A weight-restriction day is when heat thins the air so much that a departing aircraft can't reach maximum takeoff weight on the runway it has. Carriers shed payload, wait for cooler air, or cancel outright. Phoenix has published projections for…

Larger Boeing and Airbus jets almost never cancel outright because of heat. Their certified operating temperatures sit well above what regional jets can handle, so airlines usually have room to fix the problem another way — by trimming weight instead…