Pressure cooker! Washington, D.C. matches record high pressure reading in heat wave

GFS model simulation of today?s heat dome and atmospheric heights at the 500 mb pressure level. (WeatherBell.com)

An indicator of the strength of a so-called ?heat dome? ? such as the one currently baking the eastern U.S. ? is the atmospheric pressure at high altitudes. The higher the pressure, the more intense the heat dome.

On Sunday night and Monday, an indicator of pressure known as atmospheric height matched the highest level on record about midway into the lower atmosphere, said Steve Zubrick, science operations officer at the National Weather Service forecast office in Sterling, Va.

The atmospheric height essentially tells you? how much the atmosphere is bubbling up (hot air, which is less dense, occupies more space or volume than cold air): a large height measurement indicates a big heat bubble.

The record height measurements were taken by weather balloons, launched twice a day at the NWS office in Sterling. The balloons record the height in the atmosphere at which different pressures are reached.

Helpful link: Assessing atmospheric pressures and heights (WeatherPrediction.net)

On Sunday and Monday, in three straight measurements, the height of the 500 millibar (mb) pressure point (which is about midway into the troposphere, the lowest atmospheric level) was an unsurpassed 5,990 meters, or about 20,000 feet.? That?s a huge heat bubble.

?Since 1962, I?ve found 3 other instances of hitting 5,990 meters; the last time being Aug 15, 2003 [at 8 p.m.],? Zubrick said.

Atmospheric heights in decameters (dm) at 500 mb pressure level at 8 p.m. Suday night. D.C.?s height was? 599 or 5,990 meters. Amazingly, Pittsburgh touched 6,000 meters.

A more ?normal? height for the 500 mb pressure level is 5,800 meters or 19,000 feet at this time of year.

Zubrick said weather balloons launched from Sterling have never recorded a 6,000 meter reading (at 500 mb), but that there was ?a shot? we?d touch it this week. (We may have missed that shot as this morning?s 500 mb height was down to 5,970 m and models generally forecast the heat dome to weaken)

Record high pressures, however, do not always lead to record heat. Although high pressure causes the air to sink, compress and heat up, other factors, such as soil moisture, humidity levels and wind direction also affect temperature.

For this particular heat wave, the high soil moisture and humidity levels are actually making it more difficult for the mercury to rise despite the record-challenging high pressure. If we had drought conditions rather than the moist soils from the recent downpours, the air would heat more readily (but it would be less humid).

Also, our prevailing wind direction has been from the north ? which isn?t a favorable direction for heating. If it was more from a westerly direction, the air would be blowing over the mountains resulting in a ?downsloping? effect, which would enhance the sinking and warming of the air under this high pressure dome.

We should have more of a downsloping wind by mid-to-late week but, by that time, the heat dome is forecast to weaken. Hence, the heat should remain below record levels.

Jason Samenow is the Capital Weather Gang's chief meteorologist and serves as the Washington Post's Weather Editor. He earned BA and MS degrees in atmospheric science from the University of Virginia and University of Wisconsin-Madison.

Source: http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/07/16/pressure-cooker-washington-d-c-matches-record-high-pressure-reading-in-heat-wave/

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