The Air Force canceled a second test today of MOAB, officially the Massive Ordnance Air Blast, because of bad weather.

A spokeswoman says the test has been rescheduled for Thursday weather permitting.

The 21,000-pound bomb, which contains 18,000 pounds of explosives, was first tested March eleventh, eight days before the United States invaded Iraq. It shook windows miles away and sent up a huge mushroom cloud.

Air Force officials said one role for the bomb is as a psychological weapon, but it has yet to be used in combat.

It is being eyed as a replacement for the 15,000-pound BLU-82, known as the "Daisy Cutter," that was used in the 1991 Persian Gulf War to clear minefields and more recently to blast caves believed to be hiding terrorists in Afghanistan.