|MadSci Network: Earth Sciences|
Texas A&M University has a number of course instruction pages for their introductory meteorology courses on-line. This particular lesson, Winds and Pressure, is from a class for non-meteorologists and although it is written for freshman and sophomore level college students, I think you will be able to get through it and learn a great deal, including the answer to your question.
Please feel free to e-mail with follow-up questions after you have read through the lesson. Good luck.
Try the links in the MadSci Library for more information on Earth Sciences.