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GEU 0047: Meteorology Lecture 5 Condensation and Cloud. Dew. When the temperature falls below the Dew Point… Dew favors clear calm nights. Frost. When the temperature falls below the Frost(Dew)Point Occurs even if the Air Temp. is above the freezing point. Deposition.
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Dew When the temperature falls below the Dew Point… Dew favors clear calm nights.
Frost When the temperature falls below the Frost(Dew)Point Occurs even if the Air Temp. is above the freezing point.
Deposition When water vapor freezes to solid.
Condensation Nuclei Over 1,000-200,000 particles /cm3 of aerosols exist in the atmosphere. These particles provide the surface areas for condensation to occur easily. Their weights are of the order of 0.000000000001 grams and thus can remain suspended in the atmosphere.
Nuclei Types Hydroscopic: Water-seeking salts, dust, sulfuric and nitric acids Hydrophobic: Water-repellent oils, waxes < 0.2 mm Responsible for “dry” haze 0.2-1.0 mm Most favorable for cloud condensation > 1.0 mm > 10 mm liquid water droplets (precipitation seeds)
Haze • Dry Haze • “Washes out” distant objects during the day. Caused by scattering. • Wet Haze (fog-wannabe) • As the Relative Humidity drops below 75%, hydroscopic condensation nuclei are active and water vapor condenses on them.
Fog • Formed by • Cooling the air below its dew point temperature • Adding water vapor through evaporation and mixing of air parcels • Officially, fog is defined as condensed water vapor (clouds) near the ground that obscure visibility to less than 6 miles and also, the spread between dew point and air temperature is < 5 oF. • When visibility is < 0.25 miles, fog is called dense.
Radiation (Ground or Valley) Fog • Moist air near the surface cools off to a lower temperature than drier air above. Air is a bad conductor, so the layer of air near the ground cools off with the surface more rapidly. Wanted: Clear skies, calm winds, and cold temperature. Common in fall/winter.
Radiation (Valley) Fog • Cold air is denser and collects in valleys and low lying areas, therefore radiation fog is associated with mountain valleys and especially river valleys with the high moisture content. Because the fog forms from cooling near the surface, fogs develop from the ground up and are thickest in height by morning.
Burn-Off With sunrise, scattered sunlight reaches the ground and warms it up. In the warmer air, evaporation takes place and the air also rises and begins mixing with the air above. The fog is said to “lift” as it clears near the ground. Satellite photos show fog dissipates around its edges first, usually where it is the shallowest and radiation can penetrate to the ground to cause heating.
Advection Fog • Warm moist air, moving over a cold surface. If the surface is much cooler than the air above it, the air above will reach the dew point temperature to form advection fog.
Advection Fog Warm moist air from the pacific flows eastand encounters the cold waters off the California coast. Chilled from below, the air reaches the dew point and fog is carried inland.
Steam Fog • Cold air meets warm moist air. (Mixing) • your breath on a winter day • ponds and lakes after cold nights (large heat capacity of water) • air craft contrails • Arkansas Nuclear One cooling tower
Summary • Condensation • Condensation Nuclei • water vapor condensation seeds • Dew and Frost • Haze • Dry • Wet • Fog • Radiation • Advection • Steam
Cloud Types Cloud Group Cloud Base Height Cloud Types High Clouds Tropics: 6000-18000m Cirrus Mid-latitudes: 5000-13000m Cirrostratus Polar region: 3000-8000m Middle Clouds Tropics: 2000-8000m Altostratus Mid-latitudes: 2000-7000m Altocumulus Polar region: 2000-4000m Low Clouds Tropics: surface-2000m Stratus Mid-latitudes: surface-2000m Stratocumulus Polar region: surface-2000m Nimbostratus Vertical Growth Tropics: up to 12000m Cumulus Mid-latitudes: up to 12000m Cumulonimbus Polar region: up to 12000m
Stratus (St) Strata: (Latin) layers. Stratocumulus
Stratus Clouds Stratus clouds are uniform gray clouds that usually cover the entire sky. They form from very weak, upward vertical air currents causing condensation. Stratus clouds also form when a layer of air is cooled from below to its dew point temperature and water vapor condenses into liquid droplets. Stratus clouds look like a layer of fog that never reaches the ground. In fact, fog that "lifts" off the ground forms a layer of low stratus clouds. Precipitation rarely falls from true stratus clouds since the upward vertical motion needed for precipitation is very weak, but light mist and drizzle can sometimes accompany stratus clouds. • Cirrostratus (Cs) • Altostratus (As) • Nimbostratus (Ns) • Stratocumulus (Sc)
Cirrostratus (Cs) Cirrostratus: These sheet-like, thin clouds form above 18,000 feet. Cirrostratus clouds are so thin that the sun and moon can be seen clearly through the clouds forming halos (暈). Thick cirrostratus clouds give the sky a bright white appearance and often signal rain or snow within 24 hours, especially if they are followed by mid-level clouds. Cirrostratus
Altostratus (As) Altostratus: These gray or bluish-gray clouds form between 6,000 and 20,000 feet. Altostratus clouds cover the entire sky over an area that usually extends over hundreds of square kilometers. The sun may be visible under thinner sections of the cloud. Altostratus clouds usually form ahead of a storm producing widespread and mostly continuous precipitation. Altostratus clouds do not allow enough sunlight to break through the cloud to form any shadows on the ground.
Nimbostratus (Ns) NIMBUS: precipitation producer Nimbostratus: These dark gray clouds usually form below 6,000 feet and are almost always associated with continuous light to moderate precipitation. Precipitation with a nimbostratus cloud is never the heavy, showery type and often can last for several hours to more than a day. The sun and moon are not visible through a layer of nimbostratus clouds.
Nimbostratus (Ns) These clouds usually form in a stable atmosphere where warm, moist air is overrunning colder air at the surface over a relatively large area. Associated with warm fronts. Visibility is often very poor below nimbostratus clouds as fog and fast-moving, ragged clouds, known as scud or stratus fractus clouds, form in the precipitation-cooled air below the cloud.
Stratocumulus (Sc) Stratocumulus: These low, lumpy cloud layers form below 6,000 feet and can appear in rows, patches or as rounded masses with blue sky in between the cloud elements. The color of stratocumulus clouds can range from white to dark gray. Precipitation rarely falls from stratocumulus clouds. The individual cloud elements form where the air is rising while the blue sky in between the cloud elements indicates sinking air.
Cirrus (Ci) Cirrus (Latin) curl or filament. The most common variation of high-level clouds, cirrus are thin, whispy clouds composed of ice crystals. These ice crystals come from the freezing of supercooled water droplets, and usually exist at heights above 18,000 feet where the temperatures are typically below -38 degrees Celcius. Cirrus generally occur in fair weather and move across the sky, indicating the direction of the prevailing winds at their elevation.
Cirrus Clouds Cirrus-types are observed in a variety of shapes and sizes, ranging from the "finger-like" appearance of cirrus ”fall-streaks” or “mares-tails”, commonly seen during pleasant weather conditions, to a uniform texture which can be the first sign of an approaching warm front. Cirrus clouds above Arkansas Tech. Arkansas Nuclear One Cooling Tower (a.k.a. the cloud machine) producing steam fog.
Cirrocumulus (Cc) Cirrocumulus: These clouds form above 18,000 feet and appear as small, rounded white puffs that are isolated or in long thin rows. When the white puffs are in rows, they give the cloud a rippling appearance that distinguishes it from cirrus clouds. Cirrocumulus clouds rarely cover the entire sky.
Cirrocumulus The small ripples in a cirrocumulus cloud resemble the scales of a fish, which earned the nickname "mackerel sky". They also look like sheep wool.
Cirrocumulus The individual cloud elements that reflect the red or yellow light of a setting sun make this one of the most beautiful of all clouds
Contrails A contrail, also known as a condensation trail, is a cirrus-like trail of condensed vapor that is produced by jet aircraft flying at high altitudes. (Steam Fog)
Noctilucent Noctilucent clouds are caused by high cirrus-like clouds which are illuminated after sunset or before sunrise when the sun is physically well below the local horizon.
Cumulus (Cu) Cumul: (Latin) Heap. Described as having a cauliflower/popcorn look. Cumulus clouds form as water vapor condenses in strong, upward air currents above the earth's surface. These clouds usually have flat bases and lumpy tops. Cumulus clouds are usually very isolated with large areas of blue sky in between the clouds. Most cumulus clouds form below 6,000 feet and are relatively thin and associated with fairweather.
Altocumulus (Ac) These clouds form between 6,000 and 20,000 feet and appear as gray, puffy blobs, sometimes rolled out in parallel waves or bands. One part of the cloud is usually darker than the rest, which helps distinguish this cloud from the higher cirrocumulus clouds. Altocumulus clouds that look like "little castles" in the sky indicate rising air at the cloud level. The appearance of these clouds on a warm, humid summer morning are often followed by afternoon thunderstorms.
Unsettled Cumulus Clouds When the atmosphere becomes unstable and very strong, upward air currents form, cumulus clouds can grow into cumulus congestus, or towering cumulus. If the atmosphere is unstable enough, cumulonimbus (Cb) clouds, better known as thunderstorms, form. Cumulus congestus and cumulonimbus clouds can tower from below 6,000 feet to greater than 50,000 feet.
Thunderstorms The Cumulonimbus Cloud can exist as a single towering cloud, or can even develop into a line of such towers, also known as a "squall line". Fueled by vigorous convection, which leads to strong vertical updrafts, sometimes in excess 50 (mi/hr), these clouds can climb vertically in the atmosphere to an altitude of 39,000 ft (12,000 m) or higher. Some cumulonimbus clouds, which may initially appear as harmless floating puffs of cotton, can quickly develop into powerful and potentially fatal thunderstorms known as supercells.
Supercell Cumulus A cumulonimbus can grow to such heights that it actually reaches the tropopause. At this height, high winds will flatten the top of the cloud out into an anvil-like shape. Cumulonimbus clouds are associated with heavy rain, snow, hail, lightning, and tornadoes.
Mammatus The mammatus clouds are pouch-like structures and are a rare example of clouds developing as a result of sinking motion. They form underneath a thunderstorm where cooler air sinks into warmer air below the storm cloud. Mammatus clouds look threatening, but actually signal the weakening of a thunderstorm.
Cloud Type Portraits (based on height and vertical development)
Lenticular Lenticular clouds are associated with the uplift of air, usually in association with mountain peaks or other rapid ascent.
