Transcript weather maps - Yr11Geography

Climate & Weather
WEATHER MAPS
Weather and Climate
Weather is the state
of the atmosphere at
any one place or time.
This
includes:
humidity, temperature,
sunshine hours, cloud
cover, precipitation.
Climate is the
average or long
term weather
conditions of a
region. It is the
result of years of
research into data.
Barometric pressure
• Barometric pressure or atmospheric pressure
is the pressure exerted by the weight of the
air on the earth’s surface.
• Atmospheric pressure is measured in
hectopascals (hPa).
• Atmospheric pressure is mapped to form
isobars.
Isobars
• Are lines on a synoptic chart joining places of
equal atmospheric pressure.
• To estimate atmospheric pressure you need to
refer to the two nearest isobars. e.g. if point
“A” is situated half way between 1006 hPa and
1008 hPa, “A” would be > 1006 but < 1008
hPa.
Barometric Pressure readings
How do they
get the land
and sea
readings?
What do all
these dots
form when
they are
linked?
Isobars
How do you work
out which are
high pressure cells
and which are low
pressure cells?
An isobar is a line
on a map joining
places of equal
atmospheric
pressure
Synoptic Charts
• Synoptic charts or weather maps provide a snapshot of
the weather experienced at a particular place at a certain
time.
• They show information relating to air pressure, air
masses and fronts, cloud cover, wind speed, wind
direction and rainfall.
• This information allows us to make predictions relating
to temperature, humidity, ocean conditions and the
likely weather for the next few days.
Synoptic Charts
Synoptic
Charts
What is the wind speed and direction?
What is this feature?
What does the shading
refer to?
What is the air pressure?
Pressure cells -
are defined by the patterns formed by isobars.
•Are these high
pressure cell s
or a low
pressure cells?
•How can you
tell?
High pressure cells
(anticyclones)
• occur where atmospheric pressure increases
towards the centre of the system. High
pressure systems are characterised by light
winds, clear skies, dry weather and a high
diurnal (daily) temperature range.
• A ridge is an extension of high pressure from
a high pressure system.
High pressure cells
•Why does the air descend?
•Why does the air moves
outwards?
Typical weather associated
with High pressure
• light winds
• clear skies, dry weather
• higher diurnal (daily) temperature range
i.e. hot days (sun rays maximum effect)
• cool nights (because of lack of cloud
cover).
• Frosts are likely in winter.
Clear skies allow the
sun’s rays to have
maximum effect.
Daytime temperatures
are usually high.
At night, clear skies
allow heat to be lost
causing temperatures
to fall. In winter
frosts are likely.
Low pressure cells
(cyclones or depressions)
• occur where atmospheric pressure
decreases towards the centre of the
system.
• Low pressure systems are associated
with stronger winds, cloudy skies, rain
• a lower diurnal temperature range.
• A trough is an extension of low pressure
from a low pressure system.
Low pressure cells
Ascending air
•Why does the air rise?
•Why does the air moves
inwards?
Typical weather associated
with Low pressure
• strong winds, cloudy skies, rain and a lower
diurnal (daily) temperature range i.e. mild
temperatures (sun does not have maximum
effect because of cloud cover, heat trapped in
at night).
• Frosts are unlikely.
Cloudy skies block
some of the sun’s
rays. Daytime
temperatures are
relatively low.
At night, cloudy
skies prevent heat
loss into space.
Clouds reflect heat.
In winter frosts are
unlikely.
Tropical cyclones (T.C.)
• Are a very intense low pressure
system. They generate very
strong winds, produce rough
seas and heavy rains
• Wide spread damage occurs if
they make landfall (contact with
land).
A cross section of a Tropical Cyclone
Wind speed
• is determined by the closeness of the
isobars. The closer the isobars the stronger
the winds.
• A steep pressure gradient exists where
isobars are close together.
• If isobars are far apart a place will generally
experience light winds. This is called a slight
pressure gradient.
Note the spaced isobars
Note the closeness of the isobars
Wind direction
• Wind is named according
to the direction the wind is
coming from, e.g. south =
southerly.
• Wind direction is largely
determined by the location
of pressure systems.
Wind direction
Remember we always refer to where the wind is coming from!
Where has the wind come from?
1
2
Wind Direction – Anticyclones
(High Pressure cells)
• In the southern hemisphere winds blow
outwards in an anticlockwise direction from
areas of high pressure.
ANTICYCLONE = ANTICLOCKWISE WINDS.
Wind blows outwards in an anticlockwise
direction in areas of high pressure.
ANTICYCLONE = ANTICLOCKWISE WINDS.
Wind Direction – Depressions
(Low pressure cells)
• In the southern hemisphere winds
associated with low pressure systems
blow in a clockwise direction towards the
centre of the system.
LOW T. C. = CYCLONE = CLOCKWISE WINDS.
Winds blow towards the centre of a low
pressure system and in a clockwise direction
LOW P. C. = CYCLONE = CLOCKWISE WINDS.
Wind indicators
• Wind indicators show wind speed and
direction.
• The feather indicates wind speed
• The shaft indicates the direction from which
the wind is coming
• The black dot is the place that is experiencing
the windy conditions
Identifying wind direction
• First - Identify the type of pressure system.
• Southern Hemisphere
LOW =clockwise
HIGH = anticlockwise.
• (the reverse is true in the northern hemisphere)
• Wind, generally speaking, moves across the
isobars
Sea conditions
SYMBOLS FOR
SEA CONDITIONS:
VR = Very Rough
R = Rough
M = Moderate
SL = Slight
Sea conditions are •
closely related to wind •
speed or strength. The •
stronger the winds the
•
rougher the seas and
vice versa. The best
method for working
out sea conditions is
to
look
at
the
closeness
of
the
isobars.
Close isobars = strong winds = rough
seas.
FRONTS
• A front is the leading edge of change. They are
the border zone between warm and cold air.
There are two types of fronts:
• Warm fronts occur when warm air is pushed
into a cold air mass. These are very rare in
Australia. They result drizzle and light rain over
a few days.
A warm front
Cold fronts are far more common in Australia
and occur when cold air is pushed into a warm
air mass. This forces the warm air to rise causing
a number of changes:
(a) Decrease in temperature: cold air replaces
warm air.
(b) Change in wind direction and sometimes
speed.
(c) Build up of cumulonimbus or rain bearing
clouds.
(d) Increase in humidity: corresponding to a build
up of clouds.
(e) Probability of rain.
Make up an acronym to remember these changes.
A cold front
Temperature and humidity:
• are related to the wind. Winds pick up
the characteristics of where they form –
sea or land – these are called source
regions
Remember:
• Winds off the land are generally hotter
and drier.
• Winds off the sea tend to be cooler and
wetter (greater moisture = greater
humidity).
Types of rainfall: Convection
Types of rainfall: Orographic
Types of rainfall: Frontal
Seasons
• are determined by the location of various
pressure systems.
Remember:
• Summer = Lows over Northern Australia
and Highs over Southern Australia.
• Winter = Highs over Northern Australia
and Lows over Southern Australia.
Summer weather map
Where are the different cells located?
Winter weather map
Where are the different cells located?
References
• B. Crowder, The Wonders of Weather, AGPS,
Canberra, 1995.
• G. Nagle and K. Spencer, Advanced Geography,
OUP, Oxford, 1997.
• G. Underwood, Geomasters, Brooks Waterloo,
Melbourne, 1988.
• L. Bryant, Studying Contour and Weather Maps,
Jacaranda Press, Milton, 1990.
• Special Thanks – Mr N. Shute Knox 2003