On calm days, the water in the toilet looks completely still. But when it's rainy and windy out, the water looks like it moves and pulsates. Why is this?
Any siphon in Your house will be affected in this way. The inside surface (that you can see) is affected by the air pressure in Your house, the other level is connected to the sewer piping system, wich gets his pressure from a vent of the sewer system.
Both pressures can be affected in many ways by the stormy wind, depending where some passages (like windows not really tight) for the air are located , eg at the windward or leeward side of Your house. The vent of the sewer system can develop pressure variations depending on wind speed by some water-aspirator-like action.
So the siphon in Your toilet works as a pressure differential indicator.