The area's fourth air pollution alert of the year was extended yesterday for at least another day, as the highest pollution reading of the year was recorded and temperatures reached the sweltering 90s again.

Adding to the discomfort was the humidity and the depressing report that weather forecasters could see no end either to the heat or to the stagnant atmospheric conditions that have permitted the buildup of pollution.

"There's no relief in sight," said forecaster Jeffrey Bowman. Temperatures should be mostly in the 90s through Thursday, he said.

The high temperature registration yesterday at Washington's official measuring station at National Airport was 93 degrees. Thermometers recorded 95 degrees at Dulles Airport and 99 at Ft. Meade.

The readings marked the 19th day in the last 21 in which temperatures here have reached 90 degrees or above.

Forecasters associate the prolonged hot spell with atmospheric stagnation on a grand scale, with a vast mass of hot air blanketing much of the country, and demonstrating little inclination to move.

In Washington metropolitan area, forecasters said, the circulation of the atmosphere has become too feeble to disperse the pollutants that are produced each day by the action of sunlight on motor vehicle exhausts.

An air stagnation advisory issued Friday for the area by the National Weather Service was extended again yesterday for at least another 24 hours.

In the District the pollution level reached a high of 147 on the Council of Governments' air quality index. The previous high for the year was 140, which was recorded earlier this month and matched on Friday. Readings between 100 and 249 are regarded as "very unhealthy."