Temperature variations? Let’s talk about albedo

When temperatures near or exceed the century mark, I will inevitably hear from various people about how hot it got in their town or at their house.

Though the official high in Spokane or Coeur d’Alene may have only reached 98 or 99 degrees Fahrenheit, someone will tell me how “off” our temperatures were because his or her thermometer read 107 degrees.

I have lived here long enough to be familiar with the various microclimates of the region. There are many good explanations for such variations – most of which can be attributed to terrain.

Terrain doesn’t explain some of these temperature extremes, however. Extreme temperature measurements that I hear about aren’t necessarily wrong; it’s just that we’re not comparing apples to apples.

Official temperature readings are measurements of air temperature – not the temperature of the surface of some hot object that has been baking in the sun (like an exposed thermometer). In other words, official temperature readings are taken by thermometers in the shade.

I can’t change how the temperatures are reported. I can enlighten you on an interesting principle called albedo that may explain some temperature variations that can occur over a very small area, like your own back yard.

Our atmosphere is made up primarily of nitrogen (78 percent). Oxygen makes up only about 20 percent of our atmosphere by volume, with the remaining 1 percent consisting of trace gases such as carbon dioxide.

Nitrogen is a poor absorber of solar (shortwave) radiation. So, the sun does not directly heat our air. Incoming solar radiation mostly passes through our atmosphere and is absorbed by the earth’s surface, which then re-radiates in the long wave that we feel as sensible heat.

The term albedo refers to the reflectivity of various surfaces. The higher the albedo, the more solar radiation is reflected back rather than absorbed. Light-colored objects, such as fresh snow, have a high albedo and reflect 80 to 85 percent of incoming solar radiation.

Dark-colored objects have a low albedo. The albedo of blacktop is only around 5 to 10 percent. We all know how much hotter the air can be over highly paved areas. The dark-colored streets absorb a majority of the incoming solar radiation, and then re-emit that energy in the long wave, heating up the air above it.

Even with a standardized shaded and ventilated weather station, the temperature on a sunny day may show a variation from the thermometer located over a wheat field to the one over the downtown area.