Changes in weather are the result of air masses of different temperatures and pressures reacting, influenced by the terrain. Two main types of weather systems exist: high pressure and low pressure. High-pressure systems are associated with sinking air, while low-pressure systems are associated with rising air.
As air rises due to low pressure, it cools, which allows air molecules to form into clouds and release precipitation. Conversely, high-pressure systems are warmer and do not easily allow the formation of clouds.
Another factor that influences the weather is the existence of fronts, or boundaries between warm and cool air. Cool air sinks and pushes warm air higher. If there is enough moisture in the air, it results in precipitation.
Severe weather is caused by stronger or larger fronts interacting. Thunderstorms, for example, develop when clouds react along cold fronts in a electrically-charged section of the atmosphere, and hail occurs when water droplets melt and refreeze, eventually becoming too large to be lifted by updrafts.
Temperature, meanwhile, is directly affected by sunlight. As sunlight strikes Earth, it heats the atmosphere. This heat travels to the ground and disperses throughout the air. Cloud cover blocks sunlight, leading to cooler temperatures in cloudy areas.