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Unit 14

Clouds and SatelliteImagery

■ What are clouds and how do they form?■ What determines the height of a cloud

base?■ Does cloudy air have the same adiabatic

lapse rate as cloud-free air?■ What different types of clouds are there?■ How are clouds described in METAR

reports?■ How are clouds observed by satellite?

Requirements for cloud formation■ What are clouds?■ Requirements for formation:

– Abundant water vapor■ High RH, small T-DP spread

– Condensation nuclei■ Condensation needs a surface (e.g. window)■ In the atmosphere, water vapor condenses on

condensation nuclei– A cooling mechanism. What are the two ways

that cooling can occur?A. ______________ B. ______________

Condensation nuclei■ Aerosol (or particulates) - sea salt, dust,

combustion by-products, etc.■ Without these, cloud formation would be

nearly impossible■ Small, so can remain suspended for days■ Some have a special affinity for water

(hygroscopic), so condensation can start forRH less than 100%. eg - sea salt

■ Condensation and subsequent precipitationscavenge particulates from the atmosphere

Ice nuclei■ Cloud droplets are surprisingly pure and

may not freeze until cooled to -40 °C– Therefore water droplets exist at temperatures

below 0 °C--called “supercooled” water■ Supercooled droplets freeze when:

– They collide with a solid object (e.g., ground orairplane) or another cloud particle that is frozen

– Or when they contain an impurity called an icenucleus (e.g., clay particle)

■ Ice nuclei are much less abundant in theatmosphere than condensation nuclei

Supercooled water

■ Between 0°C and -15°C, most cloudparticles are supercooled waterdroplets– Many are cold, but few are frozen

■ At colder and colder temperatures,the percentage of particles that arefrozen increases

■ Colder than -40°C, all cloud water isice

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Fog■ Surface-based cloud (composed of water

droplets or ice)■ Fog is reported when clouds base < 50 ft.■ Mist when vis < 7 miles, fog < 5/8 mile■ Tends to form when T-DP is < 3°C &

decreasing■ Would you expect fog to be worse in rural or

urban areas?■ Is fog more common in stable/unstable air?■ What causes fog to form? Dissipate?■ What makes fog dissipate more slowly?

Types of Fog■ Advection fog forms by contact cooling where

warm air moving over colder surface– Occurs when warm moist air moves over colder

bodies of water (sea fog), or over cold land– Needs winds up to about 15 kt– Occurs mostly near coasts, day or night– Along CA coast, over Labrador current, S. coast

■ Radiation fog– Air near ground cools by radiation to saturation– Also called ground fog– Occurs in inland valleys (eg Fresno CA)– Needs clear nights, light breeze < 5 kts

Types of Fog (cont’d)

■ Upslope fog where warm, moist air ascendsterrain– Like advection fog, upslope fog needs wind– Dry and warmer on downwind side of mountain

■ Steam fog where very cold air moves overwarm water– Shallow and wispy– Also called sea smoke or evaporation fog

Advection fog(San Francisco)

Radiation fog

Finding cloud base - LCL■ Below cloud base,

rising dry surface aircools at DALR

■ DP decreases byabout 5/9°C/1,000 ft

■ Rising parcels aresaturated at the LCL– H=(T-DP)/2.5 (°C)– H=(T-DP)/4.4 (°F)– H in 1,000s of ft AGL– What is H if T=72°F,

DP=50°F?

Cloudbase

Dryadiabat

Dewpoint

SurfaceDP

Surfacetemp

H

Visual properties of clouds■ Two general types:

– Cumuliform (cumulus): Puffy, cauliflower-likeclouds (associated with instability/convection)

■ Can develop into TS if atmosphere conditionallyunstable

– Stratiform (stratus): Layered clouds (associatedwith stability)

■ Other visual properties:– Fall streaks: lines extending downward from

base of cloud, indicating precipitation– Boundaries: water clouds usually have distinct

edges; ice clouds usually appear wispy– Dark base: cloud has deep vertical extent

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Cloud types (pp. 6-18 & 6-19)

■ Low clouds < 6,500 ft AGL– Stratus (ST), stratocumulus (SC), nimbostratus

(NS), cumulus (CU), cumulonimbus (CB)■ Middle clouds 6,500 to 20,000 ft AGL

– Altocumulus (AC), altostratus (AS)■ High clouds > 20,000 ft AGL

– Cirrus (CI), cirrostratus (CS), cirrocumulus (CC)■ Nimbus=rain, cumulus=heaped,

stratus=layer, cirrus=high, alto=middle

What are the following cloud types?

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Cloud & ceiling - METAR reporting■ Heights AGL 100’s of ft (<50 ft=mist or fog)■ Low (< 6,500 ft), middle (6,500-20,000 ft),

high (> 20,000 ft)■ Mist: 5/8 mi < vis < 7 mi. What is haze?■ Fog: vis < 5/8 mi■ Eighths of sky cover (FEW SCT BKN OVC)■ VFR: ceiling* > 3,000 ft AGL, vis > 5 mi■ Marginal VFR: ceiling 1-3 kft, vis 3-5 mi■ IFR: ceiling < 1,000 ft and/or vis < 3 mi■ Low IFR: ceiling < 500 ft and/or vis < 1 mi

*Ceiling: lowest cloud layer having 5/8 or more coverage

Satellite imagery

■ IR imagery– Satellite detects IR radiation emitted by

earth and clouds– Works at night as well as day– Black means warm, white means cold

■ Visible imagery– Effectively a B&W snapshot of reflected

visible light– Only available during daylight hours

Infrared (IR) image

Area of sea fog(hard to see becausesimilar temp to sea)

Highclouds

Middleclouds

Cirrusclouds

Weak cold front

Land iswarm

Visible (VIS) image

Sea

Areaof fog

Cold front