The year just concluded was the warmest on record in Washington.

According to National Weather Service totals, the average temperature for 1990 soared to 60.2 degrees at National Airport, the highest since record-keeping began in 1817 and the first time the annual average has crossed the 60-degree threshold.

"It's a definite first," Weather Service spokeswoman Melody Hall said.

Washington's previous record was 59.5 degrees, set in 1980. "Normal" for the city is 57.5 degrees.

Hall said yesterday's figures are preliminary. Final calculations, she said, could change the average by as much as 0.1 degree but not enough to dislodge the new record holder.

Weather records dating back a century and a half show an irregular but definite warming trend in Washington. Annual average temperatures have pushed up from the low and mid-fifties to the upper fifties -- and now, apparently, beyond.

The lowest annual average temperature in Weather Service records was 52.2 degrees, reported in 1875 and again in 1904. Earlier but less-reliable figures from Naval Observatory, Smithsonian Institution and National Archives records show annual average temperatures of 51.4 degrees in 1836 and 51.7 degrees in 1856.

Washington wasn't the only hot spot in 1990. Record or near-record warm temperatures blanketed much of the region and the nation, guaranteeing further debate over global warming and the reputed influence of the "greenhouse effect" from fossil fuel emissions.

Whether the Earth as a whole was unusually warm in 1990 will not be known officially until the United Kingdom Meteorological Office releases its annual report later this month. However, meteorologists said preliminary indications are that 1990 worldwide could be the hottest year on record. The six hottest years for the planet were all in the 1980s.

Climatologists say it is possible that the temperatures reflect the greenhouse effect, but caution that the global warming theory has not yet been proved. Also, they say, there are other explanations, such as the so-called heat island effect in cities and the relocation of the official thermometer.

Baltimore-Washington International Airport and the city of Baltimore recorded their warmest years, 57.9 degrees at the airport, 61.1 degrees in the city. Virginia, using a statewide network of measuring stations, reported its second-warmest year, with an average of 58 degrees. Virginia's warmest year was 1921, when the average temperature was 58.3.

Although Washington's temperatures were unusual, its precipitation was not. Rain and snow totaled 40.73 inches for the year, just 1.73 inches above normal.

Despite the bone-rattling cold now gripping much of the country, including the upper Midwest and Pacific coast, the average temperature throughout the nation for all of 1990 turned out to be the third warmest on record: about 56 degrees.

Only 1922 and 1934 were warmer, said climatologist Richard Tinker at the Weather Service's Climate Analysis Center. In those years, he said, the averages were 56.2 degrees and 56.7 degrees, respectively.

Average annual temperatures are based on the cumulative total of daily maximum and minimum temperatures, divided by double the number of days in the year. The national average temperature is a composite of annual average temperatures from several hundred Weather Service stations in the contiguous 48 states. Such records have been kept since 1895.

"Normal" temperatures are based on cumulative averages of the previous 30 years of temperature records for a particular location.

For Washington, 1990 was distinctly abnormal. Nine months were above normal, with only May, August and September recording modestly below-normal temperatures. Record high temperatures were reached on eight days, including four consecutive days in March when readings leaped into the mid-eighties. Normal high temperatures in March range from the low fifties to the low sixties.

The year produced a paltry snowfall total of about 5.6 inches in Washington, well below the 10 to 20 inches common to the area. Most of 1990's snow -- three inches -- fell last week and was swiftly melted by record warm temperatures.

In fact, the temperature had a hard time getting down to freezing in 1990. Preliminary figures indicate the official thermometer at National Airport reached 32 degrees or lower only 44 times, the smallest number since the Weather Service started keeping records in 1871.

The number of days on which the temperature fell to 32 degrees or lower each year in the 1880s and 1890s frequently exceeded 90 or 100. Today, the total is more commonly in the sixties or seventies.

One factor that may explain the warm temperatures, the heat island effect, is the tendency of urban areas such as Washington to absorb more heat from sunlight during the day than rural areas and to release it at night, keeping nighttime temperatures artificially high.

Weather Service prediction branch scientist Robert E. Livezey said the buildup of heat-absorbing concrete and asphalt in the last century is bound to "drive temperatures up . . . . There is a definite heat island bias at National Airport."

Also, he said, the relocation of Washington's official thermometer from various downtown locations to the warmer environment of the airport in 1941 probably has pushed average temperatures up. (The readings taken at Dulles International Airport are not considered typical of Washington temperatures.)

Ken Shaver, forecaster at the BWI Weather Service station, has his doubts about the greenhouse effect too. "It's hard to say. You probably couldn't convince the people in California about it right now . . . . I just take the weather as it is. There's nothing much you can do about it anyway."