The atmosphere reacts to everything from rain to sunshine. A snow flurry is weather. Thunder and lightning are weather, too. Sometimes the atmosphere behaves violently, and sometimes it's peaceful and quiet. Either way, it's weather.
Meteorologists record the weather every day.
The constant recording of weather information helps to determine the climate of an area. Climate is the average weather in a location over a long period of time. A place that doesn't get much rain over many years would have a dry climate. A place where it stays cold for most of the year would have a cold climate. Climate is useful for weather forecasting.