Basically, because I also like sleeping outdoors (preferably without a tent), I do look for:
- minimal nightly temperature
- maximum daily temperature
- number of rainy days
- dew point (note that humidity is not at all good parameter, because how it feels depends on temperature!). The higher, the less comfort - then you would prefer more functional materials over cotton.
In some countries, like Spain & Morocco, the nightly temperatures could be surprisingly low. We were sleeping in tent in Spain at the beginning of April and it was only 3C! But if I go to Spain in September, I usually take only 3 t-shirts, shorts and sandals :-)
When going to tropical destinations, I'm trying to find out when there are rainy and dry seasons. It might sometimes be tricky, e.g. on Philippines, where this doesn't work. The east (Pacific) coast is almost always rainy, while the Indian (west) coasts are mild with nice weather during dry season. In the east coast, like Samar & Leyte, the Philippinos say they have only rainy season and wet season. :-)
As regards to what to take with yourself, unless you go to Sahara, always take something light and little waterproof to prevent from wind and light rain with you. Even if I go in Spain in summer, it comes in handy as there can be strong and not so hot wind near the sea.
EDIT: I forgot to add about colors: if you plan going outdoors, it's much better to have neutral colors like khaki, dull green or grey. The same applies for bag and other equipment. You will see much more animals and also less people will see you, which is good if you plan to sleep outdoors (the less people know where you sleep the better of course, especially if they are owners of the land :-)). When expecting hot weather, I prefer light colors over dark.