rainfall types chapter 2

8/12/2019 Rainfall Types Chapter 2

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cean urren s e ec on

Climate

Affect of cold ocean currents

Cools the summer temperature;

Reduces precipitation; cooler air holds less

moisture.

Maritime Climate


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Ocean Currents

1. Permanent or semi-permanent

horizontal movement of surface water (the

top 100m)

It is unusually cold or hot, when compared

with the surrounding water

2. Caused by and shaped by,

prevailing winds,

variations in temperature

density of water

Coriolis force


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Warm and Cold Currents

Cold ocean cu rrents: m ove water towards theequator.

For example the Humbolt or Peru Current carries coldwater from Antarctica toward the equator (along theSouth American coast).

Another example is the Labrador Current whichcarries cold water from the Arctic Ocean down alongthe Labrador coast towards the Grand Banks.


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Cold Ocean CurrentsPractical Examples

Marys Harbour on the

coast of Labrador is

affected by the Labrador

Current which gives Mary's

Harbour cool summer

temperatures & surprising

little precipitation for a

location right on theoceans edge.

Drying & cooling effect to

maritime climate


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Newfoundland Coast

LC brings both cool water & air south from

the Arctic

When this meets the warm Gulf Stream,

flowing north from the equator, fog forms

along our coast

There is a frontal effect created off our

coast contributing both to our precipitation

& wet / foggy weather conditions


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http://www.bgrg.org/pages/education/alevel/coldenvirons/Gulfstream.htm


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Practical Examples Newfoundland's south

coast has ice-freeports year-long while

its north coast has

heavy ice for severalmonths.

The difference in

latitude is not enoughto explain this

difference in ice.


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Warm and Cold Currents

Warm ocean currents: Move water away from warmequator ial regions.

For example the Gulf Stream moves warm water from the

Gulf of Mexico northeast toward England.

Another good example is the Japanese current whichmoves warm water from Japan northeast towardsVancouver.


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http://www.bgrg.org/pages/education/alevel/coldenvirons/Gulfstream.htm


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Forms of Condensation and

Precipitation


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Condensation

Condensationoccurs when water vapor

changes to a liquid.

For condensation to take place, the air must be

saturated and there must be a surface on whichthe vapor can condense.

In the air above the ground, tiny hygroscopic

(water-absorbent) particles known as

condensation nucleiserve as the surfaces on

which water vapor can condense.


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Clouds

Clouds, visible aggregates of minute droplets ofwater or tiny crystals of ice, are one form ofcondensation.

Clouds are classified on the basis of two criteria:form and height.

The three basic cloud formsare: cirrus(high, white, and thin),

cumulus(globular, individual cloud masses), and stratus(sheets or layers).


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Cloud Heights

Cloud heights can be either:

high, with bases above 6000 meters (20,000 feet),

middle, from 2000 to 6000 meters, or

low, below 2000 meters (6500 feet). Based on the two criteria, ten basic cloud types,

including such types as cirrostratus,

altocumulus, and stratocumulus, are

recognized.


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Cloud Types


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Cirrus Clouds


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Cumulostratus Clouds


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Movement of Storm

Cumulonimbuscloud

Positive

Charge

Negative

Charge

Lightning

Cool air descends

and replaces

warm air


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The Bergeron Process (Cold Clouds)

The Collision-Coalescence Process(Warm Clouds)

Precipitation Formation Mechanisms

The Bergeron process describes how rain or snow forms when the


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The Bergeron process describes how rain or snow forms when the

cloud temperature is below freezing.

This process where ice crystals grow at the expense of cloud droplets is called the Ice

Crystal Process. It is also named after the Norwegian researcher who discovered it (To

Bergeron, there were others).

Temperatur

eRH wrt*H2O(liq) RH wrt H2O(ice)

0C 100% 100%

-05C 100% 105%

-10C 100% 110%

-15C 100% 115%

-20C 100% 121%

*wrt = with respect to


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Three important properties of water droplets:

1. Cloud droplets do not freeze at 0.C

2. Supercooled (water in the liqu id state below 0C)

water droplets will freeze immediately if

agitated sufficiently or when they come in contact with

freezing nuclei (a crystalline structure similar to ice)

3. The saturation vapor pressure with respect to ice is

lower than the saturation vapor pressure with respect

to liquid water.


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Ice

Saturation with Respect to Ice and Water

Vapor pressure is the pressure due to water vapor molecules when the

evaporation rate is equal to the condensation rate.

Because of the crystalline structure of ice, water molecules are not ableto break free from an ice surface as easily than from a water surface.

Therefore, the saturation vapor pressure with respect to an ice surface

would be less than the saturation vapor pressure with respect to a liquid

water surface at a given temperature.

Liquid Water


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Gravitational Force = Frictional Force

Terminal Velocity occurs when: F = 0

(when Fgravity= Ffriction)

4/3r3 g = r2 v

4/3 g r = v

mass gravity = area velocity

(velocity is a function of r)


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As droplets fall they collide with

smaller droplets and coalesce.

after collecting ~1 million cloud

droplets the particle is large enough

to fall without evaporating.

Because there are a large number of

collisions needed, clouds with great

vertical extent are typically produce

precipitation by this process.

The Process from Warm Clouds:

The Collision-Coalescence Process


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Air cools as it isforced to rise

Condensation

Clouds form

Rain

Precipitation (commonly called as Rainfall)


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Rainfall is of three types.


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1. Relief Rain

2. Forced to rise over the mountains

1. Warm moist air from the sea


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Mountains on coast force the air to rise

Water vapour

Condenses toform clouds

Evaporation of

water from the

ocean

Onshore

moisture ladenwinds

Air cools

down

Further cooling

leads to

precipitation

Formation of Relief Rainfall

Occurs in the mountains

on the west coast of

Britain

1,000 mm 2,000+ mm Under 750 mm


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Mountains on the coastal beltsforce air to rise

Water vapour

Condenses to

form clouds

Evaporation of water

from the ocean

Onshore

moisture laden

winds

Air coolsdown

Further cooling

leads to precipitation

Formation of Convectional / Relief / Frontal Rainfall

Occurs in the mountains onthe west coast of Britain1,000 mm

2,000+ mm

Under 750 mm


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Water vapour

Condenses to

form clouds

Air cools down

Further cooling

leads to heavy

precipitation

Warm air rises

Ground heats up the air

Suns rays heat up

the ground

Occurs in the late afternoon after the maximum heating

In the tropical rainforest every afternoon



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Water vapour

Condenses to

form clouds

Warm air rises

Further cooling leads

to precipitation along

the Warm Front

Air cools down

Occurs mainly in

winter but can occur

any time of the year

Warm moisture laden air

from the south meets cold

air from the north and forms

the Warm Front


Warm air

Cold air


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Relief rain isquite common

in Britainespecially inthe westwhere the highland areas are


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2. Convectional Rain

1. The sun heatsthe ground

which heatsthe air

2. Warm air rises

C i l R i f ll


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Water vapour

Condenses toform clouds

Air cools down

Further coolingleads to heavy

precipitation

Warm air rises

Ground heats up the air

Suns rays heat up

the ground

Convectional Rainfall

Occurs in Britain in the late afternoon after the maximum heating

In the tropical rainforest every afternoon


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3. Frontal Rain

1. Mass of warmair meets a

mass of colderair

2. Lighter warm air rises over

heavier cold air


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Water vapour

Condenses to

form clouds

Warm moisture laden air

from the south meets cold

air from the north and forms

the Warm Front

Warm air rises

Further cooling leads

to precipitation along

the Warm Front

Air cools down

Frontal or Depression rainfall

Occurs mainly in

winter in Britain but

can occur any time

of the year

Cold airWarm air


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Frontal / Cyclonic Rainfall

Stage 1An area of warm air

meets an area of cold air.

Stage 2. The warm air is

forced over the cold air

Stage 3. Where the airmeets the warm air is cooled

& water vapour condenses.

Stage 4.


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Forms of Precipitation

(Rain , Snow, Sleet and Glaze, Hail)

Droplet size determines the type of precipitation.


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Rain is the term for drops of water that fall from a cloud

and have a diameter of 0.5 millimeter (mm). Drizzle and mist

have smaller droplets.

Rain mostly occurs in nimbostratus clouds and cumulonimbus

clouds. These clouds are capable of producing c loudburs ts.

Most rain starts as snow or ice crystals; as the snow falls through

the cloud it melts. Drizzle is a fine uniform water droplet with a

diameter less than 0.5 mm.

Rain

Temperature Profile for Rain


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Temperature Profile for Rain

S


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Snow is precipitation in the form of ice crystals (snowflakes) or

more often, aggregates if ice crystals. The size and structure of

the crystals is a function of the temperature at which they form.

When air temperatures are cold the moisture content is very

small. This results in the formation of very light fluffy snow

made up of six sided ice crystals.

When conditions are warmer,the ice crystals join together into

larger clumps consisting

interlocked aggregates of crystals.

Snow

Temperature Profile for Snow


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Temperature Profile for Snow


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Sleet and Glaze

Sleet is a wintertime phenomenon that refers to the fall of smallparticles of ice that are clear to translucent.

Sleet forms when rain

passes through a coldlayer of air and freezes

into ice pellets. This

occurs most often in

the winter when warm

air is forced over a layer

of cold air.


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Temperature Profile for Sleet and Glaze

Hail


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Hail

Hail is precipitation in the form of hard, rounded pellets or

irregular lumps of ice. The layers of ice accumulate as the

hailstone travels up and down in a strong convective cloud.

Hailstones begin as small

ice pellets that grow by adding

supercooled water droplets as

they move through the cloud. As

the ice crystal cycles up and down

in the cloud the hailstones increase

in size until they are forced out by

a downdraft or become heavyenough to fall out.


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Hail

Rime


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Rime is a deposit of ice crystals formed by the freezing of super cooled fog or clo

droplets on objects whose surface temperature is below freezing. When rime for

trees, it covers them with ice feathers; in windy conditions only the windward surf

will accumulate the layer of rime.

Approximate size of types of Precipitation


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Mist drizzlerain/sleet

0.005-0.05 mm

less than 0.5mm

0.5 5 mm

Approximate size of types of Precipitation


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Fog

Fog, generally considered an atmospherichazard, is a cloud with its base at or very nearthe ground.

Fogs formed by cooling include: radiation fog(from radiation cooling of the ground

and adjacent air),

advection fog(when warm and moist air is blownover a cold surface), and

upslope fog(created when relatively humid airmoves up a slope and cools adiabatically).


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Fog (cont.)

Those formed by evaporation are:

steam fog(when rising water vapor over

warm water condenses in cool air) and

frontal fog(when warm air is lifted overcolder air along a front).


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Dew and White Frost

Dewis the condensation of water vapor on

objects that have radiated sufficient heat

to lower their temperature below the dew

point of the surrounding air. White frostforms when the dew point of

the air is below freezing.


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Precipitation Formation

For precipitation to form, millions of clouddroplets must somehow coalesce into dropslarge enough to sustain themselves during theirdescent.

The two mechanisms that have been proposedto explain this phenomenon are: the Bergeron process, which produces precipitation

from cold clouds (or cold cloud tops) primarily in the

middle latitudes, and the warm cloud process most associated with the

tropics called the collision-coalescence process.


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Collision Coalescence Process


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Rain Measurement

Rain, the most common form of precipitation, is

probably the easiest to measure.

The most common instruments used to measure

rain are: the standard rain gauge, which is read directly, and

the tipping bucket gaugeand weighing gauge,

both of which record the amount of rain.


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A standard rain

gauge

rainfall types chapter 2

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