When to go
Following are some tips that should be useful...or you can just go straight to my climate chart.
What if you get to your dream destination, only to find that it’s freezing, raining, or unbearably hot? Bad weather can ruin a trip, so it's critical to ensure that you're going at a time when the weather should be nice.
If you’re doing a longer and/or round-the-world trip, then identify a general itinerary that enables you to “follow the sun,” i.e., get to places when the weather should be good for traveling.
For travel in the tropics, I’ve heard a few travelers say they intentionally go when the weather is “off,” as prices are lower and places are less crowded. To each his own, but I absolutely disagree. When weather is “off” in the third world, dirt roads turn to mud and transportation can become problematic; mosquitoes appear in much greater numbers; car/bus travel becomes more dangerous; your precious photos may have gray skies (or rain) instead of blue skies; etc. If you’re going to take the time and money to travel, then do it right and hit places when the weather is good.
So, with that, how do you know when the weather is good? This is a question that tends to be far more difficult to answer than you’d expect, and that is why I created my climate chart. Guidebooks these days tend to do a decent job of providing guidance on “when to go,” and this basic info is available at places like lonelyplanet.com. You can get more detailed climate info at worldclimate.com, which will tell you average temperatures and rainfall amounts.
Once you have decided where to go and when, then you need to sort-out what travel documents you need.