chapter 6 humidity, saturation, and stability. driving question how is water cycled between...
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Chapter 6
Humidity, Saturation, and Stability
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Driving Question
How is water cycled between Earth’s surface and atmosphere?
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Global Water Cycle
The supply of water is essentially fixedGlobal Water Cycle Endless flow of water between land,
atmosphere, ocean, and organisms
The driving force of this cycle is the sunOceans hold more than 97% of total water
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Transfer Process
Evaporation Ocean is principle source of
atmospheric water vapor
Transpiration Water taken up by roots that
evaporates through the leaves
Evapotranspiration Direct evaporation plus transpiration
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Transfer Process
Condensation: gas to liquidSublimation: solid to gasDeposition: gas to solid
Precipitation Water, in any form, that falls to the surface
from clouds Rain, snow, drizzle, freezing rain, hail, sleet, ice
pellets
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Global Water Budget
Net water gain over continents Precipitation > Evapotranspiration
Net water loss over oceans Evaporation > Precipitation
Balanced is achieved as land surplus flows to the ocean Runoff, rivers, ground water
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Humidity
General term describing the amount or concentration of water vapor in the airHighly variableMeasures of Humidity Vapor pressure Mixing ratio Specific, Absolute, and Relative Humidity Dewpoint Precipitable Water
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Vapor Pressure
Water vapor mixes with with other gases adding to total air pressureAmount of pressure added by water vapor is a measure of humidityVapor Pressure Pressure exerted by water vapor alone
Considerably less than 40mb
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Mixing Ratio, Specific Humidity, Absolute Humidity
Mixing Ratio Ratio of mass of water vapor per mass of
remaining dry air (g/kg)
Specific Humidity Ratio of mass of water vapor to mass of total
air, dry and moist (g/kg)
Absolute Humidity Mass of water vapor per unit volume of humid
air Density of water vapor in air (g/m3)
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Saturation (not a measure of humidity)
Air is saturated with respect to water vapor at its maximum humidityOccurs at equilibrium When rate of evaporation equals the
rate of condensation
At equilibrium the air is saturated with water vapor
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Saturation VP and MR v.Temperature
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Relative Humidity
Most commonCompares the actual amount of water vapor in the air with the amount that would be in the air if the air were saturated (%)RH is inversely proportional to temp.RH = (vapor pressure/saturation vapor pressure) * 100%RH = (mixing ratio/saturation mixing ratio) * 100%
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Dewpoint
Temperature to which the air must be cooled to reach saturationA higher dewpoint indicates a greater concentration of water vaporIf RH = 100% Air is saturated Temperature = Dewpoint
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Dewpoint
Dew: tiny droplets of water formed when water vapor condenses Water vapor deposits as frost if the
temperature of saturation is below freezing
Average dewpoint across US is between 30-45oF Can be higher than 80oF
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Precipitable Water
Depth of water that would be produced if all the water vapor in a vertical column of air were condensed into liquid water Column extends from surface to tropopause
Condensing all the water vapor would produce a 1” layer of water covering the entire earth’s surfaceValues average from 4.0cm in tropics to 0.5cm in polar regions
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Monitoring Water Vapor
Hygrometer: instrument that measures water vapor concentration of air Dewpoint hygrometer Hair hygrometer Electronic hygrometer
Hygrograph: continuous plot of relative humidity with time
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Monitoring Water Vapor
Sling Psychrometer Two thermometers
mounted next to one another
One is covered in cloth and soaked with water
Thermometers are then “whirled” causing the water to evaporate
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Monitoring Water Vapor
Dry Bulb thermometer measures actual air temperatureWet Bulb thermometer measures the wet bulb temperature Temperature to which air cools to due the
evaporation of the water in the air
Wet Bulb Depression Difference between dry and wet bulb
temperatures
Can use these numbers to find RH and dewpoint
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Monitoring Water Vapor
Water Vapor emits radiation at 6.7 micrometersSatellite imagery displays water vapor and clouds above 3000m
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How Air Becomes Saturated
Clouds Visible collections of water droplets
and/or ice crystals suspended in the atmosphere
Clouds are most likely to form as RH approaches 100%So, what causes the RH to increase?
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Warming and Cooling
Expansional Cooling As a gas expands (rises), its temperature falls
Compressional Warming As a gas contracts (falls), its temperature rises
As parcels of air move up and down in the atmosphere the temperature of that parcel changes
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Lapse RatesAdiabatic Process No heat is exchanged between a parcel and
the environment Temperature change is due to expansion and
compression only
Unsaturated Air – dry adiabatic lapse rate 9.8 oC / 1000m (5.5 oF/ 1000ft)
Saturated Air – moist adiabatic lapse rate 6.5 oC / 1000m (3.3 oF/ 1000ft) Less because expansional cooling is offset by
release of latent heat
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Problem:
Recall, DALR = 10 deg/1000m, WALR = 6 deg/1000mAssume a parcel of 15 degrees C at the surface
-5 deg C
-11 deg C
If parcel rises 2km dry adiabatically what is the new temperature?If the parcel then saturates and rises another 1000m what is the temperature?
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Stable Air Layer
A rising air parcel becomes cooler (denser) than the environment and thus sinks back to its original positionA sinking air parcel becomes warmer (less dense) than the environment and thus lifts back to its original positionVertical motion is inhibited
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Unstable Air Layer
A rising air parcel becomes warmer (less dense) than the environment and thus continues to riseA sinking air parcel becomes cooler (denser) than the environment and thus continues to sinkVertical motion is enhanced
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Types of Stability
When figuring stability it is helpful if the following are known Is the parcel saturated or unsaturated? What is the vertical temperature
profile (sounding) of the atmosphere?
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Types of Stability
Absolute Instability Saturated and unsaturated parcels are
unstable Lapse rate is greater than 10 oC / 1000 m
Conditional Instability Unsaturated parcels are stable Saturated parcels are unstable Lapse rate is between 10 oC / 1000 m and
6.5 oC / 1000 m
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Types of StabilityAbsolute Stability Saturated and unsaturated parcels are
stable Lapse rate is less than 6.5 oC / 1000 m Three types
Lapse Isothermal (temperature is constant with height) Inversion (temperature increases with height)
Neutral Air When environmental lapse rate equals dry
or moist adiabatic lapse rate Neither impedes or provokes vertical motion
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Stüve Thermodynamic Chart
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Lifting Processes
ConvectionAlong FrontsTopography (Orographic Lifting)Converging Winds
Lifting Condensation Level (LCL) The level in which rising air becomes
saturated and clouds form Marked by the base of clouds
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Convection
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Frontal Lifting
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Orographic Lifting
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Converging Winds