Question

Outdoor water pipes have to be drained or insulated in winter in a cold climate. Why?

Short answer

Outdoor water pipes have to be drained or insulated in a cold climate to prevent the water within from freezing and expanding, which could crack or burst the pipes.

Step-by-step solution

Understanding the Physics

In cold climates during winter, outdoor temperatures often drop below the freezing point of water (0°C or 32°F). Water, like most matter, undergoes thermal expansion. It expands when heated and contracts when cooled. However, when water turns into ice, it expands.

Consequence of Frozen Pipes

When water inside a pipe freezes and turns into ice, it expands. If the pipe is full of water with no room for expansion, the increasing volume of the ice can cause the pipe to crack or burst.

Explaining Insulation / Draining

To prevent this, pipes should either be drained - removing water so that there's nothing to freeze and expand - or insulated. Insulation works by slowing down the heat transfer between the pipe and its cold surroundings, consequently diminishing the risk of freezing.