Dry and warmer conditions will carry us into the weekend. A broad ridge of high pressure will set up across the west coast pushing the storm track well to our north. Temperatures will climb into the 60s and stay there through the weekend. Skies will range from mostly sunny to partly cloudy until Sunday when the next storm system will approach, kicking up winds late Sunday, and bringing a good chance of mountain rain and snow (and to a lesser extent to the valleys) late Sunday through Monday.

inches of hg

I’ve had many ask me just what is meant by an “inch of mercury,” and how that relates to pressure. Whereas a Pascal is a calculated figure equal to a Newton per square meter (you can ask, but that’s for another column), an inch of mercury is a unit derived by the readings of the first instruments used to measure air pressure.

Inch of Mercury

Keep in mind both air and liquids are “fluids” when it comes to how they exert pressure on objects. If you dive down to the bottom of a deep swimming pool, you are likely to feel the pressure increase on your ears, just as your ears pop when travelling over the mountains. A column of air an inch square extending out to the top of the atmosphere (over 20 miles) weighs about 14.5 pounds, and therefore exerts a pressure of 14.5 pounds per square inch (1 bar) at sea level. Because liquids are much denser, if you filled that same one inch square column with water, it would only take about 34 feet to weigh the same as a 20+ mile high column of air. In fact, you could make a barometer out of water by filling a 35+ foot glass test tube with water, invert it in an open pan of water, and measure how high the column of water stays in the closed end. But that wouldn’t fit into most people’s living rooms, so we use mercury, which is 13.6 times denser than water, and only takes about 30 inches in your column to weigh the same as one atmosphere. So “an inch of mercury” is the pressure equivalent of about 1/30th of an atmosphere.

 

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