Download Weather Tutorial Ex Burgess 2003

The
Weather
The Atmosphere
• The atmosphere is a
mixture of gases
• Because of mixing by
wind, the proportion
of these is relatively
constant from day to
day
• Except Water vapour
• Nitrogen 75%,
Oxygen 23%
• Varies from 0% to 4%
Air Pressure
• The Air Pressure is
simply the weight of
air pushing vertically
down on the earth.
• Pressure at sea level is
higher because there is
more air between sea
level and outer space
• At mean sea level the
air pressure averages
1013hPa, at the top of
Mt Everest the pressure
is nearer 300hPa
Air Pressure
• When air pressure is
reduced, the air cools
down
• Because air pressure is
lower at higher
altitudes, rising air
will cool as the
pressure of air around
it is less
• When you open a bottle
of soft drink a ‘cloud’
forms in the neck as the
pressure in the bottle
drops and the liquid
cools
Air Pressure
• When air is put under
pressure it is
‘compressed’ and
heats up
• Because air pressure is
higher at sea level then
sinking air will warm
up from the pressure
of air around it
• When you pump up a
bicycle tyre the pump
and tyre become hot
because you are adding
energy by compressing
the air
Water
• Water can exist in three states
Water Vapour
• Energy in the form of heat is required to
change the water from solid to liquid to gas
Water Vapour
• The amount of water
vapour in the air is
dependent on the air
temperature
• When saturated air is
cooled, some of the
vapour condenses to
form water
• Air containing the
maximum amount of
water at a given
temperature is said to
have 100% humidity
Relative Humidity
• The moisture that air
holds is referred to as
humidity.
• The ratio of water vapour
held by air compared to
the maximum it can hold
at that temperature is
called the Relative
Humidity.
• When air contains the
maximum amount of
water for its temperature
it is called saturated.
Clouds
• If air rises high
enough, expanding
and cooling, it will
reach its saturation
point
• Clouds stay in the air
because the droplets
fall slower than the air
around them is rising
• If it rises beyond this
point, the vapour will
start to condense
forming clouds
Rain
• If the water droplet
gets too big, its weight
will cause it to fall
faster than the rising
air
• When the droplets fall,
they capture smaller
droplets, growing
larger and falling
faster
• This is one of the
processes by which rain
forms - the ‘Warm Rain’
process
Latent Heat
• To change water to
vapour requires a lot
of energy
• This energy is held in
vapour as latent heat
• When boiling water, the
temperature will not
exceed 100 degrees C
no matter how much
you heat it
Latent Heat
• Water ‘collects’ this
latent heat from its
surroundings
• This is called ‘cooling
by evaporation’
• When you get out of
the water after a swim,
you will often feel cold
on a sunny day as the
water evaporates from
your skin
Latent Heat
• The amount of latent
heat released by
atmospheric processes
is huge.
• The release of latent
heat increases the
upward motion of air
that created the clouds
in the first place
• A large thunderstorm
releases enough latent
heat to power 8000
households for a year
The Water Cycle
• Our weather is an important part of the water cycle
Starting the Cycle
• What causes air to rise?
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Warming of the land
Warming of the sea
Air masses at different temperatures
Mountains
Forests
Fires
Man made objects
Warming the Earth
Air Masses
• Rising air causes low pressure on the
surface e.g. at the equator
•Descending air creates high pressure
on the surface e.g. at the poles
High Pressure Zones
Low Pressure Zones
Wind in Pressure Zones
Pressure Zones
Fronts
Warm Fronts
Cold Fronts
Occluded Fronts
Occluded Fronts
Frontal Clouds
Naming Clouds
• Cirrus (cirro-):
– ‘curl of hair’, fine or wispy
• Stratus (strato-):
– spread out or layered
• Alto:
– high
• Cumulus (cumulo-):
– piled or puffy
• Nimbus (nimbo-):
– rain or thunder
Cloud Types
• High Clouds - 6,000 to 12,000 metres
– Cirrus, Cirrocumulus, Cirrostratus
• Mid -Range Clouds - 2,500 to 6,000 metres
– Altocumulus, Alto stratus
• Low Clouds - below 2,500 metres
– Nimbostratus, Stratocumulus, Stratus
• Wide Range Clouds
– Cumulus (base to top : 2,500 to 12,000 metres)
– Cumulonimbus (base to top: 3,000 to 18,000 m)
Cirrus
Cirrocumulus
Cirrostratus
Altocumulus
Altostratus
Nimbostratus
Stratocumulus
Stratus
Cumulus
Cumulonimbus
Lenticular (Orographic Clouds)
Mountains
El Nino & La Nina
The Southern Oscillation
Blocking Highs
Weather Events
Wind
Tropical Cyclones
Rain
Dew
Frost
Snow
Fog
Hail
Mist
Thunderstorms
Tornados
Dew
At night the ground
loses heat, cooling the
air above it & raising
its relative humidity.
At the saturation point,
water vapour
condenses to form dew.
Frosts
If the temperature
near the ground
drops below zero
before dew is able
to form, then ice
crystals will grow
forming frost
Fog
Fog is simply cloud
formed at the earths
surface when humid
air next to the
ground is cooled to
its saturation point.
Mist
A fog caused
when cold air
moves over
relatively warm
water as in a
swamp
Rain - warm rain process
Large cloud
droplets fall and
capture smaller
droplets growing
larger until they
fall out of the
cloud
Rain - ice process
Ice crystals form
in the cloud and
collect water
vapour. Ice crystal
melts in warmer
air as it falls
Snow
Snow forms when
updraughts in cloud
are gentle and ice
crystals grow as
water vapour
deposits on it
Thunderstorms
Lightning
The discharge of static
electricity that has built
up in a cumulonimbus
cloud. The intense heat
causes explosive
expansion of the air thunder
Hail
Hail grows in
cumulonimbus
clouds in the
presence of
abundant water
and strong
updraughts.
Tornados
Violent funnels of air
that may extend from the
base of cumulonimbus
clouds. They usually
only last a few minutes
but are extremely
destructive.
Tropical Cyclones
Tropical cyclones are
giant heat engines
transferring warmth
from the tropical
oceans into storms
with ferocious winds
and torrential rains.
Wind
The movement of air
in the atmosphere.
It can be caused in
many different
ways and move in
different directions
at different altitudes
NZ Weather
Prevailing Weather
•New Zealand has a fairly regular
weather cycle repeating every 6-10 days.
Prevailing Weather
Westerly winds
blow onto the
western coast all
year round
bringing a great
deal of rain
Tasman Sea Effects
Warm ocean currents
from the subtropics drift
across the Tasman
affecting our weather.
Warm currents allows
plenty of water vapour to
be picked up by the air.
Mountain Ranges
A strong relationship
exists in New Zealand
between relief and
rainfall. Weather can
change rapidly in
these areas.
Sea Breeze
Off-shore breeze
Convergence Lines
Latitude Effects
Altitude Effects
The temperature
drops approximately
6.5oC for every
1000 metres of
altitude gained.
Rain Shadows
Fohn Winds
Strong, dry, warm winds that develop on
the lee side of a mountain range. Their
temperature increases as they descend.
North West Arch
Towering cumulonimbus clouds created by moist
air rising over the Southern Alps in a north wester.
Weather Maps
Isobars
Isobars are lines
on the weather
map that join
points of equal
pressure. They
are spaced at
4mb intervals.
Wind
Generally, wind blows
parallel to the isobar
lines. The direction is
anticlockwise around a
high and clockwise
around a low. The
spacing gives an
indication of wind speed.
Mountain Effects
Air flow blowing
across the
mountain ranges
creates pressure
on one side
causing a
bending of the
isobar lines.
Ridges
A ridge is associated
with High Pressure. It
is evidenced by isobar
lines stretching out
from the centre of the
high.
Troughs
Cold fronts tend to
lie in troughs.
Troughs are
elongated areas of
low pressure and
are usually marked
on the weather map
by a dashed line.
Fronts
There are four
important types of
front - warm front,
cold front,
occluded front and
stationary front.
Sources of Weather Information
•
•
•
•
Www.metservice.co.nz
• www.snow-forecastnz.com
www.nzherald.co.nz/weather • www.snowforecast.com/
resortpages/whakapapa.html
www.tv3.co.nz/weather
• www.mtruapehu.com/weather.
www.xtra.co.nz/weather
html
• www.clear.net.nz/weather
• wwwghcc.msfc.nasa.gov/goes
• www.oneweather.nzoom.com
• ftp.jcu.edu.au/jcumetsat/
sector-d.gif
Interesting Web Sites
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•
•
•
•
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•
www.ozthunder.com
www.bom.gov.au
australiasevereweather.com
www.pnr-rpn.ec.gc.ca/
air/cloudchart
www.vortex.plymouth.edu/
clouds.html
www.uen.org/utahlink/ weather
www.cloudman.com
www.usatoday.com/weather/
wworks0.htm
• Http://asd-www.larc.nasa.gov/ scool
• www.cpc.noaa.gov/products/
analysis_monitoring/lanina/
index.html
• www.cloudgazing.com/index.htm
• www.ghcc.msfc.nasa.gov/goes
• http://weather.communitycomputing.
com/owl/education/index_glossary.
asp
Other References
• The New Zealand Weather Book
– E Brenstrum
• The Wonders of the Weather
– R Crowder
• The Weather and Climate of Australia and
New Zealand
– A Sturman and N Tapper
But Remember….
• Look upon the forecast as being an idea based
on observed data
• It is only an approximation of the real world
• The MetService claims 70% accuracy for next
day forecasts
• Its accuracy for forecasts two days out is 50%
• In the mountains, three or more days
out….????
Farewell
Thank you,
and have a safe trip home.