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~ Work In Progress ~
A-C
Anvil Crawler. An exotic type of C-C lightning that crawls under the base of
an extending Cumulonimbus Anvil or slanted tower.
AEROSOL
A large molecule or
microscopic rigid body suspended in an air mass, such as CFCs.
AIR
The mixture of
free-moving gases layered above the Earth’s surface, dominantly Oxygen and
Nitrogen.
ANVIL
The common name given to the glaciated fibrous top of a Cumulonimbus cloud,
or thunder cloud, as the heaped-tower spreads outward along the stable
tropopause layer. In many cases the formation resembles a blacksmith's
anvil, hence it's name. This formation is also known as the "thunderhead"
and can appear in many shapes and sizes depending on the intensity of the
cumulonimbus. Some anvils don't resemble anvils at all and look more like
mushroom clouds.
ARCUS
A roll of dense
horizontal cloud with frayed edges often associated with a continuous
updraught along the base of deep convective cloud such as Cumulonimbus. It
occurs on the leading edge (lower front) of the cold and warm air boundary
and often has heavy rain curtains behind it's path in the downdraught
region. The cloud can appear to roll upwards into the cloud as air rises in
the updraught. Can also mark the hotspot for C-G lightning in some storms.
Other names are given to this cloud phenomena such as shelf/roll-cloud,
updraught-skirt and the "Whales Mouth" formation.
T0054 Arcus cloud
BALL
LIGHTNING (B-L)
A non-proven phenomenon
where an electric ball of plasma floats near to the ground during a
thunderstorm. No hard-evidence has been produced to this date despite
hundreds of
eyewitness accounts and unclear photographs.
BIOSPHERE
The layer of the Earth's crust and water (the oceans) that supports life.
C-A
Cloud to Air lightning, usually occurs in branches from leader strokes or
around the Anvil
C-C
Cloud to Cloud lightning, otherwise known as "Spider-Lightning" or "Streaked
Lightning".
C-G
Cloud to Ground lightning, otherwise known as "Forked".
CAPE
Convective Available Potential Energy, measured
in Joules per Kilogram (0-2500 J/kg typical value range). This is a measure
of how much potential energy is available for positive buoyancy (convection)
throughout the depth of the atmosphere. The higher the CAPE value, the
stronger the buoyancy. This is a tool used by forecasters to predict
severe thunderstorms and can be derived from tephigrams.
CCN
Cloud-Condensation Nuclei. When water vapour
condenses out of an air mass it needs to condense onto a rigid object. Cloud
droplets require a microscopic object as starting point for droplet growth,
such as dust, salt or other aerosols.
CELL
(also see SINGLECELL, MULTICELL, SUPERCELL and DAUGHTER CELL)
A cell is the name given to a column of convective cloud, or a heap of cloud with a
single updraught and downdraught system. Cumuliform layer clouds also possess cellular
structure, as one parcel of air rises another descends next to it (seen in
stratocumulus, altocumulus and cirrocumulus or general fair weather
cumulus). When talking about convective cells, you can have open-cell (free
convection with gaps in between) and closed-cell (capped convection that
fills the gaps with altocumulus or stratocumulus once it reaches an
inversion for example). More vigorous convection, i.e. cumulonimbus where the
cells reach vertical extents up to 20,000 ft to 60,000 ft, are classified by
the type of development they take on and are given three specialised names:- Singlecell, Multicell,
and Supercell.
CFC
Chlorofluorocarbon; a substance commonly used
in refrigerators and pressurised canisters. CFCs are harmful to the
environment and must be disposed of correctly. They have been found to cause
the breakdown of Ozone (O3) in the Stratosphere and have been
linked to the contribution of the Ozone hole in the Antarctic.
DAUGHTER
CELL
(also see CELL, SINGLECELL, MULTICELL, and SUPERCELL)
New convective cell
development caused by low-level uplift from the cold and dense outflow of a
dissipating convective cell. These are found most commonly in multicell
thunderstorms structures and propagating squall lines.
DEW
Water droplets that form
on cool objects on the Earth’s surface. Moist air is required to be close to
saturation (dew point) for water vapour to condensate onto objects and to
feed droplet growth. Usually occurs on the surface in autumn/winter/spring. Commonly occurs
in arid climates during night time when the air temperature drops
considerably.
DEW
POINT
The temperature in which
an air mass must reach in order to be saturated with water vapour (can not
hold any more water vapour) and hence condensation of clouds/dew/frost may
take place.
DRIZZLE
Small rain droplets of <0.5mm diameter. Can be dense and occur in or just
under stratus cloud (including dense fog).
DOWNDRAUGHT
The channel of air within a
convective cloud or thunderstorm in which cool dry or precipitation-bearing
air is falling. This is the result of rising moist-air is that has then
cooled and starts to fall back to Earth in the form of a "Hydrometeor".
These can be dry or precipitation-bearing. An example of this is the heavy
rain falling from convective clouds and thunderstorms. Strong downdrafts
near the cloud base can be dubbed as "Microbursts" which are dangerous to
aircraft with vertical winds of over 80mph.
EDDY
A non-linear motion of a fluid medium, usually
circular, when passing a body of friction or an obstacle or when induced by
shearing or instable flows. Found in turbulence.
FROST
Crystalline ice that
forms on cold objects that are 0°C or below on the Earth's surface (Ground
Frost). Moist
air is required to be close to saturation (dew point) for water vapour to
condensate onto objects and to feed crystal growth. Usually occurs in
winter. Also known as Rime.
FREEZING-RAIN / DRIZZLE / GLAZE
Supercooled liquid droplets (temperature
0°C or less but
remaining a
liquid) which freeze on impact with hard
surfaces as the water molecules find it easier to crystallise onto solid
molecules. Can lead to "ice storms" famous near North America's great lakes.
Black ice is caused primarily by freezing rain or drizzle.
G-C
Ground to Cloud lightning, otherwise known as "Up-flash", "Tower Lightning"
or "Rocket Lightning".
GREENAGE
“Greenage” is a slang name used by storm chasers and meteorologists when
describing the green-tinting colour of a cloud base. It is thought to be
caused by a large presence of hailstones held within the cloud, causing
daylight to refract in a certain way and produce a green/brown colouring.
Chasers take “Greenage” as a sign of a severe thunderstorm in progress which
can produce strong winds and possibly tornadoes, however this effect is not
always linked with Supercells.
GUST
A short but sudden
increase in wind speed in comparison to the mean strength.
HAIL
Frozen balls of ice that are generated and fall from convective cloud (i.e.
showers and thunderstorms). In thunderstorms the process of generation can
be extreme in the strong updrafts and hail can grow very large, up to
gold-ball size in some parts of the world. Hail falls out of the sky when
the rising air fails to support their suspension above the surface any
longer.
HEMISPHERE
Half a sphere. The Earth is considered to be of spherical shape and is
divided into two Hemispheres, the Northern Hemisphere which includes North
America, Europe, Russia, North Africa and Asia, and the Southern Hemisphere
which includes South America, South Africa, Australia and the Antarctic.
HYDROMETER
An instrument made out
of glass for measuring the specific gravity of liquids.
HYGROMETER
An instrument used for
measuring humidity in air. A common form of Hygrometer is known as a “psychrometer”
which uses a thermometer for dry air (Dry-Bulb Temperature Td)
and a thermometer with a dampened bulb (Wet-Bulb Temperature Tw).
The wet-bulb thermometer cools as moisture evaporates into the air, and then
a formula is used with Td and Tw to work out the
Humidity. Psychrometers are usually found in Stevenson Screens.
HYDROMETEOR
A pocket of precipitating water vapour or subsiding condensating air. This
relates to meteorology of all scales from mammatus to Microbursts
and rain-shafts.
I-C
In-Cloud lightning, also
known as "Sheet". This is lightning that occurs from within the cloud mass
out of view. The resulting effect is the cloud lights up like a bulb. If
underneath the cloud it may appear as a sheet of light.
IRIDESCENCE
An optical phenomenon
where reflected or refracted light is modulated by multiple-layered
semi-transparent surfaces. The exiting modulated-light can be seen depending on the angle from which the incident
surface is viewed. This occurs in bubbles, oil spills, and most commonly in
crystalline structures such as gemstones and in
ice-cloud formations near the sun.
KNOTS
A measure of speed widely used in meteorology, aviation and shipping. It is
equivalent to one nautical mile per hour, which is one sixtieth of a degree
(one minute) of Earth-latitude per hour.
1.152 knots = 1 mile per hour
KYRILL (name of a particular European storm)
Kyrill was the name given to an intense low pressure system that evolved
into an unusually violent wind storm in eastern Europe. The storm made
landfall in the UK on the 17th-18th January 2007 and caused some intense
damage across England and Wales as
hurricane-force gusts swept the country felling trees nationwide. The storm
proceeded eastwards to cause damage to Germany and Holland on the 18th-19th
January before proceeding to Poland and the Baltic Sea. Gusts of 130km/h
(80mph) were reported in places.
LIFTED INDEX (LI)
The lifted index is the difference between the temperature of the environmental
air mass at a height of 500mb (typically) and an air parcel's temperature
when it is lifted from the planetary boundary layer (near the surface) to
the same height. So
LI = Environmental Temp - Parcel Temp. This is used to calculate the
magnitude of convective stability in the atmosphere (from the surface). For example, LI = 0C means no
buoyancy at 500mb (statically stable), but LI = +2C means negative buoyancy
at 500mb (if the air parcel makes it to 500mb, it will SINK because it is cooler than the environment), and LI = -2C
means positive buoyancy (if the air parcel makes it to 500mb it will RISE
further because it is warmer than the
environment). The greater the magnitude of difference, the greater the force
upon the air parcel. LI is used in forecasting to calculate the
severity of convective storms.
LIGHTNING
An electrical arc or discharge between two oppositely-charged regions of the
atmosphere. Occurs around Cumulonimbus clouds as a result of a breakdown of
resistance between the two opposing charged regions.
METEOROLOGY
The study of the weather and atmosphere. Often confused with the study of
Meteorites. It isn't right, but not far off. Meteoroids are solid objects
and debris that exist in space, and a Meteor is the visible track of one
when it falls through the atmosphere (i.e. a shooting star). It becomes a
Meteorite if it hits the ground. It all extends from
the Greek word metéōros means "high in
the sky", but this
encompasses precipitation (hydrometeors) which are hydrological masses
moving through the atmosphere such as rain and cloud droplets, which are
more applicable to the study of weather.
MULTICELL
(also see SINGLECELL, CELL, DAUGHTER CELL and SUPERCELL)
Multicells, being the most common thunderstorm type, can take on various
forms and are quite often formed in lines of convergence and/or other
dynamic forcing. There is usually some element of vertical wind shear,
usually linear, which is able to displace the updraught from the downdraught
and sustain development. In the more severe multicells (referred to as
MCS/MCCs, Mesoscale Convective System/Complex) the wind shear is often
non-linear and the storm can regenerate itself in perpetual development (see
DAUGHTER CELL). As a result multicells can be
substantially electrically active over time, sometimes more so than
Supercells, and have a medium lifespan ranging between 1-5
hours depending on intensity and wind shear.
OVERSHOOTING TOP
The heaped cloud formation at the top of a cumulonimbus cloud that has enough
inertia to pierce through the stable
tropopause layer and momentarily overcome the force of static equilibrium
(becomes negatively buoyant in the atmosphere and wants to sink).
They are associated with strong updraughts and can be identified on visible
satellite imagery.
P-F
Positive Flash, an exotic form of C-G that originates from the
upper-portions of a Thundercloud, usually the Anvil. Usually 5 times as long
as a normal C-G and holding more energy, these can be very loud and
destructive and can jump out from ahead of a thunderstorm. Also known as
"Anvil Lightning" (not to be confused with Anvil Crawlers).
RADIOSONDE
A package of instruments
designed for high-altitude conditions, which are elevated by a weather
balloon (usually red in colour to distinguish against clouds) to create a
vertical profile of the immediate atmosphere after the results are radioed
back to the surface. When the balloon reaches the lower Stratosphere the air
pressure causes the balloon to burst and the Radiosonde then falls back to
earth, usually with a “return to” sticker.
SINGLECELL
(also see CELL, MULTICELL and SUPERCELL)
A singular tower of cumulus or cumulonimbus with one main updraught and
downdraught. These often form in conditions with minimal vertical wind
shear, hence the downdraught falls back through the updraught and cuts off
the inflow and kills off development. As a result the lifespan is usually 30
minutes up to an hour, but are still capable of producing lightning and
heavy rain.
SLEET
In the UK, sleet is the word the general public prefer to use for a mixture of
rain and snow. In the US sleet is the name given to Ice-Pellets which are
small translucent balls of ice (different from hail) that fall from medium
level cloud. For these reasons
meteorologists tend not to use sleet to avoid confusion for
international reports, however if you live in the UK forecasters will
often mention sleet.
STEVENSON SCREEN
A white box also known
as an Instrument Shelter, for housing the bare-basic temperature instruments
for weather observation. The screen is designed in a way to minimise
interference to results. It is painted white to reflect the sun’s rays, and
the screen’s grills point downwards at a 45-degree angle to allow airflow
through the shelter but at the same time restricting sunlight and
precipitation entering. Typically they include a minimum and maximum
thermometer, and a dry-bulb and wet-bulb thermometer. Larger screens may
also include a hygrometer (psychrometer), a thermograph and a barometer.
SUPERCELL
(also see SINGLECELL, MULTICELL, CELL and DAUGHTER CELL)
This is the most intense type of
thunderstorm, often smaller than a large Multicell but can grow up to 20km
in diameter. A storm becomes a Supercell if the core updraught becomes
organised into a large single, and sometimes rotating, long-lasting
structure (3-6 hours). They are self-sustained by strong upper level wind
shear advecting the anvil and the resulting downdraught away from the
updraught enabling an un-interrupted inflow. The updraughts can often reach
80-100mph which can pierce the tropopause layer (see "OVERSHOOTING TOP") and
support very large hail development. These storms bring with them torrential
rain, strong lightning, large hailstones, strong winds and most famously
tornadoes, which is why they are dubbed the king of thunderstorms.
SUPERCOOLED
When water droplets are
0°C or below and remain un-frozen. This is common in the middle altitudes of
the troposphere and also where cloud-condensation nuclei (CCN) are likely to
be of saline substance, for example over the sea.
SUPER-SATURATION
When relative humidity somehow exceeds 100% and water vapour is forced to
condensate. This can be associated with supercooled droplet formation.
TEPHIGRAM
(e
THUNDERHEAD
(see ANVIL)
TRANSPIRATION
The evaporation of excess water from objects such as soil, plants, leaves
and flowers. Rain-forests rely heavily on transpiration as the storm clouds
are mainly fuelled by the recycling water within the region. Deforestation
can result in an upset of the water-balance and therefore lead to a decline
in the rainforest.
UPDRAUGHT
A channel of air within a convective cloud or thunderstorm in which warm
moist air is rising caused by buoyancy and instability within the
environmental atmosphere. This is the feeding point of a convective cloud
which may in-turn produce rain and lightning.
WATER CYCLE
A simplified natural cycle a mass of water travels on Earth. Evaporation in
the oceans leads to clouds; clouds rain on the land; the land 1) replenishes
the oceans with the water through rivers and 2) releases some back into the
atmosphere through transpiration.
Text
written by Mark Seltzer © All Rights Reserved.
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