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Radar – an instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging).

Radiation – energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth's surface and the atmosphere has a long wavelength (infra-red).

Radiation fog – fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as ground fog.

Rain – liquid water droplets that fall from the atmosphere, having diameters greater than drizzle (0.5 mm). In Canada the term "rain", used alone, means liquid precipitation of significant duration and extent.

Rain gauge – an instrument used to measure rainfall amounts.

Rain showers – rain showers stop and start suddenly and vary widely in intensity, and are gone in less than an hour.

Rainbow – optical phenomena when light is refracted and reflected by moisture in the air into concentric arcs of color. Raindrops act like prisms, breaking the light into the colors of a rainbow, with red on the outer, and blue on the inner edge.

Relative humidity – the amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. It is expressed as a percentage.

Return flow – south winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).

Ridge – an elongated area of high pressure in the atmosphere. Opposite of a trough.

River flood warning – issued when main stem rivers are expected to reach a level above flood stage.

Roll cloud – a relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.