During the summer in the northern hemisphere, the sun is over the Tropic of Cancer and so the sun shines more directly at us and for a longer time in our summer. At the end of summer, when the heat has had time to warm things up and it is still hot, there is finally enough heat to produce thunderstorms. If you look at the figure at the top of the page, you can see how thunderstorms develop. As the water vapor in the air cools, it condenses and forms puffy cumulus clouds. You can see in the picture that at this point the warm air continues to rise and heat is still being released due to condensation, so the clouds can still get bigger even if it is raining.
Thunderheads often have an anvil-shaped top.
Usually we get at least one thunderstorm at the end of summer which produces hail. If you look at these towering cumulonimbus clouds, you can see that they reach way up into the sky. It gets pretty cold up there. What happens is that the water droplets in the clouds are so light at first that they are carried up with the rising air. When these water droplets get high enough they freeze.