# Atmospheric Moisture Key terms: Absolute & relative humidity Dew point temperature Precipitation.

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Atmospheric Moisture Key terms: Absolute & relative humidity Dew point temperature Precipitation

Atmospheric moisture serves 3 essential purposes: 1.Maintains life on earth flora & fauna 2. Keeps our planet at a habitable temperature (i.e. water vapour absorbs, reflects and scatters insolation) 3. Transfers surplus energy from tropical area either horizontally or vertically to balance the heat budget

Availability of water 97.2% in oceans & seas 2.1% polar ice & snow 0.7% lakes & rivers 0.6% soil moisture & groundwater 0.001% in atmosphere (only sufficient to give every where on earth 10 days’ supply of rain – so constant recycling of water between oceans, atmosphere & land is vital)

Hydrological Cycle - 84% of all evaporation is over oceans - 74% of all precipitation falls over oceans

Humidity – a measure of the water vapour in the atmosphere 2 ways to measure humidity: 1.ABSOLUTE HUMIDITY – the mass of water vapour in a given volume of air measured in grams per cubic metre. 2. RELATIVE HUMIDITY – the amount of water vapour in the air at a given temperature expressed as a percentage of the maximum amount of vapour that the air could hold at that temperature. Calculated by : what W.V. is present X 100 = ___% what W.V. could be present

Relative humidity The warmer the air temperature the more vapour it can hold RH 100% = saturated air RH 80-99% = moist, humid, clammy RH 50% = dry RH 10% recorded over hot deserts

Dew point temperature This is the critical temperature which is reached when the air becomes saturated (RH = 100%), this is caused when unsaturated air is cooled. Any further cooling past the dew point temperature will cause condensation of excess water vapour The altitude at which the dew point is reached is known as the Condensation Level

The Condensation Process Condensation RH reaches 100% saturation – dew point Relative Humidity (RH) increases Water bearing capacity decreases Air cools

Condensation = the process by which water vapour in the atmosphere is changed in to liquid (or solid if below zero), usually due to saturation as a result of cooling. Cooling due to: 1. Radiation (contact) cooling: Radiation dew or fog (or hoar frost) Vapour condenses Air cools by conduction with ground Ground loses heat rapidly Calm, clear evenings

Condensation: cooling due to: 2. Advection cooling: Warm Moist air Moves over Cooler Land or sea Advection fog forms as cooling causes condensation

Condensation: cooling due to: 3. Orographic or frontal unlift -Cooling as air forced up over mountain or cold air mass 4. Convective or adiabatic cooling Adiabatic = cooling due to fall in pressure with height rather than by loss of heat to surrounding air Convective or adiabatic cooling

Measurement of Humidity Hygrometer 2 thermometers, 1 wrapped in muslin dipped in container of water, other left dry If air not saturated water evaporates from the muslin and cools the wet bulb thermometer When air saturated no evaporation and no cooling Difference between wet & dry thermometers is indication of humidity –No difference - air is saturated –Small difference - humidity is high –Large difference – humidity is low

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