A shorthand classification scheme developed to convey to the general public the complex and often confusing levels of disturbances in the solar-terrestrial environment. Three typical events are repre
sented as: Geomagnetic Storms (G), Solar Radiation Storms (S), and Radio Blackouts (R). A numerical qualifier is added to the event type to indicate the severity of the disturbance. These qualifiers are defined as follows:
1
Minor
2
Moderate
3
Strong
4
Severe
5
Extreme
See Appendix A for complete descriptions of normal terrestrial system effects, threshold levels, and event frequency of occurrence.
(Rarely called luminous clouds.) Thin silvery-blue cirrus-like clouds frequently seen during summer twilight conditions at high latitudes (above 50?) in both hemispheres. They are the highest visible
clouds in the atmosphere, occurring in the upper mesosphere at heights of about 85 km, and are closely related to the polar mesospheric clouds seen in satellite observations at similar altitudes over the summer polar cap. Noctilucent clouds are now known to consist of tiny ice particles with dimensions of the order of tens of nanometers, growing in the extreme cold of the summer polar mesopause region. The condensation nuclei on which the particles grow are thought to be either smoke and dust particles of meteoric origin or large hydrated positive ions. Strong upwelling of air from below, associated with a pole-to-pole meridional circulation in the upper mesosphere, is responsible for both the extreme cold and the upward flux of water vapor. Although water-vapor mixing ratios are very low (less than 10 parts per million by volume) in the region, the temperatures are also low enough to produce a high degree of supersaturation at times. Anomalously strong radar echoes from the region, known as polar summer mesospheric echoes, are also associated with the clouds. Compare nacreous clouds, polar stratospheric clouds.
Thin silvery-blue cirrus-like clouds frequently seen during summer twilight conditions at high latitudes (above 50 deg) in both hemispheres. They are the highest visible clouds in the atmosphere, occu
rring in the upper mesosphere at heights of about 85 km, and are closely related to the polar mesospheric clouds seen in satellite observations at similar altitudes over the summer polar cap. Noctilucent clouds are now known to consist of tiny ice particles with dimensions of the order of tens of nanometers, growing in the extreme cold of the summer polar mesopause region. The condensation nuclei on which the particles grow are thought to be either smoke and dust particles of meteoric origin or large hydrated positive ions. Strong upwelling of air from below, associated with a pole-to-pole meridional circulation in the upper mesosphere, is responsible for both the extreme cold and the upward flux of water vapor. Although water-vapor mixing ratios are very low (less than 10 parts per million by volume) in the region, the temperatures are also low enough to produce a high degree of supersaturation at times. Anomalously strong radar echoes from the region, known as polar summer mesospheric echoes, are also associated with the clouds. (Rarely called luminous clouds.)
The cool layer of air adjacent to the ground that forms at night. At night under clear skies, radiation to space cools the land surface, which in turn cools the adjacent air through processes of molec
ular conduction, turbulence, and radiative transfer. This causes a stable boundary layer to form and grow to depths of a few hundreds of meters, depending on the season. Many interacting processes can occur within the statically stable nocturnal boundary layer: patchy sporadic turbulence, internal gravity waves, drainage flows, inertial oscillations, and nocturnal jets.
In geotechnical simulation, it is the coordinate location in space where the degrees of freedom (DOFs) of a finite element analysis are defined. In storm simulation it is a property that is associated
with a node in a [finite_element_mesh](https://n2t.net/99152/h1451). That is, it is a spatially dependent property that would normally be defined by a function.