Chapter 6 Humidity, Saturation, and Stability. Driving Question How is water cycled between Earth’s surface and atmosphere?

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 fixedGlobal Water Cycle Endless flow of water between land,

atmosphere, ocean, and organisms

The driving force of this cycle is the sunOceans 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 liquidSublimation: solid to gasDeposition: 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 airHighly variableMeasures 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 pressureAmount of pressure added by water vapor is a measure of humidityVapor 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/m3)

Saturation (not a measure of humidity)

Air is saturated with respect to water vapor at its maximum humidityOccurs 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 commonCompares 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 saturationA higher dewpoint indicates a greater concentration of water vaporIf 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 surfaceValues 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 temperatureWet 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 micrometersSatellite 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 RatesAdiabatic 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/1000mAssume a parcel of 15 degrees C at the surface

-5 deg C

-11 deg C

If parcel rises 2km dry adiabatically what is the new temperature?If the parcel then saturates and rises another 1000m what is the temperature?

Stable Air Layer

A rising air parcel becomes cooler (denser) than the environment and thus sinks back to its original positionA sinking air parcel becomes warmer (less dense) than the environment and thus lifts back to its original positionVertical motion is inhibited

Unstable Air Layer

A rising air parcel becomes warmer (less dense) than the environment and thus continues to riseA sinking air parcel becomes cooler (denser) than the environment and thus continues to sinkVertical 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 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 StabilityAbsolute 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

Stüve Thermodynamic Chart

Lifting Processes

ConvectionAlong FrontsTopography (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