Toilet Water Level

I noticed yesterday that the water level in my WC was very low - not the feeder tank but in the actual toilet. It is on the ground floor. The upstairs toilets were fine. No amount of flushing would bring it back to the normal level. It's just a U bend into the sewer drain. I can only think that it's some form of suction from the sewer. Upstairs toilets feed into a Downpipe that has an air vent. It's ok today. Any explanation would be greatly appreciated.

The level in our toilet tends to rise and fall - sounds mad I know. It also depends for some reason on the wind & if it's blowing hard outside the level seesaws up and down the toilet itself. I would be interested to hear anybody else's response on this one also!

Try these previous threads.
Leo