Environment changes affect readings
“Your temperature readings are way off!” If I had a dollar for every time a viewer called in with that comment during my television meteorology career, I’d be a rich woman.
Whenever temperatures start to get really cold or really hot – and even when they’re just average, someone will call to tell me that the temperature I’m showing on television for their city, is nothing close to what their thermometer is telling them. I think I can shed some light on this topic so folks will understand where the official temperature is coming from, what their thermometer is really measuring, and why two temperature readings in close proximity can still be significantly different from each other.
The government operates and maintains several weather reporting stations across the Inland Northwest, including places like Spokane and Coeur d’Alene. The National Weather Service is responsible for the weather station at Geiger Field in Spokane. The FAA is responsible for Coeur d’Alene’s weather station at the Coeur d’Alene airport (elevation 2,320 feet above sea level,) which actually happens to be in Hayden.
Observations such as air temperature are taken every hour, and this is where we get the information such as the official high and low for the day. The key word in the previous sentence is air temperature. Let me explain. On any given calm, sunny day I’m sure you’ve taken notice that you are much warmer standing out in the sun, than in the shade. That is because you are feeling more than the air temperature. You are feeling the effects of the sun directly heating your skin. (This has nothing to do with the “heat index.” That is a whole other issue.) A thermometer placed in direct sunlight will also “feel” that effect, which would mean we would not get an accurate measurement of the temperature of the ambient air. Thermometers used to measure the official temperatures are completely shaded, so that the sun does not directly heat the thermometer itself. These thermometers are also ventilated to keep fresh air circulating through to the instrument.
In addition to direct sunlight, many other things can affect a temperature reading. Thermometers placed over a dark surface such as asphalt will read warmer because darker objects absorb more solar radiation than light-colored objects (which reflect more). Temperatures taken at night in a low spot may read cooler partly due to the pooling of the denser cold air. Temperatures taken with a thermometer right next to your house will read warmer because of the heat radiating from your home. Differences in elevation can make a big difference in temperature, as skiers can attest. And finally, the direction of the wind, and whether it might be blowing over warmer or cooler surfaces such as water or even snow can make a difference in a particular location’s temperature.
Official temperature readings are taken over a grassy surface, about four feet off the ground, with a shaded and ventilated thermometer. Any changes in the environment surrounding the thermometer, no matter what their close proximity might be, can produce rather drastic differences in temperature readings. Now you know.
Speaking of temperatures, we got a taste of fall this past week with highs in the 50s and 60s and a few early morning readings flirting with the freezing mark. We are now officially in the autumn season, and temperatures for the next week should be seasonable with highs in the lower 70s. September will likely end on a dry note, with Coeur d’Alene receiving less than half of the normal 1.58 inches of rain.