The Metro board yesterday awarded a $485,000 contract for improvements to the cold, windy and wet Pentagon Metrobus terminal.

The terminal, which looks attractive sitting on top of the Pentagon Metro subway station, has an architectural feature a number of large openings to the sky. Not surprisingly, on days when its rains water comes through those openings and douses rush-hour riders waiting for buses.

It wasn't too long after new terminal opened last July that Northern Virginia bus commuters - who must transfer there from subway to bus and back again - began to complain.

So Metro's designers got to work on the problem and decided to close the openings in the canopy, build two enclosed rooms on both ends of the platform for added protection against the cold, screeen the escalators to the subway station below and lower the roof line to increase protection against the rain.

The Defense Department kicked in $150,000 for the work, which is expected to be completed in August. The low bidder on the contract was Williams Enterprises Inc. of Merrifield.

The board yesterday also approved a Metro staff proposal to seek bids on more subway cars. Metro has developed a plan that combines money transfered from highway projects and money generated through Metro's investments to pay for most of the car purchase.

The number that can be bought will be determined by the price per car, but Metro officials are hoping to be able to buy between 90 and 100 cars on the first order with options for more later.

Metro purchased 300 cars in its original order from Rohr Industries. That number should carry the transit system through most of 1981. But when the Huntington line from National Airport opens late in 1981, Metro estimates, more than 300 cars will be needed to provide adequate service.