Tragically, the BBC have changed the output of their weather data RSS feeds, and no longer include the information needed to produce these maps. If I have time, I'll try to find another way of doing it, but for now the weather is boring.
The number of weather symbols is dependent on the zoom of the map. To avoid drawing too many symbols (particularly at high zoom levels), symbols are only added for the visible portion of the map, meaning that while you're dragging you don't see any new ones appear - they'll be repositioned when you finish dragging. When you finish dragging or zooming you may have to wait a few seconds before the symbols update (and you get control back, at least on some browsers) - I'm filtering the number of UK weather reports to get reasonable speed, although this means that some parts of the country may be a bit thin on detail.
When you open an info window, and the map has to scroll to fit it, the symbols won't redraw. They'll redraw when you close the info window (unless you have it close automatically by opening another info window).
The weather data is grabbed periodically from the BBC using a python script, which also figures out (based on phrases in the report) which symbol to display for each point. Created using Google Maps and BBC weather data, supported by backstage.bbc.co.uk. Weather symbols from jschreiber, under the Free Art License. Thanks to Matthew Somerville for the regular expression that highlights useful bits of the weather report in bold.
There's also a world edition, and a widgetiser. Suggestions / bug reports welcome. There's a known bug in the world map that if you come up with a view that straddles the +180/-180 longitude, no symbols appear. My maths is wrong somewhere :-(
Copyright James Aylett 2006.