Oil prices drifted below $79 a barrel Tuesday in Asia as a storm threatening oil installations in the Gulf of Mexico weakened and investors eyed a volatile dollar. Benchmark crude for December delivery was down 49 cents to $78.94 a barrel at midday Singapore time in electronic trading on the New York Mercantile Exchange. Tropical Storm Ida weakened Monday from a hurricane as it neared Gulf Coast oil facilities, easing concern it could disrupt supply when it reaches land later Tuesday.