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Unit 4: Temperature-Moisture Relationship

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Air Near the Earth's Surface

Earlier we mentioned the heat gained at the earth's surface during the daytime is lost at night. To maintain a favorable climate, the daily outgoing thermal radiation must equal the incoming solar radiation on the average. How does this occur?

Nightime Surface Heat Exchange
Nightime Surface Heat Exchange

If the sky is clear, much of the heat absorbed by the surface will radiate back into space. Some heat is absorbed by clouds and other elements in the atmosphere. Clouds may reflect the rays back into the lower atmosphere and onto the surface. Thus, cloudy skies reduce the amount of heat loss at night. This is sometimes called the "greenhouse effect."

Some surface heat is transferred to the surrounding air by conduction and convection. As the surface cools, the air in contact with it also cools. Condensation of moisture in this surface air can occur with the cooling. The condensation of dew releases heat when it changes state, but that heat is soon lost by one of three heat transfer methods. In summation, air near the earth's surface is heated and cooled by conduction, radiation, and convection.

Net Radiation Radiation Reflected from Clouds
Net radiation lost to space and atmosphere Radiation reflected from clouds and absorbed by atmosphere
Greenhouse Effect Convection and Conduction
A greenhouse effect where the clouds reflect and trap radiation near the surface thereby increasing surface temperatures. Convection and conduction of air near the surface

Surface Temperatures

In the graphic above, you see again how surface properties affect surface temperatures. Here is an average summer day situation. The four surface areas and the air immediately in contact with these surfaces will have different temperatures resulting from their surface properties.

The lake has a transparent surface, thus distributing heat deeper into its mass. Its color and surface texture may reflect and/or absorb heat at the same time. The specific heat of water is much greater than that of soil; thus considerably more heat is required to raise its temperature. Water's conductivity is good. The evaporation of water will have a cooling effect.

The forested area has a canopy of vegetation with low conductivity. Its color and texture will both absorb and reflect heat rays. Much of the forest floor will be shaded, thus keeping surface temperatures lower. The transpiration or moisture loss from trees and air mixing through the canopy will have a cooling effect. Over all, temperatures will be warmer than on the lake.

Moving to the grass and brush area, temperatures will be considerably warmer because of less vegetation and less shading of the ground. The soil will receive more direct radiation and absorb considerable heat due to color, lack of transparency, lower specific heat, and lower surface moisture.

Finally, the plowed field will have the highest surface temperatures due primarily to color and texture, lack of shading, and no transparency. Soil moisture will probably be higher than in the grass and brush area due to the lack of vegetation. Soil moisture can raise the specific heat and conductivity.

What about heat losses at night? Well, you can reverse the order of these areas. The greatest heat losses and lowest nighttime temperatures will occur in the plowed field, while the lowest heat losses and higher nighttime temperature can occur on the lake. Most of the surface properties responsible for daytime heat gains will affect heat losses as well. Vegetative cover will have some insulating effect and serve to retain surface heat.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 27). Unit 4: Temperature-Moisture Relationship. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_4__Temperature-Moisture_Relationship_2.html. This work is licensed under a Creative Commons License Creative Commons License