The reason your clothes have been sticking to your back for the last three weeks is that the jet stream deserted the United States for Canada and the Bermuda High left the mid-reaches of the Atlantic for the United States.
Those explanations may be little comfort to the 160 million people living east of the Rocky Mountains, but they are why the eastern United States is steaming through one of the most humid summers in memory. Making it even worse is that the horrible weather followed a June and an early July that were among the most pleasant in memory.
"It took us all the month of June right up to June 12 to hit a 90-degree day, which was an all-tome lateness for Washington," Jerry LaRue, meteorologist-in-charge of the washington office of the National Eather Service, said Wednesday. "Since then, we've had what I can only call an oppressive summer
Oppressive it is and oppressive it will stay, at least for awhile. The jet stream shows no signs of migrating back to where it belongs, south of the Grejt Lakes, and as long as the jet stream stays where it is in Canada the high pressure air mass known as the Bermuda High will stay where it is over the southeastern United States.
The Bermuda High draws air from the sourthern oceans and the Gulf of Mexico and circulates it clockwise to the eastern parts of the United States. This air is wet, humid, soggy and muggy. So wet, in fact, that Washington has not had a day since July 12 when the relative humidity has not reached 90 percent.
"We're looking for some change in all this but its more wishful thinking than anything else," said Dr. Murray Mitchell of the National Oceanic and Atmospheric Administration. "Nothing's going to change until the sun moves south and pulls the jet stream with it."
In normal summers, the jet stream, which helps to move air from west to est, migrates north, but not as far north as it moved this summer. Not only did the jet stream move farther north this summer than usual, it's also taking a longer northern vacation than usual.
On July 12, when this humid spell began, there was no detectable jet stream anywhere in the United States. Five days later, a week jet stream had moved south into the Great Lakes region, but by July 17. It weakened again, and by july 24 it fell apart completely. The only detectable jet stream in North America today is over southern Canada.
The jet stream is the center and uppermost of the fast-moving westerly winds, usually traveling east at heights of 40,000 to 50,000 feet and speeds of 150 to 200 mph. In the summer, the jet stream weakens notably, dropping in speed to no more than 90 mph.
A normal, healthy jet stream circulates air through the entire country, carrying summer storms and cold fronts from the West, helping regulate the East's climate.
The jet stream also blocks the Bermuda High, putting enough pressure aginst it to keep it out over the Atlantic. This yar, its travel north left a vacancy that the Bermuda High filled almost naturally, bringing with it the warm, wet air that collects over the Gulf of Mexico and the sourthwest Atlantic.
"What we have here is a failure in ventilation," said Dr. Donal Gilman of the Long Range Weather Group at NOAA. "We're just not getting the storm passages and associated cold fronts that bring us any change of air."
Oddly, July saw little deviation in temperatures. The Weather Service figures this July didn't vary more than one-tenth of a degree from normal. There's another blessing we can count: July was cloudy enough to block the sun's light from making smog, which would not only make matters worse but also be downright unhealthy.