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R

RA

(abbr) Rain; as used in aviation (e.g. METAR/TAF) reports.

Radiation

This is the transmission of energy by electromagnetic waves, which may be propagated through a substance or through a vacuum at the speed of light. Electromagnetic radiation is divided into various classes on the basis of wavelengths; these are, in order of increasing wavelength: gamma radiation, X-rays, ultra-violet (UV) radiation, visible (VIS) light, infra-red (IR) radiation and radio waves.

Radiosonde

(sometimes abbreviated to 'R/S') An instrument that measures temperature, pressure and humidity of the atmosphere as it is carried aloft on a balloon. The "sonde" transmits its measurements to a ground-based radio receiver via radio signals, and by accurate tracking (radar or satellite) of the sonde unit, upper winds can be deduced.

Rain day

A period of 24hr, conventionally beginning at 09UTC, during which precipitation of 0.2mm or more has been recorded. (See also Wet day).

Relative humidity (RH)

See the main FAQ here.

Relative Vorticity

The vorticity (or tendency for air particles to 'spin') relative to the earth. It can be considered for practical purposes (and crudely assessed on meteorological charts) as the combination of two factors: (i): the 'spin' imparted due to the curved path that air takes in its passage through the atmosphere (cyclonically curved contours=positive, anticyclonically curved contours=negative). (ii) the other factor is due to the shear developed along the flow due to the differing velocities of the moving particles. Swiftly moving air will generate a 'twist' element relative to the lower-velocity flow on either side [ shear vorticity ]: where the 'twisting' generated is in a cyclonic sense, that is counted as positive; where in the anticyclonic sense then it is negative. ( See also Vorticity; Absolute Vorticity.)

Relaxation

When the amplitude of a trough decreases with time, the trough is said to have undergone relaxation. The change is usually measured in terms of a latitude change of a chosen contour or thickness line.

Retrogression

When an upper trough (or ridge) moves against the normal west-to-east flow in mid-latitudes, the feature is retrogressing, or undergoing retrogression.

RH

(abbr)Relative Humidity (expressed as a % value).

Ridge

See Upper ridge

Ridge amplification

When contour heights along the axis of an upper ridge increase, the ridge is amplifying.

Right entrance

On the warm side of the jet core, in the region of maximum acceleration of flow. Often associated with marked cyclogenesis.