Hot and cold weather can be a factor along with natural causes of fish dying although its not time to panic if you see a fish floating in your pond, its not uncommon to see this from time to time, if fish start to die on a regular basis over a period of several day then its time to be concerned.
One of the causes of fish dying is oxygen depletion mainly in the summer months, especially in a fertile pond and the dying of algae, this is important to have one end of your pond quite deep as this is where little or no dissolved oxygen will lay.
If for any reason the water is mixed with the top layer by way of wind and rain and algae/plants dying then the dissolved oxygen will start to deplete and drop to a very low level hence causing the fish to die.
When weather gets extremely hot you need to make sure every morning that the fish are not stressed or at the top gulping for air, if so you need to stop feeding and aerate the pond, if your pond is fitted with a filtration system, fountain or waterfall then you should not get a problem, if not pour water into the pond from a rain but (not tap water if you can help it) from about a metre high to create oxygen.
If you have to use herbicides or pesticides be careful as to how much you use at any one time around a pond as they can be very toxic to fish.
In extremely cold weather still ponds tend to freeze over unless you have a pond heater, once again as mentioned, one end of your pond needs to be relatively deep, this allows the fish to drop down to the area where there is little or no dissolved oxygen, hence they should be able to survive, any pumps or filtration systems are normally turned off to stop any damage occurring with the freezing water.
Return from weather to maintaining ponds