Transcript Primary Chapter 6 Notes
Chapter 6
Humidity, Saturation, and Stability
Driving Question
How is water cycled between Earth’s surface and atmosphere?
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
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
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
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
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
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
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/m 3 )
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
Saturation VP and MR v.Temperature
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%
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
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 30-45 o F Can be higher than 80 o F
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
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
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
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
Monitoring Water Vapor
Water Vapor emits radiation at 6.7 micrometers Satellite imagery displays water vapor and clouds above 3000m
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?
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
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 o C / 1000m (5.5 o F/ 1000ft) Saturated Air – moist adiabatic lapse rate 6.5 o C / 1000m (3.3 o F/ 1000ft) Less because expansional cooling is offset by release of latent heat
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
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
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
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?
Types of Stability
Absolute Instability Saturated and unsaturated parcels are unstable Lapse rate is greater than 10 o C / 1000 m Conditional Instability Unsaturated parcels are stable Saturated parcels are unstable Lapse rate is between 10 o C / 1000 m and 6.5 o C / 1000 m
Types of Stability
Absolute Stability Saturated and unsaturated parcels are stable Lapse rate is less than 6.5 o C / 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
Stüve Thermodynamic Chart
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