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Glossary

GENERAL TERMS AND ACRONYMS

AIX (Advanced Interactive Executive)
IBM's version of UNIX based on AT&T system V and runs on the IBM RS/6000 system.
Domestic Data Plus Data Circuit (DD+)
This is a composite meteorological data circuit which contains domestic data and public products (in a textual form) that has been tailored for the general public as well as the trained meteorologist. This circuit broadcasts several types of standard meteorological data such as surface observations (U.S., Canada and Mexico), upper air soundings, and radar data in addition to textual forecast, severe weather watch, model output statistics, and climatological information. This circuit also emphasizes public broadcast information style reports such as local weather warnings, sporting forecaon style reports such as local weather warnings, sporting forecasts, and earthquake reports.
FAA 604 Data Circuit (604)
A commercially available meteorological data circuit which is administered by the Federal Aviation Administration. This circuit contains broadcasts of several types of standard meteorological data such as surface observations, upper air soundings, radar summaries, aviation information (such as airport conditions), and ad hoc pilot reports.
GKS (Graphical Kernel System)
An ANSI standard specification and encoding regime for two dimensional graphics. specification and encoding regime for two dimensional graphics. This software specification contains routines to perform standard graphical output; device input; metafile generation; graphical segment generation; manipulation, and transformation; as well as language bindings for FORTRAN, C and Pascal.
GOES (Geostationary Orbiting Environmental Satellite)
These meteorological satellites are located 22,000 miles above the earth's equator in a geostationary orbit. These satellites take various types of visible and infrared pictures of the same section of the earth's surface twice an hour.
GMT (Greenwich Mean Time) or Z (ZULU) or UTC (Universal Time Coordinated)
This is the standard time zone used to coordinate time and date information for all meteorological data. Greenwich Mean Time is five hours ahead of Eastern Standard Time and eight hours ahead of Pacific Standard Time. For example, 2100 GMT is 4:00 PM EST.
High-resolution Data Service (HDS)
This is a meteorological data circuit which broadcasts model gridpoint data for various models run at NMC and ECMWF. The model data runs from initialized data fields to ten day forecasts. Data are broadcast at mandatory pressure levels on latitude-longitude grids.
HP/UX
HP's version of UNIX based on AT&T system V that runs on the HP 700 system.
IRIX
Silicon Graphics version of UNIX based on AT&T system V which runs on SGI systems.
LDM (The Local Data Manager)
This software package developed by Unidata provides an ingestor and data manager that selects and saves data from various meteorological feeds into user selectable data files.
MDR (Manually Digitized Radar)
This represents manual subjective coding of NWS radar echoes onto a regular MDR radar grid. Each grid box is 48 km on a side and the resulting grid covers the United States, lower Canada, northern Mexico, and the Gulf of Mexico. Each reporter at a radar site determines the maximum radar echo intensities for each box within its domain and reports this information as part of a SD report. This report also contains precipitation type, location, movement, coverage, change of intensity, cloud tops, and any special radar information.
MOS (Model Output Statistics)
This represents the output of model post-processors which take the gridpoint information from the model runs and derive various meteorological data for city locations out to 60 hours. Since the resolution of the model is too coarse to derive exact quantites, these data are derived from statistical models which have been developed from model comparisons to actual conditions at that station. The results are temperature, precipitation, wind, cloud forecasts for approximately 250 stations across the U.S.
MS-DOS (Microsoft's Disk Operating System)
This is the single tasking operating system most often used on IBM PC, PS/2, and compatible machines.
NFS (Network File System)
This is both a method and protocol for accessing files and data contained on separate machines connected over a network by logically mounting a remote computer's file systems on a local machine.
Solaris
Sun's version of UNIX based on AT&T System V and runs on Sun workstations.
SunOS
Sun's old version of UNIX based on BSD 4.3.
Unidata and UPC (Unidata Projects Center)
Unidata is a UCAR project with a software support center responsible for standards, data broadcast definition, and maintenance of a meteorological software library for use on workstation based systems.
WXP (The Weather Processor)
A software package that retrieves meteorological data from broadcast circuits, calculates several meteorological data from broadcast circuits, calculates several meteorological variables, and analyzes and plots the resulting data.
X Windows
This is a network-based graphical user interface developed at MIT that has gained a wide degree of acceptance by a number of hardware/software vendors. Graphics are created using a distributed computing paradigm on a X Window server and distributed over a network to a variety of client X Window machines for display.

ATMOSPHERIC VARIABLES

Temperature
The temperature is a measure of the internal energy that a substance contains. This measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.
Dewpoint
The dewpoint is the point at which air, when cooled at a specific pressure and moisture content, reaches the saturation point of water vapor. In other words, when the temperature is lowered, this is the point at which condensation occurs or dew forms.
Dewpoint depression
The dewpoint depression is the difference in degrees between the temperature and the dewpoint.
Wind Direction
This is the direction from which the air is moving. The directions are in terms of degrees from true north (0 degrees), and the angle increases in a clockwise direction. In other words, if the wind direction is 45 degrees, the wind is from the northeast.
Wind Speed
The wind speed is a measure of the average speed of movement of the wind at a specific point. When measured, the value represents an average taken over a couple of minutes.
Wind Gust
The wind gust is the maximum wind speed recorded over a specified time period. When wind speeds are measured and the peak wind speed during the measuring period is roughly 10 knots more than the average wind speed, a wind gust is reported.
Station pressure
This is the absolute air pressure at a given reporting station. The air pressure is the combined weight of all air located in a column directly above the reporting site. Consequently, the station pressure may vary tremendous in mountainous regions due to the strong variation of atmospheric pressure with height. Vertical variations of pressure range up to 150 mb per mile whereas horizontal variations are usually less than .1 mb per mile.
Sea Level Pressure
The sea level pressure is a correction of the station pressure to sea level. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in the sea level correction is a twelve hour mean, eliminating diurnal effects. Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.
Altimeter Setting
The altimeter setting is a correction of the station pressure to sea level used by aviation. This correction takes into account the standard variation of pressure with height and the influence of temperature variation with height on the pressure. The temperatures used correspond to the standard atmosphere temperatures between the surface and sea level.
Height or Geopotential Height
The geopotential height is roughly the height above sea level of a pressure level. This is an estimated height based on temperature and pressure data.
Present Weather
This is the type of weather observed at the reporting time. These conditions may include types of precipitation like light rain or heavy snow, as well as the condition of the air environment such as foggy, hazy or blowing dust.
Visibility
The visibility is the maximum distance an object may be seen considering air conditions. Precipitation, fog, haze, pollutants, and suspended dust all contribute to lowering visibility.
Cloud Cover
This is the amount of total sky coverage by clouds.
Clear = less than 10% cloud coverage.
Scattered = more than 10% and less less than 60% cloud coverage.
Broken = more than 60% and less than 90% cloud coverage.
Overcast = more than 90% cloud coverage.
Obscured = cannot determine cloud information due to obscuration by precipitation, fog, dust, etc.  Sometimes referred to as vertical visibility.
Cloud Ceiling
This is the height of the cloud base for the lowest broken or overcast cloud layer.
Pressure Tendency
This is the change of sea level pressure with time. This is usually reported as a three hour pressure change in millibars.
Precipitation
This is the amount of liquid equivalent precipitation measured over a particular range of time.  Snow is melted down and measured.  Common time periods include 1, 3, 6, 12 and 24 hours.
Maximum Temperature
This is the highest temperature recorded during a specified period of time.  Common time periods include 6, 12 and 24 hours.
Minimum Temperature
This is the lowest temperature recorded during a specified period of time.  The time period can be 6, 12 or 24 hours.
Snow Cover
This is the depth of snow in inches measured at the reporting time. Since snow may drift, this value represents an average snow depth measured at several locations.
Probability of Precipitation
The probability based on climatology that measurable precipitation will fall at that location during a specified period of time.  For example, if the probability is 30%, your location will recieve .01" or more of precipitation in 3 out of 10 similar situations.
Probability of Thunderstorms
The probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.
Quantitative Precipitation
The forecasted amount of precipitation that will fall at a particular location over a given amount of time usually either for 6 or 12 hours.
Vertical Velocity
The rate of upward or downward motion of air passing through a given pressure level.
Potential Temperature
The potential temperature is the temperature an air parcel at a specific pressure level and temperature would have if it were lowered or raised adiabatically to 1000 mb. This is defined by Poisson's equation.
Equivalent Potential Temperature
The equivalent potential temperature is the temperature a parcel at a specific pressure level and temperature would have if it were raised to 0 mb, condensing all moisture from the parcel, and then lowered to 1000 mb.
Virtual Temperature
The virtual temperature is the temperature a parcel which contains no moisture would have to equal the density of a parcel at a specific temperature and humidity.
Virtual Potential Temperature
The virtual potential temperature is the temperature a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised to 1000 mb. This is defined by Poisson's equation.
Vapor Pressure
This the partial pressure that water vapor exerts on the total air pressure.
Mixing Ratio
This the ratio of the weight of water vapor in a specified volume to weight of dry air in that same volume.
Relative Humidity
This is the ratio of actual vapor pressure to the saturation vapor pressure at a specific temperature.  
Specific Humidity
This the ratio of the weight of water vapor in a specified volume to weight of the air in that same volume.
Lifted Condensation Level (LCL)
The lifted condensation level represents the pressure level at which a parcel would first reach saturation if lifted adiabatically from a specific level in the atmosphere. As the parcel rises, its temperature decreases and moisture content remains constant. Therefore, the parcel will reach a point at which it is saturated with water vapor and condensation begins.
Wind Chill
The apparent temperature of exposed skin with a four knot wind that has the same heat loss of exposed skin at a certain certain temperature and wind speed. The higher the wind speed, the higher the heat loss from the skin's surface due to sensible heat transfer. This is calculated from heat transfer algorithms.
Heat Index
The apparent temperature of exposed skin when exposed to air with a high moisture content. Dry air allows evaporative cooling of the skin's surface. The higher the humidity, the less the skin can cool itself with evaporative cooling.
Convergence
The convergence is a measure of the rate at which air is converging along a horizontal plane. Convergence near the surface and divergence aloft can be correlated with upward vertical motion.
Vorticity
The vorticity is a measure of the rotation of air in a horizontal plane. Positive (counter-clockwise or cyclonic) vorticity can be correlated with surface low development and upward vertical motion (in areas of positive vorticity advection).
Convective Condensation Level (CCL)
The CCL is the pressure level at which a convectively mixed parcel reaches saturation when lifted. The initial parcel has the moisture content equivalent to the mean of the lowest 100 mb and retains the temperature of the surrounding atmosphere when lifted. This is usually found by determining the mean mixing ratio of the lowest 100 mb and findingtermining the mean mixing ratio of the lowest 100 mb and finding where that mixing ratio line crosses the sounding.
Convective Temperature (CT)
This is the surface temperature a parcel would have so that its temperature when lifted adiabatically to the CCL would equal the environmental temperature.  This is generally referred to as a critical surface temperature for the development of convective coulds.
Precipitable Water
This is a measure of the total amount of water vapor in a column of air and can be used to infer preciptation amounts.

ASTRONOMICAL TERMS

sunrise
The time at which the first part of the sun appears above the horizon in the morning.
sunset
The time at this the last part of the sun disappears below the horizon in the evening.
dawn or civil dawn
This is the time at which the sun is 6 degrees below the horizon in the morning. Civil dawn is defined as that time at which there is enough light for objects to be distiguishable and that outdoor activities can commence.
nautical dawn
This is the time at which the sun is 12 degrees below the horizon in the morning. Nautical dawn is defined as that time at which there is just enough sunlight for objects to be distiguishable.
astronomical dawn
This is the time at which the sun is 18 degrees below the horizon in the morning. Astronomical dawn is that point in time at which the sun starts lightening the sky. Prior to this time, the sky is completely dark
dusk or civil dusk
This is the time at which the sun is 6 degrees below the horizon in the evening. At this time objects are distinguishable but there is no longer enough light to perform any outdoor activities.
nautical dusk
This is the time at which the sun is 12 degrees below the horizon in the evening. At this time, objects are no longer distinguishable.
astronomical dusk
This is the time at which the sun is 18 degrees below the horizon in the evening. At this time the sun no longer illuminates the sky.
solar noon
This is the time at which the sun is the highest in the sky. This time varies through the year due to the change in speed of the earth's orbit around the sun.
daylength
The total time the sun is above the horizon
twilight
The average time of civil twilight which is the time between civil dawn and sunrise and sunset and civil dusk.
elevation
The angle that the sun is above the horizon (assuming no atmosphere)
elevation (r)
The angle that the sun appears to be above the horizon assuming the refraction of the earth's atmosphere.
azimuth
The direction that the sun appears to be over with 0 degrees being north, 90 being east, 180 being south and 270 being west.
zenith angle
The angle between the sun and directly overhead, the zenith
declination
The latitude that the sun is directly over which is ~23N in the summer, ~23S in the winter and 0 at the equinoxes.
right ascension
The celestial longitude of the sun. This value is 0 at the vernal equinox, 90 at the summer solstice, 180 at the autumnal equinox and 270 at the winter solstice.

ATMOSPHERIC STABILITY INDICIES

Lifted Index (LI)

This is an index used to determine the stability of the lower half of the troposphere. An air parcel is lifted from the surface with temperature and mixing ratios representative of the mean layer values of the lowest 100 mb of the atmosphere. This is done in order to capture low level boundary layer temperature and moisture conditions while reducing diurnal effects. This hypothetical parcel is then lifted dry adiabatically to the LCL and pseudo-adiabatically to 500 mb. The value of this index is the temperature of the environment subtracted from the temperature of the parcel at 500 mb. The risk of severe weather activity is defined as follows:

LI > 2 No significant activity
0 < LI < 2 Showers probable, isola 2 No significant activity
0 < LI < 2 Showers probable, isolated thunderstorms possible
-2 < LI < 0 Thunderstorms probable
-4 < LI < -2 Severe thunderstorms possible
LI < -4 Severe thunderstorms probable, tornadoes possible

Showalter Index (SI)

This is an index used to determine the stability of the lower half of the troposphere. An air parcel is lifted from an initial position at 850 mb where localized low level influences are greatly reduced. It is lifted dry adiabatically to its LCL and then pseudo-adiabatically to 500 mb. The environmental temperature is then subtracted from the parcel temperature to obtain the value of the Showalter index. The risk of severe weather activity is defined as follows:

SI > 3 No significant activity
1 < SI < 3 Showers probable, isolated thunderstorms possible
-2 < SI < 1 Showers probable, isolated thunderstorms possible
-2 < SI < 1 Thunderstorms probable
-6 < SI < -2 Severe thunderstorms possible
SI < -6 Tornadoes possible

Total Totals Index (TT)

The Total Totals index is a simple index derived from the temperature lapse rate between 850 mb and 500 mb and moisture content at 850 mpse rate between 850 mb and 500 mb and moisture content at 850 mb. It is defined as follows:

TT = T850 + Td850 - 2T500

The risk of severe weather activity is defined as follows:

44-45 Isolated moderate thunderstorms
46-47 Scattered moderate / few heavy thunderstorms
48-49 Scattered moderate / few heavy / isolated severe thunderstorms
50-51 Scattered heavy / few severe thunderstorms and isolated tornadoes
52-55 Scattered to numerous heavy / few to scattered severe thunderstorm / few tornadoes
> 55 Numerous heavy / scattered severe thunderstorms and scattered tornadoes

K Index (KI)

The K index is similar to Total Totals index except that it takes into account moist air at 700 mb contributing to air mass thunderstorm development. The K index is defined as follows:

K = T850 - T500 + Td850 - (T700 - Td700)

The risk of air mass thunderstorms is defined as follows:

K < 15 0% Air mass thunderstorm probability
15-20 <20% Air mass thunderstorm probability
21-25 20-40% Air mass thunderstorm probability
26-30 40-60% Air mass thunderstorm probability
31-35 60-80% Air mass thunderstorm probability
36-40 80-90% Air mass thunderstorm probability
K > 40 >90% Air mass thunderstorm probability

SWEAT ( Severe Weather thrEAT ) Index

This is an United States Air Force index that takes into account thermal instability, low level moisture content, vertical wind shear and horizontal wind speeds. The risk of severe weather activity is defined as follows:

SW < 300 No activity expected
300 < SW < 400 Isolated moderate to heavy thunderstorms
400 < SW < 500 Severe thunderstorms and tornadoes probable
SW > 500 SW < 500 Severe thunderstorms and tornadoes probable
SW > 500 Severe thunderstorms and tornadoes likely

Energy Index (EI)

This index compares the energy contained in the atmosphere at 850 and 500 mb to its stability. The risk of severe weather activity is defined as follows:

EI > 0 No activity expected
-2 < EI < 0 Isolated severe thunderstorms
EI < -2 Severe thunderstorms probable, tornadoes possible