Heat, Sensible

IF THE MOVEMENT of energy to or from a substance can be sensed (for example, with a thermometer) as a rise or fall in temperature, then it is referred to as [Page 501]sensible heat. By contrast, if the movement of energy to or from a substance has a different result, such as evaporation, there will be no change in temperature. In this case, the energy is held dormant in the substance for release should the substance revert to its initial state (for example, via condensation). For this reason it is called latent heat. Changes in the sensible heat content of a substance (such as air) result in temperature changes, but latent heat movements (such as by evaporation) do not. The rate of sensible heat flow (sensible heat flux) is often referred to as dry heat flux, whereas the rate of latent heat flow is often referred to as evaporative heat flux. Climate is characterized by how available energy at the Earths surface is distributed between sensible and latent heat.

Energy that drives global climate originates from the Sun as shortwave radiation. Some of it is absorbed by the Earth and heats the surface. The surface, in turn, radiates this energy as longwave (infrared) radiation to the atmosphere. Greenhouse gases in the atmosphere absorb some of this energy and radiate it back to the surface. The net result, or net allwave (long and short) radiation at the surface, is energy available to heat the air via the sensible heat flux, or to be used to evaporate water via the latent heat flux (which includes the transpiration from the leaves of plants). For example, in summer at mid-latitudes, or year round at low latitudes, more solar energy is absorbed at the surface during the day than is lost by longwave radiation. Surplus energy is transported as both sensible and latent heat from the surface to the atmosphere by eddies of air (convection).

A rise in the concentration of greenhouse gases in the atmosphere leads to more energy retained at the surface of the Earth. This additional energy could be used either to heat the atmosphere by way of the sensible heat flux (increased warming), or evaporate additional water via the latent heat flux (increased humidity). If moisture is freely available at the surface, the latent heat flux will always be larger than the sensible heat flux, meaning that energy otherwise available to heat the atmosphere is used to evaporate moisture. The resulting warming of the atmosphere would be less, in this case, than if all the additional available energy was accounted for by the sensible heat flux alone.

The heating of air by the sensible heat flux at the Earths surface causes the air to become buoyant and, as a result, rise and mix with the cooler air aloft. Moisture, as well as sensible heat from the surface, is entrained in this convectional mixing of air. As a result, the mean annual global climate impact of increased energy available from an enhanced greenhouse effect will manifest itself both as an increased sensible heat flux (warming) and increased évapotranspiration via the latent heat flux. The latter is hidden heat energy that can be transported great distances by wind and over time without loss. The impact on climate may occur in regions far from its source. This hidden heat energy does not radiate back to space until condensation occurs and returns the heat to the air.

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