Transcript Moisture, Fog, and Clouds

Clouds
ATS351
Lecture 5
Outline
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Review CCN
Fog
Clouds Types
Use of Satellite in Cloud Identification
Cloud Condensation Nuclei
• Aerosols may become CN; and CN may become CCN
– But not all CN are CCN
– Larger concentrations over land
• Cloud droplets can form on both insoluble and soluble particles
• A particle that will serve as CCN is called hygroscopic or
hydrophilic
– condensation may occur at RH <100%
• A particle that will not serve as a CCN is called hydrophobic.
– condensation may occur at RH >100%
• CCN Sources may include:
– dust, volcanoes, factory smoke, forest fires, sea salt
Cloud Condensation Nuclei
• Described by the size of the particle
Haze
• Dry haze
– Just the small particles
themselves, maybe a little
bit of condensation
• Wet haze
– Begins at RH ~ 75%
– Much more light scattering
than dry (3x)
Fog
• Going above 70% RH to 100%
• Condensation on less active nuclei
• Essentially, a thick wet haze on the ground or a low
cloud
• Called fog when visibility is less than 1 km
• 4 Main Types
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Radiation
Advection
Upslope
Evaporation/Mixing
Fog
• Type of CN affects fog
• Over the ocean
– Fewer, larger drops
• Over urban areas
– More, smaller drops
– Lower visibility
– London Fog
• Chemical reactions can
cause fog to become acidic
• If temperatures drop below
freezing, freezing fog
may result
Radiation/Ground Fog
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Surface radiatively cools
Light breeze helps more air contact cold surface
Common in the fall and winter (associated with ridges)
Common in low-lying areas
– River valleys
• Form upward from the ground
– Deepest around sunrise
– May intensify after sunrise (dew evaporation)
• “Burns Off” with more insolation
– Dissipates from bottom up
– Dissipates easily around edges (thin, mixing)
Advection Fog
• Warm air moves over a cold
surface.
• Breeze required
• Pacific Coast Cold Current
• Gulf Stream and British Isles
• May combine with radiation fog
Upslope Fog
• Moist air rises up the side of a
mountain
• Just like a cloud forming, but
touching the surface
• Would be seen on western slopes
(not around Fort Collins as much
except in the case of easterly
winds and associated upslope)
• Can last for days under
the right conditions
Evaporation (Mixing) Fog
• Just like breathing out when
it’s cold
• Cold air over warm water
(steam fog)
– Seen over a pool or spa
• Increased moisture raises the
dew point
– Maintained through vapor
pressure difference and mixing
• Steam devils on a lake
• Precipitation Fog
Foggy Weather
Annual number of days with fog in the US
Clouds
• Clouds result when saturation occurs
• Properties:
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Variety of sizes
Contain water drops and/or ice crystals
Various heights
Stratospheric clouds rare, but important for creation
of the ozone hole
Cloud Classification
Cirriform Clouds
Usually exist above 16,000 feet
Generally thin, sometimes partially translucent
Comprised of ice crystals
Absorb longwave radiation, but highly reflective of
visible radiation (high albedo)
• Rarely precipitate
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– Virga
• Examples:
Cirrus (Ci): Called “mares tails”
Wispy
Cirrostratus (Cs): Usually present when halos
around the sun are observed
Halo
Cirrocumulus (Cc): Called “fish scales” or
“mackerel sky”
Stratiform clouds
• Characterized by a horizontally uniform
base (Layered)
• May or may not precipitate
• May exist at any level
Stratus (St): Uniform and gray, maybe lifted
fog
Stratocumulus (Sc): Small “empty” spaces,
large coverage
Nimbostratus (Ns): Patchier and rainier than
regular stratus
Marine stratus
Cumuloform clouds
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Large vertical extent
May or may not precipitate
Result from vertical motion
Cumulus (Cu) Examples:
Cumulus Humilis: “Fair weather”
Cumulus Congestus: Towering Cumulus
Cumulus Congestus
Cumulonimbus: Anvil Cloud
Other cloud types
Mammatus clouds
• Precipitation
evaporates out of
anvil
• Evaporation cools
the air and it sinks
• If drops are large,
mammatus will be
long lived
Fractus Clouds
• Also known as scud
clouds
• Low, detached
clouds caught in the
outflow of a
thunderstorm
• Can also be seen
below stratus
clouds.
Pyrocumulus
• Caused by fire,
volcano or industry
• Caused by intense
heating of moist air
• Only forms in Calm
wind situations
Contrails
• Condensed
exhaust from
jet aircraft
• Important when
considering
climate effects
of clouds
Cloud Formations
• Lenticular
• Kelvin-Helmholtz
waves
• Cloud streets
• Wall Clouds
• Shelf Clouds
Lenticular Clouds
• Stationary, lens-shaped
clouds over mountains at
high altitude
• Stable, moist air flows over
mountain, creating a large
scale standing wave
• Indicates region of
turbulence
Kelvin-Helmholtz Waves
• Form when two parallel layers of air are
moving at different speeds and in different
directions
• Upper layer is usually faster
• Very short lived
Cloud Streets
• Form due to horizontal rolls in the
atmosphere
• Also due to uneven surface heating
• Clouds form over updrafts in rolls
• Occurs more frequently over the ocean
Shelf and Roll Clouds
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Low, horizontal, wedge-like cloud
Shelf: Attached to Parent Storm
Roll: Removed from Parent Storm
Formation is due to gust front from thunderstorms
Wall Cloud
• Associated with severe thunderstorms
• Indicates area of strongest updraft
• The strongest tornados form here
Really High Clouds
• Nacreous Clouds (mother of pearl)
– Form in stratosphere
– Seen best at polar latitudes in winter
– Composition not well known
• Noctilucent Clouds
– Sometimes seen in the mesosphere
– Stars shine through them
– Made of tiny ice crystals
Nacreous Clouds
Nacreous Clouds
Noctilucent Clouds
From Surface
From Space
Satellite Imagery
• Visible imagery: essentially a black and
white camera on a satellite. Measures
brightness in the visible spectrum.
• Infrared imagery: measures infrared
radiance from the surface it is pointed
at. From blackbody theory, the
temperature of the surface can be found;
since temperature changes with height,
the cloud-top height can then be
estimated.
Visible Satellite
• Pros
- good at showing low
clouds and fog
- available in high spatial
resolution
• Cons
- only works in daylight
- clouds can be confused
with reflective features
like snow
- optically thin clouds
like cirrus don’t show as
well
IR Satellite
• Pros
- available at all hours
- provides an estimate of cloud-top height
• Cons
- lower spatial resolution
- low clouds don’t show because their temperatures are close to
the surface temperature
• Color enhancement table often applied to bring out important
temperatures
Raw
Enhanced
Clouds and Satellite Imagery
• The bright, puffy areas in the visible image on the
right are cumulus and cumulonimbus clouds (the
cumulonimbus are fuzzier around the edges).
Notice how the cloud tops over the Front Range are
cold in the IR imagery
Cirrus in visible vs. IR
• Because cirrus are cold and optically thin
(meaning the sun can be seen through the
cloud), they are more easily seen in the IR than
the visible
Low clouds/fog in visible vs. IR
• Because low clouds are bright and warm, they
are easily seen in the visible, but not the IR