Horizontal Variations in Surface Air Temperatures

Surface air temperatures are strongly influenced by the differing materials that make up the earths surface as the air is primarily heated/cooled by contact with these surfaces. With the exception of water and ice, most materials that make up the earths surface have a much greater temperature range than does the air. To understand the variability of surface air temperatures, one must understand the factors influencing the temperature of the earths surface and since the majority of heat energy for the earths surface results from solar radiation, one must understand the factors that lead to variations in solar radiation reaching the earths surface.

The key factor influencing the intensity of solar radiation at the earths surface is the incidence angle of the radiation. The closer to perpendicular this angle is the more intense the radiation. Latitude and season have a pronounced large-scale influence on incidence angle, but there are also local factors such as topography to consider. The slope and aspect of the topography can significantly alter the amount of solar radiation reaching the earths surface. As the sun moves across the sky, its rays are more nearly perpendicular to different slopes and aspects at different times of the day. In the northern hemisphere, south-facing slopes receive more direct radiation than do north-facing slopes. Slope and aspect also effect timing of maximum temperature as different aspects reach their peak heating at different times. East-facing slopes reach their max temperature far earlier than those slopes facing west. Surface shading is also a factor consider in assessing local variations in temperature. Shading and scattering of radiation by any means, such as clouds, smoke, or haze in the air and objects such as vegetation, reduce the radiation reaching the ground.

The properties of the surface material also have a significant impact on surface air temperature. Dark materials generally absorb most of the radiation in the visible wavelengths, where as light materials reflect more of these wavelengths. The more radiation absorbed by a material the hotter it will get, therefore dark materials typically heat up more than light colored materials. Transparency is another factor affecting temperature. Water is fairly transparent to solar radiation. This transparency acts to spread the incoming solar radiation throughout a deeper layer of the material, slowing its surface temperature increase. Opaque substances such as soil receive all of the incoming solar radiation at their surface which more effectively heats that surface. A third surface property is conductivity, the ability of a material to transfer heat throughout the material. A material that is a good conductor will rapidly transfer heat energy from the surface to the rest of the material. A poor conductor will end up with a high concentration of heat energy at the materials surface leading to a very high surface temperature. Wood is a poor conductor. Heat applied to it is concentrated at the surface and only slowly penetrates to warm the interior. As mentioned earlier, different materials have differing heat capacities which is another surface property affecting temperature.

Moisture plays an important role in influencing surface temperatures. Water has a high specific heat capacity and is a good conductor. These properties make the moisture content of a material an important factor influencing its temperature. Compared to dry surfaces, moist surfaces will not reach as high of a maximum temperature, nor will they cool to as low of a minimum temperature. Therefore, moisture acts to moderate surface temperatures by reducing the daily temperature range. Winds also have a moderating effect. During the day, winds help transfer heat away from the surface, reducing maximum air temperatures. While at night, winds mix warmer air down to the surface thus raising minimum air temperatures. Vegetation also influences surface temperatures- usually moderating air temperatures within the vegetative layer (see: Effects of forests on surface temperatures).

The above discussion of surface properties focuses on factors that contribute to horizontal variations in surface air temperature; however, air temperature also varies with height. The processes that control the variation of surface temperatures with height are discussed in Lapse Rates and Inversions.