Lecture 3 Latent Heat Latent heat is needed to convert from a more ordered to a less ordered phase e g ice melting into liquid water or liquid evaporating to vapor and is released when the vapor condenses or liquid freezes Why does evaporation cool liquid water A water drop evaporates as the most rapidly jiggling molecules near its surface break free of their attraction to the other water molecules The less jiggly molecules are left behind so the temperature of the water drop decreases Water drop Molecule escaping as vapor Latent Heat of Phase Changes of Water EOM 2 2 620 80 540 80 540 620 1 calorie heats 1 g liquid water by 1 C It takes 540 cal to evaporate the liquid enough to bring the water from freezing to boiling five times over This latent heat is released when the water vapor condenses again The Three Modes of Heat Transfer The Weather Book p 19 Comments on Conduction Causes most heat transfer in solids Conductivity Rate of heat transfer across a 1 cm thick slice of material if one side is maintained 1 C warmer than the other Material Silver Granite rock Wet soil ice Still water dense snow Dry soil Still air Heat Conductivity cal s 1 cm 1 C 1 1 0 006 0 005 0 0015 0 0006 0 00006 from EOM Table 2 1 Comments on Convection Convection Hot air rises and cold air sinks only occurs in fluids liquids gases and only because of Earth s gravity Buoyant warmer less dense blobs of air are called thermals much loved by birds and gliders Convection does not occur if the most buoyant air is already on top gravity Cold Warm Heating from below convection Warm Cold Heating from side convection Warm Cold Heating from top no convection Comments on Radiation Every object loses heat by radiation in the form of electromagnetic waves Hotter objects radiate a lot more energy per unit of surface area and at shorter wavelengths A cooling campfire White hot Red hot Dull Red Black infrared Ultimately radiation from the sun provides the heat energy that creates air motions and weather and permit life Shortwave and Longwave Radiation The fate of incoming sunlight EOM 2 13 Of incoming sunlight 50 is absorbed at surface 20 is absorbed in the atmosphere 30 is reflected mainly by clouds Albedo TWB p 19 Albedo Fraction of solar radiation hitting an object that is reflected The remainder is absorbed e g at water or land surface or transmitted e g through a cloud Radiative Energy Balance Globally net incoming solar balances outgoing IR Air motions and ocean currents transfer energy poleward keeping poles warmer tropics cooler To balance the equator gains radiative energy and the poles lose it EOM Why winds storm weather Nonuniform Solar Heating Temperature Variations Pressure Variations Winds and Storms
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