The sight of white vapor rising from the ground can be striking, often prompting concern about an unseen problem beneath the surface. This “smoking ground” is a visual phenomenon that typically involves the rapid condensation of water vapor, though it can sometimes be true smoke or gas escaping from below. The source of this vapor or fume can range from simple, everyday weather effects to deep geological activity or failures in urban infrastructure. Determining the true origin requires assessing whether the cause is thermodynamic, geological, chemical, or man-made.
Steam Caused by Weather and Thermal Gradients
The most frequent cause of ground “smoking” is a simple meteorological event involving the physics of vapor condensation. This occurs when warm, moist air rising from the ground surface meets a much colder layer of ambient air. As the air’s temperature drops, its capacity to hold water vapor decreases sharply, causing the invisible gaseous water to condense into microscopic liquid droplets, which we perceive as fog or steam.
This effect is particularly visible over surfaces that have retained significant heat or moisture, such as roads, recently watered lawns, or fields after a rain event. Soil moisture content is a major factor, as the evaporation of water from the soil surface provides the necessary water vapor. A common time to observe this phenomenon is during a rapid temperature drop, such as on a cold morning after a relatively warm, wet day. When the ground’s surface temperature is higher than the air’s dew point, the evaporated moisture mixes with the cold air just above the ground, instantly condensing into a visible, white plume.
Geothermal Vents and Fumaroles
In specific regions of the world, especially those with active volcanism, the ground “smoke” is a manifestation of deep geological heat. This activity is known as a geothermal process, where heat generated deep within the Earth’s crust interacts with groundwater. The heat source is typically a reservoir of magma or hot, recently solidified igneous rock lying relatively close to the surface.
Groundwater percolates downward through fissures until it contacts this superheated rock. The water is heated, often reaching temperatures well above the surface boiling point due to the immense pressure underground. This superheated water and steam then rise back to the surface through natural conduits. When this mixture of steam and gas vents through the surface, it forms a feature called a fumarole. Fumaroles emit hot volcanic gases and vapors, primarily steam, but also carbon dioxide and sulfur compounds. The temperature of the gases escaping a fumarole can range from approximately 100 to 1,000 degrees Celsius.
Combustion: Underground Coal and Peat Fires
Sometimes the vapor rising from the ground is not steam, but actual smoke, fumes, and heat from sustained combustion occurring deep beneath the surface. This is seen with underground coal seam fires or deep peat fires, which involve the slow-burning of carbonaceous material. These fires can be ignited naturally through self-heating, lightning strikes, or by human activity like mining accidents.
The slow-burning process is possible because the supply of oxygen is heavily restricted by the overlying earth and rock layers. This limited oxygen supply results in a smoldering fire that can persist for decades or even centuries. The lack of oxygen causes the fire to produce significant quantities of hazardous gases, including carbon monoxide and methane. Since the fire is thermally insulated by the surrounding earth, it is extremely difficult to extinguish using surface methods like rainfall. The ground above these fires can become unstable, leading to subsidence and the formation of sinkholes, which further allows the escape of smoke and gases.
Utility Leaks and Buried Infrastructure
In dense urban environments, the appearance of “smoking” ground is often related to failures in man-made subsurface utility systems. Many older cities utilize district heating systems, which involve a network of underground pipes that transport high-pressure, high-temperature steam to heat buildings. A rupture or leak in one of these steam pipes is a common cause of white plumes rising from manholes or cracks in the pavement.
The steam escaping from a ruptured pipe can be under high pressure and temperature, sometimes reaching 170 pounds per square inch and 350 degrees Fahrenheit in city systems. Even in the absence of a direct pipe rupture, water from a broken water main or stormwater runoff can contact a hot steam pipe, instantly boiling and producing a visible steam plume.
Another source of urban steam is the sewer system, particularly in cold weather. Sewers carry warm wastewater from homes and businesses, which heats the air inside the pipes. When this warm, moist air rises through manholes or street grates and mixes with the cold ambient air, the water vapor condenses, creating a visible fog. While this is usually a benign effect, a sudden, large increase in visible vapor could indicate a significant leak in a steam line, which poses a serious hazard due to the risk of scalding and underground explosions.