Monday, October 25, 2010

Pressure: Converting between millibars and inches mercury

The two commonly-used units for measuring atmospheric pressure are millibars and inches mercury. My Davis Vantage Pro2 weather station reports pressure in inches mercury and millibars are the typical measurement used for recording hurricane pressure. A normal millibar reading is 1000; an extreme measurement is 925. A normal inches mercury measurement is 30.0; an extreme measurement is 27.3. The Wikipedia pressure page provides a relatively easy conversion path via torrs. One millimeter mercury is equal to one torr, so one inch equals 25.4 torrs. One bar is equal to 750.06 torrs, so one millibar is equal to 0.75006 torrs.

Conversion from inches mercury to millibars simply amounts to conversion to millimeters/torrs, then conversion to millibars. The first involves multiplication by 25.4; the second involves division by 0.75006. The net scaling factor is 33.864. Conversion from millibars to inches mercury is simply the reverse calculation, so it simply involves multiplication by 0.02953.

So, 1000 millibars is equal to appx. 29.53 inches mercury. 30 inches mercury is equal to appx. 1016 millibars. A half-inch of mercury is equal to appx. 17 millibars. Here's a quick lookup table:

in Hgmb

Update (10/26/10): It's useful to note that a hecto-Pascal (102 Pa) is equal to one millibar (10-3 bar). Equivalently, one bar is equal to 100,000 Pascals (105 Pa).

Sunday, October 3, 2010

Down 32 degrees in 43 hours

After watching New England weather closely for a year, I've seen some wild temperature swings. But, one thing I rarely see is wild swings in the dew point. The dew point tends to be quite a bit more steady than temperature. But, with the latest storm that rolled through, I saw an amazingly large change in dew point---30 degrees in less than 48 hours. The remains of Tropical Storm Nicole made its way to the Boston area Thursday, September 30th. This pushed the dew point up to 71.9F degrees at 12:40pm on Friday, October 1st. I had to turn the air conditioning back on even though we had seen a low temperature of 42 just a week prior! The storm dumped .64 inches of rain then departed out to sea, taking all the humidity with it! The dew point fell quickly, bottoming out at 39.6F at 7:30am on Sunday, October 3rd. So, the dew point dropped 32.3F degrees in just under 43 hours! We had to turn the heat on this morning after having the A/C on just two days ago. Phew!