Not every cat is fortunate enough to have a warm home, a full bowl of food every day, a bed near the heater, and a human's loving arms. Stray cats don't only have to worry about low temperatures, finding food is also difficult. They hunt for food, but the birds and rodents they feed on are more hidden in winter.
It's not in anyone's powers to take all these cats home, and shelters are also not capable of dealing with the situation. It should therefore be the responsibility of us all to not let uncastrated cats out so that we do not contribute to an increase in their population, otherwise we'll never get the situation under control and many cats will suffer unnecessarily. But this is a long-term plan; how can we help cats today?
It is essential to provide them with enough nutritious food to build sufficient subcutaneous fat. So if you know about a stray cat in your area, or a whole colony of cats without an owner, you can place a bowl of food in places where they appear. Cats need a lot of energy and nutrients to survive, so it will be most efficient to choose high-quality, ideally high-energy food. Because of freezing temperatures, kibble will be better than wet food, which could freeze quickly.
Water is equally important as food for stray cats, especially on days when temperatures drop below zero. If you can, you should give them fresh water twice a day. The bowl with water or food should be placed somewhere dry and downwind, and don't place it directly on the ground. A piece of polystyrene can be used as a great insulator.
Rainy and chilly weather is very unpleasant and exhausting for cats, so if you can, provide a warm and dry place for them to rest and regain their strength. If you don't have a barn or shed that you would be willing to provide for the cats, another option is to purchase a polystyrene shed. You can either make it yourself or buy it from Pražský spolek ochránců zvířat (Prague Animal Protection Society). They also recommend laying out hay in it. Old textiles are not suitable for this purpose because they retain moisture and mold.
Another certainly helpful way to help cats is to support one of the cat shelters or civic associations. A friend of mine captures stray cats for one of these associations. Some cats are adopted, others are castrated, marked and return to the place where they were found. Just remember that only a trained certified person can capture stray animals by law, anyone else faces a high fine.