Cloud Forecast Accuracy

13

Sep

Cloud Forecast Accuracy

Submitted by Phil

This set of images compares the actual infrared satellite image for midday Saturday 13th September 2008 with forecasts made on the same day and up to 5 days ahead. The forecasts made up to 2-3 days ahead are quite accurate, so that a reasonable estimate of the astronomy prospects for a Saturday night could be made on the Thursday beforehand. While the general trends are still correct in the 5 day forecast, the details for a particular location become much less accurate.

Saturday 13th September 2008	Same Day Forecast
1 Day Ahead	2 Days Ahead
3 Days Ahead	5 Days Ahead

7Timer APanel Forecast

The APanel Forecast from 7Timer is based on the same US Global Forecast System (GFS) model used to generate the cloud forecast maps above. There is however some re-intepretation of the data which results in slightly more optimistic forecast since the GFS model can be a little conservative. For the same Saturday in the forecasts you can see that APanel also shows clear sky for Melbourne at 1300.

Saturday 13th September 2008

APanel Forecast

Cloud Levels and the difference between Infrared/Visible Satellite Images

Low-level clouds are generally classified as below a height of 2,000m (6500ft), mid-level clouds from 2,000-6,000m (6,500-20,000ft) and high-level cloud generally above 6,000m (20,000ft).

This set of images shows two different types of satellite images together with forecasts for cloud at different height levels. Although the difference is not very well shown in these images, infrared satellite images show high cloud very strongly but can miss some low cloud because the temperature of the cloud is very similar to the ground surface temperature. Visible images can be more useful for showing low cloud in these circumstances. Conversely, thin high cloud can be hard to see on visible satellite images.

For astronomers looking at night time satellite images, infrared is your only option!

Visible Satellite Image

Low Level Cloud Forecast

Mid Level Cloud Forecast

Infra-red Satellite Image

High Level Cloud Forecast

US Air Force MM5 Model

The US Air Force also has a cloud forecast for the Australian region available on their website. It shows cloud top temperatures (higher cloud = colder temperatures) and appears to only plot the temperature where cloud cover is above a certain level so it tends to under-estimate cloud cover. However, it is useful for comparison with the GFS model, is much easier to access and appear to be updated more rapidly.

Saturday 13th September 2008

Air Force MM5 Model Forecast