Too much heat in the air makes it more difficult for planes to rise during liftoff

(CNN) -- Flyers expect weather-related flight delays during winter months, but they don't usually associate them with summer trips.

This year, they have learned that extreme heat also can disrupt their plans.

Too much heat in the air makes it more difficult for airplanes to rise during takeoff. Most major airlines have their own meteorology departments, which must watch for extreme heat in summer as well as for winter's icy storms.

"At the airport, they can't control the temperature and the (air's) density. They can't control the weight. So one of the things they do is to remove passengers and luggage off of the aircraft," said Jim Henderson of the Aviation Weather Center.

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Airlines must consider several factors when making decisions about planes taking off in extreme heat.

"As we're planning a flight, we take careful note of the length of the runway, the temperature at the departure airport, the performance capability of the aircraft, and don't allow the aircraft to be boarded to a weight that exceeds any of those safety perimeters," said Steve Caisse, a flight superintendent for Delta Air Lines.

During summer heat, airlines also must take care when they transport pets. Several major airlines have stopped allowing live animals to travel as checked baggage. For Delta and American Airlines, that policy lasts through mid-September. Rules vary, so flyers should check with the airline ahead of time.

Travelers can take some precautions when taking trips in hot weather. Tips for keeping your cool include: