Transcript Weather - KamoHighSchoolOutdoorEducation

Weather
The Atmosphere
 Thin layer of gas that
clings to earth’s surface
 ( as thick as the skin on
an apple )
 Water vapour varies
from less than 1% to 4%
of atmosphere. This is
because of the water
cycle – evaporation and
condensation.
Air density
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How much air is present in a
cubic metre
Varies with height above sea
level
Sea level 1.2 kg air per cubic
metre – top of Everest approx 1/3
of this.
The Earth's atmosphere is almost
completely contained within 80
kilometers, or 50 miles.
As air is highly compressible, air
at lower altitudes is compressed
by air above and so is more
dense.
From the diagram below, it can
be seen that 50% of the Earth's
atmosphere is found below 5km.
Air pressure
 Weight of the air vertically above an area
 At mean sea level the air pressure
averages just over 1000 hectopascals (
hPa) whilst at the top of Everest ( nearer
300hPa
 The decreasing pressure with increase in
height has key influence on Weather.
Expanding air
 As air rises from the surface it
experiences lower
surrounding pressure.
 This allows the rising air to
expand which in turn causes
the air to cool at a rate approx
1 C per 100m
 When air is compressed it
tends to heat up ( bike pump)
 When air descends it
experiences increased
pressure and so is
compressed and heats up.
The Suns Rays
 The amount of
sunlight hitting the
earth has an effect
on weather.
 The more sunlight
the greater the
heating that occurs
 Equator most heat,
poles least heat
How clouds are formed.
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When rising air rises and cools enough it will reach saturation point.
This is when water vapour will condense around a tiny dust speck and form a
droplet.
These will stay up in the air as the are incredibly small ( 200 per millimetre )
They fall slower than the surrounding air is rising and so stay airbourne.
As the droplets collide or increase in size they will eventually become too large to
stay airbourne and fall as rain.
This is one way rain forms and is known as warm rain.
NZ rain
 More commonly rain forms from ice
crystals.
 Once ice crystals from within a cloud
they attract water vapour much more
easily than other water droplets.
 As this happens very quickly the
water vapour cools quickly, the
saturation point falls, more water
condenses on the ice crystal which
grows – the cycle quickly gets quicker
and quicker.
 As the large crystal falls it bumps into
other droplets that freeze onto it, and
it increase in size even further.
 Eventually it falls into warmer air and
melts falling as rain.
Rain and the Weather map
 To find where rain is to fall search the
weather map for areas where air is rising
or going to rise in the atmosphere
What is a weather map
 An analysis map is a
map that shows the
CURRENT situation
at the specified time
 A prognosis map is a
prediction of the
future – a forecast
The weather map
ISOBARS
 Isobar – join places
where air pressure is the
same.
 Lines drawn at 4 hPa
intervals
 Aound NZ pressur varies
from 960 hPa ( low
pressure ) to 1040 hPa (
high pressure )
Wind
 Is caused by the movement of air from high to low
pressure.
 From the pattern of the isobars should be able to work
out what the wind is doing at different places.
 Air moves from high to low pressure
 Sometimes air is deflected by obstacles in its way.
Wind
 Wind blows almost parallel to the isobars
crossing them at a small angle at low
pressure.
 Over the open ocean the angle is approx
15 degrees
 Over the land the angle is 30 degrees
due to friction
Coriolis effect
 Lower pressure at
tropics as heat rises,
lower pressure at poles
as cold air sinks. This
continually happens.
 As this happens the
movement is deflected
westerly by the earths
rotation – this is called
the coriolis effect
Why do:
lows / depressions / Cyclones
Highs / anticylcones
form?
 Even though tropics recieve
the most sun, temperatures
do not continue to rise as
wind and oceans transport
heat towards the poles.
 This gives rise to anticyclones
and depressions that we see
on our weather map.
 Air moves from tropics to
poles due to pressure
differences.
 higher pressure at poles as
cold air sinks. This continually
happens.
 Lower pressure at tropics as
heat rises,
Covection – Tricellular model
 The tricellular model found by Ferrel is still the best model we have.
In this model the air, after crossing the warm oceans in the trade
winds and becoming warm and moist, arrives at the Equator (at the
ITCZ, inter-tropical convergence zone) and is heated, causing it to
rise.
 The unstable air rises to form very high cumulo-nimbus clouds and
afternoon thunderstorms and low-pressures are found.
 The equator is an area with very gentle winds called the doldrums.
 Polar Front Jet Stream (PFJS):Sub Tropical Jet Stream (STJS):
Wind Patterns around the world –
seasonal changes will have some
effect
Pessure gradient – wind
speed.
 In basic terms the
bigger the difference
in pressure between
the High and the Low
the stronger the
winds will be.
 The smaller the
difference the lighter
the winds.
 Winds are named according
to the compass direction of
their source. Thus, a wind
from the north blowing toward
the south is called a northerly
wind.
 The diagram describes the
sixteen principal bearings of
wind direction. Most
meterological observations
report wind direction using
one of these sixteen bearings.
Buys ballot – geostrophic winds
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In the upper troposphere the air
is unaffected by friction and we
can see that there is a balance
between the pressure gradient
force and the coriolis force. The
resultant wind is called the
geostrpohic wind.
The geostrophic wind blows
parallel to the isobars.
Buys Ballot was a Dutchman who
recognised the existence of the
geostrophic wind in 1857.
His law states that if you stand
with your back to the wind the
low pressure is always on the left
and the high pressure on the
right. (Placing of low and high
pressures tell you which direction
the pressure gradient wind is
blowing.
The coriolis is in the opposite
direction to the pressure gradient
force. So you can work out the
direction of the resulting wind.)
Wind direction
 In southern
hemisphere the wind
travel clockwise in a
low and anti
clockwise in a high
 In northern
hemisphere the flow
is reversed.
Wind strength
 In general the closer the isobars the stronger
the wind.
 Does depend on lattitude. Straight isobars at
30 degrees south would cause twice as much
wind as at 50 degrees south .
 Where winds curve tightly around a low the
wind speed may be much less than the isobars
suggest.
Low pressure
 Areas of low
pressure are marked
with an L
 Associated with
rising air
 As air rises it rapidly
expands and cools
thereby causing
clouds and usually
rain.
Coriolis and the
roundabout
 The coriolis force can be
explained by using an
example of a roundabout.
 If we consider person a
standing in the middle of the
roundabout and trying to
throw a ball to person B
standing at the edge of the
roundabout.
 If A throws the ball straight at
B then by the time the ball
has reached the edge of the
roundabout B has further
round.
 To the people on the
roundabout it looks like the
ball has curved round to the
right.
High Pressure
 Associated with high pressure
and sinking air.
 Marked with a H
 Anticyclone
 Air sunks compressing and
warming – clouds evaporate
 Produces fine settled weather
– summer
 In winter low cloud and drizzle
 Thunderstorms common in
summer
 Air diverges at high altitude,
converges low altitude.
Highs / Anti cyclone
 Formed when air converges aloft.
 The air descends, compresses and heats – pressure
increases.
 Water vapour remains the same, but this air becomes
relatively dry as warm air can hold more vapour.
 Any cloud rapidly evaporates.
 Normally air does not reach the surface but spreads
out at around 1500m leaving a layer of colder, denser
air between it and the surface
 Water vapour at the edge of the cold air often form
cloud.
 In summer this cloud burns off over the land during the
day
 In winter this cloud may stay all day as the sun is not
strong enough to heat the air to burn it off.
 This is common in Coastal areas.
Highs and mountains.
 Highest temperatures in
NZ are caused by a
combination of sunlight
heating the air plus
warm air aloft getting
even warmer as it
descends after a
mountain range
 Canterbury plains
example
Sun
heats
ground
and air
Warm air descends
to surface after
crossing mountains
Cloud
Cold dense air
is stopped by
mountain
Blocking high
 Important characteristics of highs or anticyclones is the
way they sometimes become stationary or slow moving
for a week or more.
 This is called BLOCKING
 In NZ this can have an effect on the weather
depending on where the blocking high is situated.
 If the high is centred to the west it will mean prolonged
south / southwest winds bringing cold air up from the
Antartic. Showers
 If the high is east then warm North or Northwestly
winds will be experienced. Heavy rain
 If the high is centre over the islands then weather will
be mainly dry apart from coastal drizzle in winter.
 Prolonged blocking highs in summer can lead to
drought.
Sept 1 2008
All the same info on three maps
Infra red
Satellite Photo
Synoptic chart
Fronts
 Mark the boundaries between air of
different temperatures and origin.
 Places where upward air movement
occurs.
 These cut across isobars
 Fronts have broad bands of cloud and
narrow bands of rain
Frontal symbols
Cold front
 Air marked with blue
triangles signify cold
fronts.
 Triangle points
direction that front is
travelling.
 Signifies cold air
advancing
The arrival of a cold front
Cold front in profile
Warm Front
 Shown by red half
moons side by side
and facing the same
way.
Warm front in profile
Occluded front
 Where traiangle and
half moon shapes
are side by side and
facing the same way
 No marked
difference between
temperature on
either side at sea
level
Stationary front
 Symbols are placed on either side of the
front
 Shows that the front has slowed as is not
moving.
 If the line at the front is dashed it means
it is weak.
Troughs and ridges
 Trough - Dashed line
without triangles signifies
a trough ( associated
with lows ) frequently
contains showers or
thunderstorms
 Ridge - Area of high
pressure extending away
from a high. Not usually
marked on a map.
Weather similar to that of
a high.
Who gets the rain?
 If there is a front near by or strong winds
blowing from seas to land likely to be rain.
 This is because all have or cause rising air that
cools, condenses to produce rain.
 In NZ our mountains have a lot to do with the
rainfall
 Depending on what side of the ranges you live
will determine weather you receive the rain.
Prevailing Westerlies
 Prevailing – where it
comes from most
often
 One of the most
common weather
events is a cold front
moving over NZ from
the tasman sea with
NW winds ahead
and SW winds
behind
Southern Alps
hokitika
Christchurch
What the Map shows
 Strong winds in
Canterbury– isobars
close together ahead
of the front
 Cold front pushing
warm air up, also
over the Alps will
cause heavy rain on
west coast.
Deflection due to
southern alps
Cloud types - Lenticular
 Lenticular clouds, technically
known as altocumulus
standing lenticularis, are
stationary lens-shaped clouds
that form at high altitudes,
normally aligned at rightangles to the wind direction.
 Where stable moist air flows
over a mountain or a range of
mountains, a series of largescale standing waves may
form on the downwind side.
Lenticular clouds sometimes
form at the crests of these
waves.
 Under certain conditions, long
strings of lenticular clouds can
form, creating a formation
known as a wave cloud.
High cloud
Cirrostratus”halo cloud”
Cirrus
Cirrocumulus Clouds
Mid Layer Cloud
Altocumulus Clouds
Cumulonibus (Mamma)
Clouds
Alto stratus – grey sheet
Low Cloud
Stratocumulus Clouds
Stratus
Stratus Fratus Clouds
Vertical development
Clouds ( any height )
cumulus
Growing Congest
Cumulus Clouds
Cumulonimbus
Cumulonimbus – usually
anvil shaped
Cumulus
 Shower cloud
 Look fluffy
 Can contain heavy
rain
 Last for maybe an
hour or more
 Created by strong
upward air motion
Warming by the sea
 Most commonly in
NZ when a cold air
front from the south
moves north usually
behind a cold front.
Warming by the Land
 Happens most in the
summer
 Showers can be
produced
 Favoured are places
where land can heat up
quickly – ploughed
fields, rocky surface,
cities
 Can develop into
thunderstorms
 Sometimes fires
Cooling Aloft
 Sometimes caused by divergence
common in NZ
 Winds up high often stronger than down
below.
FOG
 Simply cloud resting on earth’s surface
 Humid air next to ground is cooled enough to reach
saturation point so water vapour starts to ocndense
 Most common on clear nights with light or no wind.
 Heat radiates into space from the land cooling the air
from below.
 If the ground temperature is below freezing before the
vapour starts to condense a frost will form.
 Can also form when humid air is blown over a cold
surface – sometimes on the coast with on shore winds,
or at seas with different temperatures in currents.
 This sometimes happens in Wellington with the cooler
Cook strait.
 Fog can clear by evaporation, droplets falling to ground
without new vapour condensing to replace, wind
change
Snow
 Results from cold air from
south meeting warm air from
the north.
 Heavy snow if humid warm air
from the north is undercut by
vary cold air from the south
 Many snow storms start with
a couple of hours of rain. The
snow melting before reaching
the ground takes latent heat
from air which cools until
eventually air cools
sufficiently for snow to settle.
 The more snow falling at
higher levels the faster this
happens.
Hail
 Hail is a large frozen
raindrop produced by
intense thunderstorms,
where snow and rain can
coexist in the central
updraft.
 As the snowflakes fall,
liquid water freezes onto
them forming ice pellets
that will continue to grow
as more and more droplets
are accumulated.
 Upon reaching the bottom
of the cloud, some of the
ice pellets are carried by
the updraft back up to the
top of the storm.
Hail continued
 As the ice pellets once again
fall through the cloud, another
layer of ice is added and the
hail stone grows even larger.
 Typically the stronger the
updraft, the more times a hail
stone repeats this cycle and
consequently, the larger it
grows.
 Once the hail stone becomes
too heavy to be supported by
the updraft, it falls out of the
cloud toward the surface.
 The hail stone reaches the
ground as ice since it is not in
the warm air below the
thunderstorm long enough to
melt before reaching the
ground.
Freezing Rain
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supercooled droplets freezing on
impact
Ice storms can be the most
devastating of winter weather
phenomena and are often the
cause of automobile accidents,
power outages and personal
injury.
Ice storms result from the
accumulation of freezing rain,
which is rain that becomes
supercooled and freezes upon
impact with cold surfaces.
Freezing rain is most commonly
found in a narrow band on the
cold side of a warm front, where
surface temperatures are at or
just below freezing.
Freezing rain continued
 Freezing rain develops as
falling snow encounters a
layer of warm air deep
enough for the snow to
completely melt and become
rain.
 As the rain continues to fall, it
passes through a thin layer of
cold air just above the surface
and cools to a temperature
below freezing.
 However, the drops
themselves do not freeze, a
phenomena called
supercooling (or forming
"supercooled drops").
 When the supercooled drops
strike the frozen ground
(power lines, or tree
branches), they instantly
freeze, forming a thin film of
ice, hence freezing rain.
Sleet
 freezing rain is defined as
frozen raindrops that bounce
on impact with the ground or
other objects.
 The diagram shows a typical
temperature profile for sleet
with the red line indicating the
atmosphere's temperature at
any given altitude.
 The vertical line in the center
of the diagram is the freezing
line.
 Temperatures to the left of
this line are below freezing,
while temperatures to the
right are above freezing.
Sleet continued
 Sleet is more difficult to
forecast than freezing
rain because it develops
under more specialized
atmospheric conditions.
It is very similar to
freezing rain in that it
causes surfaces to
become very slick, but is
different because its
easily visible.
La Nina / El Nino
 La Nina – NW winds. Wet weather to
eastern areas, Bay of plenty, East cape
 El Nino – W winds. Wet weather west
coast, dry east coast - canterbury
Katabatic / adiabatic
 Katabatic where air cooled at the top of
mountains and descends
 Adiabatic where air warms at bottom of
mounatins and rises.
 Lapse rates is the term given as to the
temperature at given points of altitude.
Katabatic
 Most katabatic winds (except the Foehn) are
more or less the result of air in contact with
upper level ground is cooled by radiation,
increases in density, and flows downhill and
along the valley bottom. For example radiation
cooling during nighttime can cause a katabatic
flow in the early morning when a pool of cold,
high elevation air begins to descend beneath
warmer, less dense air.
Adiabatic lapse rate
 The temperature of the
air at sea level on the
west side of the
mountain begins at 30
degrees celsius. The air
begins to rise over the
mountain cooling at the
dry adiabatic rate of -10
degrees/1000 meters.
Measuring the weather
 Check out
http://www.metservice.co.nz
 Weather stations measure
wind speed, temp, pressure,
rainfall and othe rthings such
as humidity etc