Observing The Weather
It should be stressed from the start that besides the analysis chart, forecast charts are generated by mathematical equations. Whilst they are fairly accurate, especially in the short term, they are our best guess at what the weather will do in the future. Unfortunately the mathematical models don’t control the weather so if you want a more complete picture of what the weather is doing you should observe real data.
The internet has made it possible for anyone to make real-time observations of the weather. Commonly available weather observation tools are satellite images, ground based weather gauging station data and radar images. Data collected by weather balloons is valuable for interpreting the conditions in the upper atmosphere. The Australian Bureau of Meteorology website (www.bom.gov.au) presents most of this data as it becomes available.
Satellite Images
The satellite images available for Australia is generated by a geostationary weather satellite. The Bureau of Meteorology publishes both visible and infrared satellite images. The infrared image is more useful for assessing the weather. The infrared image depicts heat. Lighter colours represent colder areas and darker colours represent warmer areas. When you combine the current synoptic analysis with the satellite image weather features, and in particular snow bearing weather systems, are very easy to identify. Cold fronts are visible as a spiral - hook shaped cloud band and are typically followed by areas a specked cloud (cumulus), which is indicative of cold air. A low pressure system is typically surrounded by a spiral of cloud. Some satellite images showing typical winter patterns are presented below alongside a corresponding synoptic chart.
So if you know how to identify a cold front on the satellite image you can observe its movement across the alps. Unfortunately it won’t tell us how cold the front is or how much precipitation is resulting from it.
Weather Gauging Stations
Whilst satellite images are useful for observing the progress of frontal systems across the continent and identifying cloud bands that may produce substantial amounts of precipitation they don’t provide us with specific atmospheric conditions like temperatures and precipitation. In Australia, small changes in these parameters can mean the difference between precipitation falling as snow or rain. That’s where data collected at weather gauging sites comes in handy. By looking at temperature and precipitation data you can estimate how much new snow may fall (or have fallen) and what the quality may be like.
There are gauging sites scattered all over Australia. Some are automated and provide real-time data on the hour. Others are manually operated and provide data at set times throughout the day. You can find this data by following the weather observations links on the Bureau of Meteorology website. Stations that are of interest for observing weather in the alps are discussed below:
- The automated weather station at Mt William is located in central western Victoria and is at a height of approximately 1150 m above sea level. It is useful for providing an early assessment of the air temperature preceding and following a cold front and assessing the amount of precipitation that may be on its way to the Alps;The automated weather station at Lookout Hill is located further east of Mt William. It is currently off-line but similarly to Mt William, it provides an early indication of what can be expected in the Alps;The automated stations at Mt Baw Baw, Mt Buller, Mt Hotham and Falls Creek provide data for the Victorian Alps; and
- The automated stations at Thredbo, Cabramurra and Mt Ginini provide data that is relevant for the NSW Alps. However the Thredbo gauging station is notoriously bad for recording actual precipitation due to its exposed position high on the Ramshead Range. For snow bearing systems that approach the mountains from the west some of the gauging sites to the west of the divide provide a better indication of precipitation that may have fallen up in the mountains. For example, the site at Khancoban is very useful for assessing how much precipitation may have fallen on the Main Range and in the main NSW ski resorts. Similarly for systems that approach from the south or east, gauging site to the south or east of the Alps will provide relevant data.
Weather balloons are released at selected gauging sites to measure pressure, humidity, temperature and winds in the upper atmosphere. These are useful parameters in determining the strength or a cold front. The conditions in the upper atmosphere are the engine room of snow bearing systems. Unfortunately this data is only available for paying customers.
Weather Radar
The weather radar is useful in assessing where precipitation is falling and when coupled with satellite image can give an idea of which bands of cloud are actually producing precipitation at ground level. A series of radar images can also be useful in assessing the ground-speed of a frontal system.
Weather radars work in the same way in which other radars work. Instead of identifying objects they identify areas of precipitation. The radar sends out pulses which reflect off precipitation. The degree to which the pulse reflects is dependant on the size of the particles, how many particles there are, what state they are in (solid-hail, liquid-rain) and what shape they are. After making many assumptions about these factors and others, the approximate precipitation rate at the ground can be estimated and is shown on the radar image as a colour scale.
There are several weather radars located in south eastern Australia that can be used to assess the amount of precipitation heading to the mountains. However, it should be noted that these radars do not provide an accurate assessment of the alpine regions. The primary reasons for this are:
- The radars are typically located greater than 100km from the mountains. As you move further away from the radar, the returned echo becomes weaker. This occurs because as the radar beam broadens with distance, the proportion of the beam that is filled with rain lessens and reduces the echo intensity. The radar beam is also further from the ground with distance (partly because of the Earth's curvature, and partly because the beam is angled upwards by a fraction of a degree), thereby missing the lower parts of the rain. This is especially the case at the tail of a snow bearing weather system where orographic precipitation hugs the top of the peaks and isn’t high enough to be picked up by the radar; and
- The presence of mountains within the range of the radar can block part or whole of the radar beam, thus significantly reducing the echo intensity from rain on the other side of the mountains.
Although they don’t provide an accurate picture of what’s happening in the mountains they do show what’s heading towards the mountains so you can still get a good idea of when good falls may be expected.