Latent Heat is energy in the form of heat released or absorbed by a substance during a phase change of the substance. The amount of latent heat involved with condensation/evaporation and freezing/melting are different under different temperatures and pressures. As temperature increases, latent heat of vaporization decreases, but that of fusion increases. Water is not the only substance that has three different phases at a normal temperature range in the universe, but it is the most abundant substance like this on Earth. A larger amount of latent heat is involved with the phase change of water. Through this transfer of latent heat, the water cycle determines surface and atmospheric conditions, as well as atmospheric circulation. Evaporative cooling and condensation heating moderate the surface temperature. Without this cooling and heating, daily temperature range would be significant, much like that of planets such as Mars or Venus, or of blacktop asphalt in summer.
The atmosphere is a dynamic heat system designed to transfer heat from one place to another. Latent heat involved in the phase change of water plays essential roles in the transfer of heat. Just as much latent heat is released in the atmosphere through a phase change as the latent heat is absorbed at the Earth's surface. The latent heat fluxes and the transfer of sensible heat between the equator and the poles are major components of the energy balance of the Earth. The latent heat flux in the atmosphere is huge. Latent heat flux involved in the phase change of water drives the atmospheric circulation and plays essential roles in global climate.
Latent heat is also an important factor to better comprehend weather systems, because it is a primary source of energy that develops, promotes, and sustains severe weather systems, such as thunderstorms and tropical cyclones. Latent heat supplies weather energy. As water condenses, latent heat from the water molecule is released into the air, heats the air, makes it lighter, and makes it rise fast. As the air rises, more air flows in and promotes storms. In this process, some latent heat is changed into the kinetic energy that accelerates the speed of water molecules and powers up severe weather systems. Recent global warming increases the ability to evaporate and hold moisture in the atmosphere. As a greater amount of water vapor exists in the hot air, more latent energy is available to be released into severe storms. Another critical aspect of global warming is that it will increase variability of extreme weather, although the overall average of the atmospheric condition changes little. Thus rare, but extreme weather, often much beyond what we experience today, will be more common.
Controversy exists concerning if latent heat adds to global warming, increasing it, or tends to act as negative feedback for global warming, slowing it down. A small change in atmospheric composition and chemistry can induce considerable climate alteration. The proponents of global warming argue that water vapor absorbs more infrared radiation than any other atmospheric gas and induces the largest greenhouse effect of the global climate. Growing latent heat flux in the atmosphere increases the amount of clouds, helping retain more heat, which would lead to more evaporation of water and the addition of more water vapor into the atmosphere, which leads to more heat in the atmosphere, and so on, in a positive feedback cycle. Opponents of global warming argue that a growing amount of clouds would increase the Earth's albedo, which would decrease the amount of solar radiation that reaches the Earth's surface. A decrease of solar radiation received would cool the Earth's surface, instead of heating it, in a negative feedback cycle.
Clouds contribute approximately 50 percent of Earth's planetary albedo. Change in latent heat flux in the atmosphere not only determines weather, but also induces global climate change. Aside from the climate change, latent heat is also responsible for the atmospheric humidity that affects human comfort. Despite uncertainties in the understanding the global climatic system, there is evidence that indicates that the Earth's atmosphere and overall environment are warming, such as sea-level rise, polar ice caps receding, increasing weather extremes, and more severe storms. Neither positive nor negative feedback cloud forcing has reached the point where it can offset the warming or cooling effects.
sEE ALso: Cloud Feedback; Heat, Sensible; Weather.
BIBLIoGRAPHY. Roger G. Barry, Atmosphere, Weather and Climate (Routledge, 2003); Tim Flannery, The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth (Atlantic Monthly Press, 2006); Intergovernmental Panel on Climate Change, Climate Change 2007—Impacts, Adaptation and Vulnerability: Working Group II Contribution to the Fourth Assessment Report of the IPCC (Cambridge University Press, 2008).
Jongnam Choi Western Illinois University
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