The Capital Weather Gang has an entry today in the Washington Post, “Pressure cooker! Washington, D.C. matches record high pressure reading in heat wave”. The value of 5,990 meters has been matched 3 times since 1962.
This value is a pressure measurement, but is not the same as the barometric pressure value you hear on the weather report. It is a record high 500mb geopotential height.
The 500mb refers to an altitude of 18,000 feet. The geopotential height refers to the altitude you need to go to obtain the standard (average pressure) at 18,000 feet. When the air is hot (as it is now in this region of the country), it expands… you have to go higher to get a barometric pressure reading at the average level. Conversely, when it is cold, the air contracts, and you have to go lower to get the average pressure level. Hence, the geopotential height is in meters, an altitude scale.
So this height is clearly correlated to the average temperature in the column of air from the surface up to 18,000 feet. This concept is actually far more useful to meteorologists than surface pressure.
When the geopotential height is low, then the air above you is compressed. If there are toxins in that air (such as radionuclides), compressing the air column will increase the concentration of toxins in the air you are breathing. Also, there is a counterclockwise rotation around 500mb lows (in the northern hemisphere), and this tends to accumulate material into it. Think of the drain in your bathtub, and how it accumulates hair and other debris as it goes down. So specific radioactivity in the air goes up in these situations (see Miyake et al).
When the geopotential height is high, then the air above you is expanded. Therefore, the concentration of toxins is decreased. However, there are a couple of other factors involved. High pressure brings subsidence, that is, air from the jet stream level is transported down. This raises radioactivity levels if (as now) there are high concentrations of radionuclides at this higher altitude. Also, this weather pattern favors temperature inversions, in which local pollutants are trapped, and there is little wind to blow them away. People living near nuclear power plants will experience higher radiation during this weather pattern.
The recent releases from Fukushima Daiichi linger at a lower altitude than the old stuff in the jet stream that has blowing around the earth the past two years. Consider this: when the radioactive toxins rise through the air, they have a hard time getting on the faster wind track which is higher above. Think of those moving sidewalks at airports. Stepping from a slow one to a fast one takes time and effort. So it might take several months for the radionuclides to step up into the jet stream level.
Personally, this weather pattern in the DC area is an improvement. The past two days are the first time in weeks that my thyroid has NOT been bothering me. Any radioactive iodine coming down from the jet stream level has already decayed some time ago. So, I am getting a much-needed break, even though it is miserably hot and humid outside.