Download Unit 14

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?
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Unit 14
Clouds and Satellite
Imagery
Requirements for cloud formation
What are clouds?
■ Requirements for formation:
■
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Abundant water vapor
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Condensation nuclei
■ High
RH, small T-DP spread
■ Condensation
needs a surface (e.g. window)
the atmosphere, water vapor condenses on
condensation nuclei
■ In
A cooling mechanism. What are the two ways
that cooling can occur?
A. ______________ B. ______________
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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 for
RH less than 100%. eg - sea salt
■ Condensation and subsequent precipitation
scavenge particulates from the atmosphere
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Supercooled water
Ice nuclei
■
Cloud droplets are surprisingly pure and
may not freeze until cooled to -40 °C
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Therefore water droplets exist at temperatures
below 0 °C--called “supercooled” water
Supercooled droplets freeze when:
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They collide with a solid object (e.g., ground or
airplane) or another cloud particle that is frozen
Or when they contain an impurity called an ice
nucleus (e.g., clay particle)
■ Ice
nuclei are much less abundant in the
atmosphere than condensation nuclei
■ Between
0°C and -15°C, most cloud
particles are supercooled water
droplets
–
Many are cold, but few are frozen
■ At
colder and colder temperatures,
the percentage of particles that are
frozen increases
■ Colder than -40°C, all cloud water is
ice
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Fog
Types of 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?
■
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Advection fog forms by contact cooling where
warm air moving over colder surface
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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
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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)
Radiation fog
■
Upslope fog where warm, moist air ascends
terrain
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Like advection fog, upslope fog needs wind
Dry and warmer on downwind side of mountain
Steam fog where very cold air moves over
warm water
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Shallow and wispy
Also called sea smoke or evaporation fog
Advection fog
(San Francisco)
Finding cloud base - LCL
Below cloud base,
rising dry surface air
cools at DALR
■ DP decreases by
about 5/9°C/1,000 ft
■ Rising parcels are
saturated at the LCL
Visual properties of clouds
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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?
Two general types:
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Cumuliform (cumulus): Puffy, cauliflower-like
clouds (associated with instability/convection)
■ Can
Dew
point
develop into TS if atmosphere conditionally
unstable
Cloud
base
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Dry
H
adiabat
■
Other visual properties:
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Surface
DP
Surface
temp
Stratiform (stratus): Layered clouds (associated
with stability)
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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)
■
What are the following cloud types?
Low clouds < 6,500 ft AGL
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Stratus (ST), stratocumulus (SC), nimbostratus
(NS), cumulus (CU), cumulonimbus (CB)
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Middle clouds 6,500 to 20,000 ft AGL
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High clouds > 20,000 ft AGL
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Nimbus=rain, cumulus=heaped,
stratus=layer, cirrus=high, alto=middle
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Altocumulus (AC), altostratus (AS)
Cirrus (CI), cirrostratus (CS), cirrocumulus (CC)
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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
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*Ceiling: lowest cloud layer having 5/8 or more coverage
Infrared (IR) image
Satellite imagery
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IR imagery
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Satellite detects IR radiation emitted by
earth and clouds
Works at night as well as day
Black means warm, white means cold
Visible imagery
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Effectively a B&W snapshot of reflected
visible light
Only available during daylight hours
ld
co
nt
fro
k
ea
W
Area of sea fog
(hard to see because
similar temp to sea)
High
clouds
Middle
clouds
Cirrus
clouds
Land is
warm
Visible (VIS) image
nt
fro
ld
o
C
Sea
Area
of fog
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