Condensation: Dew, Fog and Clouds AT350. T=30 C Water vapor pressure=12mb What is Td? What is the...
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Transcript of Condensation: Dew, Fog and Clouds AT350. T=30 C Water vapor pressure=12mb What is Td? What is the...
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Condensation: Dew, Fog and CloudsAT350
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•T=30 C
•Water vapor pressure=12mb
•What is Td?
•What is the sat. water vapor pressure?
•What is the relative humidity?
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•T=30 C
•Water vapor pressure=12mb
•What is Td?
•What is the sat. water vapor pressure?
•What is the relative humidity?
~12/42~29%
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POLAR AIR
•T=-2 C
•Td=-2 C
•What is water vapor pressure?
•What is sat. water vapor pressure?
•What is the relative humidity?
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DESERT AIR
•T=35 C
•Td= 5 C
•What is water vapor pressure?
•What is sat. water vapor pressure?
•What is the relative humidity?
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DESERT AIR
•T=35 C
•Td= 5 C
•What is water vapor pressure?
•What is sat. water vapor pressure?
•What is the relative humidity?
~9/56~16%
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•If air is saturated at T=30 C and warms to 35 C, what is the relative humidity?
•If air is saturated at T=20 C and warms to 35 C, what is the relative humidity?
•If air is saturated at T=-20 C and warms to 35 C, what is the relative humidity?
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•If air is saturated at T=30 C and warms to 35 C, what is the relative humidity? ~75%
•If air is saturated at T=20 C and warms to 35 C, what is the relative humidity? ~43%
•If air is saturated at T=-20 C and warms to 35 C, what is the relative humidity? ~2%
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Condensation
• Condensation is the phase transformation of water vapor to liquid water
• Water does not easily condense without a surface present– Vegetation, soil, buildings provide surface for
dew and frost formation– Particles act as sites for cloud and fog drop
formation
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Dew• Surfaces cool strongly at
night by radiative cooling– Strongest on clear,
calm nights• The dew point is the
temperature at which the air is saturated with water vapor
• If a surface cools below the dew point, water condenses on the surface and dew drops are formed
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Frost• If the temperature is
below freezing, the dew point is called the frost point
• If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals– deposition
• The resulting crystals are known as frost, hoarfrost, or white frost
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Cloud and fog drop formation• If the air temperature cools below the dew point
(RH > 100%), water vapor will tend to condense and form cloud/fog drops
• Drop formation occurs on particles known as cloud condensation nuclei (CCN)
• The most effective CCN are water soluble.• Without particles clouds would not form in the
atmosphere– RH of several hundred percent required for pure
water drop formation
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Typical sizes
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Fogs
• Fogs are clouds in contact with the ground
• Several types of fogs commonly form– Radiation fog– Advection fog– Upslope fog– Evaporation (mixing)
fog
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Radiation Fog
• Surface radiation and conduction of heat away from the overlying air cool air temperatures near the ground
• A layer of air near the ground becomes saturated and fog forms
• Fog deepens as radiative cooling from the fog top continues overnight
• Solar heating warms the ground and causes the fog to “burn off” from the ground up
• What type of meteorological conditions would favor radiation fog?
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Advection Fog• Warm air moves (is advected) over cold surface
• Cold surface cools warm air
• If saturation is reached, fog forms
• Common on west coast of U.S.– Warm moist air from Pacific is advected over upwelling cold coastal waters
– As foggy air moves ashore, solar heating warms the ground and overlying surface
• Fog evaporates near ground
– Coastal advection fogs are key moisture sources for California Redwoods
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Other Fog Types
• Upslope fog– Moist air flows up along sloped
plain, hill or mountain
– Expansion of rising air causes cooling and RH increases
• Evaporation (mixing) fog– Mixing of warm, moist air
with colder air produces saturated air parcel
– Examples• Exhale on a cold day
• Evaporation of water from relatively warm, wet surface and mixing with colder air above.
• (Smokestack plume, contrails)
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Clouds• Clouds result when air
becomes saturated away from the ground
• They can– be thick or thin, large or
small– contain water drops and/or
ice crystals– form high or low in the
troposphere– even form in the
stratosphere (important for the ozone hole!)
• Clouds impact the environment in many ways– Radiative balance, water
cycle, pollutant processing, earth-atmosphere charge balance, etc….
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Cloud classification
• Clouds are categorized by their height, appearance and vertical development
– High Clouds - generally above 16,000 ft at middle latitudes• Main types - Cirrus, Cirrostratus, Cirrocumulus
– Middle Clouds – 7,000-23,000 feet• Main types – Altostratus, Altocumulus
– Low Clouds - below 7,000 ft• Main types – Stratus, stratocumulus, nimbostratus
– Vertically developed clouds (via convection)• Main types – Cumulus, Cumulonimbus
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High Clouds
• High clouds– White in day;
red/orange/yellow at sunrise and sunset
– Made of ice crystals– Cirrus
• Thin and wispy• Move west to east• Indicate fair weather
– Cirrocumulus• Less common than cirrus• Small, rounded white puffs
individually or in long rows (fish scales; mackerel sky)
– Cirrostratus• Thin and sheetlike
• Sun and moon clearly visible through them
• Halo common
• Often precede precipitation
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Cirrus
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Cirrus
Cirrus Display at Dawn
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Cirrocumulus
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Cirrocumulus
Cirrocumulus at Sunset
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Cirrostratus
Cirrostratus with Halo
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Middle Clouds• Altocumulus
– <1 km thick– mostly water drops– Gray, puffy– Differences from
cirrocumulus• Larger puffs• More dark/light contrast
• Altostratus– Gray, blue-gray– Often covers entire sky– Sun or moon may show
through dimly• Usually no shadows
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Altostratus
Alto Stratus Castellanus
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Altocumulus
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Altocumulus
Alto Cumulus Radiatus
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Alto Cumulus
Alto Cumulus Undulatus
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Low Clouds• Stratus
– Uniform, gray– Resembles fog that does not
reach the ground– Usually no precipitation, but
light mist/drizzle possible
• Stratocumulus– Low lumpy clouds– Breaks (usually) between cloud
elements– Lower base and larger elements
than altostratus
• Nimbostratus– Dark gray– Continuous light to moderate
rain or snow– Evaporating rain below can
form stratus fractus
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Stratus fractus
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Looking down on an eastern Atlantic stratus deck
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Stratiform cloud layers
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Stratocumulus cloud streets
Stratus undulatus
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Stratus
A Layer of StratocumulusCloud viewed from above
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Vertically developed
clouds• Cumulus
– Puffy “cotton”
– Flat base, rounded top
– More space between cloud elements than stratocumulus
• Cumulonimbus– Thunderstorm cloud
– Very tall, often reaching tropopause
– Individual or grouped
– Large energy release from water vapor condensation
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Cumulonimbus with Pileaus caps
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Cumulonimbus Clouds Spawn Tornadoes
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Cloud type summary
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Satellite observations
• Satellites can be– Geostationary
• Monitors fixed spot on Earth’s surface
– Polar orbiting• Orbit poles with Earth
revolving below
• Satellites observe– Clouds– Water vapor– Precipitation– Surface properties
(temperature, snow cover, vegetation, etc…)
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Visible and Infrared Satellite Photos
VisibleIR