9/22/14. Chapter 5: Forms of Condensation and Precipitation. The Atmosphere: An Introduction to Meteorology, 12 th.

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1 Chapter 5: Forms of Condensation and Precipitation The Atmosphere: An Introduction to Meteorology, 12 th Lutgens Tarbuck Lectures by: Heather Gallacher, Cleveland State University! A cloud is a visible aggregate of small water droplets. " Condensation aloft:! Adiabatic cooling causes clouds to form as water vapor condenses in the atmosphere.! The air becomes saturated and there must be a surface.! Condensation nuclei act as surfaces, on which the water vapor can condense. " Growth of cloud droplets:! Hygroscopic (water-seeking) nuclei are most effective for condensation.! Growth is rapid at first, then slows as water vapor is consumed.! Clouds are classified on the basis of two criteria. " Form:! Cirrus are high, white, and thin.! Cumulus clouds are globular, usually exhibiting a flat base.! Stratus clouds are best described as layers covering much of the sky. " Height:! High clouds are above 6000 m.! Middle clouds range between m.! Low clouds are at altitudes of less than 2000 m.! Clouds of vertical development extend upward to span more than one height range. 1

2 ! High clouds: " Cirrus " Cirrostratus " Cirrocumulus! Low clouds: " Stratus " Stratocumulus " Nimbostratus! Middle clouds: " Altocumulus " Altostratus! Clouds of vertical development: " Cumulonimbus " Cumulus humilis " Cumulus congestus 2

3 ! Cloud varieties: " Uncinus are hooked shaped clouds, often precursors to bad weather. " Fractus or fractured clouds are stratus or cumulus clouds that appear broken. " Mammatus clouds have udder-shaped protuberances on their bottom surfaces and are associated with stormy weather. " Lenticular clouds are lens shaped and are common in rugged or mountainous topographies.! Fog is defined as a cloud with its base at or very near the ground. " Radiation " Advection " Upslope " Evaporative! Radiation fog results from radiation cooling of the ground and air. " The high humidity can cause a small amount of cooling to lower the temperature to the dew point. " To be extensive, there should be a slight breeze. " It is usually thickest in valleys.! Advection fog is a blanket of fog caused by warm, moist air blowing over a cold surface. " Some turbulence is needed (10 30 kph winds). 3

4 ! Upslope fog is created when relatively humid air moves up a sloping landform or up the steep slopes of a mountain. " The upward flow causes the air to expand and cool adiabatically.! There are two types of evaporation fog. " Frontal (precipitation) fog occurs when rain falls and evaporates, saturating the cooler air below. " Steam fog occurs when cool air moves over warm water.! Moisture evaporates and saturates the air above it.! Steam fog is very common over lakes.! The Bergeron process (precipitation from cold clouds) depends on the coexistence of water vapor, liquid cloud droplets, and ice crystals. 4

5 ! The collision-coalescence process (precipitation from warm clouds) occurs as copious rainfall associated with clouds located below the freezing level (called warm clouds), especially in the tropics. " Small droplets hit other droplets and become larger. " They collide with more droplets and their falling velocity increases. 5

6 ! Rain is restricted to droplets of water with a diameter at least 0.5mm.! Cloudbursts are unusually heavy rainfalls.! Drizzle are fine, uniform droplets with a diameter less than 0.5mm.! Virga is rain that evaporates above ground.! Mist contains the smallest droplets.! Snow is precipitation in the form of ice crystals or aggregates of ice crystals.! Sleet is clear to translucent particles of ice, rain drops freeze while falling.! Freezing rain (glaze) are rain drops that become super-cooled, hit a surface, and freeze immediately. 6

7 ! Hail is precipitation in the form of hard, rounded pellets of ice. " It is produced in cumulonimbus clouds. " Hail stones begin as small ice pellets and grow as they are propelled by updrafts and downdrafts through the cloud.! Rime is a deposit of ice crystals, formed on surface objects by super-cooled fog or cloud droplets. " It occurs when the surface temperature of an object is below freezing.! Standard instruments: " A standard rain gauge catches rain water and conducts it through a narrow opening into a cylindrical measuring tube.! The gauge is 20 cm in diameter and can measure rainfall to the nearest cm.! < cm = trace " A tipping bucket has two compartments (or buckets). When one bucket fills, it tips and empties its water and the other bucket takes its place at the funnel. " A weighing gauge collects rain fall in a cylinder that rests on a spring balance. As the cylinder fills, the movement is transmitted to a pen that records the data. 7

8 ! When measuring snowfall, two measurements are normally taken. " The depth is measured with a calibrated stick. " To obtain the water equivalent, snow is melted and then weighed or measured as rain.! Weather radar uses radio waves to measure precipitation. " The radio waves penetrate small droplets, but are reflected off larger ones. Echoes are sent back and displayed.! Intentional weather modification, such as cloud seeding, is deliberate human intervention to influence processes that constitute the weather. " Snow and rain making! Silver iodide crystals act as freezing nuceli. " Fog and cloud dispersal! Cloud seeding with dry ice into super-cooled fog or stratus clouds helps to disperse them to improve visibility.! Hail suppression has been shown to be ineffective. " Anti-hail cannons produced a loud whistling noise and a large smoke ring thought to suppress hail. " Cloud seeding with silver iodide crystals was also employed to disrupt the growth of hailstones.! Several methods of frost prevention are being used. " Water sprinklers add heat from water and from the latent heat of fusion when the water freezes. " Air mixing uses wind machines to mix warm and cool air. " Orchard heaters produce the most successful results, but fuel cost can be significant. 8

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