Lecture 10-12 Water as a Major Energy Carrier

l what is air pressure l pressure exerted by a column

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Unformatted text preview: l༆  BETTER to focus on mass on both sides dims.) Specific Humidity Ra<o = mass of vapour / total air mass (g/kg or no OR Mixing or Humidity Ra<o = Mass of vapour / mass of dry air (g/kg or no dims) 11 Related to where you are l༆  At STP (1atm/25°C) 1.17kg of air present in 1m3 OR 1kg of air in 0.86m3 l༆  …so at 45° laZtude 0.86m3 contains 6.5g of water average l༆  Take a house……600m3 contains 702kg of air…..when means 4.5 kg of water vapour in that air mass 12 Air pressure l༆  mmHg, Pa, mmH2O….. l༆  What is air pressure? l༆  Pressure exerted by a column of air, P = density x gravity x h l༆  P = ρgh ……… and density (ρ) will change according to the amount of water vapour contained within that volume of air 13 Vapour Mass & Pressure l༆  Dalton’s Law of ParZal Pressure states that….. 14 Total pressure exerted by a gas = sum of pressures of component gases e.g. Pair = 10,000 Pa Air made up of 78% N2 21% O2 1% H2O So…. PN2 = 7,800 Pa PO2 = 2,100 Pa PH2O = 100 Pa l༆  And so air pressure is a direct measure of the moisture content of air E = Vapour Pressurewater When and How is the Air ‘Full’? l༆  Point of equilibrium: molecules in vs. molecules out l༆  High temperature = more volume, more energeZc water molecules E* is the saturated vapour pressure 15 When and How is the Air ‘Full’? E* is the saturated vapour pressure 16 RelaZve humidity Rela<ve humidity = actual vapour pressure / Satura<on vapour pressure RH = E/E* (%) l༆  Note also tha...
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