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Fog and Clouds Lecture

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Fog and Clouds Lecture •T=30 C •Water vapor pressure=12mb Condensation: Dew, Fog and Clouds •What is Td? AT350 •What is the sat. water vapor pressure? •What is the relative humidity? •T=30 C POLAR AIR •Water vapor pressure=12mb •T=-2 C •Td=-2 C •What is Td? •What is the sat. water vapor •What is water vapor pressure? pressure? •What is sat. water vapor •What is the relative humidity? pressure? ~12/42~29% •What is the relative humidity? DESERT AIR DESERT AIR •T=35 C •T=35 C •Td= 5 C •Td= 5 C •What is water vapor pressure? •What is water vapor pressure? •What is sat. water vapor •What is sat. water vapor pressure? pressure? •What is the relative humidity? •What is the relative humidity? ~9/56~16% 1 •If air is saturated at T=30 C •If air is saturated at T=30 C and warms to 35 C, what is the and warms to 35 C, what is the relative humidity? relative humidity? ~75% •If air is saturated at T=20 C •If air is saturated at T=20 C and warms to 35 C, what is the and warms to 35 C, what is the relative humidity? relative humidity? ~43% •If air is saturated at T=-20 C •If air is saturated at T=-20 C and warms to 35 C, what is the and warms to 35 C, what is the relative humidity? relative humidity? ~2% Condensation Dew • Surfaces cool strongly at • Condensation is the phase transformation of night by radiative cooling water vapor to liquid water – Strongest on clear, calm nights • Water does not easily condense without a •The dew point is the surface present temperature at which the air is saturated with water – Vegetation, soil, buildings provide surface for vapor dew and frost formation • If a surface cools below the dew point, water – Particles act as sites for cloud and fog drop condenses on the surface formation and dew drops are formed Frost Cloud and fog drop formation • If the temperature is • If the air temperature cools below the dew point below freezing, the dew (RH > 100%), water vapor will tend to condense point is called the frost and form cloud/fog drops point • If the surface temperature • Drop formation occurs on particles known as falls below the frost point cloud condensation nuclei (CCN) water vapor is deposited • The most effective CCN are water soluble. directly as ice crystals – deposition • Without particles clouds would not form in the • The resulting crystals are atmosphere known as frost, hoarfrost, – RH of several hundred percent required for pure or white frost water drop formation 2 Typical sizes Fogs • Fogs are clouds in contact with the ground • Several types of fogs commonly form – Radiation fog – Advection fog – Upslope fog – Evaporation (mixing) fog Radiation Fog Advection Fog • Warm air moves (is advected) over cold surface • Surface radiation and conduction of heat away from the • Cold surface cools warm air overlying air cool air temperatures near the ground • If saturation is reached, fog forms • Common on west coast of U.S. • A layer of air near the ground becomes saturated and fog – Warm moist air from Pacific is advected over upwelling cold coastal waters forms – As foggy air moves ashore, solar heating warms the ground and overlying surface • Fog evaporates near ground • Fog deepens as radiative cooling from the fog top continues – Coastal advection fogs are key moisture sources for California Redwoods 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? Other Fog Types Clouds • Clouds result when air • Evaporation (mixing) fog •Upslope fog becomes saturated away – Mixing of warm, moist air – Moist air flows up along sloped from the ground plain, hill or mountain with colder air produces • They can saturated air parcel – Expansion of rising air causes cooling and RH increases – be thick or thin, large or –Examples small • Exhale on a cold day – contain water drops and/or • Clouds impact the • Evaporation of water from ice crystals environment in many relatively warm, wet surface – form high or low in the ways and mixing with colder air troposphere above. – Radiative balance, water – even form in the cycle, pollutant processing, • (Smokestack plume, stratosphere (important for earth-atmosphere charge contrails) the ozone hole!) balance, etc…. 3 Cloud classification High Clouds • Clouds are categorized by their height, appearance • High clouds and vertical development – White in day; red/orange/yellow at sunrise and sunset – High Clouds - generally above 16,000 ft at middle latitudes – Made of ice crystals • Main types - Cirrus, Cirrostratus, Cirrocumulus –Cirrus – Middle Clouds – 7,000-23,000 feet • Thin and wispy – Cirrostratus • Main types – Altostratus, Altocumulus • Move west to east • Thin and sheetlike – Low Clouds - below 7,000 ft • Indicate fair weather • Sun and moon clearly – Cirrocumulus visible through them • Main types – Stratus, stratocumulus, nimbostratus • Less common than cirrus • Halo common – Vertically developed clouds (via convection) • Small, rounded white • Often precede • Main types – Cumulus, Cumulonimbus puffs individually or in precipitation long rows (fish scales; mackerel sky) Cirrus Cirrus Cirrus Display at Dawn Cirrocumulus Cirrocumulus Cirrocumulus at Sunset 4 Cirrostratus 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 Cirrostratus with Halo Altostratus Alto Stratus Castellanus Altocumulus Altocumulus Alto Cumulus Radiatus 5 Low Clouds Alto Cumulus •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 Alto Cumulus Undulatus form stratus fractus Stratus fractus Looking down on an eastern Stratiform cloud layers Atlantic stratus deck 6 Stratocumulus cloud streets Stratus A Layer of Stratocumulus Cloud viewed from above Stratus undulatus 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 Cumulonimbus with Pileaus caps water vapor condensation Cumulonimbus Clouds Spawn Tornadoes 7 Cloud type summary Satellite Visible and Infrared Satellite Photos 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 IR (temperature, snow cover, Visible vegetation, etc…) 8.
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