The Federal Aviation Administration imposed a ground delay at Denver International Airport early Wednesday after dense fog cut visibility on the airfield. Departures within 1,200 miles of Denver were held an average of 40 minutes and, by 7:58 a.m., FlightAware tallied 139 delays and three cancellations. The traffic-management programme, originally set to run through 1 p.m., was lifted around 11 a.m. once conditions improved, the agency said. Fog returned on Thursday, causing fresh delays at the airport. In its daily traffic advisory, the FAA again flagged low ceilings in Denver and warned that thunderstorms forecast for Florida could further disrupt flights at several major hubs. The interruptions coincide with what is expected to be one of the busiest U.S. travel periods of the year. The Transportation Security Administration forecasts that 17.4 million passengers will pass through airport checkpoints between now and 2 September, with Friday likely to see the heaviest volume. Harry Reid International Airport in Las Vegas alone anticipates about 500,000 travellers over the same stretch, with parking projected to reach capacity. Aviation officials advise passengers to arrive early, travel before midday when possible and monitor airline alerts. The FAA emphasised that weather remains the leading cause of delays nationwide and that further traffic-flow restrictions may be required to maintain safety through the Labor Day weekend.