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Copyright © 2013 Pearson Education, Inc.

The Atmosphere: An Introduction to Meteorology, 12th

Lutgens • Tarbuck

Lectures by:

Heather Gallacher, Cleveland State University

Chapter 5: Forms of Condensation and Precipitation



!  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 2000–6000 m. !  Low clouds are at altitudes of less than 2000 m. !  Clouds of vertical development extend upward to span more

than one height range.

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!  High clouds: "  Cirrus "  Cirrostratus "  Cirrocumulus

!  Middle clouds: "  Altocumulus "  Altostratus

!  Low clouds: "  Stratus "  Stratocumulus "  Nimbostratus

!  Clouds of vertical development: "  Cumulonimbus "  Cumulus humilis "  Cumulus congestus

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!  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).

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!  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.

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!  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.



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!  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.

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!  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 0.025 cm. !  < 0.025 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.

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!  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.

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