You're driving down the road on a warm summer afternoon. You pass your local bank and the display says it's 97°F. "Good grief," you think, "I didn't realize it was that hot." Well, it's probably not that hot. The thermometers they use at businesses with electronic displays are useless junk.
When people emphasize that the temperature is hot "in the shade," it invokes a certain amount of rage in people who have a baseline knowledge of weather. All temperatures recorded at well-placed weather stations are in the shade, and for good reason: the "shade temperature" is the actual air temperature. For the most part, the air is no warmer in the shade than it is out in the open. It feels warmer in direct sunlight because the sun is warming up your skin and your clothes. The same goes for thermometers. When direct sunlight shines on a thermometer, it artificially inflates the temperature 30 or more degrees higher than it really is.
That "for the most part" qualifier is key. If you stand in a parking lot in the middle of the afternoon, the air is going to be hotter where you're standing than it is if you stood 30 feet away on the grass. The air over the grass more accurately reflects the actual air temperature than the air over the pavement because of conduction; the sunlight is heating up the pavement faster than it's heating up the grass, which in turn warms the air just above it through conduction.
Official weather stations across the world follow strict standards that require their placement on a grassy surface away from pavement or buildings or trees or any other obstruction that could affect the station's exposure to wind, precipitation, and actual environmental temperatures.
The next time you're near any business that has a temperature readout on an electronic display, look around for the thermometer (it usually looks like a white plastic cylinder with horizontal slits in it). The thermometers businesses use are usually on the roof of the building near the front door or on the signpost itself near the street. A great example is in the image at the top of this post — the thermometer is hanging off the left side of the post to the left of the "ATM" sign.
The poor placement of these thermometers leads to an inaccurate reading. The thermometers can read between 5 and 15 degrees hotter than the actual air temperature because they're being influenced by the heat of everything beneath and around them such as the roof or the parking lot. In the picture at the top of this post, the 102°F reading is wrong because it's being influenced by the heat coming off of the hot metal plating on the sign.
The next time you see a bank say it's 97°F out, it's probably not unless you're lying on its roof, in which case you have bigger problems than worrying about an inaccurate temperature reading.
[Image via AP]