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GEO 211: EXAM 4
Air pressure
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The pressure exerted by the mass of air above a given point, usually expressed in millibars (mb), inches of mercury (Hg), or in hectopascals (hPa).
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Aneroid barometer
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An instrument designed to measure wind speed.
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Anticyclone
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An area of high atmospheric pressure around which the wind blows clockwise in the Northern Hemisphere.
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Barometer
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An instrument that measures atmospheric pressure.
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Contour line
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A line that connects points of equal elevation above a reference level, most often sea level.
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Hectopascal (hPa)
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1 hectopascal is equal to 100 Newtons/m2, or 1 millibar.
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Isobaric chart (constant pressure chart)
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A chart showing variables, such as temperature and wind, on a constant pressure surface. Variations in height are usually shown by lines of equal height (contour lines).
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Isobar |
A line connecting points of equal wind speed.
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Mercury barometer
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A type of barometer that uses mercury to measure atmospheric pressure. The height of the mercury column is a measure of atmospheric pressure.
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Millibar (mb)
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A unit for expressing atmospheric pressure. Sea level pressure is normally close to 1013 mb.
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Ridge |
An elongated area of high atmospheric pressure.
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Sea-level pressure
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The atmospheric pressure at mean sea level.
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Station pressure
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The actual air pressure computed at the observing station.
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Surface map (surface chart)
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A map that shows the distribution of sea-level pressure with isobars and weather phenomena.
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Trough |
An elongated area of low atmospheric pressure.
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Aerovane (skyvane)
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A wind instrument that indicates or records both wind speed and wind direction.
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Anemometer
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An instrument designed to measure wind speed.
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Chinook wind
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A warm, dry wind on the eastern side of the Rocky Mountains (similar to Foehn wind).
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Clear air turbulence (CAT)
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Turbulence encountered by aircraft flying through the cloudless skies. Thermals, wind shear, and jet streams can each be a factor in producing CAT.
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Dust devil (whirlwind)
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A small but rapidly rotating wind made visible by the dust, sand and debris it picks up from the surface. It develops best on clear, dry, hot afternoons.
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Foehn wind
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Warm, dry wind in the Alps (similar to Chinook wind).
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Haboob |
A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air.
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Katabatic (fall) wind
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Any wind blowing down slope. Usually cold.
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Lake breeze
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A wind blowing onshore from the surface of a lake.
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Land breeze
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A coastal breeze that blows from land to sea, usually at night.
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Lidar (light detection and ranging)
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An instrument that uses a laser to generate intense pulses that are reflected from atmospheric particles of dust and smoke. Used to determine the amount of particles in the atmosphere as well as particle movement that has been converted into wind speed.
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Mechanical turbulence
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Turbulent eddy motions caused by obstructions, such as trees, buildings, mountains, and so on.
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Mesoscale |
The scale of meteorological phenomena that range in size from a few km to about 100 km. It includes local winds, thunderstorms, and tornadoes.
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Microscale
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The smallest scale of atmospheric motions.
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Monsoon wind system
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A wind system that reverses direction between winter and summer. Usually the wind blows from land to sea in winter and from sea to land in summer.
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Mountain and Valley breeze
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A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.
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Offshore wind
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A breeze that blows from the land out over the water.
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Onshore wind
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A breeze that blows from the water onto the land.
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Pilot balloon
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A small balloon that rises at a constant rate and is tracked by a theodolite in order to obtain wind speed and wind direction at various levels above the earth's surface.
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Planetary (atmospheric) boundary layer
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The layer of air from the earth's surface usually up to about 1 km (3300 ft) where the wind is influenced by friction of the earth's surface and objects on it. Also called friction layer.
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Planetary (global) scale
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The largest scale of atmospheric motion.
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Prevailing wind
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The wind direction most frequently observed during a given period.
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Rotors |
Turbulent eddies that form downwind of a mountain chain, creating hazardous flying conditions.
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Scales of motion
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The hierarchy of atmospheric circulations from tiny gusts to giant storms.
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Santa Ana wind
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A warm, dry wind that blows into southern California from the east off the elevated desert plateau. Its warmth is derived from compressional heating.
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Sea breeze
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A coastal local wind that blows from the ocean onto the land.
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Sea-breeze front
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The horizontal boundary that marks the leading edge of cooler marine air associated with a sea-breeze.
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Seiches
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Standing waves that oscillate back and forth over an open body of water.
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Shelterbelt |
A belt of trees or shrubs arranged as a protection against strong winds.
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Snow rollers
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A cylindrical spiral of snow shaped somewhat like a child's muff and produced by the wind.
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Synoptic (cyclonic) scale
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The typical weather map scale that shows features such as high- and low-pressure areas and fronts over a distance spanning a continent.
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Texas (blue) norther
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A strong, cold wind from between the northeast and northwest associated with a cold outbreak of polar air that brings a sudden drop in temperature.
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Thermal circulations
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Air flow resulting primarily from the heating and cooling of air.
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Thermal lows and highs
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Areas of low and high pressure that are shallow in vertical extent and are produced primarily by surface temperatures.
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Thermal turbulence
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Turbulent vertical motions that result from surface heating and the subsequent rising and sinking of air.
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Viscosity
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The resistance of fluid flow
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Eddy |
A small volume of air (or any fluid) that behaves differently from the larger flow in which is exists.
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Wind rose
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A diagram that shows the percent of time that the wind blows from different directions at a given location over a given time.
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Wind profiler
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A Doppler radar capable of measuring the turbulent eddies that move with the wind. Because of this, it is able to provide a vertical picture of wind speed and wind direction.
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Wind shear
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The rate of change of wind speed or wind direction over a given distance.
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Wind vane
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An instrument used to indicate wind direction.
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Wind waves
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Water waves that form due to the flow of air over the water's surface.
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Aleutian low
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The subpolar low-pressure area that is centered near the Aleutian Islands on charts that show mean sea-level pressure.
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Arctic Oscillation (AO)
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A reversal of atmospheric pressure over the Arctic that produces changes in the upper-level westerly winds over northern latitudes. These changes in upper-level winds influence winter weather patterns over North America, Greenland, and Europe.
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Bermuda high
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Located over the Atlantic Ocean off the east coast of North America.
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Buys-Ballot's law
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A law describing the relationship between the wind direction and the pressure distribution. In the Northern Hemisphere, if you stand with your back to the surface wind, then turn clockwise about 30°, lower pressure will be to your left. In the Southern Hemisphere, stand with your back to the surface wind, then turn counterclockwise about 30°; lower pressure will be to your right.
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Centripetal acceleration
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The inward-direction acceleration on a particle moving in a curved path.
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Centripetal force
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The radial force required to keep an object moving in a circular path. It is directed toward the center of that curved path.
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Coriolis force effects
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An apparent force observed on any free-moving object in a rotating system. On the earth, this deflective force results from the earth's rotation and causes moving particles (including the wind) to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
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Doldrums |
The region near the equator that is characterized by low pressure and light, shifting winds.
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Ekman Spiral
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An idealized description of the way the wind-driven ocean currents vary with depth. In the atmosphere it represents the way the winds vary from the surface up through the friction layer or planetary boundary layer.
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Ekman transport |
Net surface water transport due to the Ekman spiral. In the Northern Hemisphere the transport is 90° to the right of the surface wind direction.
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El Niño
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An extensive ocean warming that begins along the coast of Peru and Ecuador and extends westward over the Tropical Pacific. Major El Niño events, or strong El Niños, occur once every 2 to 7 years as a current of nutrient-poor tropical water moves southward along the west coast of South America.
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ENSO |
A condition in the tropical Pacific whereby the reversal of surface air pressure at opposite ends of the Pacific Ocean induces westerly winds, a strengthening of the equatorial countercurrent, and extensive ocean warming.
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General circulation of the atmosphere
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Large-scale atmospheric motions over the entire earth.
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Geostrophic wind
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A theoretical horizontal wind blowing in a straight path, parallel to the isobars or contours, at a constant speed. The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force.
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Gradient wind
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A theoretical wind that blows parallel to curved isobars or contours.
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Hadley cell
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A thermal circulation proposed by George Hadley to explain the movement of the trade winds. It consists of rising air near the equator and sinking air near 30° latitude.
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Horse latitudes
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The belt of latitude at about 30° to 35° where winds are predominantly light and the weather is hot and dry.
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Hydrostatic equilibrium
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The state of the atmosphere when there is a balance between the vertical pressure gradient force and the downward pull of gravity.
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Icelandic low
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The subpolar low-pressure area that is centered near Iceland on charts that show mean sea-level pressure.
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Intertropical convergence zone (ITCZ)
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The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.
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Jet stream
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Relatively strong winds concentrated within a narrow band in the atmosphere.
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La Niña
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A condition where the central and eastern tropical Pacific Ocean turns cooler than normal.
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Low-level jet streams
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Jet streams that typically form near the earth's surface below an altitude of about 2 km and usually attain speeds of less that 60 knots.
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Meridional flow
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A type of atmospheric circulation pattern in which the north-south component of the wind is pronounced.
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Middle-latitude (extratropical) cyclone
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A cyclonic storm that most often forms along a front in middle and high latitudes.
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North Atlantic Oscillation (NAO)
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A reversal of atmospheric pressure over the Atlantic Ocean that influences the weather over Europe and over eastern North America.
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Oceanic front
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The boundary separating two masses of water with contrasting temperatures and densities.
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Pacific Decadal Oscillation (PDO)
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A reversal in ocean surface temperatures that occurs every 20 to 30 years over the northern Pacific Ocean.
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Pacific high
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Located off the west coast of North America.
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Polar front
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A semipermanent, semicontinuous front that separates tropical air masses from polar air masses.
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Polar front jet streams (polar jet)
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The jet stream that is associated with the polar front in middle and high latitudes. It is usually located at altitudes between 9 and 12 km.
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Polar easterlies
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A shallow body of easterly winds located at high altitudes poleward of the subpolar flow.
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Pressure grandient
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The rate of decrease of pressure per unit of horizontal distance. On the same chart, when the isobars are close together, the pressure gradient is steep. When the isobars are far apart, the pressure gradient is weak.
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Pressure gradient force (PGF)
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The force due to differences in pressure within the atmosphere that causes air to move and, hence, the wind to blow. It is directly proportional to the pressure gradient.
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Subtropical high
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A semipermanent high in the subtropical high-pressure belt centered near 30° latitude.
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Subpolar low
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A belt of low pressure located between 50° and 70° latitude.
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Semipermanent highs and lows
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Areas of high pressure (anticyclones) and low pressure (extratropical cyclones) that tend to persist at a particular latitude belt throughout the year. In the Northern Hemisphere, typically they shift slightly northward in summer and slightly southward in winter.
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Siberian high
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A strong, shallow area of high pressure that forms over Siberia in winter.
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Subtropical jet stream
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The jet stream typically found between 20° and 30° latitude at altitudes between 12 and 14 km.
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Southern Oscillation (SO)
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The reversal of surface air pressure at opposite ends of the tropical Pacific Ocean that occur during major El Niño events.
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Trade winds
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The winds that occupy most of the tropics and blow from the subtropical highs to the equatorial low.
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Teleconnections
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A linkage between weather changes occurring in widely separated regions of the world.
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Tropical easterly jet stream
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A jet stream that forms on the equatorward side of the subtropical highs near 15 km.
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Upwelling
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The rising of water (usually cold) toward the surface from the deeper regions of a body of water.
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Westerlies
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The dominant westerly winds that blow in the middle latitudes on the poleward side of the subtropical high-pressure areas.
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Zonal wind flow
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A wind that has a predominant west-to-east component.
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