Textbook, chapter 4, p. 100-106

Textbook, chapter 4, p. 100-106Textbook, chapter 4, p. 100-106

clouds

Cloud classification

• Originally by Luke Howard (1850’s)• Currently used nomeclature based on Abercromy & Hildebrandsson (1887) • Linnean system (genus, species)• morphological only, not genealogical

Latin Root Translation Examplecumulus heap fair weather cumulus stratus layer altostratuscirrus curl of hair cirrusnimbus rain cumulonimbus

Cloud subclassification

Further classification identifies clouds by height of cloud base.3 levels

Examples:

the prefix "cirr-" (as in cirrus clouds) refers to high levels, the prefix "alto-" (as in altostratus) refers to middle levels.

Relative heights vary zonally

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000m

eter

s ag

l

Tropics Midlatitudes Poles

H M L H M L H M L

tropopause

Cloud types and relative altitudes

High clouds

composed of ice crystals

fibrous or filamentous shape

ice crystal concentration generally very small

Cirrus uncinus (mares’ tails)Height (km): 7-10 km+Falling light snow [fallstreaks], not reaching the ground. distorted by upper-tropospheric wind shear.

Cirrus uncinus

Cirrus floccus

Cirrus uncinus

Cirrus fibratus vertibratus

Cirrus

Cirrus

Cirrus spissatus

Cirrus spissatus with virga

Cirrus uncinus

Cirrostratuscontinuoussun shines thru, no precipitation process: widespread ascent aloft, or old thunderstorm anvil

halo

Cirrocumulus Cs broken into wavessun shine thru, no precipitation

process: widespread ascent combined with convective overturning in a thin layer.

Middle clouds

Altostratus clouds occur as uniformly gray or white layers that totally or partiallycover the sky. They are usually so thick that the sun is only dimly visible, as ifviewed through frosted glass.

* occur at altitudes where temperatures range between 0 and -25ºC (32 and -13ºF).

* composed of supercooled water droplets, or are in mixed-phase (supercooled water droplets and ice crystals).

Altostratus

Height (km): base=2-6 km, can be thick, liquid or ice, no precipitation reaching the ground. Process: widespread stable ascent, often preceding a surface warm front.

Altostratus has a uniform and diffuse coverage

Iridescent Altostratus (difraction)

Altocumulusbase=2-6 km, usually thin, usually liquid, no precipitation.

Process: widespread ascent combined with convective overturning aloft (left) or with wave activity (right)

clear regions descending air cloudy regions ascending air

Altocumulus

Altocumulus castellanus

Altocumulus stratiformis

Sharp cloud boundaries indicate the presence of water droplets ratherthan ice crystals.

Altocumulus lenticularis

Low-Level Clouds

* usually at temperatures above -5ºC (23ºF)* composed mostly of water droplets.

geostationarysatellite

precipitation

Stratus

Base>0.1 km, top<3 km, liquid (or ice), may have drizzle falling. Process: stable ascent, mixing

Fog occurs when stratus meets the ground; when it lifts, it maybreak up into stratocumulus.

Nimbostratus

steady light precipitation

StratocumulusShallow, usually liquid, no precipitation Process: forced or spontaneous overturning in a shallow layer

Marine stratocumulus

cumulusBuoyantly rising air parcels (thermals) become saturatedCu cloud base … LCLShallow or deep

LCL

Cu humulis

Cu mediocris

Cumulonimbus

Fair-Weather CumulusWidely separated heap clouds of small vertical development. With flat bottoms and rounded tops, they resemble a flock of sheep grazing in a pasture.

Cu humilis or mediocris

Cumulus congestus

Larger and deeper cauliflower appearances to the tops. Base is flattish, representing the LCL. Can produce showers.

Cloud top=5-7 km, above the freezing level but usually still liquid.

CumulonimbusLook at this animation

12 June 2004, Hastings NE. Photo by Doug Raflik

Cumulonimbus: the most active member of the cumulus family

Convective family over the South China Sea

Cumulonimbus

Pop quizzes: cloud identification

A: cirrusA: cirrus B: cirrostratusB: cirrostratus C: cirrocumulusC: cirrocumulus D: altostratusD: altostratus E: altocumulusE: altocumulus F: stratusF: stratus G: stratocumulusG: stratocumulus

H: cumulus humilisH: cumulus humilis I: cumulus congestusI: cumulus congestus J: cumulonimbusJ: cumulonimbus J: nimbostratusJ: nimbostratus

a mid-level cloud in the lee of mountain ranges

cirruscirrus cirrostratuscirrostratus cirrocumuluscirrocumulus altostratusaltostratus altocumulusaltocumulus stratusstratus stratocumulusstratocumulus cumulus humiliscumulus humilis cumulus congestuscumulus congestus cumulonimbuscumulonimbus nimbostratusnimbostratus



Hint: it is overcast, but not raining




Hint: this cloud layer is low


Hint: this cloud layer is high


this is a halo


Hint:it is raining


A few other cloud visual clues 1. anvil development

process: buoyant ascentlightning usual(lightning is rare over the oceans)much precip evaporates if LCL is high

view this animation

Cumulonimbus: Severe weather

lightning, thunder, heavy rains, hail, strong winds, and tornadoes …

Model simulations:

Typical thunderstorm

Severe thunderstorm

Cumulonimbus: Shelf cloudsWe are just ahead of a severe thunderstorm. Dewpoints are in the mid-70s.

Winds gust to over 40 mph with the passage of this shelf cloud. Evaporatively cooled air is pushed out of the precipitation area by the

downdraft, warm air slides up and over the gust front forming the concave-shaped shelf cloud.

Cumulonimbus: Mammatus cloudspockets of negatively-buoyant air, filled with snowsuspended from the anvil base

Mesoscale Convective Complexes

Radar animation

Orographic and wave clouds

Orographic clouds are produced by the flow of air interacting with mountainous terrain. They often indicate areas of clear air turbulence.

mountain wave

lee wave

Altocumulus lenticularis

Lenticular clouds

Altocumulus lenticularis are the"mountain-wave clouds." As strong horizontal winds encountera mountain range, gravity wavespropagate upwards, and leeward.

Mountain waves are stationary.

Ac lenticularis elsewhere

RH stratification may yield a ‘stack of pancakes’

Plymouth, NH, 3 Dec ‘96 Which one is liquidwhich one ice ?

Unusual AC lenticularis

Mountain-wave turbulence

Föhn wall cloud, Rockies

turbulent rotor cloud in Owens Valley, CA, downwind of the Sierras.

Mountain-wave turbulence :Banner clouds

Banner cloud or lenticular cloud ? (same mountain)

Kelvin-Helmholtz waves and KH billows

KH waves form when strong wind shear overturns a stable layer (i.e., an inversion).

They are usually invisible!(CAT)

Kevin-Helmholtz instability

Breaking billows – an aviation hazard

Kelvin-Helmholtzwaves onJupiter. Colors indicate the clouds' altitudes: blue is lowestthrough red as highest.

Cap clouds

Pileus, Latin for "skullcap,"is a smooth cloud that forms when a stable, humid layer aloft forced to rise by a penetratingThunderstorm cell.

cap cloud on a Cb

Pileus cloud attached to the top of a cumuliform cloud.

“Morning glory” roll clouds

These occur when a deep stable layer overruns a shallower stable layer. Gravity waves ahead of the deep layer result in roll clouds, and may cause severe turbulence.

~1000 km

Air rises and condenses at the leading edge, and evaporates behind

These cigar-shaped clouds also occur in the US, mainly in spring

note the stably stratified layer below, and the more turbulent one above

Documents

Textbook, chapter 4, p. 100-106