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Cloud condensation nuclei (CCN) and ice-nucleating particles (INP) are aerosol particles classified by the cloud phase they help create. CCN take up water vapor and activate into liquid cloud droplets at a specified water supersaturation. INP trigger ice formation in supercooled or ice-supersaturated cloud conditions, usually through heterogeneous nucleation on a particle surface. Some particles can perform both roles, but efficient INP are far rarer than CCN. That distinction matters because CCN mainly control liquid-droplet number and growth, while INP help determine when mixed-phase clouds begin producing ice, snow, graupel, or other ice-mediated precipitation.
What are cloud condensation nuclei and ice-nucleating particles?
CCN and INP are functional categories, not two universally separate chemical families. A particle is called a CCN when it activates into a cloud droplet under a stated water-vapor supersaturation. It is called an INP when it initiates ice under stated cold-cloud conditions. The same aerosol can meet both definitions at different times or under the same cloud conditions.
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NOAA defines cloud condensation nuclei as particles on which water vapor condenses to form cloud droplets. Smoke, sea spray and wind-blown soil are examples of aerosol sources that can supply them. The World Meteorological Organization’s OSCAR definition is more specific: CCN are aerosol particles that can activate to a cloud droplet at a given water supersaturation. CCN number is therefore normally reported for a particular supersaturation rather than as a single, universal property of an air mass.
Particle size and hygroscopicity both affect activation. Hygroscopic material attracts and accommodates water more readily; NASA uses sea-salt-containing particles as an example of an effective condensation surface and contrasts them with hydrophobic particles that resist condensation. NOAA’s cloud-formation explainer gives an approximate minimum radius of 1 micrometre for a surface on which water molecules can condense into a cloud droplet in its idealized description. That figure is a teaching scale, not a universal cutoff for every atmospheric aerosol or instrument.
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Ice-nucleating particles start ice formation
INP are aerosol particles that catalyze ice formation in cold clouds. In mixed-phase clouds, a particle surface can lower the barrier for heterogeneous ice nucleation, allowing an ice embryo to form at temperatures where a pure water droplet would remain liquid. Mineral dust, biological material, soot and internally mixed particles can all show ice activity, but a material name alone does not guarantee that every particle in it will nucleate ice.
Ice activity depends strongly on temperature, ice supersaturation, surface structure and the measurement protocol. If no suitable INP are present, supercooled droplets can persist below 0 °C. Homogeneous freezing becomes important only at much colder temperatures, near approximately −40 °C according to NASA’s GPM explanation and the cloud-microphysics review by Morrison and colleagues (2020).
How do CCN and INP activation conditions differ?
CCN activation is primarily a water-vapor problem: the relevant question is whether a particle can take up enough vapor at a specified supersaturation. INP activity is primarily a cold-cloud and surface problem: the relevant questions are how cold the environment is, whether ice supersaturation is available, and whether the particle surface can initiate an ice embryo.
CCN depend on supersaturation, size and hygroscopicity
As an air parcel rises, it expands and cools. NOAA describes an idealized dry-adiabatic cooling rate of about 9.8 °C per kilometre until the parcel reaches saturation. Once the parcel is saturated, continued lifting can produce a small positive water supersaturation. Particles that are sufficiently large or hygroscopic then grow rapidly enough to become cloud droplets.
Because activation is threshold-based, a CCN count must carry its supersaturation value. Two measurements of the same aerosol can produce different CCN concentrations if one instrument operates at a higher supersaturation. IUPAC’s terminology also distinguishes general condensation nuclei from CCN by humidity: CCN are counted at relative humidity less than or equal to 102%, whereas general condensation nuclei are commonly counted around 300% relative humidity. Those conventions provide context, not a promise that every instrument or study uses identical thresholds.
INP depend on temperature and active surfaces
An INP does not simply “condense more water.” It provides a site where an ice lattice can begin, often while surrounding droplets remain liquid. Lower temperature generally makes ice formation easier, but particles differ greatly in their active-site density and in the temperatures at which they work. Surface coatings, aging, biological fragments, mineral composition and mixing with other substances can change the result.
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Measurements may expose particles to supercooled droplets, water vapor above ice saturation or other controlled conditions. Consequently, an INP concentration is meaningful only with its temperature, humidity and protocol. A particle that is inactive at one temperature can become ice active at a lower temperature without changing its chemical identity.
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Are ice-nucleating particles a type of cloud condensation nuclei?
Not exactly. CCN and INP overlap, but neither category contains the other. The IPCC states that aerosols can serve as both CCN and ice nuclei on which cloud droplets and ice crystals form. In practice, most CCN are not efficient INP, and only a small fraction of atmospheric aerosol is ice active under any particular condition.
The distinction is easiest to understand as two tests applied to the same particle. In a CCN counter, the particle must activate into a liquid droplet at a specified water supersaturation. In an ice-nucleation experiment, it must initiate ice at a specified temperature and ice supersaturation or in a specified freezing setup. A sea-salt-containing particle may readily activate as a CCN and, depending on its composition and surface history, may or may not contribute to ice formation. A dust particle may be a CCN at one humidity and an INP at a particular subzero temperature. An internally mixed aerosol can pass both tests, but its efficiency in each role remains condition-dependent.
Calling a particle an INP therefore does not identify a unique chemical species. It reports observed function under stated conditions. The same caution applies to labels such as “dust INP” or “biological INP”: they describe a source or material class, not a guarantee that every member of that class nucleates ice.
What is the difference between CCN and INP in the atmosphere?
| Axis | Cloud condensation nuclei (CCN) | Ice-nucleating particles (INP) |
|---|---|---|
| Cloud phase initiated | Liquid cloud droplet | Ice crystal or other ice embryo |
| Main activation condition | Water-vapor supersaturation; hygroscopicity and size matter | Cold-cloud temperature, ice supersaturation and active surface properties |
| Typical atmospheric abundance | Broadly common aerosol subset | Much rarer active subset |
| Main microphysical consequence | Droplet number, condensational growth and liquid-cloud optical properties | Glaciation, vapor deposition, riming, snow or graupel formation and precipitation pathways |
| Relationship | Some particles can be both CCN and INP | Some particles can be both CCN and INP |
The abundance contrast is important. A cloud can contain many potential condensation nuclei while having very few particles active as INP at its current temperature. That is why the presence of abundant aerosol does not automatically imply rapid glaciation or immediate snowfall.
Why can supercooled cloud droplets remain liquid below 0 °C?
Freezing at 0 °C is not automatic for every microscopic droplet. In the atmosphere, ice usually begins through a nucleation event: an ice embryo must form and become stable. An INP surface can make that event occur at a comparatively warm subzero temperature. Without an effective INP, a droplet can remain supercooled while the cloud extends below freezing.
NASA explains that suitable ice nuclei are much less abundant than condensation nuclei. When a droplet has no active ice-nucleating surface, the probability of freezing remains low until temperatures approach the homogeneous-nucleation range, approximately −40 °C. At that point, water can freeze without an aerosol catalyst. This explains why a cloud may contain many liquid droplets below 0 °C and why the first appearance of ice can be abrupt as the cloud enters a colder layer or encounters a new population of INP.
Supercooled liquid is not an exotic laboratory exception; it is a normal state in mixed-phase cloud physics. Once a few ice crystals appear, they can grow at the expense of nearby liquid water because water vapor is more readily deposited onto ice under many mixed-phase conditions. The resulting imbalance between liquid droplets and ice crystals drives the cloud’s subsequent evolution.
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How do CCN affect clouds and rain?
CCN influence how many liquid droplets form and how much water each droplet receives. With more activated CCN and the same available cloud water, that water is divided among more droplets, so the initial droplets tend to be smaller. Droplet number and size alter condensational growth, cloud optical properties and the efficiency of collisions between droplets.
Liquid-cloud growth and collision-coalescence
Small droplets can remain suspended longer because their fall speeds are low. For warm clouds, precipitation can begin when droplets collide and coalesce into larger drops. Changing the droplet population therefore changes the balance between condensational growth and collision-coalescence. CCN are influential in this pathway even when no ice is present.
That does not mean more CCN always produce more or less rain in every cloud. The outcome depends on the cloud’s water supply, updraft, lifetime and droplet-size distribution. The defensible statement is narrower: CCN alter the starting droplet population and therefore the microphysical conditions under which liquid-cloud precipitation develops.
Liquid-cloud persistence and optical properties
Droplet number and size also affect how a cloud reflects light and how long liquid water remains suspended. These are consequences of the same activation process, not separate particle categories. A CCN measurement is useful because it describes the aerosol population capable of entering that liquid-cloud pathway at a stated supersaturation.
How do INP affect snow, graupel and mixed-phase precipitation?
INP determine when a supercooled liquid cloud begins forming ice. After ice appears, several processes can reshape the cloud: vapor deposition onto crystals, riming as supercooled droplets freeze on impact, aggregation when ice particles collide, melting below the freezing level and secondary ice production. The NOAA-repository review by Morrison and colleagues (2020) identifies these coupled processes in its mixed-phase cloud-microphysics treatment.
Glaciation and ice growth
As ice crystals grow, they can consume water vapor and collect liquid droplets. If ice becomes sufficiently abundant, the cloud can transition toward glaciation, reducing its liquid-water content. The timing of that transition depends on the number and type of active INP, the temperature profile and the cloud’s supply of moisture.
Snow and graupel pathways
Depositional growth can produce crystals and snow, while riming can make heavily coated, dense particles that contribute to graupel. Aggregation joins ice particles into larger snowflakes. Whether any of these particles reach the ground as snow, melt into rain, or evaporate depends on the layers they cross, so INP influence precipitation pathways without uniquely determining surface precipitation type.
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Ice is not required for every rain event
NASA notes that precipitation can begin in warm clouds through droplet coalescence before temperatures ever reach the ice phase. INP are therefore important for mixed-phase and ice-mediated precipitation, but they are not required for every rain event. A statement that “aerosol affects rain” must specify whether it refers to CCN-driven liquid processes, INP-driven ice processes or both.
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How are CCN and INP reported in observations?
Read the environmental condition alongside the particle count. A CCN concentration without its water supersaturation is incomplete because activation changes with supersaturation. An INP concentration without temperature and the ice-nucleation protocol is similarly incomplete because ice activity is strongly condition-dependent.
- For CCN: note the supersaturation, particle-size distribution and whether the value is expressed as a concentration or as a percentage of total condensation nuclei.
- For INP: note temperature, ice supersaturation or droplet-freezing setup, exposure time where reported, and the definition used to count an ice-active particle.
- For either category: check whether the sample is described by source, composition, mixing state or measured function. A source label alone does not establish activation efficiency.
The WMO OSCAR definition, modified 2025-04-15, explicitly treats CCN number as activation at a given water supersaturation. IUPAC’s humidity distinction between condensation nuclei and CCN likewise shows why terminology and thresholds matter when comparing instruments. These are measurement definitions, not claims that all laboratories use identical operating points.
What common mistakes should readers avoid?
Mistake 1: treating CCN and INP as synonyms
They initiate different phases. CCN produce liquid droplets; INP initiate ice. Overlap is real, but the functional tests are different.
Mistake 2: assuming every aerosol particle is ice active
Atmospheric aerosol is much more abundant than active INP at a given temperature. Most particles do not nucleate ice under the conditions being considered.
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Mistake 3: assuming all droplets freeze at 0 °C
Supercooled droplets can persist below freezing when no suitable ice nucleus is present. Homogeneous freezing is delayed toward approximately −40 °C.
Mistake 4: comparing counts without their conditions
CCN counts require a water-supersaturation value. INP counts require temperature and a stated ice-nucleation method. Removing those qualifiers can make unlike measurements appear directly comparable when they are not.
Mistake 5: treating a material name as a universal behavior
Dust, sea salt, soot and biological particles can show different CCN and INP efficiencies depending on size, surface structure, aging and internal mixing. “Dust” or “biological” is a source description, not a guaranteed activation result.
How should you interpret the two categories together?
Start with the cloud phase you are trying to explain. If the question concerns droplet number, liquid-water growth, cloud brightness or warm-rain coalescence, CCN activation and water supersaturation are the relevant lens. If it concerns the first appearance of ice, mixed-phase glaciation, snow or graupel, examine INP activity together with temperature and ice supersaturation.
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Finally, keep the scale of the claim matched to the evidence. CCN are generally common and influence the liquid-droplet population; INP are rarer and influence the transition to ice. Neither category alone predicts an entire cloud’s lifetime or precipitation, because cloud dynamics, moisture and temperature structure control how those microphysical processes unfold.
Frequently Asked Questions
Does a CCN measurement tell me how many particles are in the air?
No. It tells you how many particles activated at the instrument’s chosen water supersaturation. Particles that remain inactive at that setting still contribute to total aerosol or condensation-nuclei counts.
Why must two CCN studies use the same supersaturation for a fair comparison?
Activation is threshold-dependent, so changing supersaturation changes which particles count. Matching supersaturation removes one major methodological difference before interpreting geographic, seasonal or source-related contrasts.
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No. Different particles have different active surfaces, and the measured concentration also depends on humidity and the ice-nucleation protocol. A temperature attached to an INP result is therefore part of the result, not a universal property of the material.
Can a cloud contain INP before any ice is visible?
Yes. An aerosol particle may be capable of nucleating ice but remain inactive until the cloud reaches the required temperature or supersaturation. Visible ice appears only after nucleation and subsequent crystal growth make the particles detectable.
Does homogeneous freezing mean aerosols never matter near −40 °C?
No. Homogeneous nucleation provides an aerosol-independent route at very low temperatures, while heterogeneous INP processes can initiate ice at warmer temperatures. Both mechanisms may matter in different parts of a deep cloud.
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