R

Radar

Coined word for radio detection and ranging. Electronic instrument, consisting of a transmitter, a receiver, an antenna and a display with associated equipment, that uses radio energy to determine at a single point, the direction and distance of an object. The scattering or reflection of radio energy permits the detection and ranging of distant objects. Distance is determined by the time taken by the emitted signal to reach the object and return as a detectable echo, in microwave wavelengths, to the station (monostatic radars). Bistatic radars have the transmitter and its antenna at one place and the receiver and its antenna at a remote one.

Radiation

Process by which energy is emitted or transferred through free space, in the form of electromagnetic waves or particles.

Radiation belt / Van Allen radiation belts

Region of high energetic charged particles trapped in the Earth’s geomagnetic field, in approximately equatorial orbits. Their distribution conform two belts, an inner belt which consists mostly of protons, and an outer belt, consisting mostly of electrons, at distances from the Earth of 1 to 2, and 3 to 4 earth radii, respectively (Van Allen belts).

Radiative zone / Radiation zone

Is a layer of a star’s interior where energy is primarily transported toward the exterior. Energy travels through the radiation zone in the form of photons (electromagnetic radiation). Within the Sun, the radiation zone is located between the core and the convection zone.

Radiosonde

Meteorological instrument, often carried up in the atmosphere by a balloon, provided with devices that measure one or several atmospheric variables, from the surface to the stratosphere, drawing a vertical profile of the atmosphere, and equipped with a radio transmitter that sends the data to the ground station, that counts with a receiving system. Typically, they measure temperature, humidity and usually also pressure. Some of them also measure wind speed and direction.

Rain

Precipitation in the form of liquid water particles, either as drops with a diameter greater than 0.5 mm or of smaller widely scattered drops.

Rainband

Complete cloud and precipitation structure associated with an area of rainfall sufficiently elongated that an orientation can be assigned.

Rain gauge

Instrument designed to measure rain amount, by measuring its depth when collected over a horizontal impervious surface and it is not submitted to evaporation. There are various types: recording, non-recording and rain-intensity gauges.

Rain shower

Brief period of rainfall, with a variable and maybe rapidly changing intensity. Usually, it has a convective nature.

Rainbow

Any of a family of large, concentric arcs, with a color range from violet to red, formed by diffraction or refraction of sunlight or moonlight on a “screen” of water droplets (rain, drizzle, fog or spray).

Rainfall

Amount of precipitation measured with a rain gauge.

Rainfall rate

Measure of the intensity of rainfall by calculating the amount of precipitation collected per unit time interval. Typically, it is expressed in terms of depth per unit time, that is millimeters per hour or inches per hour.

Ray

Imaginary bundle of propagating electromagnetic energy, with insignificant lateral dimensions. It is conceptually useful, although it is not possible to isolate a ray. A thin line or narrow beam of light or other radiant energy. Shafts of luminosity aligned in the direction of the geomagnetic field that can consist of a single ray, a small group of rays, or many scattered rays. (auroral rays)

Rayleigh scattering

Scattering of electromagnetic radiation by particles with dimensions much smaller than the wavelength of the radiation, resulting in angular separation of colors; it is responsible for the blue color of the sky.

Recurvature

Change in the direction of a tropical cyclone track, predominately from a westward to a poleward movement, and then with an easterly component, under the influence of the mid-latitude westerlies.

Red Flag Criteria

Criteria met when a geographical area has been suffering a dry spell for a week or two or even shorter, and any of the following conditions: 1) Relative Humidity 15 percent or less with either sustained winds of 25 mph frequent gusts of 35 mph or greater (for a duration of 6 hours or more). 2) Relative Humidity 10 percent or less for an extended period of time (for 10 hours duration or more). 3) Widespread and/or significant dry lightning. Once met, a Fire Weather Watch and then a Red Flag Warning will be issued.

Reflection

Change of direction and probably amplitude of any kind of wave, e.g. electromagnetic wave, propagating in a material medium, as a result of spatial variation in the properties of the medium.

Reflectivity / Same as Reflectance and Reflection Factor

Ratio of any reflected to incident irradiance. Sometimes, it is used to denote the ratio of the radiant or luminous flux reflected by a particular surface to that incident on it.

Refraction

Change of direction and probably amplitude of any kind of wave, e.g. electromagnetic wave, propagating in a material medium, homogeneous on the scale of wavelength, as a result of spatial variation in the properties of the medium. It distinguishes from reflection because the direction of propagation of the refracted wave does not travel in an opposite direction from the incident wave; it takes a different one.

Relative humidity

Ratio of the mixing ratio of an air parcel to its saturation mixing ratio. It is a measure of the quantity of water vapor in the air, usually expressed in percent.

Revolution

Orbital motion about a point. Single complete cycle of such orbital motion.

Ridge

Elongated area of relatively high pressure, often extending outward from a high pressure center. It is almost always associated with an area of maximum anticyclonic curvature of wind flow. On a synoptic chart, it is represented by a system of almost parallel isobars or contours, which are concave towards an anticyclone.

Rip current

Narrow current in the surf zone, flowing from the shore toward the sea. Usually, it can be seen as a band of agitated water. It consists of the returning water piled up on the shore by incoming waves and winds.

Riverine flood

Overflowing or burst of a river banks causing water and sediments to be transported onto the floodplain.

Rotation

Turning or spinning of a celestial body, in this case the Earth, about an axis running through it from pole to pole.