Weather 101 - Snow Cover's Effect on Air Temperature

During the period March 27, 2009 to early on March 29, 2009, a late spring storm literally paralyzed the state of Kansas with up to more than 2 feet of snow (Figure 1). In some places power was knocked out, airports closed, and highway traffic snarled.

While the storm itself was noteworthy, so, too, was the snow's effect on high temperatures on the days following the snowfall.

Reflectivity and Conductivity

Snow is very highly reflective. In fact its albedo (or reflectivity measure) is dependent upon how new the snow is and its whiteness. New snow (according to ArcticIce.org) has an albedo of about 0.80, while melting snow has an albedo of about 0.70. Older, dirty snow, especially in urban settings, can have a much lower albedo. For comparison, the Earth's overall albedo is about 0.37.

Albedo is just the fraction of light reflected divided by the amount of incident light. Thus an albedo of 0.80 means that 80% of the incident light is reflected.

Ground, asphalt, plants and other "darker" surfaces have much lower albedos, reflecting less and absorbing more. This absorbed light energy can then be transformed into heat. As the ground surfaces warm, they warm the air in contact with them through the conduction (contact heat transfer) process. This is how the Earth warms each day from overnight and early morning low temperatures.

The effect of snow cover on temperatures across Kansas was very dramatic on the afternoon of March 29, 2009. While south to southeasterly winds warmed much of the central Plains states, there was a visible cold band across Kansas and nearby snow-covered areas (Figure 2). In some places, temperatures were 10 to 20 degrees colder than they would have been without the snowpack. This was because sunlight was being used to melt the snow, not warm the snow or the ground.

National Weather Service forecasters recognized the situation and adjusted temperature forecasts to account for the snow. For example, the forecast discussion from the Dodge City weather office noted, "...KEEPING THE COOLEST TEMPERATURES WHERE THE DEEPER SNOW PACK IS (PRATT, COLDWATER)..."

It is more difficult to see this effect during the winter since air masses are already so cold and the sun is lower in the sky. But in spring, the temperature differences jump off the weather maps. That's what they did on March 29, 2009.

Climatologically Speaking

While the effect on March 29 was noteworthy unto itself, it also speaks volumes about snow and "climate change." As snow cover and glaciers retreat, the earth's albedo lowers. This allows for more absorption of sunlight and greater heating. This, in turns, allows glaciers and snow fields to melt faster. The process is an example of positive feedback within the climate system. Of course, when the reverse happens and snowy areas advance, albedo rises, temperatures cool and glaciers advance. These types of scenarios have played themselves out numerous times over geologic time.