If you have an indoor cat, you can prepare an automatic heated water bowl for it. Adding hot water will keep the bowl warm. It can be used for both food and water, but be sure to keep the water clean and not too hot, as excessively hot water can upset the cat's stomach. Water between 27-29 degrees Celsius is ideal. Diligent owners can frequently change the water to ensure it's not cold, and you can also place a heat pack under the water bowl to extend the warmth. 
For stray cats, you can use two layers of styrofoam boxes for insulation. Place a clay pot inside, wrap the clay pot with three layers of plastic bags, and then wrap the plastic bags with a layer of cotton cloth. Place a bowl of hot water in the clay pot for easy removal and cleaning. Alternatively, you can line the styrofoam box with heat packs and place the water bowl on top; this will provide continuous warmth for 10-12 hours. The thicker the styrofoam box, the better the insulation.
