Today after my shower, the drain didn't drain the water. I had several things to do, so left the water in the tub. About an inch, so what.
Home, and the drain still isn't draining. It's 8F outside, and blowing cold. I tried several things.
But what finally worked was to take a two galon water sprayer and fill it with the hottest water I could get out of the faucet. Unscrew the pop up drain plug, and put the water nozzle into the drain. Pump up the tank, and blast hot water as far down the drain as possible.
Gonna leave the hot water dripping tonight. One drip in the kitchen sink, the other drip in the bath tub.
One web site suggested some salt crystals in the drain. Both to melt the ice plug (hey, it works on sidewalks) and also to keep the drain from freezing.