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