You may not have heard of the term albedo before but you will be aware of what it measures. Like the difference between walking barefoot on a limestone footpath or a black bitumen road on a sunny day, albedo is a measure of the reflectivity of a surface. Being dark, a bitumen road surface absorbs more shortwave radiation than it reflects, resulting in the painfully high temperatures familiar to many of us and a low albedo. Conversely, the limestone path tends to reflect more radiation than it absorbs, leading to a more comfortable temperature for bare feet and a high albedo.
Albedo, measured as a ratio between zero and one, is not just important when deciding whether you can make it from the beach to your car without shoes - it has an important role in both regional and planetary climate too. For example, snow has a very high albedo - fresh snow can have an albedo of 0.95, meaning that very little of the shortwave radiation reaching a snowy surface is absorbed. This helps to keep the temperature in snow covered regions low and keeps the snow on the ground for longer. On a global scale the snow covered polar regions help to raise the average albedo of the planet. Covering most of the planet's surface, water has an albedo that can vary from one extreme to another depending on what time it is. Water tends to absorb most short wave radiation from the sun during the middle of the day however close to sunrise and sunset, the albedo of water is close to 1.
Changing land use can have a significant impact on regional albedo, and the urban heat island effect attributed to increasing temperatures in major metropolitan communities is in part caused by a reduction in albedo in urban areas.
No comments:
Post a Comment