Transcript MET 2204 METEOROLOGY

MET 2204
METEOROLOGY
Presentation 5:
Moisture, Cloud Formation, & Precipitation.
Presented by Mohd Amirul for AMC
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Recapitulate
• Lets recap what we have done last week:
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What is the Wind?
Concept of Convection
How the air move?
Use of Isobar
The force acting on the wind: PGF, GF, Friction
Major Wind: Geostrophic and Gradient Wind
Jet Stream
Local and Minor Surface Wind: mountain and Valley;
Katabatic; Land and Sea.
– Wind Shear
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Presentation Outline
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Introduction
Water Vapor
Change of State
Cloud Formation
Cloud Identification.
Precipitation
Land and Water Effects
Conclusion
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Part 2
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Learning Outcomes
• At the end of this session, student should be
able to:
– Identify the physical processes related to the
transfer of heat and moisture
– Explain change of state
– Understand the background of formation of cloud
and its classification.
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Introduction
• Moisture in atmosphere creates a variety of
hazards.
• Water can be solid, liquid, and gaseous states.
• What is the difference between moisture and
humidity?
– Moisture: caused by water(usually can be seem)
– Humidity: the amount of water vapor inside
atmosphere. (usually can’t be seem)
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Water Vapor
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Water Vapor
• Invisible
• Can be measured in TWO ways:
i. Relative Humidity
ii. Dew Points
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i. Relative Humidity
• Express in percent
• Relative humidity: express the degree of
saturation.
– 100% - Saturated.
– Below 100% - unsaturated.
• ‘Relative’ means relates the actual water vapor
present that which could be present.
– E.g. 50% humidity means the air could hold twice as
much water vapor as it actually present.
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Cont.
• In fact, warm air can hold more water than
cool air.
– Temperature determine the maximum amount of
water vapor air can hold
– Temperature increase, water vapor increase,
relative humidity decrease.
• When air is cooled, can’t hold more water
vapor (water becomes liquid/solid)
• Condensation/sublimation begins to occur -
SATURATED (100% Rel. Humidity)
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Cont.
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ii. Dew Points
• Dew Points: Temperature to which air must be
cooled to become saturated by the water
vapor already present in the air.
(Temperature at which water vapor becomes
saturated and condensation begins.)
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Cont.
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Cont.
• Temperature-dew point spread.
– Aviation weather reports normally include air
temperature and dew point temperature.
– Temperature-dew point spread: difference
between temperature and dew point.
– As SPREAD less, relative humidity increase.
– Spread will be 100% when temperature and dew
point are same.
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Cont.
• Has a little bearing about precipitation but important
in anticipating (predicting) fog.
*saturate: cause chemical compound, vapour, solution to unite
with the greatest amount of another substance.
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Change of State
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Change of State
• Change of State: a change from one state
(solid or liquid or gas) to another without a
change in chemical composition.
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Cont.
• Involving:
– Evaporation: The process of becomes a vapor from
liquid.
– Condensation: The process of change from gas to
liquid
– Sublimation: The process of change directly from solid
to gas or vice versa.
– Freezing: The process of change from liquid to solid.
– Melting: The process of change from solid to liquid.
*Example of Sublimation: Snow/Ice crystals result from sublimation of
water vapor directly to solid state.
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Cont.
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Cont.
Latent Heat
• Any change of state involves heat transaction.
• Latent heat: Heat absorbed or radiated during
a change of phase at a constant temperature
and pressure
• Gas have the high energy than solid. It can be
prove by its rapid movement of molecules.
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Cont.
• Types of latent heat:
– Latent heat of vaporization
• Involves during evaporation and condensation process
• The heat energy restored in the water vapor and release
during condensation.
– Latent heat of fusion
• Involves when melting and freezing
• Heat energy restored during melting and released during
freezing.
– Latent heat of sublimation:
• Involve sublimation
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Cont.
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Condensation Nuclei
• As air become saturated water vapor begins to
condensed (water vapor decrease) on the
nearest available surface.
• What surface in the atmosphere on which
water vapor may condense??
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Cont.
• Surface in the atmosphere on which water
vapor may condense Condensation Surface.
• Condensation Surface: the surface where
abundance microscopic solid particles
suspended in the air.
• Theses microscopic solid particles we call as
CONDENSATION NUCLEI
– E.g. salt, dust, combustion by product.
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Cont.
• As the water vapor condense or sublimates on
condensation nuclei, liquid or ice particles
begin to grow.
• The particle ice or liquid doesn’t depend
entirely on temperature
– E.g.: Liquid Water may be present at temperature
well below freezing.
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Supercooled Water
• Supercooled: Water droplet colder than 0˚C
(liquid water below freezing point)
• When these water droplets strike on expose
objectthe impact induce FREEZING!!
– May result aircraft icing.
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Cont.
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Cont.
• Some of the supercooled water left in clouds
at temperature between 0˚C to -15 ˚C
– Amount of supercooled water become less.
• At temperature colder than -15 ˚C
sublimation commonly occur.
• Sublimation will form ice crystals/clouds/fog.
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Cont.
• Strong vertical current may carry supercooled
water to great height where temperature are
much colder than -15 ˚C
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Dew and Frost
• Dew: water that has condensed on
a cool surface overnight from
water vapor in the air.
• Formation of Dew:
– During night (with wind or no wind),
Flora (vegetation) cools by radiation
(terrestrial radiation) to a
temperature at or below the dew
point of air.
*moisture form on the leaves ambiguous to moisture
form in warm room.
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Cont.
• Frost: The formation of ice on the surface.
• Formation of Frost:
– Form in much same way with dew.
– The difference is dew point of surrounding air
must be colder than freezing. In this case, water
vapor sublimates directly as ice crystals frost
rather than condensing as dew.
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Cont.
• But sometimes dew
forms and later freezes
Frozen Dew
• However, frozen dew
can easily be
distinguish from frost.
– Frozen dew is hard and
transparent.
– Frost is white and
opaque.
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Cont.
• What is the difference between frost and
snow?
– Snow-can form/fall all day long
– Frost-form in the cold of winter morning
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Clouds
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The Clouds
• Clouds: A visible mass of water or ice particles
suspended at a considerable altitude.
(A visible manifestation of condensation or
deposition in the atmosphere)
• Clouds can indicate following problem:
– Turbulence
– Visibility
– Precipitation
– Icing
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Cont.
• Turbulence: Unstable flow of a liquid or gas in
atmosphere.
• Fog: ‘cloud’ on the ground.
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Cont.
• Cloud Base
– The height of the lowest part of the
cloud above the surface.
– Similar to condensation
surface/level.
– Can be estimated using ceilometer.
• Ceilometer: Laser beam pointing
vertically and the receiver put at the
same location.
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Cloud Formation
• Clouds are formed by air being lifted and
cooled adiabatically (without heat transfer)
until condensation occur.
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Cont.
• Remember back Condensation Nuclei
– atmosphere contains particles that have an
affinity for water ---These serve as centers for
condensation.
Cloud Condensation Nuclei (CCN)
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• The atmosphere has plenty of CCN
Dust
Salt Spray from Oceans
Volcanoes
Sulfate Particles from Phytoplankton
Forest Fires
Trees
Anthropogenic Origins
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• CCN are more plentiful near the surface of the
earth.
• CCN are more plentiful over land rather than
the ocean.
• The formation of cloud droplets using CCN is
called:
Heterogeneous Nucleation.
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Cont.
• The air must become saturated for
condensation/sublimation to occur.
• Saturation can result from
– cooling temperature,
– increasing dew point
– or both
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Cont.
• But the cooling is more predominant
(frequent) for the air to become saturated.
• Cooling Process:
– 3 basics process may cool the air to saturation:
1. Air moving over a colder surface
2. Stagnant air overlying a cooling surface
3. Expansion cooling in upward moving air
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Cont.
• Expansion cooling is a major cause of cloud
formation.
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Cloud Classification
• Clouds are classify as follows:
a. Heap Clouds
b. Layer Clouds
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Cont.
a. Heap Clouds
• General Characteristics:
– Unstable
– Isolated with large vertical and small horizontal
extent.
– Associated with changeable weathers, showers,
thunderstorm and turbulence.
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Cont.
• 3 main types:
i. Cumulus (Cu)
ii. Comulonimbus (Cb)
iii. Altocumulus Castellanus (Acc)
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Cont.
i. Cumulus (Cu)
• Characteristic:
– Give heavy showers and
moderate to heavy clear
ice and turbulence.
– Brilliant (striking) white
to grey, dense isolated
clouds.
– Clumped or heaped
shapes.
– Field of Cu often have
bases all at same
(Condensation Surface)
level
– Instability cloud at
cold front.
*Cold Front: zone where cold air
replaces
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Cont.
ii. Comulonimbus (Cb)
• Characteristics:
– Thunderstorm Clouds
• Severe turbulence
and heavy clear icing.,
heavy showers, hail,
thunderstorm and
lightning.
– Occur at cold front and
major tropical storms
– Huge towering cloud,
dark base and white
sides.
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Cont.
iii. Altocumulus (Acc)
• Characteristics:
– Warning of unstable
air and
thunderstorms within
24 hours.
– white or grey, broken
cumulus-like clouds;
– Sometimes arranged
in lines.
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Cont.
b. Layer Clouds
• General Characteristics:
– Occur in stable condition
– Sheets of cloud cover a wide area
– The weather can be dull (darkened with
overcast(cover by cloud)) for lengthy periods with
continuous light to moderate precipitation.
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Cont.
• Main types:
i.
High level
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Cirrus (Ci),
Cirro Stratus (Cs),
Cirro Cumulus (Cc)
ii. Medium Level
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Alto cumulus (ac)
alto stratus (As)
iii. Low Level
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Nimbo Stratus (Ns),
Strato Cumulus (Sc),
Stratus(St)
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Cont.
i. High Level
• Cirrus (Ci)
– Characteristics:
• White, delicate, fibrous in appearance. Forms in patches(contrast)
or narrow bands.
• Cirrus clouds are formed entirely of ice crystals. These grow and
evaporate slowly, leading to soft edges to clouds.
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Cont.
• Cirro Stratus (Cs)
– Characteristics:
• Thin, transparent sheet or
veil(hide);
• sun clearly visible & casting
shadows at surface.
• A halo(circle of light) may
be seen around the sun (or
moon).
• Sheets of cirrostratus may
cover entire sky, and be up
to several 1000m deep.
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Cont.
• Cirro Cumulus
– Characteristics:
• Thin white patch or sheet of cloud;
• Appears dappled or rippled.
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Cont.
ii. Medium Level
• Alto Cumulus (ac)
– Characteristics:
• white or grey patches
arranged in sheets.
• Shape and texture are
variable.
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Cont.
• Alto Stratus (As)
– Characteristics:
• A greyish sheet of cloud, may be fibrous or uniform in
appearance.
• Thin enough in parts to make out the sun, but no halo.
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Cont.
iii. Low Level
• Nimbo Stratus (Ns)
– Characteristics:
• Dark grey, featureless, thick layer of cloud.
• Associated with prolonged precipitation.
• Commonly forms in frontal systems
*Front: zone/surface of interaction between two air masses of different
temperature.
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Cont.
• Strato Cumulus (Sc)
– Characteristics:
• white or grey sheet of cloud
• usually formed in mounds (laid on top of each others)
or rolls.
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Cont.
• Stratus
– Characteristics:
• grey featureless layer of cloud with a uniform base.
• Often associated with drizzle or snow.
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Cont.
Four latin terms form the basis for the naming of
clouds:
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Cirrus : fibrous or hair-like
Cumulus : a heap or pile
Stratus : a horizontal sheet or layer
Nimbus : rain-bearing
The prefix Alto is used to indicate medium altitude
clouds.
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Precipitation
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Cont.
• Precipitation occurs when the particles (e.g. :
water droplets or ice crystals) grow in size and
weight until the atmosphere no longer can
suspend them and they fall.
• Precipitation versus Cloud Thickness:
– To produce significant precipitation, clouds usually
are 4000 ft thick or more
– Thicker Cloud  Heavier Precipitation
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Cont.
• There is two ways of particles grows:
i.
Growth of water droplet/ice crystal
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Once water droplet or ice crystals forms, it continue to grow by
added condensation or sublimation directly into particle
ii. Collision between cloud particle/droplets
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Rapid growth process
Cloud particles collide and merge into larger drop
Produce larger precipitation particles
Upward currents enhance the growth rate and support larger
droplet.
Light rain and snow - produce by merging drops with mild
upward current.
Heavy rain, heavy snow and hail - produce by strong upward
current support larger drops and built clouds to great height.
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Cont.
Growth of raindrops by collision of cloud droplets
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Cont.
• Precipitation types are:
a) Rain
•
Water droplets are supported by upcurrents but if the
weight of water becomes too great rain or drizzle will
occur
b) Snow
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Precipitation in the form of ice crystals
c) Hail
•
Solid balls or piece of ice
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Examples of Precipitation
• Rain
– Precipitation in the form of
liquid water droplets greater
than 0.5 mm. If widely
scattered, the drop size may be
smaller. The intensity of rain is
based on rate of fall.
• The rate of fall with respect to the
intensity is as follows:
– Slight -
<0.5mm/hr
– Moderate - 0.5 – 4.0mm/hr
– Heavy -
>4.0mm/hr
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Cont.
• Drizzle
• Very small, numerous, and uniformly distributed water
drops that may appear to float while following air
currents.
• Unlike fog droplets, drizzle falls to the ground.
• It usually falls from low stratus clouds and is frequently
accompanied by low visibility and fog
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Cont.
• Hail
– Precipitation in the form of balls
or irregular lumps of ice, always
produced by cumulonimbus
cloud.
– An individual unit of hail is
called a hailstone.
– By convention, hail has a
diameter of 5 mm or more,
while smaller particles of similar
origin, formerly called small hail,
may be classed as either ice
pellets or snow pellets
– The destructive effects of
hailstorms- plant and animal life,
buildings and property, and
aircraft
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Cont.
•Shower
• Precipitation from a cloud that is characterized by its sudden beginning
and ending, changes in intensity, and rapid changes in the appearance of
the sky. It occurs in the form of rain (SHRA), snow (SHSN), or ice (SHPE). It
is reported as "SH" in an observation and on the METAR.
•The rate of fall with respect to
intensity is as follows;
Slight -
< 2mm/hr
Moderate -
2-10mm/hr
Heavy -
> 10-50mm/hr
Violent -
> 50mm/hr
*METAR is a format for reporting weather information.
A METAR weather report is predominantly used by pilots in fulfillment of a part of a
pre-flight weather briefing
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Land and Water Effects
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Cont.
• Land and water can frequently affect cloud
and precipitation.
How?
• Large body of water (e.g. oceans/large lakes)
can add water vapor to the air.
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Cont.
• Variation of season to cloudiness
– In winter:
• Cold air frequently moves over warm lakes/oceans 
warm water adds heat and water vapor to the air 
causing shower to the lake side/oceans side.
– In other seasons:
• Air may be warmer than lakes  air become saturated
by evaporation from water while also becoming cooler
in the low levels by contact with cool water  form a
fog - dense side to the lake/oceans.
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Cont.
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Cont.
• Variation of diurnal to cloudiness
– Day:
• Cool air over the lake/oceans blows toward the land
• Clouds form cloud over the land
– Night
• Reverse pattern.
• Clouds tend to form over the lake
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Conclusion
• Water can be solid, liquid, and gaseous states.
• Water Vapor can be measured in TWO ways: Relative
Humidity and Dew Points.
• Change of State: a change from one state (solid or
liquid or gas) to another without a change in
chemical composition.
• Condensation Surface: the surface where abundance
microscopic solid particles suspended in the air.
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• Clouds: A visible mass of water or ice particles
suspended at a considerable altitude.
• Clouds are formed by air being lifted and cooled
adiabatically (without heat transfer) until
condensation occur.
• Clouds are classify as follows:Heap Clouds and Layer
Clouds.
• Precipitation occurs when the particles (e.g. : water
droplets or ice crystals) grow in size and weight until
the atmosphere no longer can suspend them and
they fall.
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Key Points
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Water vapor
Measurement of Water Vapor
Change of State
Evaporation, Condensation, Sublimation, Freezing, Melting
Latent Heat
Condensation Surface and Condensation Nuclei
Supercooled Water
Dew and Frost
Clouds
Clouds Base
Clouds Classification: Heap and Layer Clouds
Precipitation
Types of Precipitation
Land and Water Effect.
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End of Presentation #5
5 Minutes for Q/A session
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Quote of the Day
• I'm a dreamer. I have to dream and reach for
the stars, and if I miss a star then I grab a
handful of clouds.
~Mike Tyson
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