GY205 Weather and GY205 Weather and ClimateClimate

Lecture 3Lecture 3

Moisture in the AtmosphereMoisture in the Atmosphere

Evaporation, Condensation,Evaporation, Condensation,and Saturationand Saturation

• Air is saturated when evaporation and condensation are in equilibrium

Saturation Vapor PressureSaturation Vapor Pressure•Pressure exerted by water vapor when the air is saturated

Temperature and Water VaporTemperature and Water Vaporin Saturated Airin Saturated Air

•As temperature increases, the amount of water vapor in saturated air also increases

Specific HumiditySpecific Humidity

• SH is the mass of water vapor present in a given mass of air

• SH is unaffected by temperature

Relative HumidityRelative Humidity• Relative humidity – amount of water vapor

present compared to the amount that COULD be present at a given temperature

• RH = SH / saturation SH x 100• RH is expressed as a %• RH tells you how close the air is to being

saturated, 100% RH = saturation• To change RH, change temperature

and/or quantity of water vapor present

Temperature Affects Relative HumidityTemperature Affects Relative Humidity

With SH held constant:•Cooling the air increases RH•Warming the air decreases RH

Specific Humidity and Relative HumiditySpecific Humidity and Relative Humidity

With temp. held constant:•Adding water vapor will increase RH•Removing water vapor will decrease RH

Humidity and ComfortHumidity and Comfort• In the winter we warm the air, lowering RH

indoors and have to use a humidifier or we get dry skin

• In the summer, we cool the air, raising RH and the excess moisture is removed by the air conditioner

• Basements are often damp because they are cooler and thus have a higher RH than the rest of the house, so we use a dehumidifier to lower RH

Dew Point TemperatureDew Point Temperature• The dew point is the temp. at which the air

will become saturated

• Always < or = to air temp.

• The closer the air temperature is to the dew point, the closer the air is to saturation, and the higher the RH

• Dew point >70°F is muggy; ~50°F is comfortable; <30°F is dry

Average July Dew Point DistributionAverage July Dew Point Distribution

Average January Dew Point DistributionAverage January Dew Point Distribution

If the air cools to the dew point temperature:If the air cools to the dew point temperature:

• RH becomes 100% (the air is saturated)

• Condensation will occur on any surface cooled to the dew point of the surrounding air

• Dew will form (cold-can, fogged bathroom mirror)

• If dew point is <32°F, frost will form by deposition

MythbusterMythbuster

• Dew does not “settle” onto a surface, it forms directly on the surface

• Frost is not frozen dew, it forms by deposition directly from water vapor

Remember Phase Changes?Remember Phase Changes?

•Evaporation absorbs heat

•Condensation releases latent heat

How Refrigerators/Dehumidifiers WorkHow Refrigerators/Dehumidifiers Work• Compressor (B) raises pressure of

refrigerant gas, causing it to heat up.• Gas flows through coils, dissipating

heat. As at cools, it condenses to liquid.

• Liquid passes through expansion valve (C) into low-pressure environment, causing evaporation. Gas passes through coils inside the frig, absorbing heat, lowering temp.

• Gas moves back to compressor to repeat the cycle.

Processes that Change Air Temp.Processes that Change Air Temp.

• Diabatic processes – heat is added to or removed from a system

• Adiabatic processes – temperature changes without adding or removing heat

• Adiabatic processes occur as a result of the compression or expansion of a gas

• Compression increases temp., expansion lowers temp.

Adiabatic Processes in the AtmosphereAdiabatic Processes in the Atmosphere

• Adiabatic processes are responsible for forming clouds, a type of fog, and some wind systems

Adiabatic Lapse RatesAdiabatic Lapse Rates

•A parcel of air rises at the DALR until it is cooled to the dew point, above that it rises at the WALR

Measuring Relative Humidity and Dew PointMeasuring Relative Humidity and Dew Point

• Sling Psychrometer – evaporative cooling on the wet bulb lowers its temp

• Drier air allows more evaporation thus a greater wet bulb depression indicates lower RH

• Psychometric tables used to convert readings to RH and dew point

• (They got the diagram wrong!)

• Hair hygrometer – hair expands and contracts in response to changes in RH (bad hair days)

Clouds and FogClouds and Fog• Clouds and fog are composed of tiny

droplets of water suspended in the air

• When air is cooled to the dew point, condensation occurs, and liquid water forms on condensation nuclei

• Condensation nuclei are hygroscopic aerosols (natural and anthropogenic)

• Condensation nuclei are always abundant

MythbusterMythbuster• Clouds are made of liquid water

droplets, NOT water vapor. Water vapor is an invisible gas.

Condensation Nuclei, Cloud Condensation Nuclei, Cloud Droplets, and a RaindropDroplets, and a Raindrop

Types of FogTypes of Fog• Fog is basically a ground level cloud• Radiation fog – forms due to overnight radiational cooling• The ground cools, and lowers the temp of the air directly

above it to the dew point

• Advection fog – warm, humid air crosses a cold surface, loses heat, and is cooled to its dew point

• Steam fog – forms when cold air moves over warmer surface, usually water

• Evaporating water rises and is cooled by the cold air to its dew point

• Upslope fog – air is forced uphill, and cools adiabatically to its dew point

• Precipitation fog – rainfall raises relative humidity to saturation due to evaporation and cooling

Cloud FormationCloud Formation

• Air is lifted and cools adiabatically

• When it is cooled to its dew point condensation occurs, forming cloud droplets

How the Air is LiftedHow the Air is Lifted

• Orographic lifting – wind rises up a topographic barrier

• Frontal lifting – air rises along, or is pushed upward by, a front

• Convergence – air flows into areas of lower pressure and rises

• Localized convection – air is warmed at the surface and rises

Convection Depends on Convection Depends on Atmospheric StabilityAtmospheric Stability

• Stability – refers to the tendency of a parcel of air to resist rising

• The air can be:

• Absolutely stable

• Absolutely unstable

• Conditionally unstable

Lapse Rates ReviewedLapse Rates Reviewed• Environmental lapse rate – change in air

temperature with height• Dry adiabatic lapse rate – change in the

temp of a rising, unsaturated air parcel• Saturated (wet) adiabatic lapse rate –

change in the temp a rising, saturated air parcel

• SALR<DALR due to the release of latent heat during condensation

Absolute StabilityAbsolute Stability

• An air parcel can not rise because it will always be colder than the surrounding environment

Absolute InstabilityAbsolute Instability

• A rising parcel of air will continue to rise, because it is always warmer than the surrounding environment

Conditional InstabilityConditional Instability

• An air parcel is stable if unsaturated; it is unstable if saturated

Temperature InversionsTemperature Inversions• Layer of air with increasing temp with height• Forms on clear nights when the heat emitted by

the surface easily escapes to space• Inversions place a cap on rising air parcels

Cloud TypesCloud Types• Clouds are classified by height and form• Heights: low, middle, high, or vertically

developed• Forms: cumulus = puffy; stratus = flat, layered;

cirrus = thin, wispy

Low CloudsLow Clouds• < 6000 feet• Stratus - flat, layered clouds, “gray skies”• Nimbostratus - stratus clouds producing precipitation• Stratocumulus - stratus with some vertical development

Stratus Stratocumulus

Medium CloudsMedium Clouds• 6000 - 19000 feet• “Alto-” prefix indicates medium• Altostratus - medium level stratus• Altocumulus - medium level cumulus; “sheep’s back”

Altostratus Altocumulus

High CloudsHigh Clouds• > 19000 feet, made of ice crystals• Cirrus - thin, wispy “mares’ tails”• Cirrostratus - filmy overcast; halos around sun & moon• Cirrocumulus - high puffy clouds; “mackerel sky”

Cirrus Cirrostratus

Clouds w/Vertical DevelopmentClouds w/Vertical Development• Cumulus - puffy clouds• Cumulonimbus - thunderstorm clouds,

“thunderheads”

CumulusCumulonimbus

GY205 Weather and GY205 Weather and ClimateClimate

End of Lecture 3