Tuesday, November 16, 2010

Don't Forget the Satellites

At a time when more Earth and weather observations are needed, not less, the U.S. government has been cutting back. Jeff Masters wrote about the demise of the QuikSCAT satellite. The NYTimes more recently raised the issue with the demise of the ICESat satellite. With the government in cut-back mode, we may soon lose more important weather satellites and lessen our ability to accurately predict weather phenomena, such as hurricanes. This is one area where the money is clearly worth it---weather satellites save lives by providing alerts of dangerous weather events. This is especially important since we are in a phase of heightened hurricane activity and global warming may be increasing the frequency and/or intensity of hurricane storms.

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!

Wednesday, January 13, 2010

No 11F Low

The National Weather Service is currently predicting a low of 11F for Bedford, MA and a low of 12F for Natick, MA. I find these forecasts hard to believe. Why? Because around 2pm today, the dew point shot up to 11F in Bedford and 13F in Natick and the dew point is currently 15F in Bedford and 17F in Natick. Yes, it is certainly possible that the dew point could drop overnight. But, I think more likely is that the jet stream shift that was due arrived a bit earlier than expected and the NWS has not used the sudden change in dew point as an indicator. Based on recent experience, late evening dew point has served as a lower bound for overnight temperature, so I'd be very surprised to see the NWS predictions come true. I'd bet that the overnight low will be around 20F in Natick and 17-18F in Bedford.

Update (1/14): I was wrong. The dew points dropped significantly this morning and temperatures fell along with them, hitting 14F in Natick and 9F in Bedford.

Tuesday, January 5, 2010

Suspicious Forecast

I was surprised and a tad suspicious when, as of 1/4/10 12pm, the overnight low for 1/4-5 forcast by NWS for Central Middlesex County was 5-10F. At that point, the dew point was around 20F. I realize the dew point can rise/fall quickly overnight, but the predicted 15F degree drop seemed unusually large, especially considering that we weren't expecting any large shifts in pressure. My suspicions were partially confirmed when, later that day, the NWS updated its overnight low forecast to 15F. But, when I saw a temp of 24F and a dew point of 17F shortly before going to bed at 10pm, I was still suspicious whether we'd really make it down to 15F.

The next morning I awoke to find that the overnight low was 22.4F and that I wasn't alone---the main airport in my prediction region, KBED, recorded a low of 25F. Though, to the NWS's credit, another nearby airport in a nearby county, KOWD, recorded a low of 16F. Still, I'm surprised the NWS prediction could be ~15F off less than 24 hours in advance, especially when daily variations in temperature are typically no more than 15 degrees! Here's hoping the NWS prediction programs noticed this anomaly...