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NOAA - Meteorology glossary
Category: Sciences > Meteorology
Date & country: 14/10/2013, US
Words: 671


Offshore Breeze
A wind that blows from the land towards a body of water. Also known as a land breeze.

Offshore Forecast
A marine weather forecast for the waters between 60 and 250 miles off the coast.

Onshore Breeze
A wind that blows from a body of water towards the land. Also known as a seabreeze.

Orographic
Related to, or caused by, physical geography (such as mountains or sloping terrain).

Orographic Lift
The lifting of air as it passes over terrain features such hills or mountains. This can create orographic clouds and/or precipitation.

Orphan Anvil
An anvil from a dissipated thunderstorm, below which no other clouds remain.

Outflow
Air that flows outward from a thunderstorm.

Outflow Boundary
A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature.

Outflow Winds
Winds that blow down fjords and inlets from the land to the sea.

Overcast
Sky condition when greater than 9/10 of the sky is covered by clouds.

Overrunning
A condition that exists when a relatively warm air mass moves up and over a colder and denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.

Ozone
A form of oxygen in which the molecule is made of 3 atoms instead of the usual two. Ozone is usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation. It is also a primary component of smog.

Ozone Hole
A thinning of the ozone layer over Antarctica, which occurs each spring.

Partly Cloudy
Sky condition when between 3/10 and 7/10 of the sky is covered. Used more frequently at night.

Partly Sunny
Similar to partly cloudy. Used to emphasize daytime sunshine.

Patches
Used with fog to denote random occurrence over relatively small areas.

Pendant Echo
Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

Permafrost
A soil layer below the surface of tundra regions that remains frozen permanently.

Polar Air
A mass of very cold, very dry air that forms in polar regions.

Polar front
The semi-permanent, semi-continuous front that encircles the northern hemisphere separating air masses of tropical and polar origin.

Polar Stratospheric Clouds (PSCs)
High altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing Southern Hemisphere spring.

Polar vortex
A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.

Pollutant
Strictly too much of any substance in the wrong place or at the wrong time is a pollutant. More specifically, atmospheric pollution may be defined as the

Polycrystal
A snowflake composed of many individual ice crystals.

POP
Probability of Precipitation. Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period.

Popcorn Convection
Clouds, showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.

Positive Area
The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel.

Positive-tilt Trough
An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Potential Temperature
The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

Precipitation
Liquid or solid water that falls from the atmosphere and reaches the ground.

Precipitation Shaft
A visible column of rain and/or hail falling from a cloud base. When viewed against a light background, heavy precipitation appears very dark gray, sometimes with a turquoise tinge. This turquoise tinge has been commonly attributed to hail, but its actual cause is unknown.

Pressure
The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

Pressure Change
The net difference between pressure readings at the beginning and ending of a specified interval of time.

Pressure Falling Rapidly
A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.

Pressure Gradient
The rate of decrease of pressure with distance at a fixed level.

Pressure Gradient Force
Force acting on air that causes it to move from areas of higher pressure to areas of lower pressure.

Pressure Rising Rapidly
An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.

Pressure Unsteady
A pressure that fluctuates by 0.03 inch of mercury or more from the mean pressure during the period of measurement.

Prevailing Westerlies
Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.

Prevailing Wind
The direction from which the wind blows most frequently in any location.

Profiler
An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

Psychrometer
An instrument used for measuring the water vapor content of the atmosphere. It consists of two thermometers, one of which is an ordinary glass thermometer, while the other has its bulb covered with a jacket of clean muslin which is saturated with distilled water prior to use.

Pulse Storm
A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

PVA
Positive Vorticity Advection. Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential precipitation.

Quality Of Snow
The amount of ice in a snow sample expressed as a percent of the weight of the sample.

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.

Radiational Cooling
Cooling process of the Earth's surface and adjacent air, which occurs when infrared (heat) energy radiates from the surface of the Earth upward through the atmosphere into space. Air near the surface transfers its thermal energy to the nearby ground through conduction, so that radiative cooling lowers the temperature of both the surface and the lowest part of the atmosphere.

Radiosonde
An instrument attached to a weather balloon that transmits pressure, humidity, temperature and winds as it ascends to the upper atmosphere.

Rain
Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle (0.5 mm).

Rain Foot
A horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.

Rain Gauge
An instrument used to measure rainfall amounts.

Rain Shadow
The region on the lee side of a mountain or mountain range where the precipitation is noticeably less than on the windward side.

Rain-Free Base
A horizontal, dark cumulonimbus base that has no visible precipitation beneath it. This structure usually marks the location of the thunderstorm updraft. Tornadoes most commonly develop (1) from wall clouds that are attached to the rain-free base, or (2) from the rain-free base itself. This is particularly true when the rain-free base is observed to the south or southwest of the precipitation shaft.

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.

Rankine Temperature Scale
A temperature scale with the degree of the Fahrenheit temperature scale and the zero point of the Kelvin temperature scale.

Rawinsonde
A balloon that is tracked by radar to measure wind speeds and wind directions in the atmosphere.

Reflectivity
Radar term referring to the ability of a radar target to return energy; used to estimate precipitation intensity and rainfall rates.

Refraction
The bending of light as it passes through areas of different density, such as from air through ice crystals.

Relative Humidity
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).

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).

RFC
River Forecast Center. The Northeast River Forecast Center is located in Taunton, MA..

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

Right Mover
A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers.)

Rime
Tiny balls of ice that form when tiny drops of water (usually not precipitation) freeze on contact with the surface.

River Flood Warning
Issued when main stem rivers (such as the Merrimack, Charles, Connecticut, etc) are expected to reach a level above flood stage.

Rope (or Rope Funnel)
A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado.

Rope Cloud
In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary.

Rope Stage
The dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.

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, a numerical model run at NCEP that focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale) weather features. Forecasts are prepared every 3 hours for the contiguous United States.

Runway Visual Range (RVR)
An instrumentally-derived value, based on standard calibrations, that represents the horizontal distance a pilot may see down the runway from the approach end.

Sandstorm
Particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.

Santa Ana Winds
Relatively warm, dry winds that blow into Southern California coastal areas from an anticyclone located over the high deserts of California or Nevada. The warmth and dryness are due to compressional heating.

Satellite Photo
A photograph of the earth taken by weather satellites that shows areas of cloud.

Saturation
A condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.

Saturation Vapor Pressure (water)
The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water. This is the maximum water vapor the air can hold for any given combination of temperature and pressure

Scattered
A cloud layer that covers between 3/8ths and 1/2 of the sky.

Scud Clouds
Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

Sea Breeze
A wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. It occurs when the land is warmer than the water.

Sea-level Pressure
The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.

Secondary Cold Front
A front that follows a primary cold front and ushers in even colder air.

Sensible Heat
The excess radiative energy that has passed from the Earth's surface to the atmosphere through advection, conduction, and convection processes.

Severe Thunderstorm Warning
Issued when thunderstorms are expected to have wind gusts to 58 mph or above or hail 3/4 inch or more in diameter.

Severe Thunderstorm Watch
Issued when conditions are favorable for the development of severe thunderstorms in and close to a defined area.

Shallow fog
Fog in which the visibility at 6 feet above ground level is 5/8ths of a mile or more.

Shear (Wind Shear)
Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Shelf Cloud
A low-level horizontal accessory cloud that appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front. The leading edge of the shelf is often smooth and at times layered or terraced. It is most often seen along the leading edge of an approaching line of thunderstorms, accompanied by gusty straight winds as it passes overhead and followed by precipitation. The underside is concave upward, turbulent, boiling, or wind-torn. Tornadoes rarely occur with the shelf cloud.

Short-Fuse Warning
A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Shortwave (Shortwave Trough)
A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.

Shortwave Radiation
The radiation received from the sun and emitted in the spectral wavelengths less than 4 microns. It is also called 'solar radiation'.

Shower
Precipitation that is intermittent, both in time, space or intensity.

Sky Condition
The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types.

Sky Cover
The amount of the sky which is covered by clouds or obscurations in contact with the surface.

Sleet
Rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air near the surface and refreezes.

Slight Chance
In probability of precipitation statements, usually equivalent to a 20 percent chance.

Sling Psychrometer
A psychrometer in which the wet and dry bulb thermometers are mounted upon a frame connected to a handle. The psychrometer may be whirled by hand in order to provided the necessary ventilation.

Small Craft Advisory
A marine advisory for winds 25 to 33 knots (29 to 38 mph) or seas of 5 feet or more, that may cause hazardous conditions for operators of small vessels.

Smog
Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health.