A couple of weeks ago, parts of interior Maine had a heat wave and heat advisories were issued. In Portland we missed an official heat wave by just a degree. Here in New England, advisories and warnings for heat are not very common and temperatures must reach certain heights before they go into effect. When temperatures will be at least 95 for two or more days, an advisory is issued. If it’s going to feel 105 or higher, an excessive heat warning is put out to the public. The threshold for a heat advisory was actually lowered this year because here in New England, heat affects us differently from those in Phoenix or Las Vegas.

Advisories and warnings for heat, cold, ice and snow are issued under different scenarios across the country because these cautionary statements are supposed to help the public be alerted to unusual conditions. It would be foolish to issue a cold advisory in Maine for temperatures in the 20s in January, but pretty important in, say, Miami.  

With this is mind as a benchmark, consider the excessive heat warnings for the southwestern part of the country today.

Tuesday was amazingly hot across the west. From Denver to Phoenix, temperatures were at or near records and Las Vegas tied its hottest day ever at any time of the year. That’s like Portland hitting 103!

Heat of this magnitude is very unusual, even in areas accustomed to triple-digit readings. The all-time high in Phoenix is 122 degrees. When you have temperatures nearing that all-time mark, it’s similar to us having temperatures of 101 here in Portland, just shy of our all-time record of 103, set back on Aug. 2, 1975.

It’s been so hot that flights were delayed in Phoenix. It’s actually harder to take off in the extreme heat because the air becomes less dense, providing less lift for the wings. You may remember this was also a concern in Iraq during the war and one reason planes took off at night, when it was cooler.

We don’t ever see this type of heat in New England.

Unlike our heat waves, which are usually part of high humidity, the air out in the desert is very dry. Evaporation, which cools the body, would be momentarily effective if you got out of a pool in Arizona today. The water would quickly disappear off your body, briefly cooling you. Running a fan through wet material can actually act like an air conditioner when the humidity is this low!

Still near 90 at 5 a.m. in Phoenix

Temperatures, even in the dry air. are having trouble falling. As of 5 a.m. Wednesday, it was still near 90 in Phoenix. That doesn’t give very much relief and makes reaching the record high today much more likely.

By the way, it’s not just Arizona that is having a heat wave. If you are planning on heading to California, inland areas are experiencing very high temperatures the rest of this week.

Why so hot?

Heat waves are caused by high pressure being stuck in a certain position. High pressure is associated with sinking air and as air sinks from above, it warms up. Think about climbing down from a high mountain; temperatures would get higher on the way down. When high pressure sits over an area for a long time and the air can continually sink, it becomes warmer and drier. This is what happened here last summer with the drought and why we had so much sunshine. It’s also why California had a multi-year drought, as high pressure just would not relinquish its grip out there for several years. Now, it’s the same thing going on across the southwest this week.  

A heat dome is created in the summer when high pressure sits in one area for an extended time.

Follow Dave Epstein on Twitter @growingwisdom