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meteorology glossary

air conductivity See conductivity
air discharge
A form of lightning discharge probably similar to a cloud discharge in which the lightning channel propagates
away from a cloud charge center into apparently clear air where it terminates. Thus, cloud charge is moved
away from its original location and space charge of opposite sign outside the cloud may be neutralized.
air–earth conduction current (Also called fair-weather current)
That part of the air-earth current contributed by the electrical conduction of the atmosphere itself. It is
represented as a downward current in storm-free regions all over the world. The conduction current is the
largest portion of the air-earth current, far outweighing the contributions made by the precipitation current and
convection current which are zero in storm-free regions.. Its magnitude is approximately 3 x 10^-12+
amp/m^2+, or about 1800 amperes for the entire earth. Such observations of the vertical variation of the
conduction current as have been made indicate that it is approximately uniform throughout the troposphere, a
condition that is consistent with the generally accepted view that the conduction current flows from a positively
charged conducting region in the lower ionosphere downward to the negatively charged earth. Only in areas of
temporarily disturbed weather does the conduction current become replaced by reverse flow. Accumulating
evidence points to the conclusion that the conduction current continues to exist only because of the action of
thunderstorms scattered at all times over the earth, which supply the positive charge to the upper atmosphere
and negative charge to the earth ( see supply current ).
air–earth current
The transfer of electric charge from the positively-charged atmosphere to the negatively-charged earth. This
current is made up of the air-earth conduction current ,a point-discharge current, a precipitation current , a
convection current , and miscellaneous smaller contributions. Of these, the first is by far the largest. The
existence of this quasi-steady current in fair weather and the observed maintenance of the earth's net negative
charge are both better established than the nature of the supply current which must replenish the positive
charge in the upper atmosphere and the negative charge on the earth.
aircraft electrification
The accumulation of a net electrical charge on the surface of aircraft; or the separation of charge into two
concentrations of opposite sign on distinct portions of the aircraft surface. Net charges appears as a result of
autogenous electrification when aircraft fly through clouds of ice crystals or dust. Charge separation occurs by
induction when aircraft fly through regions of strong atmospheric electrical field, as in thunderstorms. Charging
may also occur by the engine exhaust carrying away a net charge leaving the aircraft charged. Development of
large local charges on aircraft parts may lead to appearance of corona discharge or St. Elmo's fire and is
almost always accompanied by poor radio communication due to so-called precipitation static .
arc discharge
A luminous electrical discharge in a gas where the current flows more of less continuously along a narrow
channel of high ionization. An arc discharge requires a continuous source of electric potential difference across
the terminals of the arc. This steady-state high current discharge is to be distinguished from the low current and
visually diffuse corona discharge and point discharge, as well as from the transient, high luminosity, high-current
spark discharge.
atmospheric electric field
A quantitative term, denoting the electric field strength of the atmosphere at any specified point in space and
time. In areas of fair weather, the atmospheric electric field near the earth's surface typically is about 100 volts
per meter and is directed vertically in such a sense as to drive positive charges downward to the earth. In areas
of fair weather this field decreases in magnitude with increasing altitude, falling, for example, to only about 5
volts per meter at an altitude of about 10 km. Near thunderstorms, and under clouds of vertical development,
the surface electric field varies widely in magnitude and direction, usually reversing its direction immediately
beneath active thunderstorms. In areas of minimum local disturbance, a characteristic diurnal variation of
electric field strength is observed. This variation is characterized by a maximum which occurs at about 19 hr
UTC for all points on the earth, and is now believed to be produced by thunderstorms which, for geographic
regions, are more numerous for the world as a whole at that universal time than at any other. It is now believed
that thunderstorms, by replenishing the negative charge to the earth's surface, provide the supply current to
maintain the fair-weather electric field in spite of the continued flow of the air-earth current that tends to
neutralize that field.
atmospheric electricity - 1
Electrical phenomena, regarded collectively, which occur in the earth's atmosphere. These phenomena include
not only such striking manifestations as lightning and St. Elmo's fire , but also less noticeable but more
ubiquitous effects such as atmospheric ionization , the air-earth current , and other quiescent electrical
processes. The existence of separated electric charges in the atmosphere is a consequence of many minor
processes (spray electrification, dust electrification, etc.) and a few major processes (cosmic ray ionization,
radiactive-particle ionization, and thunderstorm electrification). The details of thunderstorm charge separation
are poorly understood at the present time, which is unfortunate in that the maintenance of the prevailing
atmospheric electric field is now widely believed to be due to thunderstorm effects.
atmospheric electricity - 2
The study of electrical processes occurring within the atmosphere.
The radio frequency electromagnetic radiation originating, principally, in the irregular surges of charge in
thunderstorm lightning discharges . Atmospherics are heard as a quasi-steady background of crackling noise (
static )on certain radio receivers, such as AM radio. Since any acceleration of electric charge leads to emission
of electromagnetic radiation, and since the several processes involved in propagation of lightning lead to very
large charge accelerations, the lightning channel acts like a huge transmitter , sending out radiations with
frequencies of the order of 10 kHz. Atmospherics may occasionally be detected at distances in excess of 2000
miles from their source. Advantage has been taken of this in using radio direction-finding equipment to plot
cloud-to-ground lightning locations, to locate active thunderstorm areas in remote regions, and in-between
weather reporting stations.
autogenous electrification
The process by which net charge is built up on an object, such as an airplane, moving relative to air containing
dust or ice crystals. The electrification is produced by frictional effects ( triboelectriciation ) accompanying
contact between the object and the particulate matter.
ball lightning
A relatively rarely seen form of lightning, generally consisting of an orange or reddish ball of the order of a few
cm to 30 cm in diameter and of moderate luminosity, which may move up to 1 m/s horizontally with a lifetime of
a second or two. Hissing noises sometimes emanate from such balls, and they can explode noisily or disappear
noiselessly. The physical nature of ball lighting is not understood. Similar phenomena occasionally occur in high
current industrial equipment, but the conditions for such production are not reproducible.
beaded lightning
A particular visual variation of the end of a normal lightning flash where periodic sections of the channel appear
to die out slowly because they have greater radius and hence lose heat more slowly, are seen end-on, or for
other unknown reasons.
Bishop wave
A striking example of an atmospheric lee wave , formed in the lee of the Sierra Nevada range near Bishop,
California. The phenomenon includes a rotor cloud and a series of lenticular clouds parallel to the crest of the
range. [Note that this is not an atmospheric electricity term. It was assigned by mistake to R. Orville. Please
reassign this word to someone in cloud dynamics]
The process by which electrically-stressed air is transformed from an insulator to a conductor. Breakdown
involves the acceleration of electrons to ionization potential in the electric field imposed by the thundercloud, and
the subsequent creation of new electrons which avalanche and expand the scale of enhanced conductivity.
Breakdown precedes the development of lightning.
breaking-drop theory
A theory of thunderstorm charge separation based upon the suggested occurrence of the Lenard effect in
thunderclouds, that is, the separation of electric charge due to the breakup of water drops. This theory, advance
by Sir George C. Simpson in 1927, was initially intended to account for a bipolar charge distribution within a
thundercloud having the main positive charge center near the base of the cloud and the main negative charge
center higher up. Simpson's theory, however, does not explain this phenomenon well because temperatures are
below the freezing point in this region of the cloud. Evidence does remain to support the weak positive charge
center that lies slightly below the lower main negative charge concentration in many, if not all, thunderclouds
where the temperature is above the freezing point. Hence the breaking-drop theory is best ascribed to this
localized secondary positive charge center. The mechanism was also proposed as an explanation for waterfall
chain lightning
Same as bead or beaded lightning.
charge separation
The physical process causing cloud electrification. The process can include particle collisions with selective
charge transfer and particle capture of small ions at the particle scale. The process can include gravity-driven
differential particle motions and convective transport of charged air parcels at the cloud scale.
Clayden effect
When a photographic emulsion is given a very brief exposure to light of high intensity, it is desensitized towards
a subsequent longer exposure to light of moderate intensity. That is, the second exposure produces less effect
than if the pre-exposure had not been given. This phenomenon was observed originally by Clayden when
photographing lightning flashes, but it can be produced equally well by any type of light source, provided the
intensity is sufficiently high and the duration short enough.
cloud electrification
The process by which clouds become electrified. This process separates positive and negative electric charge
and develops potential differences occasionally sufficient to produce lightning.
cloud flash
(also called cloud flash, intracloud flash, cloud-to-cloud flash) a lightning discharge occurring between a
positively charged region and a negatively charged region, both of which may lie in the same cloud. The most
frequent type of cloud discharge is one between a main positive charged region and a main negative charged
region. Cloud flashes tend to outnumber cloud-to-ground flashes. In general, the channel of a cloud flash will be
wholly surrounded by cloud. Hence the channel's luminosity typically produces a diffuse glow when seen from
outside the cloud and this widespread glow is called sheet lightning.
cloud-to-cloud flash
A type of cloud flash. A lightning discharge that occurs between two electrified clouds.
cloud-to-ground flash
A lightning flash occurring between a charge center (either negative or positive) in the cloud and the ground.
Negative discharges are about 10 times more common than positive. The cloud-to-ground discharge is initiated
by a downward propagating leader followed by an extremely intense upward-moving return stroke that is the
principal source of luminosity. A cloud-to-ground discharge is usually a composite event, called a flash,
composed of several distinct leader


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