Sin City is sizzling in triple-digit heat this week. It’s the worst heat wave in the Southwest in decades, and cities are reaching record high temperatures, including Las Vegas, which peaked at 117 degrees Tuesday.

Notably, this heat wave is going to last through the week, which raises concerns of heat-related illness. On Monday night, Las Vegas dropped to only 88 degrees — which also happened to be the high temperature in Washington, D.C., on Tuesday. Excessive heat warnings have been extended through at least the end of the week from Arizona to northern California.

Tucson missed its record high on Tuesday when it hit 116 degrees, but the combination of triple-digit afternoon temperature and a warm overnight low (87 degrees) set a record for average daily temperature, at 101.5 degrees. It’s the first time the average daily temperature has been above 100 degrees in Tucson, where weather records go back to 1894.

Thermal, Calif., southeast of Palm Springs, also set a record for average daily temperature, at 107.5 degrees.

Record highs set Tuesday

Las Vegas — 117 degrees (tied)
Needles, Calif. — 125 degrees (tied)
Ocotillo Wells, Calif. — 124 degrees

New records for June 20

Phoenix — 119 degrees
Tucson — 116 degrees
Yuma, Ariz. — 120 degrees
Palm Springs, Calif. — 122 degrees (tie)

In Phoenix, flights using smaller planes have been grounded because they are not allowed to take off in such hot weather. The hotter the air gets, the more it expands and the less dense it becomes. Thin air makes it difficult for planes to get lift. It may technically be possible — especially if they had longer runways — but the FAA prevents them from taking the risk:

American Airlines alerted its customers over the weekend, offering fee-free changes to upcoming flights that were departing or arriving at Phoenix between 3 and 6 p.m., when temperatures peak.
Monday and Tuesday, the Fort Worth-based airline canceled 50 flights in and out of Phoenix, according to American Airlines spokesman Ross Feinstein. Delays were expected for at least seven more flights to Sky Harbor on Tuesday, he said.
Regional flights on American Eagle were the most affected, because they use Bombardier CRJ planes that can only operate at temperatures of 118 degrees or below, Feinstein said. Flights on larger Airbus and Boeing planes were not canceled because they are able to operate at higher maximum temperatures: 127 degrees for Airbus and 126 degrees for Boeing.

Though they may not set new records, temperatures are going to stay dangerously hot through the weekend. Phoenix will climb above 115 degrees at times. On Wednesday, Tucson could break a new daily record, which currently stands at 112 degrees. Triple-digit heat is in the forecast from Southern California up through the Central Valley.

This heat wave is right on time. In the weeks leading up to the beginning of monsoon season, temperatures in Arizona, New Mexico, southern Nevada and Southern California typically climb to the highest they’ll be all year.

Southwest Monsoon, which occurs annually, is actually triggered by the very heat looming in the forecast early next week. The buildup of warmth creates a large-scale trough of low pressure that extends from Mexico to Arizona, Southern California and Nevada. Air flows away from high pressure and toward low pressure, so moisture is drawn over the Southwest.

Tucson, for example, experiences its wettest months in July and August, although the monsoon can last through September.

The reason it can get so hot in the Southwest is that the humidity is so low. Perhaps it’s counterintuitive, but in the Eastern United States — where summer humidity can be oppressive — the presence of moisture in the air keeps the temperature down. In the Southwest, there’s no moisture in the ground for the sun’s energy to evaporate. So, instead, it goes toward making the air hotter and hotter.

What makes this heat wave so dangerous isn’t necessarily the extreme afternoon temperatures. Overnight lows aren’t going to drop below the upper 80s in parts of Arizona. The inability to cool off at night puts enormous stress on the body.

The best thing you can do during a heat wave like this is stay indoors. If you’re working outside, take frequent breaks to rehydrate and cool your body. You may not realize you’re overheating, because sweat evaporates instantaneously in such dry conditions, but you can quickly suffer from heat stress in these conditions.