Why Last Night Ended Up Overcast

All forecasts stated that last night would be clear skies over Sydney and a great night for observing. However, that was not the case and the sky ended up being overcast. Pretty disappointing as I was personally looking forward to getting the telescope out.

Cloud Work Function for Last Night (Click to Enlarge)

So what went wrong with the forecasts?

The simple answer to that question is Cloud Work Function.

The Cloud Work Function  shows the difference between the moist static energy in the cloud and that in the environment. This pretty much tells us where the cloud is likely to grow. So for a great night of observing we would not want to see any sign of the Cloud Work Function anywhere near Sydney.

Sydney StarGazers looked at what happened last night and saw the culprit was the Cloud Work Function itself. If you look at the picture above, this shows the Cloud Work Function for 10pm last night. The image on the left was the forecast taken at 5pm which indicated that cloud would not be increasing in the Sydney area. The image on the right of the picture was what actually happened and you can see a ribbon stretching along the coast where cloud was actually growing. So instead of the cloud disappearing as was originally forecast, cloud was actually growing and led to the overcast conditions continuing.

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~ by sydneystargazers on April 9, 2010.

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