Heavenly Weather

The 18th Century writer Samuel Johnson is quoted as saying, "When two Englishmen meet their first talk is of the weather". It seems that little has changed since that time as the subject of the weather still inevitably crops up in everyday conversation across the nation. The British weather is not usually that kind to astronomers and a cloud-free night is a treasured commodity. To try and find out more about our restless climate, the Society invited Ross Reynolds from Reading University to talk to us about our "Heavenly Weather".

Using the latest computer technology, weather centres can predict with some degree of accuracy what we can expect in the next few days or so across the globe. The forecasters input data into a complex computer model and then fast forward the program to try and predict what will happen in a given number of days. At present the limit of the forecast's reliability is ten days. Any further than this and the errors in the model add up to make the predicted outcome too unreliable to use.

The software model breaks the atmosphere into 60 levels and each level into boxes of 40km resolution. You can appreciate that this is not a perfect simulation of the atmosphere if you understand that the size of a typical thunderstorm is only 10km across, which is far smaller than the resolving power of the model.

To try and allow for these problems, the meteorologists run 'ensemble models'. These are the same computer models but with a lower resolution of 80km boxes. Having a lower resolution means these models can be completed in a shorter time period and so the forecasters can repeat the process many times with added weather events such as thunderstorms thrown into the mix.

To obtain the data that these models rely on, a worldwide network of automated weather stations transmit their readings on a regular basis. Most of these are land-based instruments powered by solar energy that communicate their data via satellite links. Apart from these stations data is also collected via weather balloons, aircraft, ships and buoys.

With the growth of the Internet much of this information is available, usually in map form, at various websites. One valuable resource is the website of Dundee Satellite Receiving Station (www.sat.dundee.ac.uk) where you can register for free to look at some stunning satellite pictures. For a more general 5-day forecast there is the BBC weather site (www.bbc.co.uk/weather). You can find out the weather where you live by entering your town or postcode.

Another more technical site is the European Centre for Medium-Range Weather Forecasts (www.ecmwf.int), where you can examine weather charts for the next 3-6 days. Ross suggested that we could use either this site or other weather maps to keep an eye out for air currents coming from the North or Northwest. This maritime air from the north pole or Greenland would be dry, clean and cold, producing crisp clear skies where observing conditions would be at a premium.

On August 27th of this year the planet Mars will be making its closest approach to Earth for over 59 thousand years. Hopefully the fates will be kind and we will be able to view this naked-eye jewel through a window in our weather.