Why does strong black tea get cloudy when you let it sit for a while?

Tea is a “brew”. I’ve read the “cloud” is all abut tannins and junk and all you have to do is add some boiling water to your “brew” to clear it up. I’ve also heard, that like any “brew” there are bacteria in it and that can also cause cloudiness. Either way, the boiling water should clear it up and it’s safe to drink either way, although the “clouds” tend to make the tea chalky.

This is caused by precipitation of the tannins out of the solution. While the water is hot it has no trouble holding the tannins in solution, but as it cools the solution becomes super-saturated and finally the tannins are forced to solidify as minute crystals, which causes the cloudy appearance. If you reheat the tea, it will clear.