Weather Words - R

-- R --

Radar: (1) (acronym from RAdio Detection And Ranging) The term was suggested in November, 1940, by S.M. Taylor and F. R. Furth of the U.S. Navy and was officially adopted in 1943 by the Allies in World War II. (2) An electronic instrument which determines the direction and distance of objects of such composition that they reflect radio energy back to the radar site.

Radiant: The position in the sky from which, by tracing the path back, each member of a meteor shower appears to emanate.

Radiation: The process by which energy is emitted and travels through space. (In a vacuum, the movement of energy takes place at the speed of light.) Also, the energy itself.

Radiation fog: A type of fog formed over land when radiational cooling reduces air temperature to near or below its saturation point (dew point). It is therefore mainly a nighttime occurrence, though it often lingers well after sunrise.

Radiational cooling: The cooling of the earth's surface and adjacent air, accomplished mainly at night whenever the surface suffers a net loss of heat due to infrared radiation.

Radiosonde: An instrument package (carried aloft by a balloon to heights of 80,000 feet or more) that measures and transmits back to earth readings of air temperature, pressure and humidity.

Radome: The dome that covers the antenna assembly of a radar to protect it from the weather. The rigid fiberglass radome for the National Weather Service's Doppler radar is 39 feet in diameter.

Radon: A radioactive gas (emitted by the radioactive decay of radium) that exists in the atmosphere in trace amounts near the ground as a result of seepage from soil and rocks. Radon is colorless, odorless and tasteless, and is 7.5 times heavier than air.

Rain: Liquid precipitation in the form of water drops with diameters greater than 0.02 inch. Drizzle, the only other form of liquid precipitation, consists of droplets 0.02 inch in diameter or smaller.

Rain cloud: (1) Any cloud from which rain falls; a popular term having no technical connotation. (2) A colloquial, non-technical term denoting any cloud from which rain falls. In older cloud classification systems, any cloud from which rain or snow fell was termed nimbus.

Rain gauge: An instrument designed to measure the depth of rain that has fallen; also called pluviometer, ombrometer, precipitation gauge.

Rain gush: Any sudden and intense fall of rain, generally at a rate of four inches per hour or more; a cloudburst. Some research suggests a rain gush occurs immediately after a lightning discharge.

Rain making: The popular term applied to all activities designed to increase, through any artificial means, the amount of precipitation released from a cloud.

Rain prints: Markings on the surface of stratified rocks that present the appearance similar to those made by raindrops on mud and sand, and believed to have been produced in that way.

Rain producers: It is estimated that cumulus clouds (like cumulonimbus, the thunderhead) produce more than three-quarters of the world's rain.

Rain shadow: An area with relatively low average precipitation due to sheltering by mountains from prevailing moisture-bearing winds. The Rocky Mountains generate extensive rain shadows.

Rainbow: The arc of concentric prismatic bands that spans a section of the sky opposite the sun when rain is present and the sun is positioned at the observer's back. The sequence of colors is red (on the outside), yellow, green, blue, violet.

Raindrop: A drop of water that has a diameter greater than 0.02 inch when falling through the atmosphere. Large raindrops fall at up to 20 m.p.h.; average-size drops fall at about 14 m.p.h.

Raindrop fall speed: Large raindrops fall at up to 30 feet per second (20 m.p.h.), and an average-size raindrop falls about 21 feet per second (14 m.p.h.).

Raindrop shapes: (1) Raindrops are usually rendered by artists as teardrops, but they are never shaped that way. The tiniest, misty raindrops are spherical; medium-sized drops are slightly flattened on the bottom; the largest drops have bottoms that bulge upward in the middle. (2) The tiniest, misty raindrops are spherical; medium-sized drops are slightly flattened on the bottom; the largest drops have bottoms that bulge upward in the middle and the drop is bag-shaped.

Rainfall: In climatology, a generic term for the total amount of all precipitation (rain, snow, sleet, hail, etc.) normally expected at a location over an extended period of time, such as a year.

Rainforest: (1) Lush forests growing in regions of high precipitation. Tropical rainforests of the Amazon and Congo River basins are the most well known, but extensive temperate rainforests also exist: the North Pacific coastal rainforests extending from Alaska to Oregon. (2) A forest that grows in a region of very heavy annual rainfall, generally 100 inches or more per year. Two types are distinguished: tropical rain forest and temperate rain forest.

Rapid City flash flood: On June 9, 1972, a cloudburst on the eastern slopes of the Black Hills of South Dakota delivered 14 inches of rain in a four-hour period. The Canyon Lake Dam collapsed and the resulting flash flood swept down Rapid Creek and through the heart of Rapid City, drowning 238 people and injuring 3,000 others.

Raw: Colloquially descriptive of uncomfortably chilly weather, usually meaning cold, damp, cloudy and windy. Recapping 1999: A warm and a wet year -- among the warmest one-third of all years (since 1871) and among the one-quarter wettest. Average temperature 51.1 degrees F (2.1 degrees above the climatological average), total precipitation 38.34 inches (2.52 inches above average).

Record daily temperatures: (1) For a city, the very highest and lowest temperatures measured on each calendar day of the year, compiled from that city's official temperature data. Chicago's official temperature readings date from July 1, 1872. (2) The highest and lowest temperatures measured on each calendar day of the year, compiled from a city's official temperature data. Chicago's official readings date from Nov. 1, 1870.

Record event: A weather occurrence which exceeds in magnitude, intensity, severity, duration, etc. the previous most extreme occurrence at a location. Such statistics are usually presented only when at least 30 years of continuous weather data are available.

Recurvature: The change in direction of a tropical cyclone (such as a hurricane) from movement to the west or northwest to movement to the east or northeast; also known as recurving.

Red Sea: The Red Sea in the Middle East is so named because dust blown aloft from surrounding deserts produces so much haze that reflection from the tinted sky often causes the water there to appear red.

Red watch: A slang term used by meteorologists at the Storm Prediction Center in Norman, Okla., denoting a tornado watch.

Regina, Canada: The site of Canada's most deadly tornado. On June 30, 1912, a tornado rated F4 and popularly known as the "Regina Cyclone" blasted through the core of the city, killing 28 people.

Relative humidity: (1) Commonly used but often misunderstood, it is the ratio (expressed as a percentage) of the amount of water vapor actually in the air to the maximum amount the air can hold at that temperature. (2) The percentage of saturation of air; the ratio (expressed as a percentage) of the amount of water vapor actually in the air to the maximum amount the air can hold at that temperature. (3) The percentage of saturation of air; the ratio, expressed as a percentage, of the actual amount of water vapor in a given volume of air to the amount that would be present were the air saturated at the same temperature. (4) The ratio (expressed as a percentage) of the amount of water vapor actually in the air to the maximum amount the air could hold at that temperature. Because air can hold more water vapor as its temperature rises, relative humidity falls as air warms.

Reservoir: An artificial lake in which water is collected and kept in store for later use.

Residence time: The average amount of time water spends in a reservoir. The residence time of water in the atmosphere is ten day; in the oceans it is 37.000 years. Also, the amount of water in a reservoir divided by the rate of addition of water to the reservoir or the rate of loss from it.

Resistance thermometer: A type of electrical thermometer in which the thermal element is a substance whose electrical resistance varies with the temperature. Such thermometers can provide highly accurate and practically instantaneous measurements of air temperature.

Retrograde motion: The movement of a weather system in a direction opposite to that of the basic flow in which it is embedded. In the United States, it refers to a low pressure trough that moves to the west. Also known as retrogression.

Retrogression: The movement of a weather system in a direction opposite to that of the basic flow in which it is embedded. In the U.S., it refers to a low pressure trough that moves to the west.

Return period: The average time between occurrences of a specific weather event (such as a rainstorm of a certain intensity) or a weather-related event (such as a flood of a certain magnitude).

Ridge: An elongated area of high air pressure, often several hundred miles in length, containing no high pressure center and therefore no closed isobars. The term is used to distinguish from a high pressure system containing a center around which there is at least one closed isobar.

Ridge (high pressure ridge): As depicted on weather maps, a ridge is an elongated area of relatively high air pressure, often several hundred miles in length. A ridge is usually associated with fair weather and tends to be slow moving.

Rill erosion: An erosion process in which many small channels only a few inches deep are formed; occurs mainly on the bare soil of recently cultivated fields before new crops have sprouted.

Rime: A milky, granular deposit of ice on any surface, but frequently on tree branches, formed at sub-freezing temperatures by the freezing of supercooled fog droplets; not to be confused with frost.

Rime ice: A milky, opaque, granular deposit of ice formed by the rapid freezing of supercooled fog droplets when they impinge upon exposed objects like grass, trees, buildings, etc.

Rip current: A narrow, strong flow of water of short duration moving outward from the shore; the return movement of water piled up on the shore by incoming waves and wind. It is sometimes popularly (and incorrectly) called a "rip tide".

Rip tide: The popular (but incorrect) term for a rip current, A narrow, strong flow of water moving outward from the shore; the return movement of water piled up on the shore by incoming waves and wind.

Roaring Forties: The prevailing westerly winds in a region of the Southern Hemisphere below 40 degrees South latitude in which the winds circle the Earth unobstructed by land.

Rocket Lightning: A form of lightning whose luminous channel seems to advance through the air at a speed slow enough that its advance may be followed by the eye.

Rogue waves: Individual waves of exceptional height and/or abnormal shape. They are recognized as a unique and real phenomenon, but with a variety of causes.

Roll cloud: A tube-shaped horizontal cloud structure at the leading edge of the base of a thunderstorm, but detached from it. The roll cloud is associated with the storm's strong, gusty, cool outflow winds.

Rope tornado: A narrow and often contorted tornado usually associated with its dissipating stage. A colloquial term used by storm chasers.

Rouse Simmons: "The Christmas Tree Ship" that annually ferried trees from Wisconsin to Chicago via Lake Michigan sank off Two Rivers, Wisc., during a gale on Nov. 23, 1912; all on board (17) died.

Rowland, Dr. F. Sherwood: Nobel prize winning chemist (1995) who first warned (in 1974) of depletion of the atmosphere's ozone layer that protects the surface from harmful ultraviolet radiation in sunlight.

Runnel: A narrow channel or rivulet of water on a soil or snow surface.

Runoff: Water that ultimately reaches a river or stream as a result of rainfall or snowmelt in the drainage basin.