A cloud is not any single state of matter. It is a mixture: tiny liquid water droplets and ice crystals suspended in gas. The air between and around those droplets is a gas, the droplets themselves are liquid, and at higher altitudes or lower temperatures those droplets can be solid ice. Physicists classify this kind of mixture as a colloid or aerosol, meaning fine particles of one phase dispersed throughout another. That hybrid nature is what makes the question surprisingly interesting, because the answer changes depending on which part of the cloud you point at and how cold it is up there.
What Is Actually Inside a Cloud
If you could shrink yourself down and float through a cloud, you would be wading through humid air with an enormous number of extremely small water droplets or ice crystals scattered throughout it. A typical cloud droplet is roughly 10 to 20 micrometers across, far too small to see individually with the naked eye. There can be hundreds of these droplets in every cubic centimeter of cloud. The total liquid water content of a cloud is remarkably low, often just a fraction of a gram per cubic meter of air. By mass, a cloud is overwhelmingly gas. The liquid or ice component makes up a tiny percentage, but that tiny percentage is what makes the cloud visible and gives it all its interesting properties.
This means the most honest answer to the title question is that a cloud simultaneously contains at least two states of matter: the gas phase (nitrogen, oxygen, water vapor, and other atmospheric gases) and either the liquid phase (water droplets), the solid phase (ice crystals), or both. In warm, low-altitude clouds like the puffy cumulus you see on a summer day, nearly all the condensed water is liquid. In high, thin cirrus clouds, it is almost entirely ice. And in between, there are mixed-phase clouds where liquid water and ice coexist at the same altitude, sometimes in the same small volume of air.
How Droplets Form in Thin Air
Water vapor is invisible. It is a gas, dissolved seamlessly into the rest of the atmosphere. For a cloud to form, that invisible vapor has to condense into liquid droplets or deposit directly into ice crystals. But water molecules are reluctant to clump together on their own in midair. Spontaneous condensation in perfectly clean air would require extreme supersaturation that basically never occurs in the real atmosphere. Instead, water vapor condenses onto tiny airborne particles called cloud condensation nuclei.
These nuclei can be almost anything: sea salt carried aloft from ocean spray, sulfate particles from volcanic emissions, mineral dust blown off deserts, or soot from fires and combustion. The particle gives water molecules a surface to latch onto, dramatically lowering the energy barrier to forming a droplet. Recent research has shown that even soot particles with no soluble coating at all can serve as effective condensation nuclei if they have a high enough density of molecular adsorption sites on their surface, challenging the older assumption that some soluble material was required.1Atmospheric Chemistry and Physics. Heterogeneous nucleation of water vapor on different types of black carbon particles This means that wildfire smoke, diesel exhaust, and other sources of black carbon can seed new cloud droplets in ways that were underestimated until recently.
Once a droplet forms, it stays aloft because it is so small that the rising air currents inside and around the cloud, even gentle ones, are more than enough to keep it suspended. A cloud droplet falls incredibly slowly on its own because air resistance relative to its tiny mass is enormous. Only when droplets collide and merge to become much larger, eventually reaching raindrop size (roughly a hundred times the diameter of a cloud droplet), does gravity win and precipitation fall out.
Mixed-Phase Clouds and the Coexistence Problem
Between roughly 0°C and about −38°C, clouds routinely contain both liquid water droplets and ice crystals at the same time. This is the mixed-phase regime, and it is far more common than most people realize. A large fraction of the clouds covering the planet at any given moment, particularly in the mid-latitudes and polar regions, are mixed-phase. The liquid droplets and ice crystals interact in complex ways, and the balance between them has major consequences for how the cloud evolves, how much sunlight it reflects, and whether it produces rain or snow.
The reason liquid and ice can coexist is that freezing in the atmosphere is not as simple as cooling water below 0°C. Pure water droplets can remain liquid well below the conventional freezing point if they lack a suitable surface to initiate ice crystal growth. Ice formation requires its own kind of seed particle, called an ice-nucleating particle, and these are much rarer in the atmosphere than the particles that seed liquid droplets. Certain types of mineral dust, some biological particles like fragments of bacteria and fungal spores, and specific types of soot are effective ice nucleators, but most airborne particles are not.
The proportion of ice versus liquid in a mixed-phase cloud matters for climate. Ice crystals and liquid droplets interact with sunlight and thermal radiation differently, and increasing the number of ice-nucleating particles can thin a cloud and reduce the amount of heat-trapping longwave radiation it sends back toward the surface.2Geophysical Research Letters. Response of an Arctic Mixed‐Phase Cloud to Ice‐Nucleating Particle Perturbations and Warming Arctic mixed-phase clouds are a particular focus of research because even modest changes in their ice content can affect how much heat reaches the sea ice below.
Supercooled Water and the Limits of Liquid
Liquid water existing at temperatures far below freezing sounds impossible, but it is routine in clouds. Droplets as cold as −20°C or even −30°C can persist as liquid for extended periods. This supercooled water is one of the more counterintuitive aspects of cloud physics and is the reason aircraft icing is a serious aviation hazard: when a plane flies through a cloud of supercooled droplets, the impact on the aircraft’s surface provides the nudge the droplets need to freeze instantly, building up ice on wings and engine intakes.
There is, however, a lower limit. Laboratory experiments and atmospheric observations converge on roughly −38°C as the temperature at which even very small, very pure water droplets freeze spontaneously without any nucleating particle. Research into the mechanism suggests that at these extreme temperatures, the surface layer of the droplet itself begins to crystallize, triggering the entire droplet to freeze from the outside in.3PubMed Central. Surface crystallization of supercooled water in clouds This surface crystallization process helps explain why atmospheric observations consistently find very little supercooled liquid water at temperatures approaching −40°C. Below that threshold, clouds are essentially all ice.
So the answer to “what state is a cloud?” shifts with altitude and temperature. Near the base of a warm rain cloud, you are in a world of liquid droplets in gas. Climb to the top of a towering cumulonimbus thunderstorm and you are surrounded by ice crystals, some of them in elaborate hexagonal and branching shapes. In between, you pass through a zone where liquid and solid water coexist in the same cloud, sometimes only meters apart.
Why Clouds Look Solid Even Though They Are Mostly Air
Clouds appear as bright, opaque, sometimes towering masses, which is why they are so easily mistaken for a distinct “thing” made of a single material. The illusion comes from how light interacts with all those tiny droplets. Each cloud droplet scatters sunlight in all directions. Because the droplets are close to the wavelength of visible light in size (or somewhat larger), they scatter all wavelengths roughly equally, which is why most clouds look white rather than blue or red. The sheer number of scattering events as light passes through the cloud is what makes it opaque despite being almost entirely empty space.
Detailed calculations using electromagnetic scattering theory show that the angular pattern of scattered light depends strongly on the size distribution of droplets. A cloud with a peak droplet radius near 4 micrometers, for example, produces the kind of bright narrow halo around the sun (called an aureole), faint rainbows (cloudbows), and the colorful rings of glories that observers sometimes see from aircraft.4Applied Optics. Scattering and Polarization Properties of Water Clouds and Hazes in the Visible and Infrared The specific optical phenomena you see looking at or through a cloud are a fingerprint of the droplet sizes inside it, which is one reason atmospheric scientists study cloud optics so carefully.
The dark undersides of rain-heavy clouds are not a different substance; they are just so thick with droplets that very little sunlight makes it through to the bottom. The droplets at the base are still the same size as those higher up, but there are so many layers of scattering above them that the light is mostly absorbed or redirected before it reaches your eyes.
How Rain Forms From Something That Is Barely There
Given that a cloud contains so little liquid water per cubic meter, it seems almost paradoxical that it can produce a torrential downpour. The key is the collision-coalescence process: droplets bump into each other and merge, growing larger with each collision. A raindrop that hits the ground might be a million times the volume of the cloud droplet it started as. Getting droplets to collide efficiently is actually a hard physics problem, because very small droplets tend to follow air currents around each other rather than crashing head-on.
For decades, cloud physics models struggled to reproduce the speed at which real clouds produce rain. The observed time from cloud formation to first rainfall was consistently faster than models predicted when they only accounted for droplets settling under gravity and merging. The missing ingredient turns out to be turbulence. The chaotic swirling motions inside a cloud push droplets together more effectively than gravity alone, especially for medium-sized droplets in the critical early stages of growth. High-resolution comparisons between real cumulus cloud observations and simulations now confirm that including turbulent coalescence is essential to accurately match the droplet size distributions seen in nature, particularly for the drizzle-sized drops that appear near the base of warm clouds.5PubMed Central. Are turbulence effects on droplet collision-coalescence a key to understanding observed rain formation in clouds? Turbulence causes rain to form earlier and in greater amounts than gravity-only models predict.
In cold clouds, a different growth mechanism dominates. Because ice crystals and supercooled liquid droplets can coexist, and because the saturation vapor pressure over ice is lower than over liquid water at the same temperature, water vapor migrates preferentially from the liquid droplets to the ice crystals. The ice grows at the expense of the liquid, eventually producing snowflakes or ice pellets heavy enough to fall. If these pass through warm air on the way down, they melt into rain.
When Precipitation Never Reaches the Ground
Not everything that falls out of a cloud actually makes it to the surface. Virga is the meteorological term for precipitation that evaporates or sublimates before it hits the ground. You can often see it as wispy streaks trailing below a cloud base in dry climates, particularly over deserts or in the tropics. From the ground it looks like the cloud is leaking, but the rain or snow disappears partway down.
Observations over the tropical Atlantic during a recent field campaign found that roughly 42% of all detected clouds with bases below the trade-wind inversion produced precipitation that fully evaporated before reaching the surface, and more than half of that virga originated from trade-wind cumulus clouds.6Copernicus Publications / Atmospheric Measurement Techniques. The Virga-Sniffer – a new tool to identify precipitation evaporation using ground-based remote-sensing observations This means that in some environments, clouds are producing rain almost constantly, but the air below them is dry enough to reabsorb it before you ever feel a drop. From the perspective of the state-of-matter question, virga is a fascinating transition: liquid or solid water leaving a cloud, then reverting to invisible water vapor before completing its journey. The state of that falling water changes in midair.
Lightning and the Brief Appearance of Plasma
If you want to push the state-of-matter question even further, thunderstorm clouds briefly generate a fourth state of matter: plasma. Lightning is a channel of ionized gas, meaning the molecules have been stripped of electrons and the air itself becomes electrically conducting. This is plasma, and while it lasts only fractions of a second along the lightning channel, it is as real a state of matter as the liquid droplets surrounding it.
The process that leads to lightning begins with charge separation inside a cloud. Collisions between ice crystals and larger ice particles called graupel transfer electric charge, building up strong electric fields. One mechanism for lightning initiation involves large, negatively charged water drops or ice crystals becoming unstable in the cloud’s electric field. When these drops are large enough (typically more than about 30 micrometers in radius), they can break apart and emit extremely small, heavily charged daughter droplets. The electric field near the surface of these tiny fragments can be intense enough to rip electrons from air molecules, launching cascading electron avalanches that eventually form a conducting plasma channel.7Physica Scripta. The possible physical mechanism of initiation and growth of lightning In other words, a cloud can host gas, liquid, solid, and plasma all in the same storm, though the plasma is fleeting and localized to the lightning strike itself.
Volcanic and Wildfire Aerosol Clouds
The word “cloud” gets applied to more than just water. Volcanic eruptions can inject massive quantities of ash and sulfur compounds into the stratosphere, forming aerosol clouds that circle the globe and persist for months. These are distinct from water clouds, though the two can interact. During the 2019 eruption of the Raikoke volcano, the initial volcanic ash cloud contained an estimated 15 teragrams (about 15 million metric tons) of volcanic ash. Just four days before the eruption, intense forest fires in Alberta, Canada, had injected about 0.2 teragrams of biomass burning aerosol into the stratosphere, and the two aerosol plumes became difficult to distinguish from each other using ground-based instruments alone.8Copernicus Publications / Atmospheric Chemistry and Physics. The 2019 Raikoke volcanic eruption – Part 2: Particle-phase dispersion and concurrent wildfire smoke emissions
These aerosol clouds are mostly solid or liquid particles (ash, sulfate droplets, soot) suspended in gas, much like a water cloud but with very different composition. They affect climate by scattering and absorbing sunlight, and the particles within them can also serve as nucleation seeds for ordinary water clouds lower in the atmosphere. A large volcanic eruption can cool global temperatures for a year or more by injecting enough sulfur into the stratosphere to form a persistent haze of sulfuric acid droplets. The state of matter in these clouds is the same colloid concept as in a water cloud, just with different ingredients.
Fog, Mist, and Where Clouds End
Fog is simply a cloud that touches the ground. There is no physical distinction between a fog bank and a low stratus cloud other than your altitude relative to it. If you have ever walked through fog, you have walked through a cloud: the damp feeling on your skin is liquid water droplets a few micrometers across landing on you. The droplets are the same size and form by the same condensation process as the ones thousands of meters up. Mist is an informal term for very light fog with somewhat better visibility, but there is no sharp boundary between them.
Steam rising from a hot cup of coffee on a cold morning is another mini-cloud. The visible plume is not water vapor (which is invisible) but tiny liquid droplets that condensed when the warm, moist air from the cup hit the cooler surrounding air. It is the same physics, just at a smaller scale without the need for nucleation particles, because the temperature difference is so sharp that condensation happens readily. The common misconception that the white plume is “steam” in the physics sense (meaning gaseous water) is exactly backwards: the visible part is liquid. The actual steam is the invisible gap between the liquid surface and the point where droplets first appear.
Understanding that a cloud is a colloid rather than a pure substance clears up a lot of everyday confusion. When someone asks whether a cloud is a liquid, a gas, or a solid, the most helpful answer is “yes.” It is all of them at once, in proportions that shift with temperature, altitude, and the type and abundance of particles seeding the process. The gas dominates by mass, the liquid or ice is what makes it visible, and on rare occasions, plasma threads through it in bolts of lightning.