Why does it blow in?
The "Chimney Effect" theory makes a lot of sense, as everyone knows that hot air rises. However, inward movement continues even when only cold water is used. Therefore, other forces must be involved. The Bernoulli Effect refers to the flow of water downward causing an increase in velocity, which decreases air pressure within the shower. This decrease in air pressure results in an "air pressure differential". One can only assume these "effects" work together, causing lightly weighted curtains to be sucked inward.
Best option? Get a heavier curtain.
One of the best ways of putting an end to shower curtain cling is to buy a heavy weight curtain that is made to stand up to the effects shown in the picture above. These are quite cheap and highly rated.
Click here to read my blog post on heavy weight curtains (some also include built-in magnets or storage pockets)
If you like the curtain you have, check out my blog post 5 Ways To Stop Your Shower Curtain Blowing In. This is useful for people who would rather not replace their curtain. This post discusses several ideas such as adding extra weight or using devices specifically designed to hold the curtain out while you're showering. I've tracked down all of the most commonly used products and popular methods people use to solve this problem.
Temporary "quick fix" ideas:
If you're looking for a quick fix, the image below shows a commonly used technique, but it can be a bit of a pain having to do this every morning.
It's not really a "solution" as wrestling the curtain into position every morning and making the little puddle can be a pain.
These are the other "DIY" ideas I've come across. They're pretty tragic.
Another simple idea I should mention is that the air pressure and hot air rising problems can be minimized to a certain extent by raising the curtain upward a little to allow for some air movement underneath. I haven't tried this approach myself but have read that it can help to minimize the problem.
Check out the other posts in this blog for a wide range of much better ideas and solutions. The tips and tricks presented here will give you several options for eliminating this infuriating problem.