Plotting a Cold Change

Saturday 23rd January 2010 saw a classic heatwave/cold front event occurring up the eastern coast of New South Wales Australia, and I observed from Sydney, watching things progress during the day via the internet, as well as from my own home, where I have a view across parts of Sydney.

Oz Weather v2.1 introduced graphing of weather history as a new feature, and the graphs from that day show the change very clearly indeed. The following graph is a composite of the different ones available in Oz Weather, although I have overlaid a transparent bar indicating the time when the main changed occurred.

A summary of the changes:

  • Temperature dropped from about 41°C to 22°C.
  • Humidity jumped from 10% to 85%
  • Wind jumped from 30km/h to 65km/h with gusts to over 95km/h just as the change came through, and the direction shifted from NW to S.
  • Interestingly, the pressure started to rise an hour or so before the main change, and there was a little rain from some thunder cells that developed following the change.

The Doppler (wind) radar also showed the approach of the wind change very clearly. Unfortunately I didn’t save a graphic from when the change was passing right through Sydney, but an earlier shot shows the change passing through Stanwell Park, to the south of Sydney.

The key point here is to note that blue indicates wind towards the radar location (centre of crosshairs) and yellow indicates wind away from the radar location. So this is showing strong NorthWest winds (blowing offshore) over the Sydney region, but from the South at Stanwell Park and below. This picture was a lot more striking as the change passed through Sydney itself, but I’ll have to wait for another event to show that off better!

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