Cloud physics
Encyclopedia
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of clouds. Cloud
formations are composed of microscopic
droplets of liquid water (warm clouds), tiny crystals of ice (cold clouds), or both (mixed phase clouds). Cloud drops initially grow by the condensation of water vapor onto the drop when the supersaturation
of an air parcel exceeds a critical value according to Köhler theory
. Cloud condensation nuclei
are necessary for cloud drop formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the supersaturation needed for condensation to occur is so large that it does not happen naturally. Raoult's Law describes how the vapor pressure is dependent on the amount of solute
in a solution. At high concentrations, when the cloud drop is small, the supersaturation required is smaller than without the presence of a nucleus.
In warm clouds, larger cloud droplets fall at a higher terminal velocity because the drag force on smaller droplets is larger than on large droplets. The large droplet can then collide with small droplet and combine to form even larger drops. When the drops become large enough so that the acceleration due to gravity is much larger than the acceleration due to drag, the drops can fall to the earth as precipitation
. The collision and coalescence is not as important in mixed phase clouds where the Bergeron process
dominates. Other important processes that form precipitation are riming, when a supercooled liquid drop collides with a solid snowflake, and aggregation, when two solid snowflakes collide and combine. The precise mechanics
of how a cloud forms and grows is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual droplets. Advances in weather radar
and satellite
technology have also allowed the precise study of clouds on a large scale.
originated the idea that clouds were composed of water bubbles. In 1847 Agustus Waller used spider web
to examine droplets under the microscope. These observations were confirmed by William Henry Dines
in 1880 and Richard Assmann
in 1884.
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, for example the flat surface of water.
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.
of more than 1-2% relative to water is rarely seen in the atmosphere. For high levels of supersaturation there must be no condensation nuclei for the water vapor to condense on.
Supersaturation can also occur relative to ice. This is much more common in the atmosphere than supersaturation relative to water. Water droplets are able to maintain supersaturation relative to ice (remain as liquid water droplets and not freeze) because of the high surface tension
of each microdroplet, which prevents them from expanding to form larger ice crystals. Without ice nuclei supercooled liquid water droplets can exist down to about -40 C, at which point they will spontaneously freeze.
. The Bergeron process notes that the saturation vapor pressure of water, or how much water vapor a given volume can hold, 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 0 °C (32 °F) and -40 C. Below -40 C, 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 supercooled liquid water 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. 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 supercooled liquid water. An overseeded 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.
A frozen ice nucleus can pick up 0.5 inches (1.3 cm) in size traveling through one of these updrafts and can cycle through several updrafts before finally becoming so heavy that it falls to the ground. 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 7 inches (17.8 cm).
Stratus and limited convection stratocumulus clouds are seen at low altitudes of around 2 kilometres or lower. Clouds of similar shape in the topmost region of the troposphere have the prefix "cirro" added to their names ("cirrostratus" and "cirrocumulus"), appearing as light brush strokes in the blue sky. Stratiform clouds and cumuliform clouds of limited convection found at intermediate heights have the prefix "alto" added to their names ("altostratus" and "altocumulus"). All cirriform clouds are classified as high and therefore constitute a single cloud type or genus "cirrus".
Vertically developed nimbostratus, cumulus, and cumulonimbus may form anywhere from near surface to intermediates heights of around 3 kilometres and therefore, like the low clouds, have no height related prefixes. However, those capable of producing heavy precipitation or stormy weather carry a "nimbo" or "nimbus" designation. Of the vertically developed clouds, the "cumulonimbus" type is the largest and can virtually span the entire troposphere from a few hundred metres above the ground up to the tropopause. The cumulonimbus is the cloud responsible for thunderstorms.
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
formations are composed of microscopic
Microscopic
The microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...
droplets of liquid water (warm clouds), tiny crystals of ice (cold clouds), or both (mixed phase clouds). Cloud drops initially grow by the condensation of water vapor onto the drop when the supersaturation
Supersaturation
The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances...
of an air parcel exceeds a critical value according to Köhler theory
Köhler theory
Köhler theory describes the process in which water vapor condenses and forms liquid cloud drops, and is based on equilibrium thermodynamics. It combines the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface, and Raoult's Law, which relates the saturation...
. Cloud condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...
are necessary for cloud drop formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the supersaturation needed for condensation to occur is so large that it does not happen naturally. Raoult's Law describes how the vapor pressure is dependent on the amount of solute
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
in a solution. At high concentrations, when the cloud drop is small, the supersaturation required is smaller than without the presence of a nucleus.
In warm clouds, larger cloud droplets fall at a higher terminal velocity because the drag force on smaller droplets is larger than on large droplets. The large droplet can then collide with small droplet and combine to form even larger drops. When the drops become large enough so that the acceleration due to gravity is much larger than the acceleration due to drag, the drops can fall to the earth as precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
. The collision and coalescence is not as important in mixed phase clouds where the Bergeron process
Bergeron process
The Bergeron–Findeisen process , also known as the cold rain or ice crystal process, is the formation of precipitation in the cold clouds of the mid and upper latitudes by ice crystal growth. The equilibrium vapor pressure over water is greater than the saturation vapor pressure over ice, at the...
dominates. Other important processes that form precipitation are riming, when a supercooled liquid drop collides with a solid snowflake, and aggregation, when two solid snowflakes collide and combine. The precise mechanics
Mechanics
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....
of how a cloud forms and grows is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual droplets. Advances in weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
and satellite
Weather satellite
The weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while...
technology have also allowed the precise study of clouds on a large scale.
History of cloud physics
The history of cloud microphysics developed in the 19th century and is described in several publications. Otto von GuerickeOtto von Guericke
Otto von Guericke was a German scientist, inventor, and politician...
originated the idea that clouds were composed of water bubbles. In 1847 Agustus Waller used spider web
Spider web
A spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets....
to examine droplets under the microscope. These observations were confirmed by William Henry Dines
William Henry Dines
William Henry Dines BA FRS was an English meteorologist.Dines was born in London, the son of George Dines, also a meteorologist. He was educated at Woodcote House school, Windlesham, and afterwards entered Corpus Christi College, Cambridge, where he obtained a first-class in the mathematical...
in 1880 and Richard Assmann
Richard Assmann
Richard Assmann ; was a German meteorologist and physician who was a native of Magdeburg....
in 1884.
Formation
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 pressureVapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
is called saturation and the relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
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, for example the flat surface of water.
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
SupersaturationSupersaturation
The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances...
of more than 1-2% relative to water is rarely seen in the atmosphere. For high levels of supersaturation there must be no condensation nuclei for the water vapor to condense on.
Supersaturation can also occur relative to ice. This is much more common in the atmosphere than supersaturation relative to water. Water droplets are able to maintain supersaturation relative to ice (remain as liquid water droplets and not freeze) because of the high surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
of each microdroplet, which prevents them from expanding to form larger ice crystals. Without ice nuclei supercooled liquid water droplets can exist down to about -40 C, at which point they will spontaneously freeze.
Collision-coalescence
One theory explaining how the behavior of individual droplets leads to the formation of clouds 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.Bergeron process
The primary mechanism for the formation of ice clouds was discovered by Tor BergeronTor Bergeron
Tor Bergeron was the Swedish meteorologist who proposed a mechanism for the formation of precipitation in clouds. In the 1930s, Bergeron and W. Findeisen developed the concept that clouds contain both supercooled water and ice crystals...
. The Bergeron process notes that the saturation vapor pressure of water, or how much water vapor a given volume can hold, 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
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
, 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
Vapor-liquid equilibrium
Vapor–liquid equilibrium is a condition where a liquid and its vapor are in equilibrium with each other, a condition or state where the rate of evaporation equals the rate of condensation on a molecular level such that there is no net vapor-liquid interconversion...
, 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 0 °C (32 °F) and -40 C. Below -40 C, 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 supercooled liquid water 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. 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
Cloud seeding
Cloud seeding, a form of intentional weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud...
adds excess artificial ice nuclei which shifts the balance so that there are many nuclei compared to the amount of supercooled liquid water. An overseeded 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
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
storms.
Dynamic phase hypothesis
The second critical point in the formation of clouds is their dependence on updrafts. As particles group together to form water droplets, they will quickly be pulled down to earth by the force of gravity. The droplets would quickly dissipate and the cloud will never form. However, if warm air interacts with cold air, an updraft can form. Warm air is less dense than colder air, so the warm air rises. The air travelling upward buffers the falling droplets, and can keep them in the air much longer than they would otherwise stay. In addition, the air cools as it rises, so any moisture in the updraft will then condense into liquid form, adding to the amount of water available for precipitation. Violent updrafts can reach speeds of up to 180 miles per hour (289.7 km/h).A frozen ice nucleus can pick up 0.5 inches (1.3 cm) in size traveling through one of these updrafts and can cycle through several updrafts before finally becoming so heavy that it falls to the ground. 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 7 inches (17.8 cm).
Cloud Classification
Clouds are classified according to the height at which they are found, and their shape or appearance. There are three basic categories based on physical structure and process of formation. Stratiform clouds appear as extensive layers, ranging from thin to moderately thick with some vertical development. They are mostly the product of large scale lift of stable air. Cumuliform clouds are formed mostly into localized heaps, rolls and/or ripples ranging from very small cloudlets of limited convection in slightly unstable air to very large towering free convective buildups when the airmass is very unstable. Cirriform clouds are high, thin and wispy, and are seen most extensively along the leading edges of organized weather disturbances.Stratus and limited convection stratocumulus clouds are seen at low altitudes of around 2 kilometres or lower. Clouds of similar shape in the topmost region of the troposphere have the prefix "cirro" added to their names ("cirrostratus" and "cirrocumulus"), appearing as light brush strokes in the blue sky. Stratiform clouds and cumuliform clouds of limited convection found at intermediate heights have the prefix "alto" added to their names ("altostratus" and "altocumulus"). All cirriform clouds are classified as high and therefore constitute a single cloud type or genus "cirrus".
Vertically developed nimbostratus, cumulus, and cumulonimbus may form anywhere from near surface to intermediates heights of around 3 kilometres and therefore, like the low clouds, have no height related prefixes. However, those capable of producing heavy precipitation or stormy weather carry a "nimbo" or "nimbus" designation. Of the vertically developed clouds, the "cumulonimbus" type is the largest and can virtually span the entire troposphere from a few hundred metres above the ground up to the tropopause. The cumulonimbus is the cloud responsible for thunderstorms.