What is a cloud?
Dense mass
– suspended (supercooled) water or ice
particles
– on condensation / ice nuclei of solid matter
(sea salt, soil particles, organic matter,
pollutants, aerosols)
General cloud forms:
• Stratiform: layer
– layer of air forced to
rise over denser layers
• Cumuliform :
globular
– parcels of air forced to
rise above denser
surrounding air
Altitude classes:
• High Cloud (cirro-): ice crystal clouds
– > 7000 m (23,000 ft.)
– CIRRUS, CIRROSTRATUS, CIRROCUMULUS
• Middle Cloud (alto-)
• 2000 – 7000 m (6500 - 23,000 ft.)
• ALTOSTRATUS, ALTOCUMULUS
• Low Cloud (stratus)
• <2000 m (6500 ft)
• STRATUS, NIMBOSTRATUS, STRATOCUMULUS
• Vertically developed clouds (CUMULONIMBUS)
High clouds
• >23,000 ft.
• cirr- (“curl of hair”)
• ice crystals
– At high elevations in troposphere, cloud is
composed of all ice crystyals
• generally occur in fair weather and point in
direction of air movement at their elevation
• Cirrocumulus: clouds in long rows of puffy
clouds; often precursor to precipitation
“mackerel sky”
cirrocumulus
Cirrostratus
• Broad layer of air lifted by
large scale convergence;
transparent enough to
see sun or moon
• “halo” may form from
refraction of light by ice
crystals
• will thicken and lower if
warm front is approaching
cirrostratus
halo
Middle Clouds
• 6500-20,000 ft.
• alto• ice crystals and supercooled water
droplets
Altocumulus
• can be parallel bands or
rounded masses; can
distinguish from
cirrocumulus because
part of them is usually
shaded
• form by convection in an
unstable layer aloft
• may suggest an
approaching cold front
altocumulus
Altocumulus
Altocumulus
altocumulus
Altostratus
• Absence of shadows
• Sun or moon is bright
spot behind clouds, no
halo
Low Clouds
• <6500 ft.
• strat• water droplets
nimbostratus
stratocumulus
• Low, lumpy layer of
rounded masses, rolls,
sometimes with clear
sky between
Cumulonimbus
vertically developed
mammatus
Formed by sinking air in anvil
Lifting condensation level
Top & bottom elevations of clouds:
Bottom: lifting condensation level:
temperature of rising air reaches dew point
Top: Cloud rises until cloud temperature
reaches temperature of surroundings
(becomes stable).
Cloud thickness depends on thickness of
unstable layer.