Irregular bright patches in the solar photosphere forming a network in the surroundings of sunspots.


Temperature scale in which the freezing point of water at standard air pressure is 32 degrees, while the boiling point is 212 degrees.


Severe shortage of food.

Ferrell cell

Circulation cell proposed by W. Ferrell, located in the middle and high latitudes of both hemispheres. Its circulation is equatorward at an intermediate level and poleward near the surface.


Bright and transient phenomenon that occurs within the Sun’s chromosphere and corona, and produce enhanced radiation and particle emissions.

Flash flooding

Intense and localized flood that develops and falls quite rapidly, with little or no warning, usually as a consequence of intense rainfall over a small area, and that can cause severe damage.

Flood / Flooding

The overflowing of the normal confines of a stream or other body of water, or the accumulation of water, submerging an area that is not normally so.

Flow pattern

Distribution of velocities in a region of the atmosphere, at a particular time.


Downslope wind, often strong and gusty, which is warmed and dried by descent, in general on the lee side of a mountain, (the Alps) as a consequence of synoptic-scale, cross-barrier flow over the mountain range. Usually, it produces a dramatic ascent of temperature (10 to even 20 degrees) in a matter of minutes.


Very small (usually microscopic) water droplets suspended in the atmosphere, close to the surface, generally reducing horizontal visibility to less than 1 km.

Fogbow / Cloudbow, mistbow or white rainbow

Large, white circular band, usually fringed by a thin red band on the outside and a thin blue one on the inside, formed due to refraction, reflection and in less extent, diffraction, of sunlight or moonlight on small water droplets present in clouds or fog.

Forced lifting

Convection caused by mechanical forces, such as the ascending air motion caused by orography.


A person who predicts the weather.


Remains of a plant or animal that existed in a past geological age, and that has been obtained from the soil through excavation.


Process by which a substance changes phase, passing from the liquid to the solid state, involving nucleation. Same as solidification. Air temperature equal to or less than the freezing point of water (0 degree).

Freezing drizzle

Drizzle that freezes when impacting on a surface, forming a coat of glaze.

Freezing rain

Rain that freezes upon impact on the ground and on exposed objects, forming a coating of glaze on them.


Mechanical resistive force offered by one medium or body to the relative motion of a second medium or body in contact with the first.

Frictional deceleration

Loss of velocity caused by friction.


Transition zone or interface between two air masses of different density (humidity, temperature). Other than these characteristics, other features may distinguish a front, such as a pressure trough, wind rotation, moisture discontinuity and certain clouds and precipitation forms. Intersection of the surface separating two air masses with another surface or with the ground.


Process of initial formation or intensification of a front or a frontal zone, either by physical or kinematical influences. Increase in the horizontal gradient of an air mass property, and the formation of the accompanying features of the wind field that characterize a front.


Process of dissipation of a front or frontal zone by physical or kinematical influences. Decrease in the horizontal gradient of an air mass property and the dissipation of the accompanying features of the wind field that characterize a front.


Layer of ice crystals on a cold object, which forms by deposition –direct transition from water vapor to solid ice-. Condition given when the temperature of the Earth’s surface and objects fall below freezing.

Frost point

Maximum temperature of formation of hoar frost by direct sublimation of the water vapor contained in the ambient air on a cooled polished surface.


Partial freezing of exposed body parts, which causes injury to the skin and eventually to deeper tissues.

Fujita scale / F-scale

Scale that relates the intensity of a tornado to structural and/or vegetative damage. It consists of six categories, from F0 to F5, that correspond to a certain wind-speed estimate and a certain degree of damage, from light damage to incredible damage respectively.

Funnel cloud

A condensation cloud, formed at the core of a waterspout or tornado vortex, typically funnel-shaped, which may be sometimes in contact with the ground, caused by the reduction of pressure at the center of the vortex.