What is the reason why the weather always changes? Is it too polluted when it rains and sunny when it rains?

No. The sudden change of weather is mostly caused by high atmospheric instability and clear skies and showers. But because the rainfall is only a small cloud, not a cloud system, the precipitation time is very short. It will clear up as soon as the rain stops. The clearer the weather, the faster the temperature rises, which leads to a large increase in evaporation and humidity, which further enhances the unstable energy. There are two possibilities for this weather change. First, it is controlled by warm air for a long time, and there is no cold air coming. Second, the cold air is very strong and the water vapor is blown away at once.