Cooling air to its dew point in
Denmark. Nearly black color of base indicates main cloud in foreground probably
cumulonimbus.
Adiabatic cooling As water evaporates from an area of Earth's surface, the air over that area becomes moist. Moist air is lighter than the surrounding dry air, creating an unstable situation. When enough moist air has accumulated, all the moist air rises as a single packet, without mixing with the surrounding air. As more moist air forms along the surface, the process repeats, resulting in a series of discrete packets of moist air rising to form clouds. This process occurs when one or more of three possible lifting agents—cyclonic/frontal, convective, or
orographic—causes air containing invisible
water vapor to rise and cool to its
dew point, the
temperature at which the air becomes saturated. The main mechanism behind this process is
adiabatic cooling.
Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends
energy and causes the air to cool, which makes water vapor condense into cloud. Water vapor in saturated air is normally attracted to
condensation nuclei such as dust and
salt particles that are small enough to be held aloft by normal
circulation of the air. The water droplets in a cloud have a normal radius of about 0.002 mm (0.00008 in). The droplets may collide to form larger droplets, which remain aloft as long as the velocity of the rising air within the cloud is equal to or greater than the terminal velocity of the droplets. For non-convective cloud, the altitude at which condensation begins to happen is called the
lifted condensation level (LCL), which roughly determines the height of the cloud base. Free convective clouds generally form at the altitude of the
convective condensation level (CCL). Water vapor in saturated air is normally attracted to
condensation nuclei such as
salt particles that are small enough to be held aloft by normal
circulation of the air. If the condensation process occurs below the
freezing level in the troposphere, the nuclei help transform the vapor into very small water droplets. Clouds that form just above the freezing level are composed mostly of supercooled liquid droplets, while those that condense out at higher altitudes where the air is much colder generally take the form of
ice crystals. An absence of sufficient condensation particles at and above the condensation level causes the rising air to become supersaturated and the formation of cloud tends to be inhibited.
Frontal and cyclonic lift Frontal and
cyclonic lift occur in their purest manifestations when
stable air, which has been subjected to little or no surface heating, is forced aloft at
weather fronts and around centers of
low pressure.
Warm fronts associated with extratropical cyclones tend to generate mostly cirriform and stratiform clouds over a wide area unless the approaching warm airmass is unstable, in which case cumulus congestus or cumulonimbus clouds will usually be embedded in the main precipitating cloud layer.
Cold fronts are usually faster moving and generate a narrower line of clouds which are mostly stratocumuliform, cumuliform, or cumulonimbiform depending on the stability of the warm air mass just ahead of the front.
Convective lift Another agent is the buoyant convective upward motion caused by significant daytime solar heating at surface level, or by relatively high absolute humidity. The equivalent diameter of these droplets is about . If air near the surface becomes extremely warm and unstable, its upward motion can become quite explosive, resulting in towering cumulonimbiform clouds that can cause
severe weather. As tiny water particles that make up the cloud group together to form droplets of rain, they are pulled down to earth by the force of
gravity. The droplets would normally evaporate below the condensation level, but strong
updrafts buffer the falling droplets, and can keep them aloft much longer than they would otherwise. Violent updrafts can reach speeds of up to . The longer the rain droplets remain aloft, the more time they have to grow into larger droplets that eventually fall as heavy showers. Rain droplets that are carried well above the freezing level become supercooled at first then freeze into small hail. A frozen ice nucleus can pick up in size traveling through one of these updrafts and can cycle through several updrafts and downdrafts before finally becoming so heavy that it falls to the ground as large hail. Cutting a hailstone in half shows onion-like layers of ice, indicating distinct times when it passed through a layer of
super-cooled water. Hailstones have been found with diameters of up to . Convective lift can occur in an unstable air mass well away from any fronts. However, very warm unstable air can also be present around fronts and low-pressure centers, often producing cumuliform and cumulonimbiform clouds in heavier and more active concentrations because of the combined frontal and convective lifting agents. As with non-frontal convective lift, increasing instability promotes upward vertical cloud growth and raises the potential for severe weather. On comparatively rare occasions, convective lift can be powerful enough to penetrate the tropopause and push the cloud top into the stratosphere.
Orographic lift A third source of lift is wind circulation forcing air over a physical barrier such as a
mountain (
orographic lift). enhanced by the Sun's angle, can visually mimic a
tornado resulting from orographic lift
Non-adiabatic cooling Along with adiabatic cooling that requires a lifting agent, there are three other main mechanisms for lowering the temperature of the air to its dew point, all of which occur near surface level and do not require any lifting of the air. Conductive, radiational, and evaporative cooling can cause condensation at surface level resulting in the formation of
fog. Conductive cooling takes place when air from a relatively mild source area comes into contact with a colder surface, as when mild marine air moves across a colder land area. Radiational cooling occurs due to the emission of
infrared radiation, either by the air or by the surface underneath. This type of cooling is common during the night when the sky is clear. Evaporative cooling happens when moisture is added to the air through evaporation, which forces the air temperature to cool to its
wet-bulb temperature, or sometimes to the point of saturation.
Adding moisture to the air There are five main ways water vapor can be added to the air. Increased vapor content can result from wind convergence over water or moist ground into areas of upward motion. Precipitation or virga falling from above also enhances moisture content. Daytime heating causes water to evaporate from the surface of oceans, water bodies or wet land.
Transpiration from plants is another typical source of water vapor. Lastly, cool or dry air moving over warmer water will become more humid. As with daytime heating, the addition of moisture to the air increases its heat content and instability and helps set into motion those processes that lead to the formation of cloud or fog.
Supersaturation The amount of water that can exist as vapor in a given volume increases with the temperature. When the amount of water vapor is in equilibrium above a flat surface of water the level of
vapor pressure is called saturation and the
relative humidity is 100%. At this equilibrium there are equal numbers of molecules evaporating from the water as there are condensing back into the water. If the relative humidity becomes greater than 100%, it is called supersaturated. Supersaturation occurs in the absence of condensation nuclei. Since the saturation vapor pressure is proportional to temperature, cold air has a lower saturation point than warm air. The difference between these values is the basis for the formation of clouds. When saturated air cools, it can no longer contain the same amount of water vapor. If the conditions are right, the excess water will condense out of the air until the lower saturation point is reached. Another possibility is that the water stays in vapor form, even though it is beyond the saturation point, resulting in
supersaturation. Supersaturation of more than 1–2% relative to water is rarely seen in the atmosphere, since cloud condensation nuclei are usually present. Much higher degrees of supersaturation are possible in clean air, and are the basis of the
cloud chamber. There are no instruments to take measurements of supersaturation in clouds.
Supercooling Water droplets commonly remain as liquid water and do not freeze, even well below . Ice nuclei that may be present in an atmospheric droplet become active for ice formation at specific temperatures in between and , depending on nucleus geometry and composition. Without ice nuclei,
supercooled water droplets (as well as any extremely pure liquid water) can exist down to about , at which point spontaneous freezing occurs.
Collision-coalescence One theory explaining how the behavior of individual droplets in a cloud leads to the formation of precipitation is the collision-coalescence process. Droplets suspended in the air will interact with each other, either by colliding and bouncing off each other or by combining to form a larger droplet. Eventually, the droplets become large enough that they fall to the earth as precipitation. The collision-coalescence process does not make up a significant part of cloud formation, as water droplets have a relatively high surface tension. In addition, the occurrence of collision-coalescence is closely related to entrainment-mixing processes.
Primary ice production (PIP) Ice crystals in clouds and the atmosphere can be produced from
supercooled liquid or ice-
supersaturated vapour via homogeneous or heterogeneous ice
nucleation. This is referred to as primary ice production (PIP), in contrast to ice production from ice crystals that already exist, known as secondary ice production.
Homogeneous ice nucleation Homogeneous ice nucleation occurs in supercooled liquid droplets without the presence of a foreign substance that can aid the nucleation. For a typical water droplet in Earth’s atmosphere, the likelihood of homogeneous ice nucleation increases with supercooling and typically occurs between -35°C and -38°C.
Classical Nucleation Theory describes this process and how it depends on temperature, droplet volume, the
thermodynamic activity of the water, and time.
Heterogeneous ice nucleation Heterogeneous ice nucleation occurs when the nucleation is initiated by a foreign substance that reduces the supercooling (or ice-supersaturation) required ; the substances that can achieve this are referred to as
ice-nucleating particles or
ice nuclei . Some important, naturally occurring, ice-nucleating particles in the atmosphere include
mineral dust from deserts and soils, sea spray aerosols, biological material, carbonaceous particles, and volcanic ash. Man-made particles, such as
silver iodide and
Pseudomonas syringae, are also known to be effective ice-nucleating particles. Heterogeneous ice nucleation can occur via several mechanisms or modes, including immersion mode (where the substance is immersed within the droplet), deposition mode (where ice-supersaturated vapour deposits directly onto the substance), or contact mode (where freezing of a supercooled droplet occurs upon contact with the substance).
Artificially influencing ice nucleation A process whereby scientists seed a cloud with artificial ice nuclei to encourage precipitation is known as cloud seeding. This can help cause precipitation in clouds that otherwise may not rain.
Cloud seeding adds excess artificial ice nuclei which shifts the balance so that there are many nuclei compared to the amount of super cooled liquid water. An over seeded cloud will form many particles, but each will be very small. This can be done as a preventative measure for areas that are at risk for
hail storms.
Secondary ice production (SIP) Secondary ice production (SIP) generates new ice crystals through interactions with pre-existing ice particles in clouds. SIP can enhance ice particle number concentrations, often by many orders of magnitude above the expected ice particle number concentration from PIP (e.g.,). The SIP-driven enhancement of ice particle concentrations influences ice particle properties, precipitation, lifetime and radiative properties of clouds. SIP mechanisms operate under different thermodynamic and microphysical conditions, including rime splintering (e.g., the Hallett-Mossop process), collision fragmentation, shattering of freezing droplets, and fragmentation during ice sublimation. Despite research dating back to the 1940s, challenges remain in representing SIP in climate and atmospheric models, along with large uncertainties persisting around its influences on cloud properties.
Bergeron process The primary mechanism for the growth of ice cloud particles was discovered by
Tor Bergeron. The Bergeron process notes that the
saturation vapor pressure of water, or how much water vapor a given volume can contain, depends on what the vapor is interacting with. Specifically, the saturation vapor pressure with respect to ice is lower than the saturation vapor pressure with respect to water. Water vapor interacting with a water droplet may be saturated, at 100%
relative humidity, when interacting with a water droplet, but the same amount of water vapor would be supersaturated when interacting with an ice particle. The water vapor will attempt to return to
equilibrium, so the extra water vapor will condense into ice on the surface of the particle. These ice particles end up as the nuclei of larger ice crystals. This process only happens at temperatures between and . Below , liquid water will spontaneously nucleate, and freeze. The surface tension of the water allows the droplet to stay liquid well below its normal freezing point. When this happens, it is now
supercooled liquid water. The Bergeron process relies on super cooled liquid water (SLW) interacting with
ice nuclei to form larger particles. If there are few ice nuclei compared to the amount of SLW, droplets will be unable to form. This means that clouds that form in very cold air, for example in the Arctic, are particularly sensitive to ice nucleation. However, concentrations of ice nucleation particles (INPs) are relatively low in the Arctic and research has suggested that INPs such as mineral dust are likely to have been transported over long distances from lower latitudes. == Cloud classification ==