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Need to understand
Local influences on weather
Tasmania’s weather!!
Weather
Week 6,
This is the small-file version of my PowerPoint
presentation. Please e-mail me if you want the
larger file with all photos.
John Todd
We have talked about broad scale
climate patterns and introduced some of
the factors driving them, now we move
to localised weather patterns where
broad scale synoptic systems are
influenced by local topographic features
and human developments. We are also
interested in day to day variation as
well as longer term averages.
Factors influencing weather
• Variation due to synoptic patterns
– High/low pressure, wind direction
• Variation due to cold or warm
fronts.
• Variation due to land/water
influences
• Variation due to
topographic features
• Human influence
– heat islands
Tasmania – points of interest
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Rainfall – normal and extremes
Temperature
Wind - extremes
Sunshine
Frost
Thunder storms
Remember: global average
Rainfall is about 1000mm
Bureau of Meteorology
An excellent web site
www.bom.gov.au
Rainfall
Why high in West, low in East?
Prevailing westerly winds – Roaring 40s
This air picks up moisture as it passes over the ocean
Topography
Uplift  cooling  clouds
 rain
Descending air  warming
 evaporation  dryer
This illustrates one important local influence
on rainfall – orographic lifting, with a rain
shadow on the down-wind side of the
mountains
See picture on lecture handout or on page 207 of Christopherson
This is an important feature of air movements that you should
understand.
Highest ever
40.8oC
Bushy Park 1945
Hobart 1976
Lowest ever
-13.0oC
Central Plateau
1983
TEMPERATURE
Summer
Highlands cooler than lower areas
summer and winter as expected
because air temp. decreases with altitude
The summer maximum and winter
minimum temperature patterns show
some important differences. Why?
Strong coastal influence on winter
minimums (water cools slower than
the land)
Latitude and warming effect as dryer air
descends over mountains
Winter
Tasmania’s Temperature Illustrates:
• Obvious seasonal effects
• Influence of wind direction (warm off
Australian mainland, cool off Southern Ocean)
(large influence on day-to-day temp.)
• Altitude
• Moderating effect of coast
• Influence of rising (cooling) or falling
(warming) air
• Latitude (off-set by Bass Strait - Why?)
Wind
• Storm damage
• Wind resource
wind farms, King Is,
& far north-west
Influence
of cloud
Halls Creek very different pattern. Why?
Note that in summer months, not much difference, but in
winter a big difference.
Frost
• Above 300m elevation, frosts can occur
throughout the year in Tasmania
• Below 300m it is rare to have frosts in
summer
• Why do frosts occur?
Photo: Burroughs et al. 1997
Frost Formation
• Frost, clear, cold night
Heat radiating out into space
Layer of very cold air
close to the ground
No frost, under
tree or when cloudy
World-wide – 8 million lightning flashes per day
(100 every second)
Earth’s electric field of 100 volts per metre
Cold Fronts
Look at the diagram on the lecture handout or
on page 209 of Christopherson
Christopherson p 209
Moving, on average,
500km per day towards
the east
low
Visible light image, 8 April 2002, Bureau of Meteorology
Summary
• We have applied some of the basic
principles of atmospheric air movements
to gain an understanding of Tasmania’s
weather
• Temperature
• Rain
• Wind
• Frost/thunder storms
Next Week
• Last of the climate/weather lectures
• Climate change
– Human influence on global climate
• Information
– Australian Greenhouse Office
web site –www.greenhouse.gov.au
– Bureau of Meteorology
web site – www.bom.gov.au/climate/