Transcript GY205 Weather and Climate - University of Mount Union
GY205 Weather and Climate
Lecture 3
Moisture in the Atmosphere
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
bulb lowers its temp – evaporative cooling on the wet 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
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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
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Frontal lifting
– air rises along, or is pushed upward by, a front
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Convergence
– air flows into areas of lower pressure and rises
•
Localized convection
surface and rises – air is warmed at the
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
temperature with height – change in air
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 • An air parcel can not rise because it will always be colder than the surrounding environment • A rising parcel of air will continue to rise, because it is always warmer than the surrounding environment • An air parcel is stable if unsaturated; it is unstable if saturated • • • 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 • • • Clouds are classified by height and form Heights: low, middle, high, or vertically developed Forms: cumulus = puffy; stratus = flat, layered; cirrus = thin, wispy • • • • < 6000 feet Stratus flat, layered clouds, “gray skies” Nimbostratus - stratus clouds producing precipitation Stratocumulus - stratus with some vertical development Stratus Stratocumulus • • • • 6000 - 19000 feet “Alto-” prefix indicates medium Altostratus - medium level stratus Altocumulus medium level cumulus; “sheep’s back” Altostratus Altocumulus • • • • > 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 • • Cumulus - puffy clouds Cumulonimbus “thunderheads” - thunderstorm clouds, Cumulus Cumulonimbus End of Lecture 3Absolute Stability
Absolute Instability
Conditional Instability
Temperature Inversions
Cloud Types
Low Clouds
Medium Clouds
High Clouds
Clouds w/Vertical Development
GY205 Weather and Climate