Heat is a factor too. Every couple of years, there'll be a day or two in summer when temperatures at the airport in Phoenix get above the maximum takeoff tempe

Heat is a factor too. Every couple of years, there'll be a day or two in summer when temperatures at the airport in Phoenix get above the maximum takeoff temperature for a lot of the smaller regional jets, and chaos ensues as all those flights (it's a US Airways hub) delay or cancel.