Transcript Forms of Condensation and Precipitation

Humidity
 Capacity of air is primarily a function of temperature
 Relative Humidity (RH) =
(actual water vapor content)
x 100
(max. water vapor capacity of the air)
 Heated air becomes lower in RH because denominator gets larger
 Cooled air becomes higher in RH
Saturation vs Air Temperature
The actual amount of
Water air can hold changes
With air temperature
47 grams
104 F
Air at 104 F can hold 3 times
As much water as 68 F air !
(47 grams vs only 15 grams)
15 grams
Air at 68 F can hold 4 times
As much water as air at 0 F
(15 grams vs only 4 grams)
32 F
68 F
4 grams
Forms of Condensation
and Precipitation
Chapter 5
Meteorology
Clouds, Fogs, Snow and Sleet
 Clouds: visible aggregate of minute droplets of water, or tiny
crystals of ice or a mixture of both
 Indicate what’s going on in atmosphere
 For any form of condensation to occur the air must be saturated
 There generally must be a surface on which water vapor can
condense
Clouds, Fogs, Snow and Sleet
 Cloud Condensation Nuclei: tiny particles that serve as
surfaces for which water vapor condenses
 Includes dust, smoke and salt particles
 If nuclei are absent the relative humidity needs to be well above
100% in order to produce cloud droplets
Growth of Clouds
 Hygroscopic (water seeking) Nuclei:
 Particles that are effective sites for condensation
 Items that quickly absorb moisture when exposed to humid air and
become stale
 Byproduct of combustion (forest fires, automobiles, coal burning
furnaces)
 Hydrophobic (water repelling) Nuclei:
 Water will only form if relative humidity reaches 100%
 Major sources of Nuclei
 Dust storms, volcanic eruptions, pollen
Cloud Condensation Nuclei
Cloud Formation
Types of Clouds
 Identify Ten Major Types
 Where located: High, Medium, Low
 Picture
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Cloud Classification
 Cirrus:
 High, white and thin; “curl,” “filament”
 Separated and detached, form delicate veil-like patches; extended
wispy fibers; feathery appearance.
 Cumulus
 Globular individual cloud masses. Flat base and appear as rising domes
or towers (cauliflower)
 Stratus
 Clouds are best described as sheets or layers that over much or all of
the sky
 No distinct individual units
Cloud Classification: High Clouds
6000m (20,000ft) base
 High clouds: low temperatures, ice crystals, small quantities
of water vapor
 Cirrus (Ci)
 Thin delicate fibrous ice-crystal clouds (mare’s tails)
 Cirrostratus (Cs)
 Thin sheets of white ice-crystal clouds that give sky a milky look
 Cirrocumulus (Cc)
 Thin, white ice crystal clouds
 Least common
Cirrus
Cirrostratus
Cirrocumulus
Cloud Classification: Middle Clouds
2000 – 6000m
 Altocumulus (Ac)
 White to gray clouds often made up of separate globules
 “Sheep back”
 Altostratus (As)
 Stratified veil of clouds that are generally thin and may
produces very light precipitation
 Can produce a bright spot in the sky (not a halo)
 Seen with warm fronts
Altocumulus
Altostratus
Cloud Classification: Low Clouds
below 2000m (6500 ft)
 Stratus (St)
 Low uniform layer resembling fog, but not resting on the ground
 Leads to drizzles
 Stratocumulus (Sc)
 Soft gray clouds in globular patches or rolls
 May form continuous clouds
 Nimbostratus (Ns): Nimbus: “rain cloud”, Stratus: “to cover with a
layer
 Form during stable conditions
 Amorphus layer of dark gray clouds
 Chief precipitation producing clouds (light to moderatecontinuous rain)
Stratus
Stratocumulous
Nimbostratus
Cloud Classification:
Cloud of Vertical Development
 Clouds that span more than one height
 Associated with unstable air
 Cumulus (Cu): Fair Weather clouds
 Dense billowy clouds often characterized by flat base
 Seen on clear days with unequal surface heating
 Cumulonimbus (Cb)
 Towering cloud, “anvil head”
 Thunder, lightening, hail, tornadoes
Cumulus
Cumulonimbus
Lifecycle of Cumulonimbus
Cloud Varieties
 Uncinus: “hook shaped,” streaks of cirrus clouds, look like
commas
 Cirrusuncinus: bad weather is coming
 Fractus: stratus/cumulus are broken
 Mammatus: rounded protuberances on their bottom surfaces
 Associated with stormy weather and cumulonimbus clouds
 Lenticular: lens-shaped
 Found in rugged or mountainous topography
Uncinus
Fractus
Mammatus
Lenticular
Types of Fog
 Fog: cloud with base near the ground
 Physically no difference between a fog and a cloud; appearance
and structure the same
 Difference is method and place of formation
 Fog results from cooling or by addition of enough water vapor
to cause saturation
 Atmospheric Hazard:
 Light – reduces visibility (2 to 3 km, 1 to 2 miles)
 Dense – cut to a few dozen meters or less
Types of Fog
 Radiation Fog: results from radiation cooling of the ground
and adjacent air
 A nighttime phenomenon requiring clear skies and a fairly high
relative humidity
 Thickest in valleys
 Dissipates within one to three hours of sunrise, fog evaporates
from the bottom up
Radiation Fog
Radiation Fog
Types of Fog
 Advection Fog: when warm and moist air blown over a cold
surface, it becomes chilled by contact and to a certain extent,
by mixing with the cold air
 If cooling is sufficient, fog will form air moving horizontally
 Are a consequence of air giving up heat to the surface below
during horizontal movement
 Winds up to 6 to 18mph required (turbulence facilitates cooling
through a thicker layer of air but it also carries fog higher)
 Thicker than radiation
Advection Fog
Advection Fog
Advection Fog
Advection Fog
Types of Fog
 Upslope Fog: created when relative humidity moves up a
gradual sloping or up the steep slope of a mountain
 Because of upward movement air expands and cools
adiabatically
 If dew point is reached, an extensive layer of fog may form
Upslope Fog
Upslope Fog
Formation of Fog
 Steam Fog: when cool air moves over warm water; enough
moisture may evaporate from the water surface to saturate
the air immediately above
 As rising water vapor meets the cold air, it condenses and rises
 Common over lakes and rivers on clear crisp mornings in the fall
Formation of Fog
 Frontal Fog: When frontal wedging occurs, warm air is lifted
over colder air
 If the resulting clouds yield rain and the cold air below is near
dew point, enough rain can evaporate to produce fog
 Results in a continuous zone of condenses water droplets
reaching from the ground up through clouds
Dew
 Condensation of water vapor on objects that have radiated
sufficient heat to lower their temperature below dew point
 May form on some surfaces but not others
White Frost
 Forms when the dew point of air is below freezing
 Not frozen dew
 Gas to a Solid (deposition
Frost
Types of Precipitation
Atmospheric Pressure
A Crash Course
Chapter 5 and 6
Precipitation Formation
 Why do some clouds produce precipitation
and other just float overhead?
 Cloud Droplets are very tiny in comparison
to rain droplets
 For precipitation to occur cloud droplets
must grow in volume by roughly 1 million
time
Collision – Coalescence Process
 Warm clouds are those having
temperatures greater than 0 °C
throughout.
 The largest droplet (collector
drop) falls through a warm cloud
and overtakes some of the smaller
droplets because of its greater
terminal velocity
Collision – Coalescence Process
 A collector drop collides with
only some of the droplets in its
path. The likelihood of a collision
depends on both the size of the
collector and its size relative to
the droplets below.
 If the collector drop is much
larger than those below, the
percentage of collisions will be
low.
Collision-Coalescence
 The tropics are the ideal environment for
the C-C process.
 Very humid and relatively clean so fewer
condensation nuclei exists
 In these areas, large cumulonimbus clouds
form and within those clouds the larger
drops quickly gather smaller droplets to
generate the warm afternoon showers
associated with tropical climates
Bergeron Process
 A cloud at a temperature of -10oC (14oF) has ice crystals that
are surrounded by liquid droplets
 Supersaturated clouds contain more water vapor than
“regular” clouds.
 Ice Crystals collect more water than they lose through
sublimation
 Evaporation of water droplets provides a water source of
water vapor to feed the growth of ice crystals
Bergeron Process
 As the ice crystals descend they enlarge as they intercept
cloud drops that freeze on them
 B.P. produces precipitation throughout the year in the middle
latitudes (if the clouds are cold enough).
 The type of precipitation that reaches the ground depends on
the temperature profile in the lower atmosphere.
Bergeron Process
RAIN
 Rain is precipitation arriving at the surface in the form of
liquid drops, usually between 0.5 and 5 mm.
 Episodic precipitation from rapidly developing
cumuliform clouds is called showers and can occur as
either rain or snow.
RAIN
 In Mid-latitudes precipitation
leaves cold cloud as snow, if
temperatures are higher than
freezing precipitation will become
rain near the surface
SNOW
 Snow results from the growth
of ice crystals through
deposition, riming, and
aggregation.
 Ice crystals in clouds can have a
wide variety of shapes,
including six-sided plates,
columns, solid or hollow
needles, and complex
dendrites with numerous long,
narrow extensions.
SNOW
 As precipitation leaves
cold clouds, the
temperature must
remain below freezing at
or near the surface
SLEET
 Sleet (above) occurs as rain falling
from a cloud, passes through a
cold layer, and freezes into ice
pellets. This is most common
along warm fronts.
FREEZING RAIN (GLAZE)
 Freezing rain begins when a light
rain or drizzle of supercooled drops
falls through air with a temperature
at or slightly below 0 °C.
 When the raindrops hit the surface,
they form a thin film of water, but
only for a moment.
 Soon afterward the water freezes to
form a coating of ice.
Formation of Sleet
Global average annual precipitation.
HAIL
 Hail consists of ice pellets formed in roughly concentric
layers.
 Formed from updrafts that carry a particle into the colder
reaches of a cloud, and the liquid water coating the ice
freezes.
 When the stone exits the updraft and falls, it becomes
wet from its collisions with liquid droplets.
 The hailstone can be captured once again by an updraft,
and the coating of water freezes
HAIL FORMATION