If you live somewhere that gets below freezing in the winter, you’ve probably heard stories of water pipes bursting during cold weather.

It happened to my house one time. But you almost never hear about the same thing happening to fire hydrants.

Well, that is because we use different types of hydrants in different places, depending on how cold it gets. And the ones in cold places don’t have any water in them to freeze. Places with mild climates, like the southern US, tend to use what are called wet barrel fire hydrants.

Water from underground pipes flows all the way up into the tall, above-ground section of the hydrant, called the barrel, even when the hydrant’s not in use. When firefighters need water, they just turn a valve near the top, and it comes rushing out. But wet barrel fire hydrants would be terrible in places with colder winters, because freezing temperatures outside the barrel would freeze the water inside, too.

Water gets bigger when it freezes, so expanding ice inside a hydrant would push against the inside walls and squeeze any liquid water left in the hydrant, eventually making the hydrant crack or explode. This is why pipes burst in the winter: They’re not strong enough to withstand the pressure exerted by expanding ice. And it’s why wet barrel hydrants would explode in the winter, too, if we used them everywhere.

So to avoid that mess, dry barrel hydrants are used. As you might guess from the name, dry barrel hydrants are normally dry, meaning they don’t have any water in the barrel when they’re not in use. Instead, the water stays underground in water mains, which are buried below the frost line, the depth where temperatures don’t drop below freezing, even in the dead of winter.

In most places, that’s just a couple meters below your feet. In the U. K. and a few other countries, they take this to the next level, and they don’t have any part of the hydrant above ground at all.

All you see is a colorful marker or metal plate. When firefighters need water from a dry barrel hydrant, they turn a valve that lets water up into the barrel and out of the hydrant, just like with wet barrel hydrants. But then any water in the hydrant after use is drained, so there is no freezing water, and therefore no rupture.

It might seem like dry barrel hydrants would be useful everywhere, but they have a couple of downsides. Since part of the valve is so far underground, they can be harder to repair than wet barrel hydrants. And the fact that they’re basically empty when not in use can also be a problem, because things can be shoved inside them.

In cities, hydrants sometimes get clogged by people using them as makeshift trash cans. Which is why they’re not used everywhere. But in colder places like here in Montana, that is a small price to pay for hydrants that don’t catastrophically explode every winter.

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