If you are cover weather in any of your classes, the Smithsonian Weather Lab is a good place to start when looking for online activities for weather. Students predict weather patterns based on the information using ocean currents and air mass.
Following their predictions, the lab will inform them if they were correct in their prediction.
This activity would be great for introducing the basic concepts before going into further detail about meteorology.
There are resources for grades K-8, in other areas beyond weather as well.