Two years ago, the problem with the Air Force's B-2 Stealth bombers, which cost $2 billion apiece, was that their radar could not tell a rain cloud from a mountainside.

Now the problem is that the B-2 cannot go out in the rain.

The investigative arm of Congress reported this week that the B-2, the world's most expensive aircraft, deteriorates in rain, heat and humidity. It ''must be sheltered or exposed only to the most benign environments -- low humidity, no precipitation, moderate temperatures,'' said the report by the General Accounting Office.

The report said that the skin of the plane cannot handle the heat or the damp or the rain. That skin, made of thermoplastics and composites transparent to radar, is supposed to help give the bomber its much-touted radar-evading ''stealthy'' qualities.

Without that stealthiness, the plane is less than the unique technological achievement the Air Force has claimed it to be.