Download Formation of Dew and Frost

Condensation: Dew, Fog, and Clouds
Formation of Dew and Frost
• Dew - forms when the temperature cools to the dew-point
temperature
– Dew/frost often form only close to the ground, e.g., not on a bush,
why?
– Dew and frost most often form on clear, calm nights..., why?
• Dew can be an
important source
of moisture during
periods of low rain
fall.
1
If T = Td < 32°F, get frost; Dew point is called the
frost point.
Freeze and Black frost: In very dry weather, the air temperature
may become quite cold and drop below freezing without ever
reaching the frost point, and no visible frost forms.
These are the delicate ice-crystal patterns that frost exhibits on a window during
a cold winter morning
Condensation Nuclei
Key for the Formation of Haze, Fog, and Clouds
• The process of condensation of vapor -> water to
form a cloud drop is not that simple in the
atmosphere
• NEED Condensation Nuclei to form cloud drops
• Condensation Nuclei:
– small particles in air created from/by:
•
•
•
•
•
•
dust
volcanoes
factory smoke
forest fires
ocean salt
sulfate particles from phytoplankton in ocean
– They are most abundant in lower troposphere over urban
areas
– They are quite small relative to a rain drop or cloud drop
2
Sizes and Amounts of CCN
• Note: 1 cm3 is about the size of your thumb!
• Total mass of CCN put into atmosphere each
year is about 2x1012 kg!
• Two types of CCN:
– Hydroscopic - water seeking - H2O readily condenses
on these
• ocean salt is a good example (sticky salt shaker when humid)
– Hydrophobic- water repellant - H2O does not readily
condense on these (wax on car)
3
Formation of Haze
• Two types:
– dry haze - large/giant particles in the air (smoke, smog,
dust)
– wet haze - H2O condenses onto hydroscopic CCN - can
occur at RH's as low as 75%
– wet haze has a dull gray, white color
Fog introduction
• Forms as the RH increases to 100% - haze
particles grow into fog (cloud) particles near
the ground.
• Fog is really a cloud near the ground
• International definition: Visibility is less than
1 km
• NWS definition: Visibility is less than or
equal to 6 miles and T-Td < 5°F
4
Fog in polluted
areas can be a
health problem
since it becomes
acidic
Formation of Fog
• Fog forms in one of two general ways:
– Air is cooled below its saturation point
– By evaporation and mixing
• A fog droplet with a diameter of 25 μm settles
toward the ground at about 5 cm/s (2 in./s).
– Fog is maintained by new fog droplets.
• Types of Fog:
–
–
–
–
Radiation Fog
Advection Fog
Upslope Fog
Steam Fog
• Let's look at how each of these are formed......
5
Formation of Radiation Fog - Radiational Cooling
• Need:
– shallow, moist air near surface
– clear/calm nights
– although light winds will bring more air in contact with
ground
• Radiational cooling allows the temperature to drop
to the dew point.
Also called
ground fog
Formation of Radiation Fog - T = Td
• Once the temperature reaches the dew point,
radiation fog begins to develop
– Common in the fall - especially when our weather is
dominated by high pressure
– often forms in valleys first since this is where the coldest air
is - called valley fog
• Q: Do radiation fogs develop upward or downward?
• Q: When is radiation fog the thickest?
6
Visible satellite image of
dense radiation fog in the
southern half of California’s
Central Valley on the
morning of November 20,
2002. The white region to
the east (right) of the fog is
the snow-capped Sierra
Nevada range. During the
late fall and winter, the fog,
nestled between two
mountain ranges, can last
for many days without
dissipating. The fog on this
day was responsible for
several auto accidents,
including a 14-car pileup.
Formation of Advection Fog
• Common off the west coast of the U.S.
– Cold current along coastline and warm water further to the
west
– westerlies moving in from the warmer water to the colder
water
– westerlies advect warm moist air over colder water
• Heat is transferred from the warm, moist air to the
cold water near the coast via conduction
• The parcel reaches saturation - forms fog, and is
advected onshore
• NEED a light breeze for this process to occur
7
Here advection fog, having formed over the cold, coastal
water of the Pacific Ocean, is rolling inland past the Golden
Gate Bridge in San Francisco. As fog moves inland, the air
warms and the fog lifts above the surface. Eventually, the air
becomes warm enough to totally evaporate the fog.
• Advection fog can be an important source of
moisture for plant life along the CA coast - it
rarely rains there during the summer months
8
Other favorite places to observe
advection fog
• Where two ocean currents
with different temperatures
flow next to one another.
• Warm moist air from the
gulf of Mexico moves
northward over colder
land.
• Ice fog in arctic:
Along an irregular coastline, advection fog is more likely to form
at the headland where moist surface air converges and rises
than at the beach where air diverges and sinks.
9
Upslope Fog
• Take a parcel over the Gulf of Mexico and
move it towards Denver...
• As the parcel ascends up the slope, it
expands and cools to the dew point
• Upslope fog/clouds then form
– Need winds to move air up the slope
– Need a slope!
Steam Fog
• Seen over lakes or heated pools in winter
• Form it when cold air over a warm water (lakes)
• Heat and moisture are transferred from the warm water to
the cooler, drier air and the transfer occurs in a shallow
layer near the lakes surface
• Hence, you have an unstable situation with warm,
saturated air at the lake's surface below cooler air....
• the air rises forming steam fog
10
Even in summer, warm air rising above thermal pools in
Yellowstone National Park condenses into a type of steam fog.
Fog That Forms by Mixing
11
Distribution of Foggy Weather around the U.S.
• Where is it foggy?
– Pacific Coast
– Appalachian highland region
– New England
• Foggiest spot in the U.S.: Cape Disappointment, WA
– it's foggy for 2556 hours per year, or about 107 days.
• Other foggy locations:
– Mistake Island - off coast of
Maine - 1580 hours per year
– Mt. Washington!! - experience
fog on about 300 days each
year!
• Fog can be a significant
forecast problem:
– aviation
– ground transportation
– maritime activities
Introduction to cloud types
• Introduced in 1803 by Luke Howard, an English naturalist;
modified by Abercromby and Hildebrandsson in 1887.
• Clouds are comprised of liquid droplets of various sizes and/or
ice crystals
• They are characterized according to their height location in the
atmosphere and their vertical development:
12
Cloud base height changes with latitude !
High Clouds - Cirrus (Ci)
• High clouds are comprised
largely of ice
• Cloud-base heights for high
clouds:
– Tropical Region (6-18 km)
– Middle Latitudes (5-13 km)
– Polar Regions (3-8 km)
• Cirrus Clouds - high, thin
wispy clouds up at jet stream
level in the upper
troposphere
• They are almost always
comprised of ice
• associated with fair weather
13
Cirrostratus Clouds
• High, thin, sheet-like clouds
• Produce halos around the sun/moon
• Rain or snow is often 12/24 hours away
Cirrocumulus clouds.
• High, thin clouds appear as small, rounded,
white puffs.
• May occur individually or in long rows.
• When in rows, is has a rippling appearance.
14
Middle Clouds - Altocumulus (Ac)
• Middle clouds are composed of water and/or ice
• Cloud-base heights for middle clouds:
– Tropical Region 2-8 km
– Middle Latitudes 2-7 km
– Polar Regions 2-4 km
• Altocumulus
Clouds - shallow,
puffy or wave-like
in appearance
• appear the size of
your thumbnail
when holding your
arm up to the sky
Middle Clouds - Altostratus (As)
•
•
•
•
Altostratus Clouds - grayish/blue-gray
thin layer covering entire sky uniformly
found ahead of storms
can seen the sun through altostratus, but NO
halo will be observed
15
Low Clouds - stratus (St)
• Cloud-base heights for middle clouds:
– Tropical Region 0-2 km
– Middle Latitudes 0-2 km
– Polar Regions 0-2 km
• Stratus Clouds - Uniform grayish cloud covering the
entire sky
• Common in the Northeast Kingdom
Low Clouds - Nimbostratus (Ns)
• Nimbostratus Clouds - darker gray, "wet" looking
low clouds
• they produce light/moderate precipitation over a
large region
16
Low Clouds - Stratocumulus (Sc)
• Stratocumulus Clouds - low, lumpy, puffy clouds
in patches or rounded masses
• Visually appear larger than altocumulus
• Appear the size of your fist when holding your arm
up to the sky
Vertically Developed Clouds - Cumulus (Cu)
• Cumulus Clouds - Look like cotton
balls/cauliflower in the sky
• sub categories of cumulus:
– cumulus humilis - slightly developed Cu
– cumulus congestus - moderately developed
17
Vertically Developed Clouds - Cumulonimbus (Cb)
• Cumulonimbus Clouds - thunderstorms
– develop from growing Cu
– can extend up to the troposphere
– can contain both water and ice
• contains precipitation
(rain, snow, hail, etc)
• produce lightning and
severe weather
• form a distinctive
"anvil" cloud at the top
of the storm
Anvil
18
Summary of Cloud Types
Other unusual clouds - Lenticular Clouds
• Lenticular Clouds - form as air flows over
mountains
• look like pancakes, UFOs even
19
Other unusual clouds - Cap Cloud
Cap Cloud - forms as air is forced up and
over a mountain
Other unusual clouds - Pileus
• Pileus - forms as a growing thunderstorm
deflects moist air up and over the top of the
building cumulus congestus or cumulonimbus
20
Other unusual clouds - Mammatus
•
•
•
•
Mammatus - look like cow's udder
visually very impressive
usually form underneath Cb
visual manifestation of sinking air, not rising air!
Other unusual clouds - Contrails
• Contrails: are formed from the exhaust of highflying aircraft
• are similar in makeup to cirrus clouds
• have an important impact on the radiation balance
for the earth.
21
Nacreous clouds.
• They form in the stratosphere and are
most easily seen at high latitudes.
Noctilucent clouds
They are usually observed at high latitudes, at altitudes between
75 and 90 km above the earth’s surface.
22
Other unusual clouds - Scud
• Scud - ragged low clouds beneath actual
cloud base
• Often form due to turbulent mixing of air:
– warm air from the updraft
– cool air from the downdraft
Ground-Based Description of Sky Conditions
23
Cloud base measurements with laser ceilometer
Satellite Observations of Clouds
• First Meteorological Satellites were launched in late
1950's
• Today, we use two types of satellites to monitor
weather:
– Geostationary
– Polar orbiting
24
Geostationary Satellites - GOES
•
•
•
•
Geostationary satellites are positioned over the equator
They orbit the earth at the same rate as the earths rotation
This allows for a continuous view of the same location
The U.S. has two geostationary satellites:
– GOES-E -->>
• located at 0°N, 75°W
– GOES-W -->>
• located at 0°N, 135°W
http://goes.gsfc.nasa.gov/
25
Q: What are the advantages and disadvantages of
the GOES satellites?
Polar Orbiting Satellites
•
•
•
•
Orbit in a nearly N-S manner around the globe
Nearly pass over the poles
1 orbit takes about 120 minutes
Q: What are the advantages and disadvantages
of the polar orbiting satellites?
Time 1
Time 2
26
Types of Satellite Data
• Satellites collect two primary types of
meteorological imagery:
– Visible
• basically a picture of the clouds - much like taking
a picture with your camera
– Infrared - tells you the temperature of what
the satellite is sensing, whether its the ground
or the tops of clouds
• very useful for determining the height of the cloud
(low, middle, high)
Infrared image of the
eastern Pacific
27
A visible image of the
eastern Pacific
Ocean surface temperature
measurements with satellite
28
Infrared water vapor image
Get Hurricane cloud and
precipitation structure from Satellite
29