Weighted average of station data

Hi there!
I'm not too good at statistics (tho I try!) so I hope I'm not asking a really dense question. I'm trying to do my dissertation at the moment and I'm studying the island of Tenerife - a small island in the Atlantic - and daily precipitation measurements taken at four stations on that island over a 30 year period. I want to find the average daily precipitation for that 30 year period. I've done a bit of research and ruled out most methods (e.g. Thiesson's method) so I'm really left with taking some sort of average or mean. But the stations are not evenly distributed across the island. Can I take a weighted average of the data? If so, how?! Obviously the precipitation received at any station is more than just a function of its geographical location, but also its elevation and other factors. Is it normal to factor these in? Again, if so, how?

ANY help would be greatly appreciated - I'm really struggling here! If you think this makes matters too complex (bearing in mind I have 4 stations on an island of quite a small area, if varied climate) and I should just stick with taking an arithmetic mean, please let me know!

Many thanks,