met 2204 meteorology presentation 5: moisture, cloud formation, & precipitation. 1presented by...

MET 2204METEOROLOGY

Presentation 5: Moisture, Cloud Formation, & Precipitation.

1Presented by Mohd Amirul for AMC

Recapitulate• Lets recap what we have done last week:

– 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


Presentation Outline

• Introduction• Water Vapor • Change of State • Cloud Formation • Cloud Identification. • Precipitation • Land and Water Effects • Conclusion

Part 1

Part 2


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.


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)


Water Vapor


Water Vapor

• Invisible

• Can be measured in TWO ways:i. Relative Humidityii. Dew Points


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.


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)


Cont.


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.)


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.

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.

Cont.


Change of State


Change of State• Change of State: a change from one state

(solid or liquid or gas) to another without a change in chemical composition.


• 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.

Cont.


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.

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

Cont.


Cont.


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??


• 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.

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.

Cont.


Supercooled Water

• Supercooled: Water droplet colder than 0˚C (liquid water below freezing point)

• When these water droplets strike on expose objectthe impact induce FREEZING!!– May result aircraft icing.


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.

Cont.


• Strong vertical current may carry supercooled water to great height where temperature are much colder than -15 ˚C

Cont.


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.


• 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.

Cont.


• But sometimes dew forms and later freezesFrozen Dew

• However, frozen dew can easily be distinguish from frost.– Frozen dew is hard and

transparent.– Frost is white and

opaque.

Cont.


• What is the difference between frost and snow?– Snow-can form/fall all day long– Frost-form in the cold of winter morning

Cont.


Clouds


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


• Turbulence: Unstable flow of a liquid or gas in atmosphere.

• Fog: ‘cloud’ on the ground.


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.


Cont.

Cloud Formation• Clouds are formed by air being lifted and

cooled adiabatically (without heat transfer) until condensation occur.


• Remember back Condensation Nuclei

– atmosphere contains particles that have an affinity for water ---These serve as centers for condensation.

Cloud Condensation Nuclei (CCN)


Cont.

• The atmosphere has plenty of CCN

Dust Salt Spray from OceansVolcanoes Sulfate Particles from PhytoplanktonForest Fires TreesAnthropogenic Origins


40

• 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.

Presented by Mohd Amirul for AMC

• The air must become saturated for condensation/sublimation to occur.

• Saturation can result from – cooling temperature, – increasing dew point – or both


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 surface2. Stagnant air overlying a cooling surface3. Expansion cooling in upward moving air


Cont.

• Expansion cooling is a major cause of cloud formation.


Cont.

Cloud Classification

• Clouds are classify as follows:a. Heap Cloudsb. Layer Clouds


a. Heap Clouds

• General Characteristics:– Unstable– Isolated with large vertical and small horizontal

extent.– Associated with changeable weathers, showers,

thunderstorm and turbulence.


Cont.

• 3 main types:i. Cumulus (Cu)ii. Comulonimbus (Cb)iii. Altocumulus Castellanus (Acc)


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 warm air 47Presented by Mohd Amirul for AMC

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.


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.


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.


Cont.

• Main types:i. High level

• Cirrus (Ci), • Cirro Stratus (Cs), • Cirro Cumulus (Cc)

ii. Medium Level• Alto cumulus (ac)• alto stratus (As)

iii. Low Level• Nimbo Stratus (Ns), • Strato Cumulus (Sc), • Stratus(St)


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.


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.


Cont.

• Cirro Cumulus– Characteristics:

• Thin white patch or sheet of cloud; • Appears dappled or rippled.

Presented by Mohd Amirul for AMC 54

Cont.

ii. Medium Level

• Alto Cumulus (ac)– Characteristics:

• white or grey patches arranged in sheets.

• Shape and texture are variable.


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.


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.


Cont.

• Strato Cumulus (Sc)– Characteristics:

• white or grey sheet of cloud• usually formed in mounds (laid on top of each others)

or rolls.


Cont.

• Stratus– Characteristics:

• grey featureless layer of cloud with a uniform base. • Often associated with drizzle or snow.


Cont.

Four latin terms form the basis for the naming of clouds:

– 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.


Cont.

Precipitation


• 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


Cont.

• There is two ways of particles grows:i. Growth of water droplet/ice crystal

• 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 • 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.


Cont.

Growth of raindrops by collision of cloud droplets


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• Precipitation in the form of ice crystals

c) Hail• Solid balls or piece of ice


Cont.

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


• 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


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


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 69Presented by Mohd Amirul for AMC

Cont.

Land and Water Effects


• 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.


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.


Cont.


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


Cont.

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.

• 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.


Key Points


• 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.

End of Presentation #55 Minutes for Q/A session


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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