Severe weather is caused by a process called convection, which occurs when warm air is forced to rise to accommodate incoming cold air. Because cold air has a higher density than warm air, when warm air meets a cold front, the air from the incoming cold front displaces the warm air by pushing it upwards, creating the necessary conditions for severe weather or a storm.
As ice crystals and water droplets move chaotically within a cloud, their movement causes the atoms near the top of the cloud to become positively charged, while atoms lower in the cloud's middle and bottom become negatively charged. The movement between the opposing charges creates voltage. The voltage builds up within the system until it reaches a large enough point to force a discharge, resulting in lightning and thunder. Lightning is the visual result, and it releases large amounts of heat. The released heat expands, causing thunder. After the discharge, the process repeats. The rain during severe weather results from the uplifting of warm air. As the warm air rises, it begins to cool and condense. The water droplets resulting from the condensation process are denser than the surrounding air, so they fall to the ground as rain.