Subj : Weather Glossary (X)
To   : All
From : Daryl Stout
Date : Mon Mar 25 2019 08:28 am

  This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather
advisories, watches, and warnings, for the United States, and its
territories.

  Many of these terms and abbreviations are used by NWS forecasters to
communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the
purpose of this glossary to aid you in better understanding NWS products.

***

X-Band
A frequency band of microwave radiation in which radars operate.

X-Ray Background
In solar-terrestrial terms, a daily average background X-ray flux in
the 1 to 8 angstrom range. It is a midday minimum designed to reduce
the effects of flares.

X-Ray Burst
In solar-terrestrial terms, a temporary enhancement of the X-ray emission
of the sun. The time-intensity profile of soft X-ray bursts is similar to
that of the H-alpha profile of an associated flare.

X-Ray Flare Class
In solar-terrestrial terms, rank of a flare based on its X-ray energy
output. Flares are classified by the order of magnitude of the peak burst
intensity (I) measured at the earth in the 1 to 8 angstrom band as
follows:

Class   Intensity (in Watts/m2)                                          
B       I < 10-6                                                         
C       10-6 <= I < 10-5                                                
M       10-5 <= I < 10-4                                                
X       I >= 10-4                                                       


X-Rays
Very energetic electromagnetic radiation with wavelengths intermediate
between 0.01 and 10 nanometers (0.1-100 Angstroms) or between gamma rays
and ultraviolet radiation. Essentially all X-Rays from space are absorbed
in the Earth's upper atmosphere.

XBT
Expendable Bathythermograph

XCITED
Excited

XCPT
Expecting

XPC
Expect

XSEC
Cross Section

1-2-3 Rule
A means of avoiding winds associated with a tropical cyclone by taking
into account the forecast track error of the National Weather Service
over a 10 year period which is approximately 100 nm in 24 hours, 200 nm
for 48 hours and 300 nm in 72 hours. The forecast track error is added
to the 34 knot wind radii to compute the danger area. The wind radii may
be found within Tropical Cyclone Forecast Advisory (TCM) forecasts.

100-year Flood
A statistic that indicates the magnitude of flood which can be expected
to occur on average with a frequency of once every 100 years at a given
point or reach on a river. The 100-year flood is usually developed from a
statistical distribution that is based on historical floods. This is also
called a base flood.

100-year Flood Plain
The flood plain that would be inundated in the event of a 100-year flood.

500 hPa
Pressure surface (geopotential height) in the troposphere equivalent to
about 18,000 feet above sea level. Level of the atmosphere at which half
the mass of the atmosphere lies above and half below, as measured in
pressure units. This area is important for understanding surface weather,
upper air storms tend to be steered in the direction of the winds at this
level and are highly correlated with surface weather.

500 mb
Pressure surface (geopotential height) in the troposphere equivalent to
about 18,000 feet above sea level. Level of the atmosphere at which half
the mass of the atmosphere lies above and half below, as measured in
pressure units. This area is important for understanding surface weather,
upper air storms tend to be steered in the direction of the winds at this
level and are highly correlated with surface weather.

88D
Doppler Radar currently used nationwide by the National Weather Service.
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