ducation from the National Weather Service Office in Missoula, Montana Topics More Links Air Pressure Clouds Inversions Lightning Mountain and Valley Winds Jet Streams NOAA Weather Radio Severe Weather Awareness Week Watches and Warnings Weather Balloons Thunderstorms Tornadoes Weather Satellite and Radar FEMA for Kids NOAA Kids' Link Web Weather for Kids

Educational Guide

• JetStream - NWS Southern Region Weather School

Helpful Links

• El Nino/La Nina - educational links.
  
• Severe Wx Safety Guide - how to prepare for and what to do during severe weather events.
  
• NOAA Weather Tales
  
• NOAA History
  

Hydrologic Glossary
Storm Spotters Glossary
Western Regional Climate Center Glossary

-A-

ASOS

• Stands for Automated Surface Observing System. [picture]
• First system deployed as part of the NWS modernization and is the observing system for 1000 airports across the country.
  
• Supports the aviation observation programs of NWS, FAA, and the Department of Defense (DOD).
  
• Updates obervations every minute, 24-hours a day, much more data than ever before.
  
• Less work involved for observing weather, more time available for tasks associated with forecasts, watch/warning program, NOAA Wx Radio, etc.

AWIPS

• Stands for Advanced Weather Interactive Processing System.
• Cornerstone of the NWS modernization and restructuring.
  
• A high-speed computer system that integrates all meteorological and hydrological data, all satellite and radar data, and enables the forecaster to prepare and issue more accurate and timely forecasts and warnings.
  

Advection

• The transfer of atmospheric properties by horizontal movement of air.
• Most commonly used in reference to transfer of warmer or cooler air.

Avalanche

• A mass of snow, perhaps containing ice and rocks, moving rapidly down a steep mountain slope.
• Characterized as loose and turbulent, usually from dry snow, or slab, usually from wet snow.
• Avalanche Bibliography and Glossary

-B-

Blizzard

• A severe weather condition characterized by winds of 35 mph or higher, and visibility reduced to less than 1/4 mile due to blowing and/or falling snow.
• Usually considered a blizzard when conditions prevail for three hours or more.

-C-

Chinook

• A foehn wind that occurs on the eastern side of the Rocky Mountain front.
• The chinook usually blows from the southwest and is sometimes called the "snow eater" since temperatures may rise as much as 20 to 40 degrees in a short amount of time, melting and evaporating snow.
• The rise in temperature is due to three causes: the replacement of a cold air mass by advection of warmer air; dynamic heating by subsidence; and the inhibition or destruction of the normal night ground inversion.

Chinook Arch

• A foehn cloud formation over the Rocky Mountains, generally a flat layer of altostratus, heralding the approach of a chinook.

Cold Front

• The leading edge of a cold air mass as it moves toward warmer air; its movement is characterized by a drop in temperature and humidity after the front passes.

CRS

• Stands for Console Replacement System.
• Automatic weather broadcast console which provides a more efficient means of disseminating severe weather watches, warnings, and emergency information via NOAA Weather Radio
• CRS allows forecasters to broadcast information as quickly as possible using live voice or from text derived from AWIPS, along with activation of the NOAA Weather Radio tone-alert and Specific Area Message Encoder (S.A.M.E.) codes.

-D-

Dense Fog Advisory

• Statement issued by the NWS when widespread visibilities equal or less than 1/8 mile are imminent or occurring.

Dew Point

• The temperature to which a certain amount of air must be cooled in order for saturation to occur (a cloud or fog to form); the drier the air the lower the dew point and the more air is needed to be cooled for a cloud or fog to form.

Doppler Effect

• Same as Doppler shift.
• The change in frequency with which energy reaches a receiver when the receiver and the the energy source are in motion relative to each other.

Doppler Weather Radar

• Also known as WSR-88D (Weather Surveillance Radar 1988 Doppler), and NEXRAD (Next Generation Weather Radar). [picture]
• State-of-the-art technology in weather surveillance radar.
• Doppler Weather Radar calculates both the speed and direction of motion within thunderstorms.
• It significantly aides forecasters with short-fused warnings by allowing detailed analysis of wind shear, wind strength, precipitation intensity, and rainfall rates.
• WSR-88D is one part of the NWS modernization.

Drizzle

• A form of precipitation that consists of water droplets less than 0.5mm.

Dry-adiabatic lapse rate

• A lapse rate of temperature, defined as the rate of decrease of temperature with height of a parcel of dry air lifted adiabatically through an atmosphere in hydrostatic equilibruim.
• Lapse rate equal to about 5.4 degrees F per thousand feet.

Dust Devil

• A rapidly rotating column of air carrying dust, leaves and other light-weight material; dust devils usually develop during hot, sunny days over dry and dusty or sandy areas.

-E-

El Nino

• A large-scale warming of the tropical eastern Pacific waters near the the coast of South America, typically felt during winter months, with global weather impacts. Occur in cycles varying in length from one to ten years.
• Signals from past El Nino events show trends for above normal precipitation and above normal temperatures in winter months over the Pacific Northwest.

-F-

Flash Flood

• A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure.

Flood

• High flow, overflow or inundation of a normally dry area which causes or threatens damage.

Flood Stage

• The level of a river or stream at which considerable inundation of surrounding areas will occur.

Foehn

• A warm, dry wind on the lee side of a mountain range. Warmth and dryness of the air is due to adiabatic compression upon descending the mountain slopes. Also known as a chinook in the Rocky Mountains, east of the continental divide.

Fog

• The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Essentially a cloud whose base is at the earth's surface, limiting visibility.

Freezing Level

• The altitude in the atmosphere at which the temperature drops to thirty-two degrees Fahrenheit.

Freezing Rain

• Precipitation that falls in liquid form but freezes upon contact with cold objects.

Funnel Cloud

• A funnel shaped cloud that extends from a cumulus or cumulonimbus cloud; this cloud is related to a rotating column of air that is not in contact with the ground.

-G-

Gradient

• (pressure gradient) Change of value of the atmospheric pressure per unit of distance.
• The greater the change per unit of distance, the stronger the gradient, and the stronger the winds.

-H-

Heavy Snow

High Wind

• The sustained wind of forty mph or greater and/or gust greater than fifty-eight mph.

Humidity

• See relative humidity.
• A measure of the water-vapor content of air.

-I-

Ice Jam

• An accumulation of broken river ice caught in a narrow channel that frequently produces local floods during a spring break-up.

Instability

• (unstable air mass) A state in which the vertical distribution of temperature is such that an air particle, if given either an upward or downward impulse, will tend to move away with increasing speed from its original level.

Inversion

• An atmospheric layer where temperature increases with height in contrast to a normal decrease in temperature with height.
• Inversion is more common under high pressure with clear skies and light winds.
• Warm air above the surface of the ground acts as a lid, trapping colder air in low-lying valleys. Temperatures can be cooler in valleys under the inversion than higher elevated mountain slopes.
• An extended duration of inversion can lead to increased air pollution.

-J-

Jet Stream

• A zone of strong winds concentrated in a narrow band in the upper atmosphere; these winds are often referred to as the storm track since the jet stream often "steers" atmospheric storms.

-K-

Katabatic Wind

• Wind blowing down an incline, such as down a hillside; downslope wind.

Knot

• The unit used to measure wind speed, equal to 1.15 statute miles per hour.

-L-

La Nina

• Unusually cold ocean temperatures in the tropical eastern Pacific waters near the coast of South America, typically felt in winter months with global weather impacts.
• Signals are weaker than with El Nino, however winter months in the Pacific Northwest tend to be cooler and with near or above normal precipitation.

Lapse Rate

-M-

Macro Burst

• A large downburst within a 2.5 mile or larger outflow diameter and damaging winds lasting five to twenty-five minutes.

Micro Burst

• A strong localized downdraft from a thunderstorm with peak gusts lasting 2 to 5 minutes.

Monsoon

• A seasonal change in wind direction bringing dry air or heavy rain.

Meteorology

• The study of phenomena of the atmosphere and all the processes that take place in the atmosphere and their relationships with processes at the surface of the earth.

-N-

NCDC

• Stands for National Climatic Data Center. (Ashville, NC)
• Agency that archives climatic and forecast data from the National Weather Service.

NCEP

• National Centers for Environmental Prediction. Central computer and communications facility of the National Weather Service; located in Washington, DC.

NHC

• National Hurricane Center. The office of the National Weather Service in Miami that is responsible for tracking and forecasting tropical cyclones.

NOAA

• National Oceanic and Atmospheric Administration; parent agency of the National Weather Service in the Department of Commerce.

-O-

Occluded Front

• A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion.

Orographic Uplift

• The vertical forcing of air by terrain features such a hills or mountains. This can create orographic clouds and/or precipitation.

-P-

Partly Cloudy

• State of the sky when 3/8 to 5/8 covered with opaque clouds.

POPs

• Probability of Precipitation. Used in National Weather Service zone forecasts.

Pressure

• A measure of the weight of the air, that is usually measured with a barometer in meteorology.

-Q-

QPF

• Quantitative Precipitation Forecast

-R-

Radiosonde or Rawinsonde

• An instrument attached to a weather balloon that transmits pressure, humidity, temperature and winds as it ascends.

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).

RFC

• River Forecast Center. Specialized centers monitoring river data to support National Weather Service hydrologic programs.

-S-

Saturation-adiabatic lapse rate

• A lapse rate defined as the rate of decrease of temperature with height of an air parcel lifted in a saturation-adiabatic process through an atmosphere in hydrostatic equilibruim.

Sleet

• A type of frozen precipitation, consisting of small transparent ice pellets.

Slight Chance

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

Snotel

• Observing sites mainly in remote, high-elevation locations.
• Operated and maintained by the Natural Resources Conservation Service (NRCS)
• Data reported: 24-hour max/min temperatures and precipitation
• Frequency of reports: 6-12 hours

Snow Advisory

• Statement issued by the NWS when accumulating snow is expected to create a nuisance or inconvenience, usually for elevations below 6000 feet, with 2 to 5 inches in 12-hours.

Snow Level

• The altitude in the atmosphere where frozen precipitation changes to liquid as it falls, not necessarily at 32 degrees F (0 degrees C), and usually at a level below (warmer than) the freezing level.

Southern Oscillation

• A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events.

SPC

• Stands for Storm Prediction Center, located in Norman, OK.
• Monitor national weather conditions for development of severe thunderstorms.
• Issue severe thunderstorm and tornado watches.
• Provide NWS offices with meteorological reasoning during heavy rain and/or heavy snow events.

Squall Line

• A non-frontal band, or line, of thunderstorms.

Stable Air

Stability

• (stable air mass) A state in which the vertical distribution of temperature is such that an air particle will resist displacement from its level. An inversion is an example of a very stable condition.

Straight Line Winds

• Thunderstorm winds most often found with the gust front. They originate from downdrafts and can cause damage which occurs in a "straight line", as opposed to tornadic wind damage which has circular characteristics.

-T-

Thunder

• The sound caused by a lightning stroke as it heats a column of air, causing it to rapidly expand.

Thunderstorm

• A storm with lightning and thunder, produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail.

Tornado

• A violent, rotating column of air in contact with the ground, a pendant from a cumulonimbus cloud.

-U-

Unstable Air

-V-

Virga

• Precipitation falling from the base of a cloud and evaporating before it reaches the ground.

-W-

Warm Front

Warning

• Statement issued by tbe NWS when a particular hazard is "imminent" or already occurring (e.g., severe thunderstorm warning, winter storm warning).

Watch

• Statement issued by the NWS in advance of a storm system to alert the public of the possibility of a particular hazard (e.g., severe thunderstorm watch, winter storm watch).

Waterspout

• A column of rotating air over a body of water (ie. a tornado over the water).

Wet-Bulb Temperature

• The temperature an air parcel would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it.

Wind Advisory

• Statement issued by the NWS when sustained wind speeds of 30 to 40 mph and/or with gusts 40 to 49 mph are imminent or occurring.

Wind Chill

-X-

-Y-

-Z-

Zulu Time

• The time in all hydrometeorological products is expressed according to a single standard, which is the Universal Coordinated Time (UTC) (formerly known as Greenwich Mean Time [GMT] or Zulu {zero} Time [Z]). Midnight (0000 UTC) starts the 24 hour clock at the zero meridian.

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National Oceanic and Atmospheric Administration
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Missoula Weather Forecast Office
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Missoula, MT 59808-9381

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