Plumes of white vapor often rise from manhole covers and utility vents in cities on cold days. This visible cloud of water vapor, or mist, escapes from the subterranean infrastructure. The vapor signals a temperature imbalance between the warm, moist underground environment and the cooler air above the street. Understanding how this moisture becomes visible explains this common urban sight.
Infrastructure That Uses Real Steam
In a few older, high-density urban areas, the visible vapor is actual steam used for district heating systems. These extensive underground networks transport high-pressure steam from a central plant to multiple buildings for heating, cooling, and hot water production. This steam is often several hundred degrees Fahrenheit and under immense pressure for efficient distribution.
The vapor seen at street level is often “secondary vapor.” This occurs when ambient groundwater or rainwater seeps into utility vaults and contacts superheated steam pipes, causing the water to flash-boil into vapor. Utility companies also intentionally vent the system through stacks or manholes to relieve pressure or perform maintenance, releasing plumes of true, high-temperature steam. Accidental leaks in the conduit piping can also allow the high-pressure steam to escape.
Heat Sources That Vaporize Groundwater
In most cities without dedicated district steam networks, the visible vapor is caused by heat generated by other underground utilities. This heat warms collected rainwater and groundwater, causing it to evaporate and rise. This process is the most frequent cause of manhole “steam” seen worldwide.
Electrical Vaults and Conduits
High-voltage electrical cables and transformers in underground vaults are a significant heat source. The continuous flow of electricity through these conductors generates heat due to electrical resistance, known as Joule heating. This heat is radiated into the surrounding vault space, which often collects water from street drainage or leaks. As the electrical equipment warms the standing water, the water vaporizes and escapes through manhole access points or utility conduits.
Sewer Lines
Another widespread thermal source is the municipal sewer system, which transports warm effluent from homes and businesses. Wastewater from showers, laundry, and industrial processes enters the pipes at an elevated temperature, often ranging from 50 to 80 degrees Fahrenheit. This warm liquid heats the air inside the sewer pipes and the surrounding soil and groundwater. The resulting warm, moist air rises toward the surface, amplified in cold weather by the “chimney effect,” where warm air ascends to displace colder ambient air.
The Science of Visibility
The escaping vapor becomes a white cloud due to the temperature difference between the underground air and the street-level atmosphere. The warm air rising from the utility vaults and sewers is highly saturated with water vapor, meaning it holds a large amount of moisture in its invisible gaseous state.
When this warm, saturated air escapes, it immediately mixes with the cold ambient air above the manhole. This rapid cooling causes the temperature of the escaping air to drop below its dew point. Once the dew point is reached, the excess water vapor condenses, turning from an invisible gas into microscopic liquid water droplets. These droplets are visible as fog or mist. If the outside air were warmer, the moisture would still escape but would remain an invisible gas.