Hot air rises because when a substance is hot, its molecules are farther apart, which makes the hot air less dense and, therefore, lighter than cooler air. Air is generally warmer nearer the surface of the Earth because of the sun's radiating heat. When hot air rises, it starts to get cooler and eventually it sinks back down to the surface.
Heat itself does not rise, but hot air does. The cycle of rising hot air and cool sinking air is what creates many weather phenomenon, such as tornadoes and hurricanes as well as everyday thunderstorms and breezes. With global warming, the warmer seas and ground will cause this cycle to become exacerbated. There is a good chance that in the future, stronger winds, thunderstorms, tornadoes and hurricanes will become stronger and more frequent.
When the hot air rises, it creates thermals which are pockets rising upward. During the mornings, these thermals do not rise very high before cooling as the air is cooler after a night without radiating heat. Later in the day, the air near the ground gets warmer throughout the day and if the thermal rises high enough, some of the water vapor in the thermal builds up and forms into a cloud.