if you're in a training place that doesn't use airconditioning (this is my preference) then weather changes and the general climate is definitely going to affect how hard and what kind of training you can do, and for how long. This variation itself is great training, and it also helps to put a perspective to your ability to go for any length of time under different conditions. So, if you are feeling really down in the prevailing weather conditions, then stick to training that has a pick-up effect.... or train with a partner that makes it fun. Let's say you are working on conditioning exercises - one can go around checking your alignments and gving you feedback, and then you swap which gives you a rest and mental break to recover.