|ALBEDO AND THE ENERGY BUDGET
METEOROLOGIST JEFF HABY
The albedo of the earth's surface and the atmosphere is important to forecasting. A substance with a high albedo
reflects a significant amount of incoming radiation. Two examples include clouds and
surface snow cover. This acts
to keep the earth's surface cooler than it otherwise would have been. A substance with a low albedo reflects very
little incoming radiation. In fact, the object absorbs much of the energy. Three examples of substances with a low
albedo include forests, concrete and dirt. All else being equal, temperatures will be warmer over a low albedo
surface than over a high albedo surface.
The urban heat island is
partially caused by changes in albedo. The concrete,
buildings, and metal have a lower albedo than rural areas with trees and vegetation. There is also less evaporative
cooling or transpirational cooling from concrete, buildings or metal. When forecasting, temperatures in urban areas
(especially large urban areas with greater than 1 million in population) will be a couple to a few degrees warmer
than rural areas on sunny days with light to moderate wind.
Surface snow cover will produce lower temperatures
than without surface snow cover, especially on clear days and nights. This cooling will occur in the day as well
as night. The snow has a high reflectivity of shortwave radiation during the day and radiates longwave radiation
efficiently away from the surface snow cover at night. Sublimation, melting and evaporational cooling from the
snow also cools the air, especially if the
Relative Humidity is low.
As you well know, clouds during the day will
keep temperatures cooler since they are efficient at reflecting the sun's radiation back into space. Clouds can
also produce differential heating at the surface. A clear region will warm more at the surface than a cloudy region.
The transition zone between the cloudy and clear region can act as a
trigger mechanism for convection, similar to
that of an outflow boundary if the right
moisture are in place.
There are also albedo changes between
land and water. Water is unusual in that it has a lower albedo when the sun's radiation is coming toward the water
surface at a high angle (sun high in the sky) but has a much higher albedo when the sun's radiation is coming in
at an angle (sun close to the horizon). This is one reason the sun is
inefficient at warming the polar regions.
Evaporative cooling and the water's high heat capacity are most important in keeping temperatures cooler over a
water surface. Differential heating occurs between the water and the land surface. This transition zone can also
act as an axis of instability if the atmosphere is unstable and moisture is present
(i.e. sea breeze thunderstorms).