We're not air-traffic controllers, nor are we meteorologists, but our understanding is that there are several factors possibly at work here.
First up, back in early 2007, the FAA changed a longstanding rule that said airplanes taking off from McCarran International Airport had to turn left once airborne, a path that took them over mostly uninhabited desert. However, apparently a 30-percent increase in traffic from the beginning of the decade led to runway and airspace congestion and consequent flight delays (in 2010 McCarran ranked 9th in the world for aircraft movements, with 505,591 takeoffs and landings). So, the rule was reversed back to as it had been in 2001, allowing planes to make a right-hand turn after takeoff, meaning they pass over residential areas, including Summerlin. As a compromise, the FAA stated at the time that planes would fly about 1,000 feet higher, in the hopes of cutting down on noise, but there still have been complaints, of course.
Another factor affecting flight paths at McCarran has been the interruption caused by the "McCarran 2000" plan, an ongoing three-stage expansion and upgrade project adopted back in 1978, which to date has included runway improvements that have affected takeoffs and landings from time to time.
The third consideration is the weather. When air density is low -- i.e., in hot weather -- airplanes need longer runways to take off and land and don't climb as quickly as when the air's density is high. Hence, seasonal considerations may well affect the takeoff and landing paths at McCarran, which of course experiences extremely high temperatures in the summer months, countered by some pretty chilly days and nights in the winter.
