You're right. It ain't the heat, it's the humidity.

For the last 12 days, muggy, energy-sapping air has hung like a dead weight over the Washington area, creating the illusion of great heat but the reality of great discomfort.

While temperatures have been running about average or slightly below, humidity readings have been far above normal for this usually humid month, according to National Weather Service records at National Airport.

Since July 13, the humidity has pushed repeatedly into the 90 percent range in the early mornings - usually the dampest time of the day - and dropped only to the 65-to-80-percent range by early afternoon. The normal humidity level for early moring (7 a.m.) is 75 percent. Normal afternoon humidity (1 p.m.) is 52 percent.

On six of the last 12 days, the humidity stood at a sticky 94 percent at 7 a.m. Yesterday, it was 91 percent at 7 a.m. and dropped only to 82 percent by 1 p.m.

At the same time, temperatures have been moderate, at least for July. The thermometer at National Airport has reached 90 degrees only six times this month, with the highest reading of 93 degrees on July 12.The temperature normally hits 90 or higher 13 times in July, frequently pushing into the upper 90s and occasionally topping the 100 mark.

It all started about two weeks ago when warm, moist air rolled in from the south, bringing an end to prolonged springlike weather for the area. Both temperature and humidity climbed. The dog days of July arrived.

Now, a huge, nearly stagnant high-pressure area is sitting Buddha-like over the Atlantic coast, blocking the movement of cooler air into the Washington area from the west. A cool front stretching from Labrador southwest across New York and western Pennsylvania has not made a dent. The only air here is in the form of sluggish, clockwise currents from the high-pressure area, bringing damp heat from the Gulf of Mexico and Florida.

"And there's no relief in sight," says National Weather Service observer Marge Grbich. The high-pressure area over the Atlantic - a typical summertime phenomenon called a "Bermuda high" - shows no sign of moving on, Grbich said. The outlook calls for continued humid and warm conditions through at least Saturday with a chance of thunderstorms each day, she said.

There was one bit of good news. Even with the stagnant atmosphere, air quality in the area has remained relatively good. The Metropolitan Washington Council of Governments, which maintains seven air pollution monitoring stations in the area, said the air quality has generally been in "good" or "fair" range all summer.

"This is the first year we've gone this far without declaring a health advisory or pollution alert," said COG's Dennis Bates. The highest air quality index reading so far was 95 earlier this month, Bates said. A health advisory is issued when the index reaches 100 and a pollution alert at 200. CAPTION: Illustration, Readings for last 12 days show above normal humidity. The Washington Post