Low pressure weather systems. Warm, wet air begins to rise into the air, and the more it rises, the cooler it becomes, and water vapor in the air forms drops- this is called condensation. The drops join together to form rain, hail, sleet or snow.
For a thunderstorm to form, the air has to be full of moisture, and there has to be an approaching cold front. Finally, the warm air that rises has to be warm enough to stay warmer than the air is passes through. When these three conditions are met, clouds form and a storm begins.