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Lecture 10-12 Water as a Major Energy Carrier

25 specic humidity mass of vapour total air mass gkg

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Unformatted text preview: t RH = Mixing ra<o / saturated mixing ra<o (%) l༆  Recall that Mixing or Humidity RaZo = Mass of vapour / mass of dry air (g/ kg or no dims) 17 RelaZve humidity changes daily 18 Why? The ability of the air to hold moisture changes Sweaty nights…..no room for your sweat to evaporate Boiling l༆  Why does water ‘boil’? l༆  CharacterisZc of boiling is formaZon of bubbles at the surface l༆  BEFORE boiling point, molecules escape and bubbles do not form: Pbubbles < Pair At boiling point Pvap = Pair When pressure of vapour being evaporated from water = atmospheric pressure, boiling occurs….and bubbles do not burst as their vapour pressure = atmospheric pressure 19 Air pressure with AlZtude 20 l༆  We now know boiling point relates to atmospheric pressure and vapour pressure l༆  Also know pressure decreases with alZtude l༆  So impact on boiling point… Air pressure with AlZtude 21 22 23 Lecture Outline l༆  Psychrometrics and its uses and pracZcal effects with our weather l༆  Will cover all of chapter 4 (except last pages on ‘measuring humidity – for interest only) 24 How to Define Humidity? 25 Specific Humidity = mass of vapour / total air mass (g/kg or no dims.) OR Humidity Ra<o/Mixing Ra<o = Mass of vapour / mass of dry air (g/ kg or no dims) Recap l༆  Atmospheric pressure – varies with atmospheric humidity l༆  SubstanZally easier to measure than any mass- related measurement (Dalton’s Law of...
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