Forms of Condensation and Precipitation

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Forms of Condensation and Precipitation Chapter 5

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5.1 Cloud Formation

+Cloud Formation

A cloud is a visible aggregate of small water droplets or ice crystals that are suspended in the atmosphere above the earth’s surface

Lifting condensation levelThe ascending parcel of air is cooled to its dew

point temperature and triggers condensation

+Cloud Formation

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.

+Cloud Formation

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.Hygrophobic (water-fearing) nuclei are not

efficient condensation nuclei but could droplets will form on them when the relative humidity reaches 100%

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5.2 Cloud Classification

+Cloud Classification

Clouds are classified on the basis of two criteria.

1.Form:

Cirrus are high, white, and thin forming delicate veil like patches or wisplike strands often have a feathery appearance

Cumulus clouds are globular, usually exhibiting a flat base and appear as rising domes or towers.

Stratus clouds are best described as layers or sheets covering much of the sky.

+Cloud Classification

2. 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.

+Cloud Classification

+Cloud Classification

+Cloud Classification- High Clouds

Low temp and small amount of water vapor at high altitudes equal high clouds, thin, white and made of ice crystals

CirrusDelicate, icy filaments. Winds cause the

filaments to bend or curl

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+Cloud Classification- High Clouds

CirrostratusTransparent, whitish cloud veils with fibrous or

sometimes smooth appearance covering whole sky.

Produce halos around the the sun or moon

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+Cloud Classification- High Clouds

CirrocumulusWhite patches composed of small cells or

ripples resembling fish scales

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+Cloud Classification- Medium Clouds

AltocumulusTend to form in large patches composed of

rounded masses or rolls that may or may not merge

Composed of water not ice dropletsThe individual cells have a more distinct outline

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+Cloud Classification- Medium Clouds

AltostratusFormless layer of grayish clouds that cover all

or portions of the sky.Sun is visible as a big bright spot with the edge

of it’s disc not discernible; no halosInfrequent snow or drizzle.Commonly associated with approaching warm

fronts thicken into nimbostratus (which = lots of rain)

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+Cloud Classification- Low Clouds

StratusUniform layer covers much of the skyOnes with scalloped bottom and appears as long

parallel rolls or broken patches are called stratocumulus

+Cloud Classification- Low Clouds

NimbostratusRain clouds

+Clouds of Vertical Development

Cumulus humilisDense, billowly clouds with tops resembling

cauliflower. Often form on clear daysKnown as fair weather cloudsHumilis are smaller

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+Clouds of Vertical Development

CumulonimbusLarge dense billowly clouds of considerable

vertical extent in the form of huge vertically towers

Tops spread out into an anvil shapeTowers produce heavy precipitation with

lightening, thunder, and occasionally hail

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+Cloud Classification

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.

+Cloud Classification

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5.3 Types of Fog

+Types of Fog

Fog is defined as a cloud with its base at or very near the ground.

Fog results from cooling or when air becomes saturated through the addition of water vaporRadiationAdvection UpslopeEvaporative

+Fog Formed by Cooling

Radiation fog results from radiation cooling of the ground and adjacent air.Night time phenomena requiring clear skies and

relatively high humidityThe 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.

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+Fog Formed by Cooling

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).

Turbulence facilitates cooling through a thicker layer of air and carries it to greater heights

Wintertime fog in the midwestThis fog is thick and produces hazardous driving

conditions

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+Fog Formed by Cooling

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 resulting in fog.

+Fog Formed by Cooling

There are two types of evaporation fog.Frontal (precipitation) fog

Occurs when rain droplets falling from relatively warm air above a frontal surface evaporates into the cooler air below and causes it to become saturated.

+Fog Formed by Cooling

Steam fog Occurs when cool air moves over warm water. Moisture evaporates and saturates the air above

it. Rising water vapor meets the cool air,

condenses and rises. Looks like steam.Steam fog is very common over lakes.

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5.4 How Precipitation Forms

+How Precipitation Forms

Cloud droplets are about 20 micrometers in diameter. A human hair is about 75 micrometers

A rain droplet is about 2 mm or 100 times the average cloud dropletVolume is 1 million times greater

+How Precipitation Forms

+How Precipitation Forms

Process that generates precipitation in the middle latitudes

The Bergeron Process depends on the process of the coexistence of water vapor, liquid cloud droplets and ice crystals. Cloud droplets do not freeze at 0°C. It freezes at -

40°C. it is super cooled.The saturation vapor pressure above ice crystals is

slightly lower than above super cooled liquid droplet

+How Precipitation Forms

So, imagine a cloud at -10°C where each ice crystal (snow crystal) is surrounded by thousands of liquid droplets. Because air is saturated at 100% with respect to liquid water, it will be super saturated (above 100%) with respect to the newly formed ice crystals.

At this result of this supersaturation, the ice crystals collect water, lowering relative humidity, the water droplets shrink to replenish what was lost

+How Precipitation Forms

So the growth of ice crystals is fed by continued evaporation of liquid droplets

When ice gets large enough they fall, they grow as they intercept cloud droplets on their fall. A chain reaction ensues and produces many snow crystals= snow flakes

+How Precipitation Forms

The type of precipitation (snow, sleet, rain, or freezing rain) depends on the temperature in the lower few km of the atmosphere

When the surface temperature is above 39°F snow usually melts before it hits the ground

+How Precipitation Forms

The Bergeron process (precipitation from cold clouds) depends on the coexistence of water vapor, liquid cloud droplets, and ice crystals

+How Precipitation Forms

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.

+How Precipitation Forms

+How Precipitation Forms

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5.5 Forms of Precipitation

+ Forms of Precipitation

+Forms of Precipitation

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. (not really considered rain)

+Forms of Precipitation

Virga is rain that evaporates above ground.

Fall streaks are ice crystals that sublime in the dry air below

Mist contains the smallest droplets.

+Virga

+Forms of Precipitation

Snow is precipitation in the form of ice crystals or aggregates of ice crystals.

Size, shape and concentration depend to great extent on the temp at which they formLow temperature, the moisture is low = very light

fluffy snow made up of individual 6 sided crystalsWarmer temperature at about 23°F, ice crystals join

together into large clumps of tangled crystals

+Forms of Precipitation

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.

+Forms of Precipitation

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.

+Forms of Precipitation

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.

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5.6 Precipitation Measurement

+Precipitation Measurement

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

+Precipitation Measurement

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.

+Precipitation Measurement

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.

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5.7 Intentional Weather Modification

+Intentional Weather Modification

Intentional weather modification, such as cloud seeding, is deliberate human intervention to influence processes that constitute the weather.Snow and rain makingSilver iodide crystals act as freezing nuceli.Fog and cloud dispersalCloud seeding with dry ice into super-cooled fog

or stratus clouds helps to disperse them to improve visibility.

+Intentional Weather Modification

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.

+Intentional Weather Modification

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.