Although a frigid January brought record business volumes on several days recently to Washington Gas Light Co., the natural gas distributor's profits for all of 1981 were up only modestly because weather was warmer--on average--for that year.

The D.C.-based utility reported yesterday that net income rose to $19.2 million from $18.1 million in 1980, as revenues increased $94 million to $558 million. Per-share earnings declined by 15 cents to $3.36 from $3.51, because more common shares were outstanding after WGL sold 1 million new shares last May.

In the fourth quarter alone, the utility company's profits rose to $11.7 million from $11.1 million as revenues increased to $196 million compared with $159 million. Per-share earnings in the October-December quarter fell to $2.04 from $2.37 with the average common shares outstanding up to 5.46 million from 4.4 million.

Weather during the 1981 heating season was 5 percent warmer than normal and about 3 percent warmer than 1980, WGL noted.

Higher revenues and the relatively small increase in overall profits were attributed to higher customer rates and a larger volume of natural gas sold. But expenses rose sharply, too. In the fourth quarter alone, interest costs jumped $1.14 million from the 1980 level to $7.57 million.

Part of the increased sales volume reflects more customers in an era when natural gas supplies have been plentiful.

USF&G Corp., a Baltimore-based insurance company, said operating profits fell in the fourth quarter and for the full year.

Not counting capital gains or losses, USF&G (U.S. Fidelity & Guaranty) earned $41.2 million ($1.46) in the recent quarter, compared with $45.5 million ($1.62) a year earlier. Net income was $36 million ($1.28) vs. $45.9 million ($1.64).

In the year, operating earnings declined to $168.6 million ($5.99) from $228.5 million ($8.18). After capital losses in both years, based on investment transactions, net income fell to $167.1 million ($5.94) from $223.1 million ($7.99).

Chairman Jack Moseley said yesterday that underwriting profits on property casualty insurance declined and that fourth-quarter results were adversely affected by strengthening USF&G's existing statutory loss reserves by approximately $50 million because of adverse developments in prior accident years.

Total revenue increased slightly for the fourth quarter and for the year. Pretax investment income for the fourth quarter and the year 1981 increased 23.3 percent and 20.2 percent respectively over the comparable periods of 1980.

Robertshaw Controls Co., a Richmond manufacturer of environmental and timing-control devices, said profits fell in the final quarter and for all of 1981.

The firm also revealed that Ametek Inc., a New York instruments and plastics firm, has purchased almost 10 percent of Robertshaw common stock, and Robertshaw went to court in Richmond yesterday to get an injunction to stop Ametek.

President Ralph Thomas said fourth-quarter profits were $1.7 million (48 cents) compared with $4.1 million ($1.11) a year earlier, as sales declined to $83 million from $92 million. Profits for the year dipped to $7.9 million ($2.25) from $10.2 million ($2.61) as sales rose slightly to $351.6 million from $349 million.

The Robertshaw chief blamed high interest rates, inflation and near record lows in auto and housing sales for the declines. In the early weeks of 1982, shipments in appliance and temperature-control markets "were at extremely low levels," and if the trend continues "first-quarter operations will likely result in a loss," he added.

On the Ametek stock purchases, Ametek told the Securities and Exchange Commission it plans to contact Reynolds Metals Co. of Richmond, which owns about 30 percent of Robertshaw. But Reynolds Chairman David Reynolds said his firm has no intention of selling its interest, and Thomas said Robertshaw has no interest in a sale or merger while charting "an independent course."

Planning Research Corp., a McLean-based professional services firm, reported record profits for the quarter and six months ended Dec. 31. In the October-December period, net income was $1.76 million (26 cents) compared with a loss of $1.25 million in the same months of 1980, as sales rose to $79.9 million from $77.8 million.

In the six months, profits were $3.6 million (53 cents) vs. $217,000 (3 cents) and sales rose to $158.9 million from $153.7 million.