Saturday, October 17, 2009

The Balloon Goes Up

Twice each day, at 00Z and 12Z, weather balloons are released at a multitude of locations around the globe. As they rise, they are tracked by radar to obtain data on winds aloft. Each balloon carries a sensor package (called a "radiosonde") that records air data and transmits it to the ground.

At NOAA/ESRL Radiosonde Database Access you can access the archived "RaOb" data. The data can be had in various output formats including a very nice Skew-T Plot. ("ESRL" stands for NOAA Earth System Research Laboratory.)

Here are the Skew-T plots for 00Z last night derived from the 00Z soundings at Brookhaven, NY (OKX, on Long Island) and Sterling, VA (at IAD, Dulles Airport):

The plot for OKX shows the atmosphere as it stood a couple hours after I'd have left KBDR (if I hadn't decided that AmTrak was a better idea). The IAD plot shows what I'd have found on arrival at VKX. On both plots the red data curve is temperature and the blue one is dew point. The isotherms (lines of constant temperature) are tilted 45 degrees to the right (hence the term Skew-T) and the height scale is calibrated in millibars. The 850 Mb pressure level is about 5,000 feet altitude and the 700 Mb level is about 10,000 feet. (The stuff on the right side is a hodograph giving wind speed and direction vs. pressure level.)

Look at the OKX sounding. You can see that the temperature and dew point plots merge at about 900 Mb (around 3,500 feet). That makes it very likely that there would have been clouds at that level. The curves hit the 0 degree Celsius isotherm at about the same height. That says those clouds would probably have ice in them.

A look at the IAD sounding shows a slightly warmer lower atmosphere. The temperature and dew point curves don't reach the freezing point until about 850 Mb (5,000 feet). But they are really close together from a very low altitude. That says to me that low ceilings are a distinct possibility.

These two plots leave me convinced that if I'd attempted to fly from KBDR to VKX last evening I'd have probably gotten involved with serious icing. And if that's not enough, I can show you the METAR for Andrews AFB (four miles away from VKX) at about the time I'd have arrived:
KADW 162347Z AUTO 36010KT 7SM R01R/5000V5500FT R01L/4500V5500FT +RA OVC005 05/05 A2995 RMK AO2 UPB2315E2346RAB2258E2315B2346DZE2311 SLP147 $
If I'd somehow managed to cope with the icing I'd have arrived to find heavy rain and a 500 foot overcast. Probably a missed approach and a diversion to Manassas (KHEF). Here's the KHEF weather about an hour later:
KHEF 170055Z AUTO 35007KT 5SM BKN008 OVC014 06/03 A2998 RMK AO1
A bit better. No precip, five miles visibility under an 800 foot ceiling. Still, all in all I'm glad I was on the train.

Incidentally, you can go to Soundings From ESRL/GSD to get forecast Skew-T sounding plots for just about any airport. A very useful weather resource.

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