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WeatherChannel - Weather Terms Category: Meteorology and astronomy > Weather Glossary Date & country: 30/09/2008, USA Words: 751

middle clouds
A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be ...

middle latitudes
The latitude belt roughly between 35 and 65 degrees North and South. May be referred to as the temperate region.

millibar (mb)
The standard unit of measurement for atmospheric pressure used by the National Weather Service. One millibar is equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2 millibars.

minimum
The least value attained by a function, for example, temperature, pressure, or wind speed. The opposite of maximum.

mist
A collection of microscopic water droplets suspended in the atmosphere. It does not reduce visibility as much as fog and is often confused with drizzle.

mixed layer
It is the upper portion of the boundary layer in which air is thoroughly mixed by convection. In oceanography, it is the layer of the water that is mixed through wave action or thermohaline convection.

mixed precipitation
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.

moist adiabat
The line on a Skew T-Log P chart that depicts the change in temperature of saturated air as it rises and undergoes cooling due to adiabatic expansion. As saturated air rises, the temperature changes at a rate of 0.55°C per 100 meters (2-3°F per 1,000 feet). Related term: dry adiabat

moisture
Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

monsoon
The seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season. Th...

mountain breeze
A katabatic wind, it is formed at night by the radiational cooling along mountainsides. As the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. It may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. It blows in the opposite ...

mountain wave
A wave in the atmosphere caused by a barrier, such as a mountain. Sometimes it is marked by lenticular clouds to the lee side of mountain barriers. May be called a standing wave or a lee wave.

mud slide
Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain.

muggy
A subjective term for warm and excessively humid weather.

multicell storm
A thunderstorm made up of two or more single-cell storms.

nadir
The point on any given observer's celestial sphere diametrically opposite of one's zenith.

national weather association (nwa)
An organization whose membership promotes excellence in operational meteorology and related activities, recognizing the professional as well as the volunteer. For further information, contact the NWA. Related term: American Meteorological Society

national weather service (nws)
A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings. For further information, contact the NWS.

nautical mile
A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a sphere. One international nautical mile is equivalent to 1,852 meters or 1.151 statue miles. Related Term: sea mile

nautical twilight
The time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars. Related term: twilight

neap tide
A tide of decreased range, which occurs about every two weeks when the moon is at one quarter or three-quarters full. Related term: spring tide

negative vorticity advection
The advection of lower values of vorticity into an area. Related term: positive vorticity advection

nephelococcygia
A term applied when people find familiar objects within the shape of a cloud.

newhall winds
The local name for winds blowing downward from desert uplands through the Newhall Pass southward into the San Fernando Valley, north of Los Angeles.

newton
The unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one meter (1 yard) per second per second.

nexrad
http://www.weather.com/glossary/r.html

night
The period of the day between dusk and dawn.

nimbostratus
This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. They are generally associated with fall and winter conditions, but can occur during any season.

nitrogen (n2)
A colorless, tasteless, odorless gas that is the most abundant constituent of dry air. It comprises 78.09%.

noctilucent clouds
Rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth's surface. They have been seen only during twilight (dusk and dawn) during the summer months in the higher latitudes. They may appear bright against a dark night sky, with a blue-silver color or orange-red.

nocturnal thunderstorms
Thunderstorms which develop after sunset. They are often associated with the strengthening of the low level jet and are most common over the Plains states. They also occur over warm water and may be associated with the seaward extent of the overnight land breeze.

nor'easter
A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its nam...

normal
The recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. Normals are concerned with the distribution of data within limits of common occurrence. The parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and d...

north pacific high
A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate. Related terms: Azores High and the Bermuda High

nowcast
A short-term weather forecast for expected conditions in the next few hours.

obscuration
Any phenomena in the atmosphere, excluding precipitation, that reduces horizontal visibility. According to the National Weather Service, some of the obstructions to visibility include blowing and widespread dust, fog (including freezing fog and patchy fog), haze, mist, sand and blowing sand, smoke, blowing spray, and volcanic ash. It is reported as...

observation
In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. An observer is one who records the evaluations of the meteorological elements.

occluded front
Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. It develops when three thermally different air masses conflict. The type of frontal boundary they create depends on the manner in which they meet. Related terms: cold front and warm front

ocean
The intercommunicating body of salt water occupying the depressions of the earth's surface, or one of its major primary subdivisions, bounded by the continents, or the equator, and other imaginary lines. A sea is subdivision of an ocean.

oceanography
The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.

omega block
A warm high aloft which has become displaced and is on the polarward side of the jet stream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts. Related term: blocking high

opaque
A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.

orographic lifting
Where the flow of air is forced up and over barriers such as highlands or mountains. Moist air being forced aloft begins to cool, consequently condensation forms, and rain or snow begins to fall. By the time the air reaches the leeward side of the barrier, it sinks and warms, resulting in decreasing relative humidity, cessation of precipitation, an...

outflow
Also referred to as an outflow boundary, it is the outward flow of air from a system, such as a thunderstorm. It is the result of cold downdrafts and its passage includes a wind shift and temperature drop. Related terms: bubble high and meso high

overcast
The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer.

overrunning
This occurs when a relatively warm air mass is forced above a cooler air mass of greater density. Weather generally associated with this event includes cloudiness, cool temperatures, and steady precipitation.

oxygen
A colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, comprising 20.946%.

ozone (o3)
A nearly colorless gas and a form of oxygen (O₂). It is composed of an oxygen molecule made up of three oxygen atoms instead of two.

palmer drought index
A long-term meteorological drought severity index produced by the NOAA/USDA (Department of Agriculture) Joint Agricultural Weather Facility. The index depicts prolonged times, as in months or years, of abnormal dryness or wetness. It responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep perco...

palouser
A strong, dangerous, katabatic wind that descends from the mountains into the Palouse River valley in northern Idaho and eastern Washington. May be called a Cow-Killer.

parcel
A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.

parhelion
The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. T...

partial obscuration
Denotes that 1/8th or more of the sky, but not all of the sky, is hidden by any surface-based phenomena in the atmosphere, excluding precipitation. It often reduces horizontal visibility but not the vertical. It is reported as 'X' in an observation and on the METAR. Related term: obscuration

partly cloudy
The state of the weather when the clouds are conspicuously present, but do not completely dull the sky or the day at any moment. The National Weather Service does not have an amount of sky cover for this condition. Related terms: clear, few, scattered, broken, and overcast

pascal
The unit of pressure produced when one newton acts on about one square meter.

pascal's law
When an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure applied. It means that the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface which is exposed to the atmosphere. ...

peak gust
The highest instantaneous wind speed observed or recorded.

perigee
The point nearest the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of apogee.

perihelion
The point of the earth's orbit that is nearest to the sun. Although the position is part of a 21,000 year cycle, currently it occurs around January, when the earth is about 3 million miles closer to the sun than at aphelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of aphelion.

photometer
Any of a number of atmospheric phenomena which appear as luminous patterns in the sky. They do not directly cause adverse weather. They include halos, coronas. rainbows, and fogbows.

photosphere
The intensely bright portion of the sun visible to the unaided eye; the 'surface' of the sun. Reaching temperatures estimated at about 11,000°F, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation.

pilot balloon
A small balloon whose ascent is used to determine the direction and speed of low level atmospheric winds. Also known as a pibal.

pilot report
A report of in-flight weather by an aircraft pilot or crew member. Often referred to as a PIREP.

plan position indicator
Also known as a PPI Scope, it is a radar indicator scope displaying range and azimuth of targets in polar coordinates.

plow/plough wind
The spreading downdraft and strong straight-line winds preceding a thunderstorm. So named in the American Midwest because of its ability to flatten tall grasses as it passes. Related term: first gust

poles/polar
The poles are the geographic point at 90 degrees latitude North and South on the earth's surface. They are equal distance from the equator. The polar region is considered to be that area between 60° and 90° latitude, both North and South.

polar air mass
An air mass that forms over a high latitude region. Continental polar air (cP) is formed over cold surface regions and is typically very stable with low moisture. Maritime polar air (mP), produced over warmer waters, is less stable with high moisture.

polar front
A semi-continuous, semi-permanent boundary between polar air masses and tropical air masses. An integral part of an early meteorological theory known as the Polar Front Theory.

polar jet
Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the mid...

polar-orbiting satellite
A satellite whose orbit passes over both of the earth's between poles. Related term: geostationary satellite

pollutant
Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. Something unfavorable to health and life that has been added to the environment.

positive vorticity advection
The advection of higher values of vorticity into an area. It is also known as cyclonic vorticity. related term: negative vorticity advection

pounds per square inch (psi)
A unit for measuring pressure. One PSI equals the pressure resulting from a force of one pound force acting over an area of one square inch.

precipitation
Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over...

pre-frontal squall line
A line of thunderstorms that precedes an advancing cold front. Related term: squall line

pre-frontal trough
An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction. Related term: trough

pressure
The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface. Related terms: atmospheric pressure and barometric pressure

pressure altimeter
An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.

pressure altitude
The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.

pressure change
The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.

pressure characteristic
The pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.

pressure gradient
The amount of pressure change that occurs over a fixed distance at a fixed altitude.

pressure jump
A sudden increase in the observed atmospheric pressure or station pressure.

pressure tendency
The pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation.

prevailing wind
A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

prevailing visibility
It is considered representative of visibility conditions at the observation station. It is the greatest distance that can be seen throughout at least half the horizon circle, but not necessarily continuous.

profiler
A type of Doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere.

prognostic chart
A chart of forecast predictions that may include pressure, fronts. precipitation, temperature, and other meteorological elements. Also known as a prog.

psychrometer
An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.

radarsonde observation
An upper air observation used to determine winds and other meteorological data, by tracking the range, elevation, and azimuth of a radar target carried aloft. A type of rawinsonde.

radial velocity
A type of velocity that expresses motion toward or away from a given location. In Doppler radar, it is the component of motion that is parallel to the radar beam.

radiation
The process by which energy is propagated through any medium by virtue of the wave motion of that medium. Electromagnetic radiation, which emits heat and light, is one form. Sound waves are another.

radiational cooling
The cooling of the earth's surface and the adjacent air. Although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation. Related terms: terrestrial radiation

radiation fog
Fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its dew point. Formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. This primarily occurs during the night or early morning. Related...

rain
Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. It is reported as 'R' in an observation and on the METAR. The intensity of rain is based on rate of fall. 'Very light' (R--) means that the scattered drops do not completely wet a surface. 'Light' (R-) means it is greater than ...

rainbow
A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow i...

rainfall
The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured by a rain gauge. Related terms: rain and quantitative precipitation

rain forest
A forest which grows in a region of heavy annual precipitation. There are two major types, tropical and temperate.

rain gauge
An instrument used to measure the amount of rain that has fallen. Measurement is done in hundredths of inches (0.01').

rain shadow
Also referred to as a precipitation shadow, it is the region on the lee side of a mountain or similar barrier where the precipitation is less than on the windward side. For example, the relatively dry Washoe Valley of western Nevada is in the rain shadow of the Sierra Nevada.