Humidity, Saturation, and Stability

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Chapter 6

• Humidity, Saturation, and Stability

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Driving Question

• How is water cycled between Earths surface and
  atmosphere?

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Global Water Cycle

• The supply of water is essentially fixed
• Global Water Cycle
• Endless flow of water between land, atmosphere,
  ocean, and organisms
• The driving force of this cycle is the sun
• Oceans 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 liquid
• Sublimation solid to gas
• Deposition 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 gt Evapotranspiration
• Net water loss over oceans
• Evaporation gt 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 air
• Highly variable
• Measures 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 pressure
• Amount of pressure added by water vapor is a
  measure of humidity
• Vapor 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 humidity
• Occurs 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 common
• Compares 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 saturation
• A higher dewpoint indicates a greater
  concentration of water vapor
• If 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 earths
  surface
• Values 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
  temperature
• Wet 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 micrometers
• Satellite 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 Rates

• Adiabatic 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/1000m
• Assume a parcel of 15 degrees C at the surface

If parcel rises 2km dry adiabatically what is the
new temperature?
-5 deg C
If the parcel then saturates and rises another
1000m what is the temperature?
-11 deg C
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Stable Air Layer

• A rising air parcel becomes cooler (denser) than
  the environment and thus sinks back to its
  original position
• A sinking air parcel becomes warmer (less dense)
  than the environment and thus lifts back to its
  original position
• Vertical 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 rise
• A sinking air parcel becomes cooler (denser) than
  the environment and thus continues to sink
• Vertical 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 Stability

• Absolute 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

• Convection
• Along Fronts
• Topography (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