The temperature in Washington topped off at a blistering 98 degrees yesterday afternoon, matching a record for the date set in 1952, the National Weather Service reported. Yesterday's high was recorded at 3:45 p.m., and it served to mark the day as the hottest of 1981 so far.

But relative humidity declined during the course of the day, dropping the discomfort index to a point where conditions were bearable for most residents. The humidity reading dropped from 88 percent at 6 a.m. to 42 percent at 4 p.m.

Brief thunderstorms hit the area at about 8 o'clock last night, soaking the area with 1/2 to 1 inch of rain mixed with hail, according to the weather service. Temperatures dropped from 92 degrees to 76 within about a half hour. s

Storm damage in the area appeared to be minimal. The C&P Telephone Co. reported few problems and Pepco said service was interrupted in about 1,200 households in Montgomery and Prince George's counties and in the District. Virginia Electric & Power Co. said about 7,000 of its customers were affected by power failures.

Prince George's County firemen reported that lightning struck two single family houses, causing a small fire in one home and no problems in the other. In Washington, branches were downed in scattered areas along the George Washington Parkway.