Cloud physics
Cloud physics is the science that explains how water vapor becomes clouds, precipitation, and ice. Cut a hailstone in half and you find onion-like layers of ice inside. Each layer marks a distinct passage through a zone of supercooled water inside a storm. Hailstones have been found with diameters of up to 7 inches.
At the opposite extreme, a typical cloud droplet has a radius of about 0.002 millimetres. Modern cloud physics traces its roots to the 19th century. Otto von Guericke first proposed that clouds were composed of water bubbles. In 1847, Augustus Waller used spider web to examine droplets under a microscope. William Henry Dines confirmed those observations in 1880, and Richard Assmann did the same in 1884.
The questions those early researchers raised still drive the field. Why does liquid water persist well below 0 degrees Celsius inside a cloud? What tips a mild afternoon shower into a violent storm? The answer begins with a nearly invisible speck of dust.
Supersaturation of more than 1-2 percent relative to water is rarely seen in the atmosphere. Cloud condensation nuclei are almost always present before that threshold is crossed, triggering droplet formation. These nuclei, known as CCN, are microscopic particles of dust, smoke, salt, and other matter that circulate in the air. At typical CCN concentrations, when droplets are still small, the supersaturation needed for condensation falls below what would otherwise be required.
The Kelvin effect describes how saturation vapor pressure changes at a curved surface. Raoult's law describes how dissolved substances reduce the vapor pressure of a solution. Together, they explain why a soluble or partially soluble particle makes an effective condensation site. When CCN are absent, rising air stays supersaturated and cloud formation is inhibited. In cleaner air, much higher supersaturation is possible; this is the principle behind the cloud chamber used in physics research.
Cloud drops can begin to freeze at temperatures as high as -5 degrees Celsius if suitable ice-nucleating particles are present. Those particles determine whether a cloud remains liquid or transforms into an entirely different structure made of ice.
Three lifting agents can drive moist air upward until it cools to its dew point: cyclonic and frontal lift, convective lift, and orographic lift. As air ascends, atmospheric pressure decreases and the air expands. That expansion costs energy, cooling the air in a process called adiabatic cooling. For non-convective clouds, the altitude where condensation begins is called the lifted condensation level, which roughly marks the cloud base.
Warm fronts associated with extratropical cyclones tend to generate mainly cirriform and stratiform clouds across wide areas. Cold fronts move faster, producing a narrower band of clouds ranging from stratocumuliform to cumulonimbiform, depending on air mass stability.
Convective lift operates through a different mechanism. Intense daytime solar heating warms the surface, steepening the temperature gradient between ground and upper atmosphere. When the resulting instability becomes extreme, updrafts can reach speeds of up to 180 miles per hour. On comparatively rare occasions, convective lift is strong enough to push a cloud top through the tropopause and into the stratosphere.
Orographic lift occurs when wind forces air over a physical barrier such as a mountain. In stable conditions, the result is only a lenticular cap cloud. If the air is moist and unstable, orographic showers or thunderstorms can develop. Radiational cooling near the surface, common on clear nights, can bring air to its dew point without any lifting at all, forming fog. When air rises far enough to pass the freezing level, it enters a zone where the physics of ice takes over.
Homogeneous ice nucleation occurs in supercooled liquid droplets without any foreign substance to assist. For a typical atmospheric water droplet, it becomes likely at temperatures between -35 and -38 degrees Celsius. Classical Nucleation Theory describes how this probability depends on temperature, droplet volume, the thermodynamic activity of the water, and time.
Heterogeneous ice nucleation is far more common and requires a foreign substance that lowers the supercooling threshold. Naturally occurring ice-nucleating particles in the atmosphere include mineral dust from deserts, sea spray aerosols, biological material, carbonaceous particles, and volcanic ash. Silver iodide and Pseudomonas syringae are among the man-made substances also known to be effective. Nucleation occurs in three modes: immersion, deposition, or contact. In immersion mode, the nucleus sits inside the droplet. In deposition mode, ice-supersaturated vapor deposits directly onto the substance. In contact mode, freezing occurs when the substance touches a supercooled droplet.
A process called cloud seeding introduces artificial ice nuclei to encourage precipitation in clouds that might not otherwise rain. Adding excess nuclei shifts the balance so many nuclei compete for a limited amount of supercooled liquid water. The result is many small particles rather than fewer large ones. Cloud seeding can serve as a preventative measure in areas at risk of hail.
Secondary ice production, known as SIP, generates new ice crystals through interactions among ice particles that already exist. SIP can boost ice particle concentrations by many orders of magnitude above what primary nucleation would produce alone. Mechanisms include rime splintering, known as the Hallett-Mossop process; collision fragmentation; shattering of freezing droplets; and fragmentation during sublimation. Despite research dating to the 1940s, large uncertainties about SIP and its influence on cloud properties persist in climate models. Once ice crystals exist in a cloud, whether from primary nucleation or SIP, a different process governs how they grow.
Tor Bergeron identified a key asymmetry in how water vapor interacts with different surfaces inside a cloud. The saturation vapor pressure over ice is lower than the saturation vapor pressure over liquid water. A parcel of air just saturated relative to a water droplet is, at the same moment, supersaturated relative to any ice particle nearby. The water vapor migrates toward the ice and deposits there, causing the ice crystal to grow while the liquid droplet shrinks.
Between 0 and -40 degrees Celsius is the only temperature range where the Bergeron process operates. Below -40 degrees Celsius, liquid water freezes spontaneously, eliminating the mixed-phase conditions the process requires. Ice-nucleating particle concentrations are relatively low in the Arctic, where research suggests mineral dust must be transported over long distances from lower latitudes. Clouds forming in very cold Arctic air are particularly sensitive to the availability of ice nucleation sites. Without sufficient nuclei, the Bergeron mechanism stalls. The properties of those Arctic clouds depend on a supply that originates far away.
The warm-rain process builds precipitation entirely within liquid water, without any ice. Droplets suspended in a cloud interact constantly, colliding and either bouncing apart or merging into a larger drop. Different-sized drops fall at different speeds, increasing the chance of collision; in-cloud turbulence also drives droplets together. Once a drop grows to a radius greater than about 0.1 millimetres, it is large enough to fall as precipitation.
Riming and aggregation produce precipitation in clouds that contain ice. Riming occurs when a supercooled liquid drop collides with a solid snowflake and freezes onto it. Aggregation occurs when two solid snowflakes collide and join into a single particle. A frozen nucleus cycling through a violent updraft can accumulate about 0.5 inches of additional ice per pass. Repeated cycles produce hail too heavy to stay aloft.
What kind of precipitation a cloud produces depends partly on which cloud type it is and how high it forms.
Five structural forms describe the full range of cloud shapes in the troposphere. Cirriform clouds are high, thin, and wispy, appearing most prominently along the leading edges of organized weather disturbances. Stratiform clouds form as non-convective sheet-like layers, the product of large-scale lifting of stable air. Cumuliform clouds build into localized heaps through unstable free convection. Stratocumuliform clouds show a mix of heap and sheet characteristics, appearing as rolls or ripples. Cumulonimbiform clouds are the most convectively active and often include cirriform tops.
Ten genus types in the troposphere result from cross-classifying those five structural forms against three altitude bands. High-level clouds form at altitudes of 5 to 12 kilometres. All cirriform clouds at this level belong to the genus cirrus. High-level stratiform and stratocumuliform types carry the prefix cirro-, giving cirrostratus and cirrocumulus. Mid-level equivalents, at altitudes of 2 to 7 kilometres, carry the prefix alto-, giving altostratus and altocumulus. Low-level stratiform and stratocumuliform clouds, forming at around 2 kilometres and below, are called simply stratus and stratocumulus.
Cumulonimbus is the tallest cloud type, capable of spanning from a few hundred metres above the ground up to the tropopause. It is the cloud responsible for thunderstorms. Above the troposphere, polar stratospheric clouds appear rarely in winter at altitudes of 18 to 30 kilometres. Noctilucent clouds appear in summer at high latitudes at altitudes of 76 to 85 kilometres. They display many of the same structural forms as clouds found much lower in the troposphere.
Research aircraft carry cloud probes on wing pods and fuselage mounts to study cloud microphysics from inside. These instruments determine droplet size distributions and cloud liquid water content. They also measure raindrop sizes and concentrations, and the shape, size, and concentration of ice particles, including graupel. Statistical flight patterns cover fields of roughly homogeneous cloud. Sawtooth flight patterns gather multiple vertical profiles through shallower cloud layers. Many projects combine aircraft with ground-based instruments, drones, radars, and satellite instruments.
ACTRIS, the Aerosol, Clouds, and Trace Gases Research Infrastructure, operates sites across Europe with Doppler cloud radars, lidars, and microwave radiometers. These instruments produce continuous vertical profiles of cloud fraction and of water and ice cloud properties, processed through standardized Cloudnet procedures. Weather surveillance radars transmit microwave pulses at centimetre wavelengths to track the location, intensity, and motion of hydrometeors within clouds.
MODIS, POLDER, CALIPSO, and ATSR are among the satellite instruments that measure cloud radiances at the planetary scale. From those radiances, researchers retrieve cloud amount, height, temperature, pressure, and infrared emissivity. Other retrieved quantities include visible optical depth, effective particle size, and the cloud water path for liquid and ice phases. Cloud icing poses a significant aviation hazard, and different cloud types create icing over different temperature ranges. Low-level stratocumulus and stratus produce icing conditions between 0 and -10 degrees Celsius. Cumulonimbus and other vertically developed clouds extend that risk to -25 degrees Celsius. No instruments yet exist to directly measure supersaturation inside a cloud. That gap is one of the field's open frontiers, driving the next generation of cloud observation tools.
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Common questions
What is cloud physics?
Cloud physics is the study of the physical processes that lead to the formation, evolution, and impacts of atmospheric clouds, including precipitation and cloud radiative effects. It covers cloud particles ranging from liquid droplets a few micrometres in diameter to hailstones with diameters of up to 7 inches. Modern cloud physics traces its roots to the 19th century.
What are cloud condensation nuclei and why do they matter for cloud formation?
Cloud condensation nuclei (CCN) are microscopic particles of dust, smoke, salt, or other matter that allow water vapor to condense into cloud droplets at the low levels of supersaturation observed in the atmosphere. Without them, rising air becomes supersaturated and cloud formation is inhibited. The combined effects of the Kelvin effect and Raoult's law explain why a soluble CCN lowers the supersaturation threshold for condensation.
Who discovered the Bergeron process and what does it describe?
The Bergeron process was discovered by Tor Bergeron. It describes how ice crystals in mixed-phase clouds grow preferentially because the saturation vapor pressure over ice is lower than over liquid water, causing water vapor to migrate from liquid droplets to nearby ice particles. The process operates only between 0 and -40 degrees Celsius.
How do hailstones form inside a thunderstorm?
Hailstones form when rain droplets are carried above the freezing level by convective updrafts and freeze into small hail. A frozen nucleus can accumulate roughly 0.5 inches of additional ice per pass through an updraft, and repeated cycling through updrafts and downdrafts builds the onion-like layers of ice visible when a hailstone is cut in half. Hailstones have been found with diameters of up to 7 inches.
What is secondary ice production in cloud physics?
Secondary ice production (SIP) generates new ice crystals through interactions among ice particles already present in a cloud, rather than through fresh nucleation from supercooled liquid or vapor. SIP can enhance ice crystal concentrations by many orders of magnitude above what primary ice nucleation would produce. Mechanisms include the Hallett-Mossop rime splintering process, collision fragmentation, shattering of freezing droplets, and fragmentation during sublimation.
How are clouds classified by type and altitude?
Clouds in the troposphere are classified into ten genus types by cross-classifying five structural forms against three altitude bands. High-level clouds form at 5 to 12 kilometres, mid-level clouds at 2 to 7 kilometres, and low-level clouds at around 2 kilometres and below. The cumulonimbus, the tallest cloud type, spans from near the surface to the tropopause and is responsible for thunderstorms.
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