GY205 Weather and Climate Lecture 3

Moisture in the Atmosphere To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Evaporation, Condensation, and Saturation Air is saturated when evaporation and condensation are in equilibrium

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

Temperature and Water Vapor in Saturated Air As temperature increases, the amount of water vapor in saturated air also increases

Specific Humidity SH is the mass of water vapor present in a given mass of air SH is unaffected by temperature

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

Specific Humidity and Relative Humidity With temp. held constant: Adding water vapor will increase RH Removing water vapor will decrease RH

Humidity 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 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 Distribution

Average January Dew Point Distribution

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

Mythbuster 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? Evaporation absorbs heat Condensation releases latent heat

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

Adiabatic 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 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 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

Mythbuster Clouds are made of liquid water droplets, NOT water vapor. Water vapor is an invisible gas.

Condensation Nuclei, Cloud Droplets, and a Raindrop

Types 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 Formation Air is lifted and cools adiabatically When it is cooled to its dew point condensation occurs, forming cloud droplets

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

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

Conditional Instability An air parcel is stable if unsaturated; it is unstable if saturated

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

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

High 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” Cirrostratus Cirrus

Clouds w/Vertical Development Cumulus - puffy clouds Cumulonimbus - thunderstorm clouds, “thunderheads” Cumulus Cumulonimbus

GY205 Weather and Climate End of Lecture 3