I don't think there is anything that actually repels the ticks. They still get their heads into the skin of the animals and fill up on blood. The best defense I have found for my pets is the flea comb. While they are still on the patio I run the flea comb through their fur after they have been outside to get the loose ticks off. If I find any attached to the skin I pull them off with the tick puller. I'm not going to put additional chemicals that do not even work on my pets when I can get better results getting rid of the foul creatures myself.
Hard ticks have a 4 stage life cycle. They start out in the spring as a small spider like bug. After feeding on an animal they go into their next stage. They require a blood meal in order to complete each stage of their life cycle. That is why certain time of the season you may see more ticks than at other times. It has little to do with the weather and more to do with the life cycle and how well they are able to find their next meal to go on with the life cycle.
CDC - Tick Life Cycle and Hosts - Ticks