Transcript Weather
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 • • • • • • 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/