Thunderstorm - a thunderstorm in
which new development takes place on the upwind side (usually the west or southwest
side), such that the storm seems to remain stationary or propagate in a backward
Winds - winds which shift in a counterclockwise
direction with time at a given location (e.g. from southerly to southeasterly),
or change direction in a counterclockwise sense with height (e.g. westerly
at the surface but becoming more southerly aloft). In storm spotting,
a backing wind usually refers to the turning of a south or southwest surface
wind with time to a more east or southeasterly direction. The opposite
of veering winds.
Anvil - [Slang], a thunderstorm
anvil which spreads upwind, against the flow aloft,
often implying that a very strong updraft and,
hence, a high severe weather potential exist
- The range of frequencies (in
Hertz) between the limits of a frequency band. Bandwidth is a measure of
how well radio energy input is passed through the receiver without distortion
or loss of data. It is one of the variables determining the minimum detectable
signal of a radar unit. The shorter the pulse duration,
the larger the bandwidth required to preserve the same quality of receiver output
- an instrument for determining the pressure
of the atmosphere
Zone - a region in which a temperature
gradient exists on a surface of constant pressure;
not barotropic. Baroclinic zones are favored areas
for strengthening and weakening weather systems. Wind
shear is characteristic of a baroclinic zone.
(or baroclinicity) - A measure of the state of
stratification in a fluid in which surfaces
of constant pressure (isobaric) intersect
surfaces of constant density (isosteric).
Pressure - see atmospheric pressure
System - the term barotropic system usually is used in a relative
sense to describe systems in which the isotherms
and height contours are nearly parallel everywhere
on a surface of constant pressure; directional
wind shear is weak; as a rule, a true equivalent
barotropic system can never be achieved in the real atmosphere
- The state of a fluid in which surfaces
of constant density (or temperature) are coincident with surfaces of constant
pressure; it is the state of zero baroclinity.
Reflectivity - one of the three fundamental quantities (along with
base [radial] velocity and spectrum width)
that a Doppler radar measures. Reflectivity
is related to the power, or intensity, of the reflected radiation that is sensed
by the radar antenna. Base reflectivity is expressed on a logarithmic scale
in units called dBZ. The term "base" refers to the
product being "basic", with little advanced processing performed on the data.
Base reflectivity is related to rainfall intensity
(e.g., drop size and rainfall rate) and hail size
(for large values of reflectivity).
Velocity - one of the three fundamental quantities (along with base
reflectivity and spectrum width) that a Doppler
radar measures. Base [radial] velocity
is the average velocity (towards or away from the radar looking in a specific
direction) of the hydrometeors detected in the radar pulse volume. Base velocity
is expressed as being positive or negative, with positive values (warm colors)
being interpreted as flow away from the radar and negative values (cool colors)
being interpreted as flow towards the radar. The term "base" refers to the product
being "basic" with little advanced processing performed on the data.
Width - the angle between the center of the radar
beam and the point in the beam where the power of the transmitted energy
is one-half of the power at the center's maximum. A WSR-88D
radar's beam width is approximately 1 degree.
High - The semipermanent atmospheric subtropical anticyclone (high
pressure system) over the North Atlantic Ocean, so name especially when it is
located in the western part of the ocean, near Bermuda (near 30° N).
Principle - Air flowing over an airfoil results in an increase in
flow speed over the upper curved surface. Since a velocity increase in fluid
flow results in a corresponding pressure decrease, the increased airflow over
the upper surface of the airfoil produces a lift on the airfoil because of lower
pressure exerted on the upper surface. Named for Daniel Bernoulli (1700 -1782),
a Swiss physicist who discovered the effect.
- A hypothetical "body" that absorbs all of the electromagnetic
radiation striking it - it does not reflect or transmit any of the incident
radiation. The radiation emitted is consistent with Planck's
law. In accordance with Kirchhoff's law,
a blackbody not only absorbs all wavelengths, but emits at all wavelengths with
the maximum possible intensity for any given temperature. Contrast with whitebody
Radiation - The electromagnetic
radiation emitted by an ideal blackbody adhering
to the radiation laws; it is the theoretical maximum
amount of electromagnetic radiation of all wavelengths that can be emitted by
a body at a given temperature.
- severe weather condition characterized by low temperatures,
winds of 32 mph or higher, and sufficient snow
for visibility to be reduced to less than 500
Point - the temperature at which a liquid boils
Constant - The ratio of the universal gas
constant to Avogadro's number; equal to 1.38062
X 10-23 joules per Kelvin. Named for Ludwig Boltzmann (1844 -1906),
an Austrian physicist.
Layer - in general, a layer of air adjacent to a bounding surface.
Specifically, the term most often refers to the planetary boundary layer,
which is the layer within which the effects of friction are significant. For
the earth, this layer is considered to be roughly the lowest one or two kilometers
of the atmosphere. It is within this layer that
temperatures are most strongly affected by
daytime solar heating and nighttime radiational
cooling, and winds are affected by friction with
the earth's surface. The effects of friction die out gradually with height,
so the "top" of this layer cannot be defined exactly.
Echo - a radar echo which is linear but bent outward in the shape
of a bow (i.e., used by an archer). Damaging straight-line
winds often occur near the "crest" or center of a bow echo. The left (usually
northern) end of the bow is a preferred location for the formation of tornadoes.
Ratio - For any moist surface, the ratio of heat
energy used for sensible heating (conduction
and convection) to the heat energy used for latent
heating (evaporation of water or sublimation of snow). The Bowen ratio ranges
from about 0.1 for the ocean surface to more than 2.0 for deserts; negative
values are also possible. It is named for Ira S. Bowen (1898-1978), an American
Law - The empirical generalization that for many so-called perfect
gases, the product of pressure and volume is constant in an isothermal
process. Named for Robert Boyle (1627-1691), a British chemist who formulated
Band - a distinct feature observed by a radar
that denotes the freezing level of the atmosphere. The term originates from
a horizontal band of enhanced reflectivity
that can result when a radar antenna scans vertically through precipitation.
The freezing level in a cloud contains ice particles
that are coated with liquid water. These particles reflect
significantly more radiation (appearing to the radar as large raindrops) than
the portions of the cloud above and below the freezing layer. The bright band
can affect the ability of the NEXRAD algorithms
to produce accurate rainfall estimates
at far ranges because the algorithm may interpret reflectivity from the bright
band as an overestimate of precipitation reaching the surface.
- a basic visual sensation describing the amount of light that appears
to emanate from an object, or more precisely, the luminance
of an object
Temperature - the apparent temperature of a celestial object, based
on the assumption that it radiates as a blackbody
- a classification for sky cover used
when 0.6 (six tenths) to 0.9 (nine tenths) of the sky is covered by clouds
- see Bulk Richardson Number
High - a mesoscale area of high pressure,
typically associated with cooler air from the rainy downdraft
area of a thunderstorm or a complex of thunderstorms.
A gust front or outflow boundary separates a
bubble high from the surrounding air.
Richardson Number (or BRN) - a non-dimensional (i.e., no units) number
relating vertical stability to vertical shear
(generally, stability divided by shear). High values indicate unstable and/or
weakly-sheared environments; low values indicate weak instability and/or strong
vertical shear. Generally, values in the range of around 50 to 100 suggest environmental
conditions favorable for supercell development.
- the tendency of a body to float or to rise
when submerged in a fluid;
the power of a fluid to exert an upward force on a body placed in it
(Bounded Weak Echo Region) - also known as
a vault; a radar signature within a thunderstorm
characterized by a local minimum in radar reflectivity
at low levels which extends upward into, and is surrounded by, higher reflectivities
aloft. This feature is associated with a strong updraft
and is almost always found in the inflow region of a thunderstorm. It cannot
be seen visually.