How much different in temperature and pressure can a microclimate be than the surrounding region? In other words, if you had a dry region but then a valley within a ring of mountains that isolated the local environment could the valley have distinctly different humidity, temperature and pressure and how far much different could it get?
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.