The current heat wave that is taking its toll in drought, air pollution, broken air-conditioners and general human misery is just an added squiggle to a gradual but definite century-long warming trend in the Washington area.

Historical weather data assembled by The Washington Post show that the summers are getting hotter and the winters are getting milder. Over the last 100 years, the average annual temperature has risen 7 degrees from a low of 52.2 degrees in 1875 to a high of 59.4 degrees in 1975.

There have been fluctuations a along the way, buut the general directionn has been upward.

The temperature reaches 90 degrees more often now than in the past. It drops to freezing - 32 degrees - less often, notwithstanding last winter's freakishly cold weather.

The last time the temperature here reached zero was in 1935. In the 19th century, it frequently fell below zero.

Between 1872, when the National Weather Service began keeping records here, and1924, the temperature hit 90 degrees or higher an average of 25.8 days per year. Between 1925 and 1976, the average jumped to 33.6 days.

Similarly, each year between 1872 and 1924 average 90.7 days during which the temperature fell to freezing or colder. Between 1925 and 1976, the average number of freezing-days dwindled to 73.3 days.

Climatologists blame the increased temperature here on a global warming trend affecting the entire nothern hemisphere between the end of the 19th century and about 1950 and on the urban "heat island" phenomenon affecting all cities where concentrations of concrete and asphalt as well as heat produced by factories and machines tend to warm the surrounding air.

Washington's current heat wave is caused by amuch more short-lived factor, according to the Weather Service: a subtropical high pressure belt of wind that usually lies across the southern tier of states in the summertime has pushed farther north than normal.

This in turn has pushed the jet stream - the high altitude band of westerly winds that control much of the weather here - farther north into Canada. The cool frents normally borne by the jet stream to the Washington area are not penetrating this far south, thus affording no relief.

Though the smothering, humid heat of the last weeks continued yesterday, it did not reach the extremes of previous days.A string of predrawn thundershowers followed by partial cloud cover during the rest of the day held the thermometer to a high of 92 degrees.

The Metropolitan Washington Council of Governments extended its air pollution alert for the sixth consecutive day through 3 p.m. today after reporting an air quality index reading of 110 at 3 p.m. yesterday. The index climbed to 120 an hour later. Readings of 100 or more are described by COG as "very unhealthy."

Forecasters predicted more high temperatures today, possibly up to 100 degrees but said Friday may not be so bad with highs reaching only 86 to 92. There is a chance of move thunderstorms tonight with overnight lows of 70 to 75 degrees.

While sporadic thundershowers peppered some parts of the eastern states yesterday, other areas in the East went rainless.

Virginia Gov. Mills E. Godwin Jr. said in Richmond that conditions in some counties are reaching the point that water rationing may have to be imposed.

He granted a request for a local emergency declarations in the Shenandoah Valley town of Woodstock to permit the town to institude mandatory water conservation measures. Godwin said he expects to receive similar requests from other localities.

The National Weather Service's long-range outlook for the D.C area calls for a continuation of above normal temperatures and below normal rainfall through mid-August.

James Wagner, a meteorologist with the Weather Service's long-range prediction group, said yesterday it is difficult to pinpoint specific historical reasons for climatic changes.

The hemispheric warming trend that began in the late 19th Century and began to reverse itself between 1940 and 1950, he said, is blamed by some climatologists on increases in industry-related carbon dioxide in the atmosphere.

"We started having a slight cooling trend in 1960's, but increases in carbon dioxide were still being made in the atmosphere," he said. He said it is difficult to tell if the cooling tendency of the 1960s is continuing today.

In any event, he said, the "heat island" effect of urban areas superimposes an additional warming factor, which in the case of Washingon and other large cities could offset a hemispheric cooling trend.

While carbon dioxide can heat the air, Wagner said, other factors - such as ashes and dust from volcanic eruptions - can have a cooling effect on parts of the world by literally screening the sun's rays.

Also he siad, in drought areas like those surrounding Washington, the parched ground "heats up faster than usual, and the ground in turns warms the air" by releasing heat into it.

The Weather Service's official recorling station for Washington since World War II has been at National Airport, one of the hottest locations in the area. Prior to the war, the official recording station was at 24th and M Streets NW and at other downtown locations, also relatively warm places.

So far, the Weather Service has recorded 18 days this month on which the temperature reached 90 degrees or higher, nearing the record of 21 days for any single month set, oddly ebnough, in July, 1872, ans again in July, 1878, when temperatures were otherwise much cooler.

The all-time annual record for 90 degree days was 59 set in 1966, foolowed by 58 in 1872. The smallest number of 90-degree days was seven, occuring in 1886 and in 1905.

On the cold side, the greatest number of days with freezing temperatures - 118 - occurred in 1904. The smallest number was 46 in 1974. The last year that had more than 100 freezing days was 1917.