Washington Gas Light Co., which supplies natural gas to the Washington metropolitan area, has boosted the gas it has in storage substantially to supplement the supplies it gets from two interstate pipelines -- Columbia Gas Transmission Co., its major supplier, and Transcontinental Gas Pipeline Co.

The gas company has increased the amount it can draw from its storage field in Hampshire, W. Va., from 1.8 billion cubic feet of natural gas to 2.4 billion cubic feet. It also has increased the amount of gas it owns in a storage field maintained by Transcontinental Pipeline from 623 million cubic feet last year to 1.485 billion cubic feet this year.

Transco's Washington storage field is in Louisiana.

The company also has increased the horsepower at the underground propane storage facility in Ravensworth, Va. Propane gas can be mixed with natural gas as a supplemental supply on days when customer demands exceed the amount of gas Transco and Columbia can send to Washington.

A company spokesman said Washington Gas plans further increases in its storage facilities for the 1978-79 heating season.

The company now has storage facilities for about 3 billion cubic feet of gas, or the equivalent. In the November-through-March heating season last winter, Washington Gas sold 75.8 billion cubic feet of gas. The year before, in a warm winter, Washington Gas sold 65.5 billion cubic feet of gas.

Meteorologists and others use a measure called a degree-day to determine how hot or cold a season is. Degree days list the amount the temperature varies from 65 degrees. In the warm five-month heating season of 1975 and 1976, Washington Gas recorded 2,975 degree days. A normal Washington winter has 3,446 degree days. Last winter, there were 3,898 degree days, Washington Gas said. That means last winter was 31 per cent colder than the year before and 13 per cent colder than normal.

Washington Gas's biggest day was Jan. 17, when it sold 866 million cubic feet of natural gas.

A spokesman said that in 1975-76 the company had available supplies of 76.4 billion cubic feet (excluding storage) when customers used 65.5 billion cubic feet. Last winter, suppliers could have shipped 76.9 billion cubic feet over the five-month period, and customers actually used 75.8 billion cubic feet.

The gas company expects to have about the same supply from Transco and Columbia together that is had last winter and has nearly 2 billion cubic feet more storage than last year.