CLOUDS

Clouds are visible aggregates of minute particles of water or ice, or both in the free air. They form when air becomes supersaturated with respect to water or ice.

Aggregate may include larger particles of water or ice and particles such as those present in fumes, smoke or dust

Cloud shapes are largely a product of patterns of rising and falling air and winds

Ordinary cloud droplets are less than 20 microns in diameter rather than 2000 microns for a raindrop

Typically there are 3000 cloud droplets in each cubic inch

Clouds are white because of confused reflection

CLOUD OBSERVATIONS

Cloud types are determined by visual observations with accuracy dependent on experience and knowledge of the processes of cloud formation

Sky cover is obtained visually by viewing the clouds against the dome of the sky as a background and estimating by tens the sky area covered.

Four terms are in common use

Clear- no clouds or less than 1/10th obscured by clouds

Scattered- .1 to .5 inclusive covered

Broken- .5 to .9

Overcast- greater than .9

Cloud Ceiling-the height above the ground at which the cloud cover appears broken or overcast. If less than .6 of the sky is covered the ceiling is considered "unlimited" regardless of the cloud heights. If clouds are present at different levels the ceiling is reported as the lowest level at which the accumulated cloud cover appears broken or overcast to an observer at the ground

Major Genera (mutually exclusive)

Greater than 6 km above earth

-cirrus fibrous

-cirrocumulus--sheet of very small elements

-cirrostratus--transparent halo phenomena

2 to 6 km above earth

-altocumulus

-altostratus

Near Surface

-stratus

-nimbostratus

-stratocumulus

-cumulus

Vertical extent

-cumulonimbus

May be further divided into species and varieties

Cirrus

-greater than 6 km above surface

-fibrous

-shape is result of winds

-major part of cloud is trail of falling ice crystals twisted by wind

-ice evaporates where trail ends

-Cirrocumulus-shallow patch or sheet broken into more or less regular, unshaded and small blobs or ripples

-Cirrostratus-shallow sheet of white cloud, thin enough to be largely transparent and produce haloes around sun and moon

Altostratus

-greyish or bluish cloud sheet or layer of striated, fibrous

or uniform appearance.

-these clouds totally or partly cover the sky

-they have parts thin enough to reveal the sun at least

vaguely

Altocumulus

-white or grey, or both white and grey, patch, sheet or layer

of clouds, generally with shading

-more or less regular rounded patches or rolls

-composed of laminae, rounded masses, rolls, etc. which are

sometimes partly fibrous or diffuse and which may or

may not be merged

Stratocumulus

-failed cumulus

-common at sunset

-often found where high pressure exists and convection is

limited i.e. inversion

-may be gray or whitish with dark spots

-may or may not be merged

Stratus

-no color variations

- may be hundreds ofkm in length

-produced by

-surface cooling - sea ice- cold water

-low and structureless

-lifting of entire layer of air at gradual rate

-may slowly grade to altostratus and cirrostratus

-distinguishing feature is the fact that there are no distinguishing features

Nimbostratus

-gray cloud layer, often dark, the appearance of which is rendered diffused by more or less continuously falling rain or snow which in most cases reaches the ground. It is thick enough throughout to blot out the sun.

Cumulus (Latin for hill)

-consist of several thermals

-become visible at lifting condensation level

-important in heating the atmosphere between 2 and 15 km

-usually start to appear in clear skies at midmorning

-very dynamic

Cumulonimbus

-in severe thunderstorms the underside of the cloud sometimes

huge and ominous protuberances called mamma

-usually a sign of a severe thunderstorm

-sometimes appear on the underside of the anvil

-top of anvil usually flat indicating updraft halts at definite altitude

SPECIES

fractus-broken or ragged st and cu

lenticularis-elements shaped like a lens or almond of sc, ac and cc

humilis-of only slight vertical extent or cu

congestus-(of cu) growing markedly, often by bulging like heart of cauliflower

SPECIAL CLOUDS

Wave Clouds

-Mountain Wave Cloud

-altocumulus lenticularis

-smooth and lenslike

-usually stationary

-may occur in a series of waves caused by rising and sinking air

-rotor lee side of mountain like breaking wave

Mammatus clouds form through what can best be described as "upside-down"convection. Ice crystals on the underside of the anvil fall earthward due to gravitation. On the outside of the cloud, the air is drier; thus these crystals sublime. The sublimation process cools the air on the underside of the anvil, creating an inversion. In this case, the warm air inside the cloud "sinks" and the cold ambient air "rises". The reason why you see mammatus

in strong storms is simple. The strength of the storm's updraft throws a bunch of ice crystals in the anvil. You'd need a lot of sublimating ice crystals to generate the temperature profile necessary to produce mammatus

-Gravity Wave

-areas of wind shear and usually perpendicular to wind

-when too high produces CAT and not clouds

Open and Closed Convection Cells Over Water

-closed

-rising air with clouds in center and fringes clear

-tend to form in stratus over cold water

-open

-warm ocean waters with rings of thunderstorms