Terry’s Take: Does the thermometer lie?

Posted on: 10:08 pm, July 30, 2013, by

Terry Swails Weather Blog

I’ve recently been asked by viewers why the temperatures I show on TV are often different than the ones they see in their backyard?

First of all, know that the temperature at your house is generally different than what is reported at the airport. In fact, the temperature you are measuring is often slightly different than what your neighbor shows just a block away. Sometimes the air temperatures, rainfall, snow, and even wind, can vary dramatically over a relatively small area.

SEEMS LIKE IT'S ALWAYS WARMER OR COLDER ON THE BANK

SEEMS LIKE IT’S ALWAYS WARMER OR COLDER AT THE BANK

On air, I show the “official” temperature, meaning the gauge that measured them was calibrated by the National Weather Service for accuracy.

By the way temperature is always measured in the shade, at a standard height above the ground, and should be taken over grass-covered ground.

NWS OFFICIALLY CALIBRATED MAX MIN THERMOMETER

NWS OFFICIALLY CALIBRATED MAX MIN THERMOMETER

Some put an inexpensive, non-calibrated thermometer on the back porch and expect it to be the same as the “official” temperature. The reality is, if they were the same, I would be very surprised.

PROPERLY VENTILATED AND ELEVATED NWS INSTRUMENT SHELTER

PROPERLY VENTILATED AND ELEVATED NWS INSTRUMENT SHELTER

Most of the time a $20 thermometer is not going to be as accurate as a several hundred dollar scientific instrument that gets calibrated on regular intervals. And don’t even get me started on bank sign thermometers and car thermometers!

When you are looking for a place to put a home thermometer, do not attach it to your brick house, or place it over concrete. These can cause your readings to be too high.  At this time of the year, the last thing you want to see is a measurement of 105 when it should be 95.

INTERIOR VIEW OF AN INSTRUMENT SHELTER

INTERIOR VIEW OF AN INSTRUMENT SHELTER

Make sure the thermometer never receives direct sunlight; this can cause temperatures to spike well above the “official” temperature.

For consistency, temperatures around the world are all measured in the shade. If you get even a hint of direct sunlight the temperature can be 15 to 20 degrees hotter. The thermometer must also be properly ventilated to allow any build up of heat.

Over the years, I’ve been asked, “Why do they show the temperature at the airport, no one lives at the airport?” Good point, but airports were a popular choice for thermometers because in the days before automation, employees were there to manually take and record the hourly observations. For consistency, most of these sites remain active today.

So, know the temperature at your house will likely be different than what the National Weather Service reports and what I show on tonight’s weathercast. Also know that I only show bank sign thermometers for amusement!