RA – Rain.
RADAR – An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging)
RADIATION FOG – See ground fog.
RADIOSONDE– An instrument attached to a weather balloon that measures and transmits pressure, humidity, temperature, and winds as it ascends.
RAIN – Precipitation of liquid water particles which, in contrast to drizzle, are widely separated and larger in size.
RAINBOW– Optical phenomenon when light is refracted and reflected by moisture in the air into concentric arcs of color.
RELATIVE HUMIDITY (RH) – The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage).
RETROGRADE– To move backwards. It relates to a ridge or trough moving from east to west. Typically in the mid-latitude, troughs and ridges move west to east. Retrograde motion is the opposite movement of normal.
RFC – River Forecast Center. The California/Nevada River Forecast Center is located in Sacramento.
RIDGE – An elongated area of high pressure in the atmosphere.
RING OF FIRE– This refers to thunderstorms and rain across the edges of a high pressure ridge. If a ridge occurs across the central US in summer, the ring of fire will extend across the west coast, up into Canada and across the east coast. Underneath the main core of the ridge weather will be stable.
ROSSBY WAVES – long waves that form in air or water that flows almost parallel to the equator, which results form the effect of the earth’s rotation.
RUC – Rapid Update Cycle model generated every 3 hours. The model is run each hour, with its initial conditions updated by the inclusion of the latest surface observations available. Its primary usage is for aviation and severe weather forecasting.