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.
Clouds consist of particles that include liquid droplets, raindrops, ice particles made of pristine ice crystals, aggregates, graupel and hail. Cloud droplets can be a few micrometres in diameter, whilst graupel can be several millimetres, and hail can be larger still. The aggregate effect of these particles results in the properties of clouds we commonly observe, including their brightness (i.e. how they reflect or absorb sunlight), or precipitation development. Clouds can consist solely of liquid droplets (often called "warm" clouds) or ice particles (cold clouds), or a combination of both (mixed-phase clouds), along with microscopic particles of dust, smoke, or other matter, which can act as condensation nuclei.
Cloud condensation nuclei (CCN) are necessary for cloud droplet formation at supersaturation values observed, allowing droplets to form as a result of the combined effects of the Kelvin effect and Raoult's law. The Kelvin effect describes the change in saturation vapour pressure due to a curved surface. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At concentrations typical of CCN, when the cloud droplets are small, the supersaturation required for condensation to occur is smaller than without the presence of a soluble (or partially soluble) CCN. Cloud drops can freeze to become ice crystals at temperatures as high as -5 oC if suitable ice nucleating particles (INP) are present. Ice particles grow at lower temperatures where the Bergeron process dominates.
The warm-rain process involves the collision and coalescence of cloud drops of different sizes. Collisions can take place as a result of either different drop fall speeds or in-cloud turbulence. When the drops become large enough (greater than around 0.1 mm), they can fall as precipitation. 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 evolves is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual cloud particles. For example, in-situ airborne measurements have shown that ice particle concentrations can be many orders of magnitude greater than expected from pristine ice formation, so research has identified a number of potential secondary ice production (SIP) mechanisms. Advances in weather radar and satellite technology have also allowed the precise study of clouds on a large scale.
History of research
Modern cloud physics began in the 19th century and was described in several publications. erratum/corrigendum: "1358" Otto von Guericke originated the idea that clouds were composed of water bubbles. In 1847 Augustus 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.
Cloud formation
Cooling air to its dew point
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. Incoming short-wave radiation generated by the sun is re-emitted as long-wave radiation when it reaches Earth's surface. This process warms the air closest to ground and increases air mass instability by creating a steeper temperature gradient from warm or hot at surface level to cold aloft. This causes it to rise and cool until temperature equilibrium is achieved with the surrounding air aloft. Moderate instability allows for the formation of cumuliform clouds of moderate size that can produce light showers if the airmass is sufficiently moist. Typical convection upcurrents may allow the droplets to grow to a radius of about before precipitating as showers. 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). If the air is generally stable, nothing more than lenticular cap clouds will form. However, if the air becomes sufficiently moist and unstable, orographic showers or thunderstorms may appear.Pidwirny, M. (2006). "Cloud Formation Processes" , chapter 8 in Fundamentals of Physical Geography, 2nd ed.
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.Ackerman, p. 109 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.