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Clouds & Precipitation
What is required?
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Humidity
Measuring humidity – Relative
humidity –
•Ratio of the air's actual water vapor
content compared with the amount of
water vapor required for saturation at
that temperature (and pressure)
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• Relative humidity can be changed in
two ways
•Add or subtract moisture to the air
•Adding moisture raises the relative
humidity
•Removing moisture lowers the
relative humidity
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• Changing the air temperature
• Lowering the temperature raises the relative
humidity
• Dew point temperature
•Temperature to which a parcel of air would
need to be cooled to reach saturation
• Cooling the air below the dew point causes
condensation
• e.g., dew, fog, or cloud formation
• Water vapor requires a surface to condense on
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Typical daily variations in
temperature and relative humidity
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Adiabatic heating/cooling
Adiabatic temperature changes occur when
1. • Air is
a. • Motion of air molecules
b. • Air will
c. • Descending air is compressed due to
air pressure
2. • Air
3. • Air parcel does work on the surrounding air
4. • Air will
5. • Rising air will expand due to
pressure
air
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Adiabatic cooling of rising air
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Other Processes that lift air
lifting
• Elevated terrains act as barriers
• Result can be a rainshadow desert
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Frontal
• Cool air acts as a barrier to warm air
• Fronts are part of the storm systems called
middle-latitude cyclones
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Processes that lift air where the air is flowing
together and rising (low pressure)
Localized
lifting
• Localized convective lifting occurs where
unequal surface heating causes pockets of
air to rise because of their buoyancy
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Stability of air
Types of stability
• Stable air
• Resists vertical displacement
• Cooler than surrounding air or
•Denser than surrounding air
• Wants to sink
• No adiabatic cooling
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stability
• occurs when the
environmental lapse
rate is less than the
wet adiabatic rate
• Stable air
instability
• Environmental lapse
rate is greater than the
dry adiabatic rate
• Often results in
widespread clouds with
little
• Continues to rise until
it reaches an altitude
with the same
temperature
• vertical thickness
• Adiabatic cooling
• Precipitation, if any, is
light to moderate
• Clouds are often
towering
• Warmer & Less dense
than surrounding air
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Conditional instability
• occurs when the atmosphere is stable for an
unsaturated parcel of air but unstable for a
saturated parcel
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Condensation and cloud formation
• Water vapor in the air changes to a liquid
and forms dew, fog, or clouds
• Water vapor requires a surface to
condense on
• Possible condensation surfaces on the
can be the grass, a car window, etc.
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• Possible condensation surfaces in the
are tiny bits of particulate matter
• Called
• Dust, smoke, etc
• Ocean salt crystals which serve as
hygroscopic ("water seeking") nuclei
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Clouds
• Made of millions and millions of
minute
droplets, or
• Tiny crystals of
• Classification based on
and
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Form (three basic forms)
•
– high, white, thin
•
- globular cloud masses often
associated with fair weather
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– sheets or layers that cover much
of the sky
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Height
• High clouds - above 6000 meters
•Types include cirrus, cirrostratus, cirrocumulus
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Height
• Middle clouds – 2000 to 6000 meters
• Types include altostratus and altocumulus
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Height
• Low clouds – below 2000 meters
• Types include stratus, stratocumulus, and
nimbostratus (nimbus means "rainy")
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• Clouds of
development
• From low to high
altitudes
• Called
• Often produce
rain showers and
thunderstorms
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Cirrus Clouds - thin and wispy
•The most common form of high-level clouds are
thin and often wispy cirrus clouds.
•Typically found at heights greater than
(6,000 meters),
feet
•cirrus clouds are composed of ice crystals that
originate from the freezing of super-cooled water
droplets.
•Cirrus generally occur in
weather and point in
the direction of air movement at their elevation.
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Cirrus can form from almost any cloud that has
undergone
and can be observed in a
variety of shapes and sizes.
Possibilities range from the "finger-like"
appearance of cirrus fall streaks to the uniform
texture of more extensive cirrus clouds
associated with an approaching warm front.
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Clouds
parallel bands or rounded masses
Altocumulus clouds are composed
primarily of
droplets and are located
between
and
feet (2,000 to
6,000 meters) above the ground.
Altocumulus clouds usually form by
in an
unstable layer aloft, which may result from the gradual
lifting of air in advance of a
.
The presence of altocumulus clouds on a warm and
humid summer morning is commonly followed by
thunderstorms later in the day.
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Clouds
•dark, low-level clouds with
•Nimbostratus are dark, low-level
clouds accompanied by light to
moderately falling precipitation.
•Low clouds are primarily composed
of water droplets since their bases
generally lie below
feet (2,000
meters).
•However, when temperatures are cold
enough, these clouds may also
contain
particles and
.
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•The sun (or moon) is not visible through
nimbostratus clouds, which distinguishes them from
mid-level altostratus clouds.
•Because of the fog and falling precipitation
commonly found beneath and around nimbostratus
clouds, the cloud base is typically very diffuse and
difficult to accurately determine.
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Fair Weather Cumulus Clouds
•puffy cotton balls floating in the sky
•Fair weather cumulus have the appearance of
floating cotton and have a lifetime of
minutes.
• Known for their flat bases and distinct outlines, fair
weather cumulus exhibit only slight vertical growth,
with the cloud tops designating the limit of the rising
air.
•Given suitable conditions,
however, harmless fair weather
cumulus can later develop into
towering
associated with powerful
thunderstorms.
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•Fair weather cumulus are fueled by buoyant bubbles of air,
or thermals, that rise upward from the earth's surface.
•As they rise, the water vapor within cools and condenses
forming cloud droplets.
•Young fair weather cumulus have sharply defined edges
and bases while the edges of older clouds appear more
ragged, an artifact of cloud erosion.
•Evaporation along the cloud edges cools the surrounding
air, making it heavier and producing sinking motion (or
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subsidence) outside the cloud.
Clouds
•reaching high into the atmosphere
•Cumulonimbus clouds (Cb) are much larger and
more
developed than
•They can exist as
towers called a
or form a line of
.
•Fueled by vigorous convective
(sometimes in excess 50
knots), the tops of cumulonimbus
clouds can easily reach
feet (12,000 meters) or higher. 36
•Lower levels of cumulonimbus clouds consist mostly of
water droplets while at higher elevations, where
temperatures are well below 0 degrees Celsius, ice crystals
dominate.
•Under favorable atmospheric conditions, harmless
clouds can quickly develop into large
cumulonimbus clouds associated with powerful
thunderstorms known as
.
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are large thunderstorms with deep rotating
updrafts and can have a lifetime of several hours.
•Supercells can produce frequent
, and
.
, large
,
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Fog
Considered an atmospheric hazard
with its base at or near the ground
Most fogs form because of
• Radiation
, or
• Movement of air over a
surface
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Precipitation
Cloud droplets
• Less than micrometers (0.02
millimeter) in diameter
• Fall incredibly
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Particle sizes involved in condensation and precipitation
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Rain develops when growing cloud droplets
become too
to remain in the cloud and as
a result, fall toward the surface as rain.
Rain can also begin as
crystals that collect
each other to form large snowflakes.
As the falling snow passes through the freezing
level into warmer air, the flakes melt and
collapse into rain drops.
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Hail
• Hard
•
pellets
shells
• Most diameters range from
to
cm
• Formation - Occurs in large
clouds with violent up- and down drafts
• Layers of
rain are caught in up- and
down drafts in the cloud
• Pellets fall to the ground when they become
too
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Hail is a large frozen raindrop produced by
intense
, where snow and rain can
coexist in the central updraft.
oAs the snowflakes fall, liquid water freezes
onto them forming ice pellets that will continue
to grow as more and more droplets are
accumulated.
oUpon reaching the bottom
of the cloud, some of the
ice pellets are carried by
the updraft back up to the
top of the storm.
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•As the ice pellets once again fall through the
cloud, another layer of ice is added and the hail
stone grows even larger.
• Typically the
the updraft, the more
times a hail stone repeats this cycle and
consequently, the
it grows.
•Once the hail stone becomes too heavy to be
supported by the updraft, it falls out of the cloud
toward the surface.
•The hail stone reaches the ground as ice since it
is not in the warm air below the thunderstorm long
enough to melt before reaching the ground.
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Freezing Rain
droplets freezing on impact
•Ice storms can be the most devastating of winter
weather phenomena and are often the cause of
automobile accidents, power outages and personal
injuries.
The vertical line in the center
of the diagram is the freezing
line.
Temperatures to the left of
this line are below freezing,
while temperatures to the
right are above freezing.
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Ice storms result from the accumulation of
, which is rain that becomes supercooled and
freezes upon
with cold surfaces.
Freezing rain is most commonly found in a narrow
band on the cold side of a
, where
surface temperatures are at or just below freezing.
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•Freezing rain develops as falling
encounters
a layer of warm air deep enough for the snow to
completely melt and become
.
•As the rain continues to fall, it passes through a
thin layer of cold air just above the surface and
cools to a temperature below freezing. However, the
drops themselves do not freeze, a phenomena
called
(or forming "super-cooled
drops").
•When the super-cooled drops strike the frozen
ground (power lines, or tree branches), they
instantly
, forming a thin film of ice, hence
freezing rain.
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•Sleet is less prevalent than freezing rain and
is defined as frozen raindrops that bounce on
impact with the ground or other objects.
•Sleet is more difficult to forecast than freezing
rain because it develops under more specialized
atmospheric conditions.
• It is very similar to freezing rain in that it causes
surfaces to become very slick, but is different
because its easily visible.
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Snow
•an aggregate of ice crystals
•Progressing even further away from the warm
front, surface temperatures continue to
decrease and the sleet changes over to snow.
• Snowflakes are simply aggregates of ice crystals
that collect to each other as they fall toward the
surface.
•Since the snowflakes do not pass through a layer of
air warm enough to cause them to melt, they
remain in tact and reach the ground as snow.
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Measuring precipitation
•Rain – Standard Rain Gauge
•Snow - has two measurements
• Depth
• Water equivalent
• General ratio is 10 snow units to 1 water
unit
• Varies widely
• Radar is also used to measure the rate of
rainfall
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Other Weather Instruments
WIND
direction
TEMPERATURE
AIR PRESSURE
WIND
speed
HUMIDITY
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