We’re getting into that stretch of the summer when in much of the world it’s downright miserable to sit near a window, let alone go outside and let the Sun beat down on us. Pretty soon we’ll start to hear people exclaim that it’s 100°F in the shade. Talking about “the temperature in the shade” lets us emphasize how hot it is outside while also pointing out how hot it feels when you’re not in the shade. But meteorologically speaking, there is no such thing as the temperature in the shade. That’s just the actual air temperature.
When you look at a weather report during the summer, you typically see three readings reported: air temperature, dew point, and the heat index. The dew point is the temperature at which the air would reach 100 percent humidity, which is the best way to measure how muggy it is outside. The heat index is what it feels like to your body when you combine both the temperature and the dew point—a heat index of 110°F, for instance, means that the current air temperature plus the humidity has the same effect on your body as an actual air temperature of 110°F.
Bring your own shade, as these women knew to do in 1939 London when they donned their Dolly Varden Sun Bonnets, which consisted of a large non-inflammable eye shade and a georgette tie. Image credit: George W. Hales/Fox Photos/Getty Images
Direct sunlight makes those unbearable temperatures even worse. If you’ve ever seen a thermometer exposed to direct sunlight on your deck or vehicle, you know what I mean—even if the actual air temperature is only 90°F, the thermometer bathing in the sun will read dozens of degrees higher, sometimes hotter than the hottest natural temperature ever recorded (134°F in Death Valley, California, in 1913). That’s not the real temperature, of course; it’s the temperature of the air combined with the temperature of the sensor itself warming up in the sunlight.
Thermometers used in official weather stations maintained by airports and weather agencies around the world are built with plastic or wooden shelters around them to prevent contamination from sunlight. The enclosure that surrounds the thermometer allows the sensor to measure the actual air temperature; in other words—drum roll—the temperature in the shade.
Even though “the temperature in the shade” is little but hyperbole, the sun really does have an impact on your body beyond a painful sunburn. Just like your deck thermometer, standing out in broad daylight on a blistering day can make your body have to work much harder than it already does—potentially to the point that you can get sick. That 90°F temperature is already made worse by humidity (preventing your sweat from cooling you off), but standing in the sun heats up your skin and your clothes even more, making it harder to cool off unless you find air conditioning or cover from the sun.