MICHAEL DAVISPHD CANDIDATE

THE OHIO STATE UNIVERSITY

Clouds

Altocumulus (Ac)Midlevel

2400-6100 m (6500-20000 ft)

Appear in either masses or rolls and are similar to cirrocumulus but the masses are usually larger and darker

Normally signal convection and will take on a wavy sheet-like appearance

Occur before a cold front and may signal thunderstorm development later in the day

Altostratus (As)Midlevel

2400-6100 m (6500-20000 ft)

Appears as a gray sheet and is typically lighter than nimbostratus but darker than cirrostratus

The sun may be seen through the cloud and appear as a ‘bright spot’

Formed by the elevated condensation of an air mass usually a frontal system and can be very expansive

May produce very light rainfall

Arcus

A low level cloud usually on the front edge of a thunderstorm with the outflow boundary

2 types of arcus clouds:

Shelf cloud – low wedge shaped cloud that is attached to the thunderstorm

Roll cloud – detached, tube shaped cloud that appears to roll along a horizontal axis

Cirrocumulus (Cc)High level

+6000 m (+20000 ft)

Convective environment and composed primarily of ice and supercooled water droplets.

Generally short lived as they may evaporate or precipitate as virga in either rain or snow

Patterns may look like fish scales so sometimes called “mackerel sky”

Cirrostratus (Cs)

High level

+6000 m (+20000 ft)

Thin and mostly composed of ice crystals

May produce an ice halo as seen in photo

Possible indication of upper level moisture

Form in front of warm fronts and rain may be expected in the next 12-24 hours

Cirrus (Ci)High level

+7000 m (+23000 ft)

Thin and wisplike, sometimes called mare’s tails

Composed of ice crystals

Can indicate wind shear based on the streaks

Cover approximately 30% of the Earth and warm the planet via IR absorption

Usually indicate a change in the weather in the near future or blow off from a thunderstorm

ContrailsArtificial clouds created by passing aircraft exhaust

As hot exhaust is expelled from the engine, the water vapor freezes quickly in the surrounding cold air and forms ice in trails

Time duration is highly variable

May act to warm the planet by trapping outgoing longwave radiation

Cumulonimbus (Cb)Vertical

2000-16000 m (6500-60000 ft)

Tall cloud that signifies the presence of an intense thunderstorm

Can form alone, in clusters, or along a cold front

Form from convection and strong updrafts which elevate the cloud top to the tropopause

Jet stream shears the top creating an anvil like appearance

Cumulus (Cu)Low to mid level

Below 2000 m (Below 6500 ft)

Appears as heaps and may form alone, in lines, or in clusters

Often precede other types of clouds as they encompass a large variety of cloud types

Forms when warm air rises and condenses

May appear flat bottomed if at the lifted condensation level or form ‘cloud streets’ in a windy environment

Cumulus Congestus (TCu)

Vertical

Up to 6000 m (up to 20000 ft)

Form in unstable conditions usually undergoing convection

May appear like broccoli

Usually signal the progression from cumulus to cumulonimbus

FogGround level

Low level cloud that reaches the ground resulting in a decrease in visibility

Forms when saturation occurs and little wind

Many different types of fog

Grand Banks, Newfoundland is recognized as the foggiest place in the world averaging 200 days of fog annually

Kelvin-Helmholtz

High level

Cirrus like

Forms when there is a stark difference in velocities between two layers of fluid (remember, the atmosphere is a fluid!)

This causes motion between two stratified layers creating a shallow layer of instability

LenticularMid level

6000-12000 m (20000-40000 ft)

Lens shaped cloud typically forming over areas of orography

Forms when stable moist air flows up the orography, creating standing waves on the leeward side. If the temperature reaches the dew point and condenses, a lenticular cloud forms

They may be mistaken for UFOs

Mammatus

Low level

Forms on the underside of a violent thunderstorm and through descending air

Sometimes called a breast cloud

May appear as smooth, ragged, or lumpy lobes averaging 1-3 km wide and lasting 10-15 minutes

Nimbostratus (Ns)

Low level

Below 3000 (Below 10000 ft)

Appears uniform and dark

Will be precipitating as ‘nimbo’ is Latin for precipitation. If not, then it is stratus

Precipitation is moderate to heavy and may last for several days

PileusSometimes called a scarf cloud or cap cloud

Horizontal cloud that appears on top of cumulus or cumulonimbus

Formed by strong updrafts uplifting moist low levels causing the air to cool to the dew point as it ascends

Indicates strong updrafts as they tend to be associated with thunderstorms

Stratocumulus (Sc)Low level

Below 2400 m (8000 ft)

Appear rounded, dark masses that usually form in lines or clusters

Weak updrafts and dry, stable air above keep clouds low

Don’t usually precipitate but can produce light rain or snow. However, they are seen at either the front or end of thunderstorms

Stratus (St)

Low level

Below 2000 m (Below 6000 ft)

Horizontal uniform gray layer of clouds that looks like a bed sheet

Usually obscure the sky thus blocking out the sun

Essentially above the ground fog