Lecture10 - EAS 1600 Introduction to Environmental Sciences...

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EAS 1600 Introduction to Environmental Sciences Class 10- Hydrologic Cycle Water comprises only a few percent of the atmosphere, but still plays a very important role. Why? Because water is the only significant constituent of the atmosphere that changes phase under the conditions that are encountered in the atmosphere. The phase change is important in the weather because it results in rain, snow, etc. But the phase change is also important because it involves the conversion of energy between sensible and latent heat . Thus water can play a role in the overall energy budget of the earth/atmosphere. In this lecture we will explore this role . .. But first let’s learn a little about water and its phases. To do this we will consider a new principle: The Second Law of Thermodynamics The three phases of water (H 2 O) Each phase change requires the absorption or release of “ latent energy or heat .” (NOTE: Latent heat is a form of energy distinctly different from sensible heat!) O = H + H + O = H + H + O = H + H + Intermolecular Bonding
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The amount of latent energy is referred to as the: heat of condensation/evaporation heat of freezing/melting heat of deposition/sublimation Here are the values associated with each process: Process Changes Heat gained or lost by air From To Condensation vapor liquid 2500 J/g or 600 cal/g Evaporation liquid vapor -2500 J/g or -600 cal/g Freezing liquid ice 333 J/g or 80 cal/g Melting ice liquid -333 J/g or -80 cal/g Deposition vapor ice 2833 J/g or 680 cal/g Sublimation ice vapor -2833 J/g or -680 cal/g Note: The heat of condensation + the heat of freezing = heat of deposition. Which law of nature would be violated if it did not? Saturated vapor pressure/density of water vapor and relative humidity: When the RH (we use RH as shorthand for relative humidity) is 100%, we say that the air is saturated with water. When RH < 100%, the air is undersaturated When RH > 100%, the air is supersaturated) Imagine a simple experiment: (temperature held constant) At all times, water is evaporating and condensing. Depending upon the RH, the rate of evaporation can be less than, equal to, or greater than the rate of condensation. What determines this? Imagine a simple experiment: 1. If RH < 100%: evaporation will be faster than condensation, until RH = 100%. 2. If RH > 100%: condensation will be faster than evaporation, until RH = 100%. 3. If RH = 100%
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This note was uploaded on 01/16/2012 for the course EAS 1601 taught by Professor Lynch during the Summer '08 term at Georgia Institute of Technology.

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Lecture10 - EAS 1600 Introduction to Environmental Sciences...

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