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Clouds and Fog
Stability summary
• 3 types of environmental stability:
– Absolutely stable
– Absolutely unstable (very uncommon)
• Does occur in few tens of feet above parking lots &
roads in summer (mirages are often co-located)
– Conditionally unstable
• Conditional on when the rising air parcel reaches
saturation (i.e., the height of the LCL)
– If parcel can maintain its temperature WARMER than
environment, then it remains unstable
Examples in stability
1. Assume the environmental temperature cools at a rate of 4°C per 1000 m.
If the surface temperature is 24°C and the surface dewpoint temperature is
19°C, determine the atmospheric stability for surface air parcels lifted to 3
km. Will clouds form? If so, at what level?
2. Now assume the environmental temperature cools at a rate of 9°C per
1000 m. If the surface T is 31°C and the surface Td is 16°C, determine the
atmospheric stability for surface air parcels lifted to 3 km. Will clouds form?
If so, at what level?
3. Finally, assume the env. cools at 12°C/km. If sfc T is 11°C, sfc Td is 11°C,
determine stability. Will clouds form? If so, at what level?
Water vapor in the atmosphere and the concept of relative
humidity are important when we talk about the formation of
fog, clouds and precipitation
Fog - is actually a cloud formed or lying on
the ground
 Fogs form when air saturates (its relative humidity reaches
around 100 percent), and the water vapor within the air mass
condenses on small particles in the air to form liquid cloud
droplets.
 The air may become saturated either by lower its
temperature to its condensation (dew) point or by adding
water vapor into the air until it reaches saturation.
There are actually different types of fog in the sense that fog
can form under a variety of conditions
Radiation fog generally
forms when the air near
the surface cools to its
saturation temperature due
to radiational cooling at
night when the sun has set.
Advection fogs are
fogs formed when air
moves either over a
cooler surface or over
a warmer, moist
surface, and as a result
the air mass reaches
saturation.
Most often this occurs when a moist
air mass moves over a cold surface,
such as a large, cold body of water or
snow/ice cover, whose temperature is
below the dew point of the advecting
air mass, and its lowest reaches are
cooled to condensation.
Upslope and valley fogs
are two special cases of
advection fogs particular
to hilly terrain. They form
when air moving in hilly
or mountainous terrain
cools to condensation.
Frontal fog, which may also be called precipitation fog.
This fog type generally occurs when rain falling from warm air aloft
evaporates at or near the surface under light wind conditions. The
evaporating precipitation as it falls through colder air thus increasing
the surface air's moisture until condensation is achieved. Such fogs
are most common in the vicinity of warm or stationary fronts, but
they can form at cold fronts as well. However, cold fronts generally
move and mix too quickly to allow the condition to persist for long.
CLOUDS
In 1802 an Englishman by the name of Luke Howard
invented the cloud naming system that is still in use today.
Howard used Latin names to describe clouds.
• part of a cloud's name describes height
• part tell us something about the cloud’s shape
 The prefixes denoting heights are: cirro, high clouds above
20,000 feet; alto are mid level clouds between 6,000 - 20,000
feet. There is no prefix for low level clouds.
 The names denoting shapes are:cirrus mean curly or
fibrous, stratus means layered, while cumulus means lumpy
or piled. Nimbo or nimbus is added to indicate that a cloud
can produce precipitation.
Cumulus clouds have a flat base and fluffy tops, and range in size from
very shallow to very large, with a dense, mound-shaped top that looks
like a large cauliflower. Where the sun shines on these clouds, they are
a brilliant white; while their sides opposite the sun are darker, because
they are in shadow. Cumulus clouds are often at least as tall as they are
wide, if not taller.
Cumulus clouds are
classified as low
clouds
Stratus clouds are usually gray seen from below, fairly uniform
horizontally, and lie close to the surface of the Earth. They usually
look like a solid layer but are sometimes found in patches. Stratus
clouds can produce light precipitation. They are usually water
clouds. When the sun is visible through stratus clouds, its edges are
distinct. Stratus is classified as a low cloud.
Nice, flat, uniform lighting
throughout the cloud and snow, and
trees and building make a good
contrast. Photo from Pequot Lakes
Elementary School, Pequot Lakes,
MN. January 2003
Stratocumulus clouds are gray or whitish. Both the tops and bottoms
of these bases are rounded. They can look like waves on the ocean,
joined flat cumulus, or a continuous but irregular layer. They can form
from spreading out cumulus clouds, or from a stratus layer. The
individual cloudlets are wider than two fingers at arm's length.
Stratocumulus is classified as a low cloud.
Very opaque and mostly cloudy
stratocumulus. Notice how the cloud bases
are all about the same height. Photo taken
by Doug Stoddard in March 2002, in St.
Thomas.
Generally translucent stratocumulus clouds.
Mostly cloudy to overcast cloud cover. Photo
by Kevin Larman.
Nimbostratus
A dark, gray cloud
characterized by more or
less continuously falling
precipitation. It is not
accompanied by lightning,
thunder, or hail.
Altocumulus clouds take many forms. They can look like waves on
the sea, tiny cumulus, balls of cotton, or even little castle towers. The
individual "cloudlets" are usually between one and two fingers wide
at arm's length. They are thick enough to have gray bases.
Altocumulus clouds are almost always made up of water droplets, but
sometimes contain ice crystals as well.
Altocumulus is
classified as a
middle cloud.
Photo by Mandana Khaiyer,
March 1997, in San Antonio,
TX. This is a nice picture of
altocumulus, showing a
tendency to form cloud 'streets'
(i.e., rows of cloud), which is
an indicator that there is some
wave phenomenon occurring in
the atmosphere.
Altostratus clouds form a bluish or grayish veil that totally or
partially covers the sky. The sun can barely be seen through
altostratus and its edges are not distinct. Altostratus clouds are water
or ice clouds, and they do not produce halos.
Altostratus
is classified
as a middle
cloud.
Compare to stratus and
cirrostratus photos to
contrast the opacity
(amount of light
reduction) by these
clouds. Photo by Lin
Chambers.
Cirrus clouds look like white delicate feathers or horse tails. They
are generally white, wispy, and fibrous. Cirrus clouds are made up of
ice crystals.
Cirrus is classified as a high cloud.
This photo shows transparent to translucent cirrus
streaks, and illustrates why cirrus clouds are often
described as mare's tails. The streaks come from
falling ice particles that make up the clouds.
Photo by Lin Chambers, August, 2002.
Feathery cirrus turned pink by the setting
sun, with some very distinct persistent
contrails running through the sky. Photo by
Ed Donovan, South Carolina, July 2003.
From http://asd-www.larc.nasa.gov/
Cirrocumulus clouds are thin white layers that look like patches of
cotton or ripples. They are not shaded. They are usually made up of
ice crystals.
Cirrocumulus is classified as a high cloud.
Photo by Jeff Caplan, NASA Photographer in
1997. This is an example of fairly largecelled cirrocumulus, mostly cloudy and
pretty much translucent.
Photograph taken just after sunset, about 8:30
pm, on July 8, 2005 by Dave Kratz in Poquoson,
Virginia.
Cirrostratus clouds are a thin, transparent, whitish layer made up
of ice crystals. They may partially or totally cover the sky.
Cirrostratus clouds sometimes produce haloes around the sun or
moon. Sometimes patches of rainbow-colored light called
"sundogs" occur on the halo on both sides of the sun. Cirrostratus
is classified as a high cloud.
Photo taken by Doug Stoddard in
March of 2002 in Puerto Rico
The upper right of the picture shows a
cirrocumulus, but the portion where the sun
is shining is a nice uniform cirrostratus.
Photo from Dr. Francisco Vazquez Colon
School, Manati, Puerto Rico. January 2003
The most likely cause of the
cloud lines stems from the
exhaust of ocean going
vessels. Large numbers of
Aitken (CCN) nuclei form in
this exhaust. These are
carried upward by the
buoyancy of the hot gases
and ship’s air wake to form
droplets at slight
supersaturation. The
phenomenon does not appear
related to special
characteristics of the vessel's
power plant but to a critical
condition of the atmosphere.
This condition may be
described as having 1) a
convectively unstable layer
from the surface to a lowlevel stable layer, 2)
saturation or slight
supersaturation near the top
of the convective layer, and 3)
a convective layer,
presumably deficient in CCN
Cumulonimbus are large clouds, with a generally flat, dark base and large tops
with the shape of an anvil or giant cauliflower starting to flatten and become
fibrous at the top or sides. Mature cumulonimbus clouds have strong updrafts; the
flat top of the anvil indicates where the updraft is no longer buoyant and the air
spreads outward horizontally. The temperatures at the top of the cumulonimbus are
well below freezing: the fibrous structure indicates the presence of ice crystals.
Cumulonimbus clouds are made up of a mixture of water droplets (lower parts of
the cloud), ice crystals (upper parts of the cloud), and precipitation.
Cumulonimbus are classified as "clouds with vertical development"
since their tops extend quite high.
Unusual Cloud Formations
NASA photo
Rows of clouds stream over
the Bering Sea from the
edge of the sea ice. Called
cloud streets, these cumulus
clouds form when cold air
from the ice blows over the
open ocean, chilling the
moist air. As the temperature
drops, water freezes into tiny
clouds, which are arranged
in neat rows in line with the
powerful sweep of the wind.
Though some clouds form
over the cracking sea ice on
the right side of the image,
most are over the unfrozen
water.
Unusual Cloud Formations
Lenticular Clouds
These clouds are
formed by mountain
waves of air created
by strong winds
forced over high
mountains.
A stack of lenticular clouds caps and blocks the view of the summit
of Mount Rainier. From http://www.livingwilderness.com/
mountains
Unusual Cloud Formations
Wave Clouds
Kelvin-Helmholtz wave clouds are formed when there are two parallel layers of air that are
usually moving at different speeds and in opposite directions. The upper layer of air
usually moves faster than the lower layer because there is less friction. In order for us to
see this shear layer, there must be enough water vapor in the air for a cloud to form. Even
if clouds are not present to reveal the shear layer, pilots need to be aware of invisible
atmospheric phenomenon.
PRECIPITATION
The average cloud
droplet is small
enough to stay
suspended in the
atmosphere by the
motion of the air.
In order to fall as
precipitation, these
liquid (or solid) water
particles must grow
much larger so that
gravity dominates.
But how do we go
from something the
size of a cloud
droplet to something
2 orders of
magnitude larger?
Cloud droplets are so small it may take a million or
more to produce a single raindrop.
How do clouds really form?
Humid air, purified of all foreign nuclei, can be expanded in
cloud chambers to relative humidities of the order of 400%
without any condensation taking place!! Why? Surface tension of
a water molecule is very high and tension must be overcome to
form small cloud droplet. This type of condensation is called
homogeneous nucleation.
However, cloud condensation occurs in our atmosphere at relative
humidities near 100% only because nature provides an abundance
of condensation nuclei. This process is called heterogeneous
nucleation.
• 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
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)
Type of
particle
Approximate
radius (μm)
# of particles per cubic
centimeter*
Range (typical)
Small (Aitken)
< 0.2
1000 – 10,000
(1000)
Large
0.2 – 1.0
1 – 1000
(100)
Giant
> 1.0
<1 – 10
(1)
Fog & cloud
> 10.0
10 – 1000
(300)
*Note: 1 cm3
is about the
size of your
thumb!!!
There are two basic mechanisms by which precipitation
forms in clouds:
 “Warm rain“ or collision-coalescence
 “Ice crystal" or Bergeron processes
The term "warm rain" was derived after scientists noticed
that rain in tropical regions often fell from clouds with
temperatures never colder than 32°F (0°C).
1) There must be a high liquid water content within the
cloud.
2) There must be sufficiently strong and consistent updrafts
within the cloud.
3) A large range of cloud droplet sizes is very helpful.
4) The cloud must be thick enough so that the cloud
droplets have enough time to gather surrounding smaller
droplets.
Typical raindrop path:
Starts as very small cloud
droplet
Rises (relatively slowly)
through the cloud
Collision-coalesence occurs
both in the UP and DOWN
paths of water drop
Collision-coalesence
-Occurs in clouds
with temps > -15C
-Larger drops fall
faster than smaller
ones
-Drops collide &
merge (coalesce)
The Bergeron Process, named after its discoverer, Tor
Bergeron, involves supercooled water droplets.
 The term supercooled, refers to water that is still a liquid
at temperatures below the freezing point
 Cloud droplets have been observed to remain liquid at
temperatures well below freezing (as low as -40 C)! Why??
Large amount of kinetic energy must be removed to initially
form the lattice structure of ice crystal (to transform liquid
water into ice).
But: ice crystals routinely form between -5C and -15C.
Why? Presence of ice nuclei (larger than CCN). Ice
nucleus impacts supercooled cloud droplet, droplet
immediately freezes onto it.
This process assumes that ice crystals and very small pure
water cloud droplets (supercooled liquid) and water vapor all
are present in the same region of a cloud.
 Tiny ice crystals, in a
supercooled cloud, grow larger at
the expense of the surrounding
liquid cloud droplets.
 After growing large enough,
the crystals fall from the clouds
as precipitation.
 This precipitation formation
process is more common at
higher altitudes as well as at
middle and high latitudes.
Bergeron Process
-- If we can get small ice
crystals to form (not trivial,
needs presence of “ice nuclei” in
upper parts of cloud), then
--water moves, in vapor phase,
from supercooled liquid droplet
onto crystal structure
Notice the small droplets have evaporated and redeposited
onto the ice crystal
Four types of “winter precip”
1. Snow
2. Sleet
3. Freezing rain
4. Rain
The type of precip we see at the
surface is highly dependent on
the vertical temperature
structure of the atmosphere in &
below the cloud:
-Air is < 0C entirely: snow
-Air is > 0C near sfc: rain
-Warm layer above sfc: sleet or
freezing rain
SNOW
The green dashed
line is the
temperature in
respect to elevation.
The surface
temperature is 25°F
and increases with
height before
decreasing.
However, since the
temperature remains
below freezing any
precipitation that falls
will remain as snow.
Snow along the Trans Labrador Highway in Quebec, Canada
SLEET
Note that the surface
temperature in this figure it
27°F. Also as elevation
increases, the temperature
increases to a point where some
of the atmosphere is above
freezing before the temperature
lowers again below freezing.
As snow falls into the layer of air
where the temperature is above
freezing, the snow flakes
partially melt. As the
precipitation reenters the air that
is below freezing, the
precipitation will re-freeze into
ice pellets that bounce off the
ground, commonly called sleet.
The most likely place for
freezing rain and sleet is to the
north of warm fronts. The cause
of the wintertime mess is a layer
of air above freezing aloft.
FREEZING RAIN
Freezing rain will occur if the
warm layer in the atmosphere
is deep with only a shallow
layer of below freezing air at
the surface. The precipitation
can begin as either rain and/or
snow but becomes all rain in
the warm layer. The rain falls
back into the air that is below
freezing but since the depth is
shallow, the rain does not have
time to freeze into sleet.
Upon hitting the ground or
objects such as bridges and
vehicles, the rain freezes on
contact. Some of the most
disastrous winter weather
storms are due primarily to
freezing rain.