•T=30 C • =12mb

Condensation: , and •What is Td? AT350 •What is the sat. pressure? •What is the relative ?

•T=30 C POLAR AIR •Water =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? 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%

•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% Dew

• Surfaces cool strongly at • Condensation is the transformation of night by water vapor to water – Strongest on clear, calm nights • Water does not easily condense without a • The is the surface present at which the air is saturated with water – Vegetation, soil, buildings provide surface for vapor dew and formation • If a surface cools below the dew point, water – Particles act as sites for 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 , the dew (RH > 100%), water vapor will tend to condense point is called the frost point and form cloud/fog drops • 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 – deposition • Without particles clouds would not form in the • The resulting crystals are known as frost, hoarfrost, – RH of several hundred percent required for pure or white frost water drop formation Typical sizes • Fogs are clouds in contact with the ground • Several types of fogs commonly form – Radiation fog – fog – Upslope fog – (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 near the ground • If saturation is reached, fog forms • A layer of air near the ground becomes saturated and fog • Common on west coast of U.S. forms – Warm moist air from Pacific is advected over cold coastal – As foggy air moves ashore, solar heating warms the ground and overlying surface • Fog deepens as radiative cooling from the fog top continues • Fog evaporates near ground overnight – Coastal advection fogs are key moisture sources for Redwoods • 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 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 environment in many relatively warm, wet surface and mixing with colder air – form high or low in the ways above. – Radiative balance, water • (Smokestack plume, – even form in the cycle, processing, ) (important for -atmosphere charge the hole!) balance, etc….

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 – Made of ice crystals – High Clouds - generally above 16,000 ft at middle latitudes – Cirrus • Main types - Cirrus, Cirrostratus, Cirrocumulus • Thin and wispy – Cirrostratus – Middle Clouds – 7,000-23,000 feet • Move west to east • Thin and sheetlike • Main types – Altostratus, Altocumulus • Indicate fair and moon clearly – Low Clouds - below 7,000 ft – Cirrocumulus visible through them • • Less common than cirrus Main types – Stratus, stratocumulus, nimbostratus • Halo common – Vertically developed clouds (via ) • Small, rounded white puffs individually or in • Often precede • Main types – Cumulus, Cumulonimbus long rows (fish scales; mackerel ) Cirrus Cirrus

Cirrus Display at Dawn

Cirrocumulus Cirrocumulus

Cirrocumulus at Sunset 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 Cirrostratus with Halo


Alto Stratus Altocumulus Altocumulus

Alto Cumulus Radiatus

Low Clouds Alto Cumulus • Stratus – Uniform, gray – Resembles fog that does not reach the ground – Usually no precipitation, but light / possible • Stratocumulus – Low lumpy clouds – Breaks (usually) between cloud elements – Lower base and larger elements than altostratus • Nimbostratus – Dark gray – Continuous light to moderate or – Evaporating rain below can Alto Cumulus Undulatus form stratus fractus Stratus fractus

Looking down on an eastern Stratiform cloud layers Atlantic stratus deck 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 – cloud – Very tall, often reaching – Individual or grouped Cumulonimbus with Pileaus caps – Large energy release from water vapor condensation Cumulonimbus Clouds Spawn Tornadoes

Cloud type summary Visible and Satellite Photos observations • 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…)