A wave cloud is a cloud form created by atmospheric internal waves.
In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.
Internal waves are gravity waves that oscillate within a fluid medium, rather than on its surface. To exist, the fluid must be stratified: the density must change with depth/height due to changes, for example, in temperature and/or salinity. If the density changes over a small vertical distance, the waves propagate horizontally like surface waves, but do so at slower speeds as determined by the density difference of the fluid below and above the interface. If the density changes continuously, the waves can propagate vertically as well as horizontally through the fluid.
The atmospheric internal waves that form wave clouds are created as stable air flows over a raised land feature such as a mountain range, and can form either directly above or in the lee of the feature. As an air mass travels through the wave, it undergoes repeated uplift and descent. If there is enough moisture in the atmosphere, clouds will form at the cooled crests of these waves. In the descending part of the wave, those clouds will evaporate due to adiabatic heating, leading to the characteristic clouded and clear bands. The cloud base on the leeward side is higher than on the windward side, because precipitation on the windward side removes water from the air.
A mountain range or hill range is a series of mountains or hills ranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arisen from the same cause, usually an orogeny. Mountain ranges are formed by a variety of geological processes, but most of the significant ones on Earth are the result of plate tectonics. Mountain ranges are also found on many planetary mass objects in the Solar System and are likely a feature of most terrestrial planets.
An adiabatic process occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as work. The adiabatic process provides a rigorous conceptual basis for the theory used to expound the first law of thermodynamics, and as such it is a key concept in thermodynamics.
It is possible that simple convection from mountain summits can also form wave clouds. This occurs as the convection forces a wave or lenticular wave cloud into the more stable air above.
Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, including molten rock (rheid). Convection includes sub-mechanisms of advection, and diffusion.
Wave clouds are typically mid- to upper-tropospheric ice clouds. They are relatively easy to study, because they are quite consistent. As a result, they are being analyzed to increase our understanding of how these upper-level ice clouds influence the Earth's radiation budget. Understanding this can improve climate models.
The streamlines in these clouds have the steepest slope a few kilometers downwind of the lee slope of a mountain. It is in these regions of highest vertical velocity that sailplanes can reach record-breaking altitudes.
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word soaring is also used for the sport.
In an ideal model, a wave cloud consists of supercooled liquid water at the lower part, a mixed phase of frozen and liquid water near the ridge, and ice beginning slightly below the ridge and extending downstream. However, this doesn't always occur. Wave cloud structure ranges from smooth and simple, to jumbled phases occurring randomly.Often, ice crystals can be found downwind of the waves. Whether this happens depends on the saturation of the air. The composition of the ice is currently an active topic of study. The main mechanism for ice formation is homogeneous nucleation. The ice crystals are mostly small spheroidal and irregular-shaped particles. Ice columns make up less than 1%, and plates are virtually nonexistent. Multi-level mountain wave clouds form when the moisture in the air above the mountain is located in distinct layers, and vertical mixing is inhibited.
Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled below 0° C, it will tend to freeze into ice. Volumes of water cooled only a few degrees below 0° C often stay completely free of ice for long periods. At these conditions, nucleation of ice is either slow or does not occur at all. However, at lower temperatures ice crystals appear after little or no delay. At these conditions ice nucleation is fast. Nucleation is commonly how first-order phase transitions start, and then it is the start of the process of forming a new thermodynamic phase. By contrast, new phases at continuous phase transitions start to form immediately.
Cirrus is a genus of atmospheric cloud generally characterized by thin, wispy strands, giving the type its name from the Latin word cirrus, meaning a ringlet or curling lock of hair. This cloud can form at any altitude between 16,500 ft and 45,000 ft above sea level. The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of "mares' tails".
Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin cumulo-, meaning heap or pile. Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.
Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Fog can be considered a type of low-lying cloud, usually resembling stratus, and is heavily influenced by nearby bodies of water, topography, and wind conditions. In turn, fog has affected many human activities, such as shipping, travel, and warfare.
Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds that are formed by rising thermals. More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color from dark gray to nearly white. The word "stratus" comes from the Latin prefix "strato-", meaning "layer". Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes over a region. Some call these clouds "high fog" for the fog-like cloud. While light rain may fall, this cloud does not indicate much meteorological activity.
The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, changes with altitude. Lapse rate arises from the word lapse, in the sense of a gradual change. It corresponds to the vertical component of the spatial gradient of temperature. Although this concept is most often applied to the Earth's troposphere, it can be extended to any gravitationally supported parcel of gas.
In meteorology, precipitation is any product of the condensation of atmospheric water vapour that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates". Thus, fog and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called "showers."
A föhn or foehn is a type of dry, warm, down-slope wind that occurs in the lee of a mountain range.
Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.
Mammatus, meaning "mammary cloud", is a cellular pattern of pouches hanging underneath the base of a cloud, typically cumulonimbus rainclouds, although they may be attached to other classes of parent clouds. The name mammatus is derived from the Latin mamma. According to the WMO International Cloud Atlas, mamma is a cloud supplementary feature rather than a genus, species or variety of cloud. They are formed by cold air sinking down to form the pockets contrary to the puffs of clouds rising through the convection of warm air. These formations were first described in 1894 by William Clement Ley.
In meteorology, lee waves are atmospheric stationary waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Krkonoše. They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range. They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street.
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of 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 droplets are small, the supersaturation required is smaller than without the presence of a nucleus.
Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. There are also intermediate formations.
A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure. Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.
In meteorology, the various types of precipitation often include the character or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain, such as a mountain.
Atmospheric convection is the result of a parcel-environment instability, or temperature difference layer in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to increased winds, cumulus cloud development, and decreased surface dew points. Moist convection leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.
Atmospheric instability is a condition where the Earth's atmosphere is generally considered to be unstable and as a result the weather is subjected to a high degree of variability through distance and time. Atmospheric stability is a measure of the atmosphere's tendency to discourage or deter vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air at altitude. Because it is warmer, it is less dense and is prone to further ascent.
The levant is an easterly wind that blows in the western Mediterranean Sea and southern France, an example of mountain-gap wind. In Roussillon it is called "llevant" and in Corsica "levante". In the western Mediterranean, particularly when the wind blows through the Strait of Gibraltar, it is called the Viento de Levante or the Levanter. It is also known as the Solano.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.