A cloud is not purely a solid, liquid, or gas. It is a visible mass of tiny water droplets or ice crystals, sometimes both at once, suspended in the surrounding air. The air itself is a gas, but the particles that make a cloud visible are condensed water in liquid or solid form, each droplet or crystal far too small to see individually. So the honest answer is that a cloud is a mixture of phases, and which phases are present depends on the cloud’s altitude, temperature, and internal dynamics.
What a Cloud Is Actually Made Of
When you look at a cloud, you are seeing billions of water droplets or ice crystals packed closely enough to scatter light. A typical cloud droplet is roughly 10 to 20 micrometers across, small enough that about 50 of them lined up would span a single millimeter. These droplets are liquid water, not water vapor. Water vapor is an invisible gas that exists throughout the atmosphere. The moment vapor cools enough to condense into droplets, it becomes visible, and that visible collection is what we call a cloud.
The gas component matters too. The droplets or crystals are suspended in ordinary air, which is a mixture of nitrogen, oxygen, and trace gases. By mass, the liquid or solid water in a cloud is a tiny fraction of the total. A cubic meter of cloud typically contains only about 0.3 grams of liquid water, while the air in that same cubic meter weighs over a kilogram. Clouds look substantial from the ground, but they are overwhelmingly gas with a sparse scattering of condensed water particles floating within it.
Liquid Clouds, Ice Clouds, and Everything in Between
Not all clouds are made of the same stuff. Low-altitude clouds, like the flat gray blanket of stratus or the puffy cumulus you see on a summer afternoon, tend to be composed entirely of liquid water droplets. The air at those altitudes is warm enough that water stays in its liquid phase.
High up in the atmosphere, the story changes. Cirrus clouds, the thin wispy streaks you see at high altitude, form in the upper troposphere at temperatures well below −40°C and are composed entirely of ice crystals.1Cirrus. Ice Crystals in Cirrus These crystals come in a variety of shapes. In midlatitude and Arctic regions, cirrus clouds often contain bullet rosette–shaped ice crystals, intricate structures that look like clusters of hexagonal columns radiating from a common center.2Journal of Applied Meteorology and Climatology. The Asymmetry Parameter of Cirrus Clouds Composed of Hollow Bullet Rosette–Shaped Ice Crystals from Ray-Tracing Calculations The specific crystal shape affects how the cloud scatters sunlight, which is partly why cirrus clouds look so different from the thick, opaque clouds at lower altitudes.
Between the purely liquid low clouds and the purely ice high clouds lies a broad zone where both phases coexist. These are called mixed-phase clouds, and they exist at temperatures between 0°C and roughly −38°C. In that temperature range, you find supercooled liquid water droplets sitting alongside ice crystals within the same cloud.3Journal of Geophysical Research: Atmospheres. What Is Triggering Ice in Mixed‐Phase Clouds: A Process Analysis With ECHAM6.1‐HAM2.3 Using the Factorial Method Supercooled droplets are liquid water that has cooled below freezing without actually turning into ice. This sounds impossible, but it happens routinely in the atmosphere because tiny pure water droplets can resist freezing for a surprisingly long time without something to trigger the process.
Why Liquid Water Survives Below Freezing
Water does not automatically freeze the instant it hits 0°C. Freezing requires either reaching extremely cold temperatures or having a surface that ice crystals can latch onto and begin growing. In the atmosphere, certain aerosol particles known as ice-nucleating particles can trigger ice formation in cloud droplets at temperatures warmer than the roughly −38°C threshold where water freezes on its own.4PubMed Central. Mineral and biological ice-nucleating particles above the South East of the British Isles These particles include mineral dust, certain bacteria, and fragments of biological material. Only a small fraction of all the particles floating in the air have this ability, which is why supercooled liquid water can persist in clouds even at temperatures as cold as −20 or −30°C.
Over Antarctica, researchers have found that some clouds remain entirely liquid despite being well below freezing, with no ice forming at all. These are sometimes called unglaciated supercooled liquid clouds, distinguished from mixed-phase clouds where ice is also present.5Atmospheric Chemistry and Physics. Antarctic clouds, supercooled liquid water and mixed phase, investigated with DARDAR: geographical and seasonal variations The distinction matters because the presence or absence of ice dramatically changes how a cloud behaves, how long it lasts, and how it affects the energy balance of the planet.
Why Clouds Float If They Contain Liquid and Solid Particles
If clouds are made of water droplets and ice crystals, both of which are denser than air, why don’t they just fall to the ground? The short answer is that the individual particles are so small and so light that they fall extremely slowly, and the rising air currents inside and beneath a cloud are strong enough to keep them aloft. A cloud droplet falls through still air at only about one centimeter per second. Updrafts inside a cloud commonly move at speeds of one meter per second or more, easily overwhelming that gentle settling.
The relationship between updrafts and droplet fall speed is central to how clouds maintain themselves. Research into warm convective clouds has shown that the interplay between the speed of rising air and the effective fall speed of the droplets determines how water is distributed throughout the cloud, where it concentrates, and where it thins out.6Atmospheric Chemistry and Physics. Quantifying the effect of aerosol on vertical velocity and effective terminal velocity in warm convective clouds When the updraft weakens or the droplets grow large enough, they begin falling faster than the air can carry them. That is the beginning of precipitation.
How Clouds Become Rain
Cloud droplets are far too small to fall as rain. A raindrop is roughly a million times the volume of a typical cloud droplet. To make rain, cloud droplets need to collide and merge with each other, growing step by step until they are heavy enough to fall through the updraft and reach the ground. This collision-coalescence process sounds straightforward, but for decades it presented a puzzle: the basic physics of droplets bumping into each other in calm air is too slow to explain how rain forms as quickly as it does in real clouds.
One leading explanation is that turbulence inside clouds dramatically speeds things up. The chaotic, swirling air motions in a convective cloud push droplets together more frequently and more forcefully than would happen in still air. Recent high-resolution observations of cumulus clouds, compared with detailed simulations, have provided strong evidence that turbulence has a significant effect on how droplet size distributions evolve and how rain forms.7PubMed Central. Are turbulence effects on droplet collision-coalescence a key to understanding observed rain formation in clouds? In ice-containing clouds, a different mechanism also operates: ice crystals grow at the expense of surrounding supercooled droplets, because the air around an ice crystal is slightly drier than what a liquid droplet needs to survive. The ice crystal effectively steals water vapor from nearby droplets, growing large enough to fall as snow or, if it melts on the way down, as rain.
The Energy Hidden Inside Phase Changes
Every time water changes phase inside a cloud, energy is either released or absorbed, and these energy exchanges drive much of what clouds do. When water vapor condenses into liquid droplets, it releases latent heat, warming the surrounding air and fueling the updraft that keeps the cloud growing. This is a big part of why thunderstorms can build to enormous heights: each round of condensation adds energy that pushes air higher, where more vapor condenses, releasing more heat in a self-reinforcing loop.
The reverse processes matter just as much. When droplets evaporate, when ice melts, or when ice sublimates directly into vapor, all of these absorb heat from the surrounding air, cooling it. This latent cooling is considered a primary driver of the strong downdrafts inside storms and helps explain why storms can sustain themselves for hours.8PubMed Central. Interpreting the dominant signature of inhomogeneous mixing resulting from dry-air entrainment in clouds The tug-of-war between warming from condensation and cooling from evaporation is what gives thunderstorms their characteristic cycle of powerful updrafts and downdrafts.
When dry air from outside the cloud gets mixed in at the edges, something interesting happens to the droplets. Rather than all droplets shrinking a little as they partially evaporate, observations from cumulus clouds show that the mixing tends to be strongly inhomogeneous: a subset of droplets evaporate completely while the rest survive more or less intact.8PubMed Central. Interpreting the dominant signature of inhomogeneous mixing resulting from dry-air entrainment in clouds This uneven evaporation pattern affects the cloud’s optical properties, how much sunlight it reflects, and how quickly it dissipates.
Fog Is Just a Cloud on the Ground
If you have ever walked through fog, you have walked through a cloud. Fog is a cloud that forms at or very near the ground surface, composed of the same tiny liquid water droplets as any low-altitude cloud. Meteorologists treat fog as a well-defined cloud that forms within the atmospheric boundary layer and varies on horizontal and temporal scales that belong to mesoscale meteorology.9International Geophysics. Fogs and Stratocumulus Clouds The difference between fog and a stratus cloud overhead is purely one of altitude. When the base of a stratus layer descends to ground level, it becomes fog. When fog lifts, it becomes a cloud.
This makes fog a useful intuitive reference point. When you feel fog on your skin, that dampness is contact with liquid water droplets, not with water vapor. You are standing inside a cloud and experiencing firsthand that it is made of condensed liquid, not gas. The droplets are small enough that they feel like humidity rather than rain, but they are genuinely liquid water.
Contrails and Human-Made Clouds
Aircraft flying at high altitude create their own clouds. Contrails, short for condensation trails, form when the hot, humid exhaust from jet engines mixes with the extremely cold ambient air, causing water vapor to condense and then freeze almost immediately into ice crystals. In a sense, a contrail is an artificial cirrus cloud.
When the surrounding air is humid enough, contrails can persist and spread, eventually becoming indistinguishable from natural cirrus. The ice crystals in these persistent contrails form initially around soot particles emitted by the engines. Research into contrail formation has shown that the number of ice crystals in a contrail is linked to the number of soot particles the aircraft emits, though the relationship is not straightforward: reducing soot emissions by a moderate amount does not proportionally reduce ice crystal numbers, because fewer nucleated crystals also means fewer are lost to sublimation in the early stages of contrail development.10CrossRef API / Geophysical Research Letters. Susceptibility of contrail ice crystal numbers to aircraft soot particle emissions This is an active area of research because contrail cirrus may have a substantial warming effect on the climate, potentially comparable to the warming from the CO₂ that aircraft emit.
How Scientists Tell a Cloud’s Phase from Space
Determining whether a cloud is made of liquid droplets, ice crystals, or both is not just an academic question. The phase composition of clouds affects how much sunlight they reflect back to space and how much infrared heat they trap, making it a critical input for climate models. But you obviously cannot reach up and grab a sample from every cloud on Earth, so scientists have developed remote sensing techniques to figure out a cloud’s phase from orbit.
One powerful approach uses lidar instruments on satellites, which send pulses of laser light down through the atmosphere and analyze the signal that bounces back. When laser light hits liquid water droplets, it scatters in a particular pattern. When it hits ice crystals, the scattering pattern is different, especially in how much the light’s polarization changes. By measuring the ratio of polarized to unpolarized light returning from a cloud, satellite instruments can distinguish water clouds from ice clouds.11Geophysical Research Letters. Depolarization ratio–effective lidar ratio relation: Theoretical basis for space lidar cloud phase discrimination The CALIPSO satellite, which carried a lidar instrument designed specifically for this task, has been used to build global maps of cloud phase by analyzing the backscattered and depolarized light from clouds around the world.12Journal of Geophysical Research: Atmospheres. Global analysis of cloud phase and ice crystal orientation from Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data using attenuated backscattering and depolarization ratio
These measurements have revealed that ice crystals in some clouds tend to orient themselves horizontally as they fall, like leaves settling through still air. Horizontally oriented crystals produce a distinctive bright backscatter signal that looks different from randomly oriented crystals, and the satellite data can pick this up. The orientation matters because it changes how the cloud interacts with radiation, both reflecting sunlight and trapping outgoing heat in slightly different ways depending on how the crystals are aligned.
Why the Question Itself Is a Little Misleading
Asking whether a cloud is a solid, liquid, or gas assumes that everything must be neatly sorted into one of those three bins. In everyday life, that framework works well: an ice cube is solid, a glass of water is liquid, the steam from a kettle is gas. But a cloud does not fit into any single category because it is not a single substance in a single state. It is a system of multiple phases coexisting. The gas phase (air and invisible water vapor) provides the medium. The liquid phase (tiny droplets) or the solid phase (ice crystals) provides the visible cloud itself. Sometimes all three exist simultaneously in the same cloud volume.
If forced to pick a classification, the closest everyday analogy is an aerosol: a suspension of fine particles (liquid or solid) in a gas. Hairspray is an aerosol. So is mist from a spray bottle. A cloud is a natural aerosol on a much grander scale. But even this label is a simplification. Clouds are dynamic. They are constantly forming, evaporating, freezing, melting, growing, and dissipating. The phase of the water inside them shifts in response to temperature, air motion, and the particles suspended in the air. Calling a cloud “liquid” would be accurate for some clouds at some altitudes, while calling it “solid” would be accurate for others. Calling it “gas” would be wrong in every case, because gas is invisible and a cloud, by definition, is something you can see.