April 14, 2015: Did the air feel really dry today to you? It did in Chicago today, where the dry air tied a record at O’Hare airport. The relative humidity dropped to 13%, which tied April 8, 1971, April 11, 1956, and May 10, 1934 for the driest relative humidity on record for Chicago. For reference, the human body generally feels comfortable at 45% relative humidity, so 13% is very dry!
Relative humidity readings at 4 pm today
If you are wondering what or how we figure out the relative humidity of the air, here’s a crash course (get ready for a lot of science!): humidity describes how much water vapor is in the air. We can measure this with absolute humidity, which is the mass of water vapor divided by the mass of dry air at a certain temperature. The hotter the air, the more water vapor it can hold, and so the value for absolute humidity is higher.
To get to relative humidity, we use the ratio of the current absolute humidity to the highest possible absolute humidity at that temperature. At 100% relative humidity, the air is completely saturated and can’t hold any more water, usually creating rainfall as the moisture falls out of the air.
An area of high pressure results in downward movement in the atmosphere. This dries the air out, and is why we don’t usually have clouds or active weather under high pressure.
Why did the air feel so dry today? We did have an area of high pressure right overhead, which may have contributed to the dry conditions. High pressure promotes downward movement in the atmosphere, which counteracts what you may have learned in elementary school about the water cycle- instead of the air rising, then cooling and condensing into clouds, downward moving air dries out. This is one of the reasons why the air was so dry today.
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