurized water, and as the water is exposed to normal atmospheric pressure when flowing into your glass, the gases come out and temporarily "cloud" the water.

And now you know. Join us again every week for a new CoreFact. For other CoreFacts or for CoreCast, our in depth science podcast, go to If you'd like to have questions featured on our show, give us an email at or a phone call at 703-648-5600. Remember, long distance fees do apply.

CoreFacts is a product of the U.S. Geological Survey, Department of the Interior.

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