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 instability and 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). |