The Washington area endured an unusually warm and dry year in 1981, the second consecutive year of below-normal precipitation and above-normal temperatures, according to preliminary year-end figures supplied by the National Weather Service.

Rain and snow totaled only 30.67 inches for the year at the weather service's official measuring station at National Airport. That was 8.22 inches below the long-term average here of 38.89 inches, forecasters said yesterday.

The temperature for the year averaged59 degrees, 1.7 degrees above normal, according to unofficial totals. Seven of the12 months of the year were above normal, but the summer months turned out to be relatively mild, with the area escaping the prolonged heat waves of previous summers.

The sparse precipitation in 1981, combined with a similarly slim total of29.32 inches in 1980, brought near-drought conditions for much of the Washington area. Authorities imposed water restrictions on 700,000 northern Virginia residents for26 days in January and February, when the Occoquan Reservoir dropped to less than a third of its 11-billion-gallon capacity.

Also, the Potomac River, which supplies water for the District of Columbia and much of suburban Maryland, ran 24 percent below its normal level of 5.9 billion gallons per day. It fell below 1 billion gallons a day briefly in August, the first time it had done so since 1977. Municipal water supplies were never endangered, however, with daily consumption ranging from 300 million to 400 million gallons, compared to a daily flow of about 900 million gallons on Aug. 29 and 30, the low point of the year.

As for the temperature in 1981, the highest readings occurred on June 25 and July 9, when the thermometer hit 98 degrees, and the lowest came on Jan. 12 and 13 with readings of 11 degrees on both days. None of the temperatures set records, but the 98-degree mark on June 25 tied an 1894 record for that date.

The weather service has maintained temperature and precipitation records here since 1871. Forecasters note that the official measuring station, located at National Airport since 1941, is in the center of Washington's so-called urban "heat island" and has recorded increasingly warmer temperatures than the surrounding suburbs and rural areas as the urban buildup around the airport has expanded in the last 20 to30 years.

Even so, things were not all bad last year. Officially, there were only 33 days during which the temperature hit 90 degrees or higher, slightly below the normal number of 38 for National Airport. There were far fewer 90-degree days elsewhere in the area.

At the other extreme, the airport recorded 61 days during which the thermometer dropped to the freezing point of 32 degrees or colder, also below the normal number of 73 such days for the airport.