Humidity, Saturation, and Stability 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 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 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

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 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)

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 30-45oF Can be higher than 80oF

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 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

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

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 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

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

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

Convection

Frontal Lifting

Orographic Lifting

Converging Winds