Weather changes are due to differences in temperature, moisture content and pressure between the atmospheric air masses that circle the planet. Generally speaking, weather is determined by the current state of the atmosphere in a particular area, and is also affected by seasonal changes due to the Earth's rotation around the Sun.
Atmospheric pressure is the most important factor in weather change, as high pressure causes the air to slowly flow downwards and spread out, whereas low pressure causes the air to rise, where water vapor then condenses into clouds. High pressure weather systems tend to result in mostly clear, cloudless days, as the sinking air prevents clouds from forming. On the other hand, the air cools down as it rises in low pressure systems, causing the water vapor to turn into clouds and eventually be released in the form of precipitation.
The Earth's atmosphere is formed of these different air masses, which are constantly moving at different speeds. In the northern hemisphere, the air masses move in a clockwise direction, whereas the air in the southern hemisphere moves in the opposite, counterclockwise, direction. The moving and/or collision of these air masses leads to wind and storms, which result in changing weather patterns.