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Humidity, Saturation, and Stability

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Presentation on theme: "Humidity, Saturation, and Stability"— Presentation transcript:

1 Humidity, Saturation, and Stability
Chapter 6 Humidity, Saturation, and Stability

2 Driving Question How is water cycled between Earth’s surface and atmosphere?

3 Global Water Cycle The supply of water is essentially fixed
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


5 Transfer Process Evaporation Transpiration Evapotranspiration
Ocean is principle source of atmospheric water vapor Transpiration Water taken up by roots that evaporates through the leaves Evapotranspiration Direct evaporation plus transpiration

6 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

7 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

8 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

9 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

10 Mixing Ratio, Specific Humidity, Absolute Humidity
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)

11 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

12 Saturation VP and MR v.Temperature

13 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%


15 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

16 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

17 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 surface Values average from 4.0cm in tropics to 0.5cm in polar regions

18 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

19 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

20 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

21 Monitoring Water Vapor
Water Vapor emits radiation at 6.7 micrometers Satellite imagery displays water vapor and clouds above 3000m

22 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?

23 Warming and Cooling Expansional Cooling Compressional Warming
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

24 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



27 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

28 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


30 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


32 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?

33 Types of Stability Absolute Instability Conditional 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

34 Types of Stability Absolute Stability Neutral Air
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



37 Stüve Thermodynamic Chart

38 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

39 Convection

40 Frontal Lifting

41 Orographic Lifting

42 Converging Winds

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