Glossary of Terms Used on The Buxton Weather Site, and
sometimes in Forecasts and Weather Reports:Term Definition
DENSITY The ratio of the mass of a substance to the volume it occupies. In
oceanography, it is
equivalent to specific gravity and represents the ratio of the weight of a given
volume of sea water to that of an equal volume of distilled water at 4.0 degrees
Celsius or 39.2 degrees Fahrenheit.
PRESSURE See barometric pressure
pressure exerted by the atmosphere as a consequence of gravitational attraction
exerted upon the "column" of air lying directly above the point in
question. The measurement is expressed in millibars. Also known as atmospheric
BEAUFORT WIND SCALE A
system of estimating and reporting wind speeds. It is based on the Beaufort
Force or Number, which is composed of the wind speed, a descriptive term, and
the visible effects upon land objects and/or sea surfaces. The scale was devised
by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.
ERROR The inaccuracy that the manufacturer permits when the unit is
calibrated in the factory.
BASE For a given cloud or cloud layer. The lowest level in the atmosphere at
which the air contains
a perceptible quantity of cloud particles. The weather station estimates the
cloud base on temperature and humidity readings, using the following
Base (ft) = 250(Temperature - Dew Point)
DEGREE DAY A cooling degree day is given for each degree that the daily mean
temperature departs above
the baseline of 75 degrees a given temperature It is used to estimate the energy
requirements, and is an indication of fuel consumption for air conditioning or
refrigeration. Refer to degree day or heating degree day.
POINT The temperature to which a sample of air must be cooled, while the
mixing ratio and barometric
pressure remain constant, in order to attain saturation by water vapor. When
this temperature is below O°C, it is sometimes called the frost point.
These were virtually unknown until 2013 when a major ice storm and a numbing
deep freeze struck eastern Canada in December 2013.
Residents were alarmed to hear deep bangs in the middle of the night. Emergency
phone lines were swamped as people panicked at the prospect of earthquakes or
mystery bomb blasts.
In fact, the bangs were caused by frostquakes (also known as cryoseisms) — when
water freezes deep underground and turns into ice that expands so powerfully
that it forces huge cracks in subterranean rocks.
They are caused when heavy rain is followed by air temperatures plunging far
Frostquakes do little real damage, bar putting a few cracks in people’s
The rapid appearance of perishingly cold winter storms can in turn cause a new
traffic peril called flash-freezing.
A flash freeze will occur when sub-zero wind blasts over a wet road and
instantly turns the water to ice. It can be lethally unexpected, because it
doesn’t have to be snowing, sleeting or raining for the road surface to turn
into a rink.
That wind can also be cold enough to freeze your car doors shut.
Flash freezing is a cousin of deadly black ice, which forms most often when it’s
raining and air temperatures are 32f (0c) or below.
This is cold enough to freeze the rain on impact. It’s called black ice because
it is so transparently thin that it can look just like ordinary road tarmac.
A sudden significant increase in or rapid fluctuations of wind speed. Peak wind
must reach at
least 16 knots (18 miles per hour) and the variation between peaks and lulls is
at least 10 knots (11.5 miles per hour). The duration is usually less twenty
INDEX The combination of air temperature and humidity that gives a
description of how the temperature
feels. This is not the actual air temperature.
HEATING DEGREE DAY One
heating degree day is given for each degree that the daily mean temperature is
below 65 degrees a given temperature. It is used as an indication of fuel
consumption. Refer to degree day or cooling degree day.
amount of water vapor in the air. It is often confused with relative humidity or
dew point. Types of humidity include absolute humidity, relative humidity, and
The location north or south in reference to the equator, which is designated at
zero (0) degrees.
Parallel lines that circle the globe both north and south of the equator. The
poles are at 90 degrees North and South latitude.
The location east or west in reference to the Prime Meridian, which is
designated as zero (0)
degrees longitude. The distance between lines of longitude are greater at the
equator and smaller at the higher latitudes, intersecting at the earth's North
and South Poles. Time zones are correlated to longitude. See Greenwich Mean
PHASE The moon phase is caused by sun rays reflecting off the moon's surface
while it moves around
the earth. The sun illuminates half of the moon at any time while the moon
orbits around the earth. The variation in the angle made by the earth-moon line
with respect to the earth-sun line causes changing phase of the moon. The moon
completes one revolution around the earth in 27.322 days with respect to the
background stars. This is called the SIDERIAL period of the moon. During this
same time the earth moves about 27 degrees along its orbit around the sun. As a
result, the moon takes about two extra days to complete the cycle with respect
to the sun-earth line. This longer cycle of the moon that takes about 29.57 days
is called SYNDONIC period of the moon. The longer cycle is considered as Lunar
Until recently, you’d have had to search through an 1853 almanac called the
Living Age to find any mention of the phenomenon of a polar vortex. But in
January 2014, this once-obscure weather event escaped from its natural home in
the North Pole to wreak destruction much further south in the U.S.
Now it’s back and being blamed for the deep freeze engulfing much of America’s
Mid-west, killing at least 12 people and inflicting temperatures colder than
So what is it? The vortex is a wind current that sits like a spinning ‘hat’
above the North Pole.
Normally, the Jet Stream — a narrow band of fast flowing air at high altitude
that marks the boundary between cold polar air and warmer air to the south —
traps the vortex above the pole.
But in recent years, the Jet Stream current has weakened, allowing freezing
polar vortex winds to blast southwards.
In early January, meteorologists warned that another factor was coming into play
— a sudden warming of air in the stratosphere above the North Pole.
They predicted, apparently correctly, that this warm air would further weaken
the integrity of the polar vortex and cause it to split off into smaller ‘sister
vortices’ that could travel south far beyond their typical range — as has indeed
ALTITUDE Atmospheric or barometric pressure expressed in terms of altitude
which corresponds to that
pressure in the standard atmosphere.
OF CHANGE The derivative or change in a parameters value with respect to
time. The station
calculates the rate of change by calculating the derivative of a parameter, and
then filtering it over one hour. Thus, the rate of change equation factors all
of the measurements taken in the last hour, and may not exactly match the change
in one hour.
HUMIDITY A type of humidity that considers the ratio of the actual vapor
pressure of the air to the saturation
vapor pressure. It is expressed in percentage.
LEVEL PRESSURE The atmospheric pressure at mean sea level either directly
measured by stat ions at sea level
or empirically determined from the station pressure and temperature by stations
not at sea level. Used as a common reference for analyses of surface pressure
These are caused by wild winds spinning over surfaces that are covered in snow
and are also known as snow devils, ice devils or willy-willys.
They form when a mass of cold air passes over a warmer surface, heated by
Wind shear — a variation in wind velocity at right angles to the direction of
the wind — then causes the rising air to spin. In January 2017, a 100ft-high
snownado was filmed by a holidaymaker near Aviemore in the Scottish Highlands.
‘Beautiful but scary’ was how Lynn Godfrey described the spiralling pillar of
snow and ice as it engulfed nearby skiers and walkers.
What about Wind Chill?
Unless you were an Arctic explorer, once all you needed to know about the wind
was how hard it would be blowing.
Now, when cold weather strikes, wind chill is a crucial factor, because while
the thermometer might read minus 2c (28f), wind chill makes it feel like minus
Cold air blowing past our bodies takes heat from the skin and reduces our core
The stronger the wind, the faster is the cooling and the lower the temperature,
the more impact the wind has.
In freezing weather, wind chills can quite rapidly lead to hypothermia.
Snizzle or Frizzle
You may not yet have come across these words but you soon will, according to
etymologists at Rice University in Texas, who note they are increasingly popular
with weather forecasters.
Snizzle describes a mixture of snow and drizzle. Frizzle is freezing drizzle —
although one could argue until the cows come shivering home about whether a
downpour of sleet is actually either snizzle or frizzle.
Our changing climate is not only creating wild new weather it is also vastly
increasing the chances of one event colliding with another.
Scientists use the term Frankenstorm to describe such monster mixes of lethal
It was first employed by the U.S. National Weather Service’s Hydrometeorological
Prediction Centre in October 2012, when meteorologists realised that Hurricane
Sandy was roaring in from the Atlantic to merge with an inland snowstorm to
create a superstorm.
The result was the deadliest and most destructive hurricane seen that year,
responsible for killing more than 70 people across eight U.S. states.
The daily appearance of the sun on the eastern horizon as a result of the
earth's rotation. In the
United States, it is considered as that instant when the upper edge of the sun
appears on the sea level horizon. In Great Britain, the center of the sun's disk
is used instead. Time of sunrise is calculated for mean sea level. See sunset
The daily disappearance of the sun below the western horizon as a result of the
In the United States, it is considered as that instant when the upper edge of
the sun just disappears below the sea level horizon. In Great Britain, the
center of the sun's disk is used instead. Time of sunset is calculated for mean
sea level. See sunrise for comparison.
HOURS - Daily and Cumulative Are collected by a digital sun duration
sensor which is equivalent to a Campbell Stokes recorder. It has a resolution
to 0.1 hours daily. As records build Monthly and Annual hours will be
A once-rare occurrence, this is a type of thunderstorm in which snow or hail
falls instead of rain.
This is because temperatures at the centre of the storm clouds have plummeted to
freezing and below. Often, people are unaware they are in the middle of
Snow dampens sound, so you have to be almost right beneath a lightning strike to
hear the following thunderclap.
Britain suffered its first recorded snowbomb, also known as ‘bomb-ogenesis’, in
The term describes a dramatically rapid plunge in atmospheric pressure that
sends barometers plunging down by at least 24 millibars within 24 hours.
It’s been around since the 1940s but was taken up by forecasters in 2014 to
describe a fast-forming catastrophic snowstorm that was heading rapidly towards
the East Coast of America.
Our own British snowbomb caused heavy blizzards and transport chaos. Up to 12
inches of snow fell in some places.
Again, the Jet Stream was implicated. ‘Wobbles’ in its flow had prompted that
sudden fall in atmospheric pressure over the Atlantic. This in turn sucked in
Arctic air from Scandinavia and the East — to create a whirlpool of vicious
winds and stormy weather that swept eastwards into Britain.
The Beast from the East — the cold blast which hit Britain in February 2018 and
in a weaker form in March — had its origins in similar but less dramatic
atmospheric falls over a longer period of time.VAPOR PRESSURE The pressure exerted by water vapor molecules in a given volume
TEMPERATURE Virtual temperature is a fictitious temperature that takes
into account moisture in the air. The
formal definition of virtual temperature is the temperature that dry air would
have if its pressure and specific volume were equal to those of a given sample
of moist air. Virtual temperature allows meteorologists to use the equation of
state for dry air even though moisture is present.
WIND CHILL INDEX The
calculation of temperature that takes into consideration the effects of wind and
temperature on the human body. Describes the average loss of body heat and how
the temperature feels. This is not the actual air temperature.
DIRECTION The direction from which the wind is blowing. For example, an
easterly wind is blowing from
the east, not toward the east. It is reported with reference to true north, or
360 degrees on the compass, and expressed to the nearest 10 degrees, or to one
of the 16 points of the compass (N, NE, etc.).
RUN The distance the Wind has travelled. i.e. the Wind Run for a constant
wind speed 20mph for 2 hours is 40 miles. Wind Run = Wind Speed X
SPEED The rate of the motion of the air on a unit of time. It can be
measured in a number of ways. In
observing, it is measured in knots, or nautical miles per hour. The unit most
often used in the UK & United States is miles per hour.
Beaufort scale is a system of recording wind velocity (speed) devised in 1806 by
Francis Beaufort (1774–1857). It is a numerical
scale ranging from 0 for calm to 12 for a hurricane as follows:r
calm smoke rises vertically; water smooth 0–2 0–1
light air smoke shows wind direction; water ruffled 2–5 1–3
light breeze leaves rustle; wind felt on face 6–11 4–7
gentle breeze loose paper blows around 12–19 8–12
moderate breeze branches sway 20–29 13–18
fresh breeze small trees sway, leaves blown off 30–39 19–24
strong breeze whistling in telephone wires; sea spray from waves 40–50 25–31
near gale large trees sway 51–61 32–38
gale twigs break from trees 62–74 39–46
strong gale branches break from trees 75–87 47–54
storm trees uprooted; weak buildings collapse 88–101 55–63
violent storm widespread damage 102–117 64–73
hurricane widespread structural damage above 118 above 74
Heat Stress Index calculates the body's reaction to heat and humidity, or the
Heat Index. High temperatures and humidity stress
the body's ability to cool itself, and heat illness becomes a special concern
during hot weather .Do not base important decisions on this heat stress
index! Everybody's reaction to heat is different. :Scale
Index <26 °C
< Heat Index < 32 °C
Caution 32 °C
< Heat Index < 40 °C
< Heat Index > 55 °C
Danger Heat Index > 55 °C
comfort index calculates the body's reaction to heat, cold, humidity and wind
chill. High temperatures and humidity stress the body's ability to cool itself,
and low temperature can stress the body's ability to heat itself. Do not base
important decisions on this comfort index! Everybody's reaction to heat and cold
Cold Wind chill < -20 °C
Cold -20 °C
< Wind chill < 0 °C
< Wind Chill < 16 °C
< Temperature < 27 °C
< Temperature < 32 °C
Hot Temperature > 32 °C
and Heat Index < 38 °C
Hot Heat Index > 38 °C