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John W. Enz, Published July 14 2012

Measuring temperatures no simple task

Monica Bruhn (July 3 letter) said The Forum article (June 27) comparing air temperatures from public signs was a waste of time, money and energy. I disagree.

I’m sure many readers have often wondered why air temperatures vary so much from sign to sign. However, Mark Ewens’ comments in Marino Eccher’s original article may have been too brief for most people to fully understand why air temperatures can be variable.

First some facts: On a clear day, the sun heats the Earth’s surface, and the surface heats the overlying air. The sun does not heat the air directly. This means the surface is hotter than the air. As a result, air temperature decreases as you move up (away) from the surface. Surface temperatures also depend on surface type, color and wetness. As a result, official air temperatures are measured in the shade, 5 feet above a grass-covered surface, which is nearly always cooler than pavement.

Think about the last time you walked across a dry asphalt parking lot on a hot summer day and compare that to a green grass surface. If the grass is well-watered, it will be even cooler because evaporating water uses most of the energy. Without this official National Weather Service standard, air temperatures from different places would not be comparable.

Some reasons why temperatures can vary so much from sign to sign is because of the sensor’s height above the surface and its exposure. Ideally, as Ewens said, the temperature sensor must be shaded and ventilated with a fan. If not shaded, the sensor is like the surface, and the sun heats the sensor surface, and the surface heats the air. If not ventilated, it will be like the inside of a closed car on a sunny day. Furthermore, surface color will make a difference; the darker the sensor’s cover the hotter it will be regardless of ventilation. Have you ever worn a black shirt on a sunny day?

Finally, the temperature will also depend on the type and color of the surfaces surrounding the sign. Dry black pavement or other dry black surfaces will probably always be the hottest surfaces on a sunny day.

Measuring air temperature accurately is far more difficult than it appears.

Enz is professor emeritus in soil science at North Dakota State University and agricultural climatologist with the North Dakota Climate Office and founder of the North Dakota Agricultural Weather Network.