Concrete and pavement are warm, while grassy or snowy areas are cool. That's an observation we are familiar with. Have you ever wondered why each material has a different temperature? Why aren't all substances as warm as the air temperature?
The main source of heat on our planet is infrared radiation, from the sun. All things, living and nonliving absorb and reflect the sun's infrared heat differently. These differences arise from several factors, including heat capacity and albedo.
Heat capacity measures a substance's resistance to heat loss or gain. For example, water has a higher heat capacity than the atmosphere, which is why land heats and cools more rapidly throughout the seasons than the ocean or lakes.
Albedo, which is what we will focus on in this article, is a measure of reflectivity. This is measured on a scale of 0 (0%) to 1 (100%), 0 being black (absorbs all infrared light) and 1 being white (reflecting all infrared light). A low albedo substance is asphalt and a high albedo substance is fresh snow.
Asphalt has an albedo of about 0.05, meaning it absorbs 95% of incoming light. This will warm asphalt over the air temperature. Cue the flashbacks to summer and running barefoot on blazing hot pavement.
On the opposite spectrum, fresh snow can have an albedo as high as 0.9, reflecting 90% of incoming light. This keeps snow cooler than the air temperature, thus staying frozen even on a 50° day. Another important factor in this process is the high heat capacity of water, which helps keep the temperature steady. As snow begins to melt, the process is a positive feedback system, which accelerates the process. As the snow becomes dirty and patches form in the grass, the albedo lowers, which in turn increases heat absorption and speeds up melting.
So next time you get a sunburn while skiing, you can blame good ol' albedo.
