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 – 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
• An
air parcel can not rise because it will always
be colder than the surrounding environment
Absolute Instability
•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
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•
•
•
< 6000 feet
Stratus - flat, layered clouds, “gray skies”
Nimbostratus - stratus clouds producing precipitation
Stratocumulus - stratus with some vertical development
Stratus
Stratocumulus
Medium Clouds
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•
•
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6000 - 19000 feet
“Alto-” prefix indicates medium
Altostratus - medium level stratus
Altocumulus - medium level cumulus; “sheep’s back”
Altostratus
Altocumulus
High Clouds
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•
•
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> 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 Development
• Cumulus - puffy clouds
• Cumulonimbus - thunderstorm clouds,
“thunderheads”
Cumulus
Cumulonimbus
GY205 Weather and
Climate
End of Lecture 3