|Average annual temperature map from rsc.org|
The area around the equator is the part of the world that changes the least in temperature. It just varies a lot in temperature at different altitudes, but around sea level at the equator the temperature is usually around 30 degrees (celsius) by day and around 25 degrees at night. The temperature North America and Northern Asia drops as we are getting close to winter in Europe (Yes, most of China is also ice cold around New Years. You would be surprised to know how many people don’t know that!). For the countries furthest south in the world (the southern parts of Argentina and Chile, South Africa, Australia and New Zealand) they can run around in swim shorts and enjoy summer weather while we freeze our asses off. So, the recipe for having 365 days of warm weather is quite simple: stick to the area near the equator or visit the Northern Hemisphere in european summer months and Southern Hemisphere in our winter months.
BUT temperature should not be the only criteria for good weather. Rainfall is another very important aspect of world climate, and this has a bit more complicated explanation including changing of wind patterns which I will not try to explain (then I would only make a fool of myself trying), but I hope the below maps from weatherandclimatechange.eu can give you a rough indication about where and when it is wettest:
In the European spring(left) the rainfall increases in the European and North American region. In the fall(right) it rains the most in Central and West Africa, in addition it is the end or the turning point for the rain season in South East Asia and Central America.
These maps might be a bit confusing and provide little information about when the turning points for the rainy seasons are, but a good page to find out when the best time to go is actually a page called thebesttimetogo.com! If it is live/day to day information about the weather you seek, then you should check out the page yr.no.
Finally the best page to find out the information you need regarding average temperature, humidity, rainfall etc is weatherbase.com. This is a page I use daily and once you get used to it, it quickly becomes the easiest and quickest way of finding out what kind of weather you can expect in the places you plan to visit!