GEU 0047: MeteorologyLecture 5

Condensation and Cloud

DewWhen the temperature falls

below the Dew Point…

Dew favors clear calm nights.

FrostWhen the temperature falls below the

Frost(Dew)Point

Occurs even if the Air Temp.

is above the freezing point.

DepositionWhen water vapor freezes to solid.

Ice Crystals

Condensation NucleiOver 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 TypesHydroscopic: Water-seeking

salts, dust, sulfuric and nitric acids

Hydrophobic: Water-repellent

oils, waxes

< 0.2 m Responsible for “dry” haze

0.2-1.0 m Most favorable for cloud condensation

> 1.0 m

> 10 m 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-OffWith 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 FogWarm moist air from the pacific flows east and 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

Average Fog Days per Year

Summary• Condensation

– Condensation Nuclei• water vapor condensation seeds

– Dew and Frost– Haze

• Dry

• Wet

– Fog• Radiation

• Advection

• Steam

Clouds

Cloud TypesCloud Group Cloud Base Height Cloud TypesHigh 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 GrowthTropics: up to 12000m Cumulus

Mid-latitudes: up to 12000m Cumulonimbus

Polar region: up to 12000m

Stratus (St)Strata: (Latin) layers.

Stratocumulus

Stratus CloudsStratus 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 CloudsCirrus-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.

CirrocumulusThe small ripples in a cirrocumulus cloud resemble the scales of a fish,

which earned the nickname "mackerel sky". They also look like sheep wool.

CirrocumulusThe individual cloud elements that reflect the red or yellow light of a

setting sun make this one of the most beautiful of all clouds

ContrailsA 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)

NoctilucentNoctilucent 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.

Noctilucent

Noctilucent

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 fair weather.

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 CloudsWhen 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.

ThunderstormsThe 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 CumulusA 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.

Supercell thunderstorm

Supercell thunderstorm

MammatusThe 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.

Mammatus

Cloud Type Portraits (based on height and vertical development)

LenticularLenticular clouds are associated

with the uplift of air, usually in

association with mountain peaks

or other rapid ascent.

Lenticular

Lenticular

Lenticular

Lenticular Formation Where stable moist air flows over a mountain or a range of mountains,

a series of large-scale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves. Under certain conditions, long strings of lenticular clouds can form.

Lenticular clouds have been often mistaken for UFOs

Banner cloudsBanner clouds are found in the lee of isolated mountains, especially sharp mountain peaks. It is as if the mountain carries a flag or banner.

Wind ShearKelvin-Helmholtz or

Billow clouds formed

by wind shear and air

layers of differing

temperatures.

Kelvin-HelmholtzThe kelvin-helmholtz clouds are formed between two layers of air, with

different densities, traveling at different speeds.

Therefore, if a warm, less dense layer exists over a layer of colder,

denser air, and if the wind shear across the two layers is strong enough,

eddies will develop along the boundary.

Orographic UpliftOrographic clouds are associated with uplift much like lenticular clouds.

They can sometimes form waves “downstream” of the mountains.

Clouds on the Web

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/home.rxml

http://www.windows.ucar.edu/cgi-bin/tour.cgi?link=/earth/Atmosphere/clouds/cloud_types.html

http://www.floridalightning.com/

http://www.atmos.washington.edu/gcg/Atlas/