How different can microclimates be than their surrounding areas?
Elevation is often responsible for the biggest differences; you can have snow and glaciers on the top of mountains in warm climates. South America can show you how extreme the differences in precipitation can be: rain forest on one side of the Andes Mountains, world's driest (non-polar) desert on the other. So really it depends on how high and complete your mountain ring is.
Here is a decent book on the subject.