The white cloud that billows off a block of dry ice is not smoke at all. It is fog, made of tiny water droplets suspended in air, produced when the extreme cold of sublimating carbon dioxide chills the surrounding atmosphere below the dew point. The process behind this everyday spectacle turns out to be more interesting than most people realize, and the popular explanation taught in many classrooms gets a key detail wrong.
Sublimation Instead of Melting
Dry ice is solid carbon dioxide, and at the temperatures and pressures we live with every day, it does something unusual: it skips the liquid phase entirely and transforms directly into gas. This direct solid-to-gas transition is called sublimation, and it happens because the pressure required for liquid CO₂ to exist is far higher than normal atmospheric pressure. Under standard conditions, dry ice sublimation is continuous and unavoidable.1International Communications in Heat and Mass Transfer. Experimental and theoretical investigation of the dry ice sublimation temperature for varying far-field pressure and CO2 concentration That is why you never see a puddle under a block of dry ice the way you do under regular ice. The solid just shrinks and disappears.
The surface temperature of dry ice sits around −78.5°C (−109.3°F).2PubMed. A novel mouse model for frostbite injury That is cold enough to freeze ethanol and far colder than anything you encounter in a home freezer. As the solid sublimes, it releases carbon dioxide gas at roughly the same frigid temperature. This gas is invisible on its own. So what, then, is the white cloud?
Where the Fog Actually Comes From
If you ask people why dry ice produces fog, most will say something like: “the cold CO₂ gas hits the air and condenses the water vapor around it.” That explanation is intuitive but incomplete, and researchers have shown it is partly wrong. The fog you see when dry ice is placed in water, for instance, does not form simply by atmospheric water vapor condensing onto cold CO₂ gas as it rises through the water. The real mechanism involves the cold gas interacting with both the water surface and the moist air above it in ways that are more complex than the standard textbook answer suggests.3ACS Publications. A Molecular Explanation of How the Fog Is Produced when Dry Ice Is Placed in Water
What actually happens involves mixing. When the extremely cold CO₂ gas mixes with warmer, moisture-laden air, the temperature of the mixture drops low enough that the air can no longer hold all of its water vapor. The excess moisture condenses into tiny liquid water droplets, and those droplets scatter visible light to create the white cloud. The phenomenon falls into a category physicists call “mixing clouds” or “mixing fogs,” which arise whenever two air masses at different temperatures and humidities combine.4ACS Publications. Fog Machines, Vapors, and Phase Diagrams The same basic principle explains the visible cloud of your breath on a cold morning and the fog that hovers over a heated swimming pool.
So the white cloud is water, not carbon dioxide. The CO₂ gas itself disperses invisibly. It just acts as the delivery mechanism for extreme cold, forcing the water already in the surrounding air to condense into visible droplets.
Why Hot Water Makes More Fog Than Cold Water
Drop a chunk of dry ice into a glass of cold water and you get a gentle stream of fog. Drop it into hot water and you get a dramatic, roiling cloud that spills over the edge and creeps along the floor. The reason is straightforward once you understand the mixing-cloud mechanism. Hot water accelerates the sublimation rate because the temperature difference between the water and the dry ice is larger. More CO₂ gas is released per second, and it carries more thermal energy away from the water surface as it exits. Meanwhile, the air directly above the hot water is already warm and moisture-rich. When a large volume of very cold gas surges up through that humid layer, the mixing effect is amplified. More water condenses, and you get a thicker, more impressive fog.
The fog also behaves differently depending on how it’s produced. Because the mixture of cold CO₂ and condensed water droplets is denser than the surrounding room-temperature air, the fog sinks and flows downward across any surface it encounters. This is why dry ice fog in a cauldron at a Halloween party pools along the floor rather than rising like steam. The carbon dioxide gas is roughly one and a half times denser than air to begin with, and when chilled far below room temperature it becomes even heavier. Combined with the mass of the suspended water droplets, the fog behaves almost like a slow-motion liquid spilling over the rim.
The Role of Latent Heat
Sublimation is an energy-hungry process. Every kilogram of dry ice that transitions from solid to gas absorbs roughly 574 kilojoules of heat from its surroundings.5ScienceDirect (Elsevier). A combined experimental-mathematical study on the kinetics of dry ice sublimation under different airflow velocities and blowing modes That energy has to come from somewhere, and in practice it comes from the air, the water, or whatever surface the dry ice is sitting on. The result is that the immediate environment gets extremely cold very quickly. This rapid, intense cooling is what makes dry ice so effective at generating fog: it yanks heat out of the nearby air faster than the air can replenish it from the broader room, keeping the local temperature low enough that moisture stays condensed into visible droplets for several seconds before the fog warms and evaporates.
This same property is why dry ice works so well as a coolant. Unlike regular ice, which melts at 0°C and can only absorb about 334 kilojoules per kilogram during melting, dry ice operates at a much lower temperature and absorbs its energy entirely as sublimation. The cooling effect is both colder and, gram for gram, more intense. It also leaves no liquid behind, which is why it’s used for situations where wetness would be a problem.
Safety and the Invisible Danger
The fog itself is harmless. It’s just cold water droplets, and it dissipates in seconds once it warms. The real hazards are the temperature of the dry ice and the gas it releases.
Direct skin contact with dry ice causes frostbite almost immediately. At −78.5°C, even brief contact can damage tissue. Researchers studying frostbite injury models have used dry ice precisely because of how reliably and rapidly it freezes skin.2PubMed. A novel mouse model for frostbite injury Handling dry ice with bare hands for even a few seconds produces burns that feel and look similar to thermal burns from heat. Always use insulated gloves or tongs.
The carbon dioxide gas is the less obvious hazard. In a well-ventilated room, the CO₂ disperses and is no concern. But in a small, enclosed space, sublimating dry ice can displace enough oxygen to create a dangerous atmosphere surprisingly quickly. A single kilogram of dry ice produces about 500 liters of gas at room temperature. A cooler full of dry ice left in a sealed car, or a large quantity used in a poorly ventilated basement, can push oxygen levels low enough to cause dizziness, unconsciousness, or worse. Because CO₂ is colorless and odorless at low concentrations, and because it sinks to the floor, people in the space may not notice anything until symptoms hit. The rule of thumb is simple: use dry ice where air moves freely, and never transport large quantities inside the passenger compartment of a vehicle.
Practical Uses Beyond the Fog
The dramatic fog effect gets all the attention, but dry ice is more commercially important for its cooling and cleaning properties. Cold chain logistics rely on it heavily: vaccines, biological samples, frozen foods, and transplant organs all travel in dry-ice-cooled containers because the material keeps temperatures extremely low, leaves no meltwater residue, and sublimes away completely at the destination.6Processes. A Comparative Review on Dry Ice Production Methods: Challenges, Sustainability and Future Directions
Dry ice blasting is a cleaning technique that has been used industrially since the 1980s. Small pellets of dry ice are fired at a surface using compressed air. When the pellets strike the surface, they sublime on contact, and the rapid expansion of the gas lifts contaminants off without leaving any blast media behind. Unlike sandblasting, which leaves grit everywhere, dry ice blasting produces no secondary waste other than whatever was removed from the surface.7ScienceDirect (Elsevier – Wear). Dry-ice blasting for cleaning: process, optimization and application This makes it popular for cleaning delicate equipment, food processing machinery, and electrical components where you cannot afford to leave residue or moisture.
In entertainment, the floor-hugging fog from dry ice remains a staple of theater, concerts, and film sets. Machine-generated fog from glycol-based fluids tends to float and fill a room evenly, but dry ice fog clings to the ground and rolls outward, which produces a very different visual effect. Production crews typically place dry ice in heated water reservoirs and use fans or hoses to direct the fog where they want it.
Common Misconceptions Worth Clearing Up
One persistent myth is that the white cloud is carbon dioxide gas becoming visible. Carbon dioxide gas is always invisible, whether it is cold or warm. What you see is condensed water, not CO₂. If you could somehow sublimate dry ice in a perfectly dry atmosphere with zero humidity, there would be no visible fog at all. The same block of dry ice that produces billowing clouds in a steamy kitchen would produce almost nothing in the middle of a bone-dry desert.
Another misconception is that dry ice is dangerous to store in a regular freezer. In fact, a home freezer at around −18°C is far warmer than dry ice, so the dry ice will continue to sublime inside the freezer. Putting dry ice in a freezer does not preserve it; it just slows the sublimation rate slightly compared to leaving it on a countertop. The gas can also build up pressure if the freezer is sealed tightly. The best storage is in an insulated but not airtight container, like a thick styrofoam cooler with a loose-fitting lid, in a ventilated area.
People also sometimes confuse the fog from dry ice with the vapor trails from liquid nitrogen. Liquid nitrogen (at about −196°C) creates a similar-looking fog by the same general mechanism, but the physics differ. Nitrogen is a gas at room temperature and pressure too, but unlike CO₂ it does pass through a liquid phase at atmospheric pressure. When liquid nitrogen boils off, the escaping nitrogen gas chills the surrounding air and condenses its moisture into fog, much like dry ice does, but the nitrogen is colder and the evaporation is faster. The visual result looks similar, but the underlying transitions are different.
How Humidity and Airflow Change the Effect
Because the fog is condensed water vapor, environmental conditions have a huge impact on how dramatic the display is. On a humid summer day, a small piece of dry ice can produce a surprisingly large and long-lasting fog because there is so much moisture available in the air to condense. On a dry winter day, the same piece produces a thin, wispy stream that vanishes almost immediately.
Airflow matters too. A gentle breeze can carry the fog across a wider area and actually make the effect look more impressive by spreading the cold zone and condensing moisture over a larger volume of air. But strong airflow dilutes and warms the CO₂ gas quickly, causing the fog to dissipate before it has a chance to pool. Researchers studying sublimation rates have found that increasing airflow velocity significantly accelerates how fast dry ice disappears, which means the fog effect at any given moment may look thinner even though the total gas output is higher.5ScienceDirect (Elsevier). A combined experimental-mathematical study on the kinetics of dry ice sublimation under different airflow velocities and blowing modes The best visual effects come from warm, humid, relatively still environments, which is why the classic demonstration of dry ice in a bowl of hot water indoors looks so much more dramatic than anything you would see outdoors on a breezy day.
Dry Ice on Mars
Earth is not the only place where CO₂ sublimation shapes the environment. On Mars, carbon dioxide is the primary atmospheric component, and it freezes directly onto the surface during the Martian winter, forming seasonal ice caps of solid CO₂. When spring arrives, this dry ice sublimes back into gas, and the process drives a range of surface changes that have no equivalent on Earth.8Scientific Reports. Experiments On Sublimating Carbon Dioxide Ice And Implications For Contemporary Surface Processes On Mars
Researchers have proposed that the sublimation of CO₂ ice is responsible for some of the most distinctive features on the Martian landscape, including certain types of gullies carved into dune slopes. These gullies were originally assumed to require flowing liquid water, but laboratory experiments and modeling suggest that blocks of CO₂ ice breaking free from dune surfaces and sliding downhill can carve similar channels as they sublime and release gas beneath them.9Icarus. A new dry hypothesis for the formation of martian linear gullies The gas creates a low-friction cushion under the sliding block, allowing it to travel farther and dig deeper than a simple ice block rolling down a slope. Other work has concluded that debris flows triggered by CO₂ sublimation can explain gully formation without invoking liquid water at all, a finding that shifts how scientists think about the recent geological activity on Mars.10Nature Geoscience. Formation of gullies on Mars by debris flows triggered by CO2 sublimation
This is a striking example of how the same physical process that makes a Halloween cauldron fog also sculpts the landscape of another planet. The scale is different, the atmosphere is different, and the consequences are wildly different, but the core phenomenon is the same: solid CO₂ absorbing energy and transitioning directly to gas, moving material and reshaping its surroundings in the process.