Get in your car on days like these and it sure feels like the surface of the sun! And the car thermometer confirms what you’re thinking: it’s 105º, 108º even 110º! So why is that guy on TV (me) saying it’s only 100º?
Official temperature readings are measured using consistent standards that car thermometers and bank thermometers just don’t meet. There are specific rules for what kinds of instruments are used, where they are placed and what they’re close to.
National Weather Service weather instruments are typically placed inside a vented, white box called a “Stevenson Screen.” (There are even rules about what kind of white paint should be used!)
The Stevenson Screen should be around five feet off the ground (give or take a foot). It’s supposed to be placed in a flat, open area, well away from trees and buildings that could affect the reading. And, probably most importantly, the box must be at least 100 feet from any paved or concrete surface. If you’ve ever walked across black top on a summer day, you know exactly why!
Bottom line: your car thermometer may give you a good idea about how miserably hot it is near your car’s engine and the baking black top beneath it, but don’t expect the “official” temperature to be nearly as high.