dBZ - Nondimensional "unit" of radar reflectivity which
represents a logarithmic power ratio (in decibels, or dB) with
respect to radar reflectivity factor, Z.
The value of Z is a function of the amount of radar beam energy that
is backscattered by a target and detected as a signal (or echo).
Higher values of Z (and dBZ) thus indicate more energy being
backscattered by a target. The amount of backscattered energy
generally is related to precipitation intensity, such that higher
values of dBZ that are detected from precipitation areas generally
indicate higher precipitation rates. However, other factors can
affect reflectivity, such as width of the radar beam, precipitation
type, drop size, or the presence of ground clutter or AP. WSR-88D
radars can detect reflectivities as low as -32 dBZ near the radar
site, but significant (measurable) precipitation generally is
indicated by reflectivities of around 15 dBZ or more. Values of 50
dBZ or more normally are associated with heavy thunderstorms,
perhaps with hail, but as with most other quantities, there are no
reliable threshold values to confirm the presence of hail or severe
weather in a given situation. See VIP for threshold dBZ values
associated with each VIP level.
*Debris Cloud - A rotating "cloud" of dust or debris, near or
on the ground, often appearing beneath a condensation funnel and
surrounding the base of a tornado.
This term is similar to dust whirl, although the latter typically
refers to a circulation which contains dust but not necessarily any
debris. A dust plume, on the other hand, does not rotate. Note that
a debris cloud appearing beneath a thunderstorm will confirm the
presence of a tornado, even in the absence of a condensation funnel.
Delta T - A simple representation of the mean lapse rate
within a layer of the atmosphere, obtained by calculating the
difference between observed temperatures at the bottom and top of
the layer. Delta Ts often are computed operationally over the layer
between pressure levels of 700 mb and 500 mb, in order to evaluate
the amount of instability in mid-levels of the atmosphere.
Generally, values greater than about 18 indicate sufficient
instability for severe thunderstorm development.
Derecho - (Pronounced day-RAY-cho), a widespread and usually
fast-moving windstorm associated with convection. Derechos include
any family of downburst clusters produced by an extratropical MCS,
and can produce damaging straight-line winds over areas hundreds of
miles long and more than 100 miles across.
Dew Point (or Dew-point Temperature) -
A measure of atmospheric
moisture. It is the temperature to which air must be cooled in order
to reach saturation (assuming air pressure and moisture content are
Differential Motion - Cloud motion that appears to differ
relative to other nearby cloud elements, e.g. clouds moving from
left to right relative to other clouds in the foreground or
background. Cloud rotation is one example of differential motion,
but not all differential motion indicates rotation. For example,
horizontal wind shear along a gust front may result in differential
cloud motion without the presence of rotation.
Difluence (or Diffluence) - A pattern of wind flow in which
air moves outward (in a "fan-out" pattern) away from a central axis
that is oriented parallel to the general direction of the flow. It
is the opposite of confluence.
Difluence in an upper level wind field is considered a favourable
condition for severe thunderstorm development (if other parameters
are also favourable). But difluence is not the same as divergence. In
a difluent flow, winds normally decelerate as they move through the
region of difluence, resulting in speed convergence which offsets
the apparent diverging effect of the difluent flow.
Directional Shear - The component of wind shear which is due
to a change in wind direction with height, e.g., south-easterly winds
at the surface and south-westerly winds aloft. A veering wind with
height in the lower part of the atmosphere is a type of directional
shear often considered important for tornado development.
Diurnal - Daily; related to actions which are completed in
the course of a calendar day, and which typically recur every
calendar day (e.g., diurnal temperature rises during the day, and
diurnal falls at night).
Divergence - The expansion or spreading out of a vector
field; usually said of horizontal winds. It is the opposite of
convergence. Divergence at upper levels of the atmosphere enhances
upward motion, and hence the potential for thunderstorm development
(if other factors also are favourable).
Doppler Radar - Radar that can measure radial velocity, the
instantaneous component of motion parallel to the radar beam (i.e.,
toward or away from the radar antenna).
*Downburst - A strong downdraft resulting in an outward burst
of damaging winds on or near the ground. Downburst winds can produce
damage similar to a strong tornado. Although usually associated with
thunderstorms, downbursts can occur with showers too weak to produce
thunder. See dry and wet microburst.
Downdraft - A small-scale column of air that rapidly sinks
toward the ground, usually accompanied by precipitation as in a
shower or thunderstorm. A downburst is the result of a strong
Downstream - In the same direction as a stream or other flow, or
toward the direction in which the flow is moving.
Dry Adiabat - A line of constant potential temperature on a
Dry Line - A boundary separating moist and dry air masses,
and an important factor in severe weather frequency in the Great
Plains. It typically lies north-south across the central and
southern high Plains states during the spring and early summer,
where it separates moist air from the Gulf of Mexico (to the east)
and dry desert air from the southwestern states (to the west). The
dry line typically advances eastward during the afternoon and
retreats westward at night. However, a strong storm system can sweep
the dry line eastward into the Mississippi Valley, or even further
east, regardless of the time of day. A typical dry line passage
results in a sharp drop in humidity (hence the name), clearing
skies, and a wind shift from south or southeasterly to west or
southwesterly. (Blowing dust and rising temperatures also may
follow, especially if the dry line passes during the daytime; see
dry punch). These changes occur in reverse order when the dry line
retreats westward. Severe and sometimes tornadic thunderstorms often
develop along a dry line or in the moist air just to the east of it,
especially when it begins moving eastward. See LP storm.
Dry-line Bulge - A bulge in the dry line, representing the
area where dry air is advancing most strongly at lower levels (i.e.,
a surface dry punch). Severe weather potential is increased near and
ahead of a dry line bulge.
Dry-line Storm - Generally, any thunderstorm that develops on
or near a dry line. The term often is used synonymously with LP
storm, since the latter almost always occurs near the dry line.
Dry Microburst - A microburst with little or no precipitation
reaching the ground; most common in semi-arid regions. They may or
may not produce lightning. Dry microbursts may develop in an
otherwise fair-weather pattern; visible signs may include a cumulus
cloud or small Cb with a high base and high-level virga, or perhaps
only an orphan anvil from a dying rain shower. At the ground, the
only visible sign might be a dust plume or a ring of blowing dust
beneath a local area of virga. Compare with wet microburst.
Dry Punch - [Slang], a surge of drier air; normally a
synoptic-scale or mesoscale process. A dry punch at the surface
results in a dry line bulge. A dry punch aloft above an area of
moist air at low levels often increases the potential for severe
Dry Slot - A zone of dry (and relatively cloud-free) air
which wraps east- or north-eastward into the southern and eastern
parts of a synoptic scale or mesoscale low pressure system. A dry
slot generally is seen best on satellite photographs.
Dry slot should not be confused with clear slot, which is a
Dust Devil - A small atmospheric vortex not associated with a
thunderstorm, which is made visible by a rotating cloud of dust or
debris (dust whirl). Dust devils form in response to surface heating
during fair, hot weather; they are most frequent in arid or
*Dust Plume - A non-rotating "cloud" of dust raised by
straight-line winds. Often seen in a microburst or behind a gust
If rotation is observed, then the term dust whirl or debris cloud
should be used.
*Dust Whirl - A rotating column of air rendered visible by
dust. Similar to debris cloud; see also dust devil, gustnado,
Dynamics - Generally, any forces that produce motion or
affect change. In operational meteorology, dynamics usually refer
specifically to those forces that produce vertical motion in the