Have you ever been walking down a busy city street and see steam rising up from the sewer? This phenomenon is sometimes replicated on movies for authenticity and captured in photography for interesting lighting effects. But why does steam come out of the sewer at all? In part, the question depends on what kind of climate you’re living in. Let’s explore all of the possible reasons that you may see steam coming out of your city’s sewer.

In Cold Climates

Sewer steam is more common in cold climates. This is simply because the sewers are warm. After all, they are transporting water that was just in warm homes and buildings. That heat rises up out of the sewer and turns into steam when it meets the cooler temperatures of the air. Of course, that means the steam is more noticeable in the cooler months. In fact, in major cities, the homeless will often harness this steam to heat themselves up.

Have you noticed that there is more steam today than yesterday? Big increases in steam are often from increases in precipitation. If it just rained or snowed, that extra moisture makes its way down to the sewers, where it heats up and fuels more steam.

In Hot Climates

What about if you’re seeing steam rise from the sewer in the middle of summer, especially if you’re in California or somewhere with a warm climate? Then the answer might be that the steam is being pumped alongside the sewers. This is a way of creating and delivering energy throughout the city. In some areas, utility companies supply steam to a few blocks at a time. This is sometimes called “district steam.” Buildings connected to these systems don’t need their own boilers to generate steam for heat. Sudden bursts of steam may be leaks in the pipes delivering steam to these areas.

However, shared steam delivery isn’t very common, as steam quickly loses its heat during anything but short-term transport. If steam isn’t produced in your area,the steam coming from sewers in hot climates may be a result of the very high heat conditions underground. Rainwater falling onto the hot steam pipes may turn into steam itself and rise up to the street level. This can happen even when temperatures above ground are very hot.