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Lectures_4

# Lectures_4 - Hydrological cycle condensation Rain cooling...

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Water in the Atmosphere Water in the Atmosphere

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Fig. 4-1, p.78 Hydrological cycle clouds Heat release Vapour - GHG Evaporation condensation Rain - cooling Runoff Glaciers
Fig. 4-CO, p.76

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Water vapour in the atmosphere Mixing ratio Troposphere,10 -2 to 10 -5 by volume or mass, with most in the lower troposphere global average 2.5x10 -3 Stratosphere 4x10 -6 by volume Latent (hidden) heat Water can exist in all 3 phases Vapour, liquid, ice Energy is required/released for a phase change (at constant temperature) Heat storage when liquid converted to vapour Heat release vapour condenses to liquid
Water in the atmosphere ctd Lv - vaporization - 2.5x10 6 J/kg Lf - fusion/freezing - 3.3x10 5 J/kg Ls = Lv+Lf – sublimation/deposition 2.8x10 6 J/kg Energy required/released when 1 kg of material changes state Example: How long would it take to evaporate 2 cm of water that could later fall as rain? Assume that it is overhead Sun at the tropics.

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Water in the atmosphere ctd 6.3 mb
Water in the atmosphere ctd We first calculate the amount of energy required for evaporation Need mass of water = volume x density 2 cm = 0.02 m rain (over each m 2 ) Volume = 0.02x1m 3 = 0.02 m 3 Density water, ρ w = 1,000 kg/m 3 mass = 20 kg/m 3 Heat release = mass x L v = 20x2.5.10 6 J • Time = heat required/delivery rate (E ) = 5.0x10 7 J/m 2 /1.38x10 3 J/(m 2 .s) = 3.6x10 4 s = 10.0 hours which would actually be a bit longer since the Sun is not incident at full strength all day 0.02 m

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Water in the atmosphere ctd Q: Water evaporates/condenses/rains etc. How long does it stay (turn over time) in the atmosphere? One measure of this time, Mass(air)/m 2, M = 1x10 4 kg/m 2 [ = P/g] Mean water mixing ratio, w 2.5x10 -3 kg/kg Mass water = M.w = 25 kg/m 2 2 residence 2 Amount in atmosphere (kg/m ) = Removal rate (kg/(m .s) τ
Water in the atmosphere ctd Removal Rate is estimated from the globally average rainfall 1 m/year This is equivalent to about 1m 3 /m 2 .yr Mass over 1 m 2 vol. ρ w / m 2 = 1x10 3 kg/m 2 Turnover time, Of course this is a very approximate estimate as the turnover time can be much shorter (tropics) or much longer (deserts) 2 residence 3 2 25 (kg/m ) = 0.025y 9d 1x10 (kg/(m .year) τ = =

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Question: What do we mean by temperature? In physics it is a measure of the average speed of the atoms/molecules higher temperatures correspond to faster average speeds. Motion is random spectrum of speeds The energy associated with this motion is Kinetic Energy. The temperature of the air is a measure of its average kinetic energy. Caveat: For molecules that rotate and vibrate we also have vibrational and rotational temperatures. For all that we do all 3 temperatures will be the same.
Temperature ctd The lowest temperature possible (i.e.

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