rainfall types chapter 2
TRANSCRIPT
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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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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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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
Formation of Convectional / Relief / Frontal Rainfall
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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
Formation of Convectional / Relief / Frontal Rainfall
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