CHICAGO – O’Hare International Airport again ranked worst among major U.S. airports for on-time departures during the first half of the year, according to the U.S. Department of Transportation.

Less than 65 percent of flights left O’Hare on time between January 1 and June 30, placing it last among 32 major U.S. airports, according to federal statistics released Monday.

O’Hare also was worst during the same period last year, but its percentage of on-time flights declined from 70.4 percent to 64.6 percent.

Meanwhile, Midway International Airport saw improvement.

In the first half of 2006, the South Side airport ranked second-to-last behind O’Hare for on-time departures. It jumped six places this year after nearly 73 percent of flights left on time, an improvement of just more than 1 percent.

Storms and air traffic volume are primarily to blame for O’Hare’s ranking, said Karen E. Pride, spokeswoman for the Chicago Department of Aviation, which manages both airports.

O’Hare sees between 96 and 100 arrivals an hour, Pride said, making it difficult for the airport to rebound from storms. Midway averages 32 to 36 arrivals an hour, she said.

O’Hare is in the midst of a $15 billion expansion project, which the FAA has said will reduce delays by 68 percent.