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| Weather Terminology |
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A
Anemometer: An anemometer is a device for measuring the velocity or the
pressure of the wind, and is one instrument used in a weather station. The term is derived from the Greek
anemos meaning wind.
Apparent Temperature: The apparent temperature is a measure of relative discomfort due to the
combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological
studies of evaporative skin cooling for various combinations of ambient temperature and humidity. The
apparent temperature equals the actual air temperature when the dew point temperature is 14°C
(57.2°F). At higher dew points, the apparent temperature exceeds the actual temperature and measures
the increased physiological heat stress and discomfort associated with higher than comfortable humidities.
When the dew point is less than 14°C, on the other hand, the apparent temperature is less than the
actual air temperature and measures the reduced stress and increased comfort associated with lower humidities
and greater evaportive skin cooling.
Apparent temperatures greater than 27°C are generally associated with some discomfort. Values approaching
or exceeding 40°C are considered life-threatening, with severe heat exhaustion or heatstroke possible if
exposure is prolonged or physical activity high. The degree of heat stress may vary with age, health and body
characteristics.
Atmospheric Pressure: Atmospheric pressure is the pressure at any point in the Earth's atmosphere. In
most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the
weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location,
whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases
there is less overlying atmospheric mass, so that pressure decreases with increasing elevation.
Mean sea level pressure is the pressure at sea level or (when measured at a given elevation on land) the
station pressure reduced to sea level assuming an isothermal layer at the station temperature. This is the
pressure normally given in weather reports. When barometers in the home are set to match the local weather
reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure.
B
Barometer: A barometer is an instrument used to measure atmospheric
pressure. A barometer is commonly used for weather prediction, as high air pressure is a region indicates fair
weather while low pressure indicates that storms are more likely. Simultaneous barometric readings from across
a network of weather stations allow maps of air pressure to be produced. Isobars drawn on such a map links
sites with the same pressure and give, in effect, a contour map of areas of high and low pressure. Localised
high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low
atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making
it more likely that low pressure will be associated with increased storm activities. If the barometer is
falling then bad weather or some form of precipitation will fall, however if the barometer is rising then
there will be nice weather or no precipitation.
Beaufort Scale: In 1805 Commander, later Admiral Sir Francis Beaufort
published a method of measuring the wind at sea based on what sails a frigate could safely hoist. The
Beaufort Scale, as it came to be known, was adopted by the Royal Navy in 1838 when it became mandatory for
all ship's log entries.
The scale had 13 steps; from force 0, where the wind was calm, to force 12, where the steady wind would be at
least 64 knots or 75 miles per hour. With this scale also came descriptions of the sea. From this standard,
sailors were able to predict how ships would react in certain wind speeds.
It was quite a lot later in the century that the Beaufort Scale was adapted for use on land. In many aspects
it, and the descriptions that go with it, are still used today.
A further set of Beaufort numbers from 13 to force 17 was added by the United States Weather Bureau in 1955
to cope with the exceptional winds that they can get during a hurricane.
|
| Force |
Wind Speed (mph) |
Description |
Effect on Land |
| 0 |
0 |
Calm |
Calm; Smoke rises vertically. |
| 1 |
1 - 3 |
Light Air |
Direction of wind shown by smoke drift, but not by wind vanes. |
| 2 |
4 - 7 |
Light Breeze |
Wind felt on face; leaves rustle; ordinary vanes moved by wind. |
| 3 |
8 - 12 |
Gentle Breeze |
Leaves and small twigs in constant motion; wind extends light flags. |
| 4 |
13 - 18 |
Moderate Breeze |
Raises dust and loose paper; small branches are moved. |
| 5 |
19 - 24 |
Fresh Breeze |
Small trees in leaf begin to sway; crested wavelets form on inland waters. |
| 6 |
25 - 31 |
Strong Breeze |
Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty. |
| 7 |
32 - 38 |
Near Gale |
Whole trees in motion; inconvenience felt when walking against the wind. |
| 8 |
39 - 46 |
Gale |
Breaks twigs off trees; generally impedes progress. |
| 9 |
47 - 54 |
Severe Gale |
Slight structural damage occurs (chimney-pots and slates removed) |
| 10 |
55 - 63 |
Storm |
Seldom experienced inland; trees uprooted; considerable structural damage occurs. |
| 11 |
64 - 72 |
Violent Storm |
Very rarely experienced; large trees uprooted. |
| 12 |
73+ |
Hurricance |
Wide-spread damage.
|
C
Cold Front: The leading edge of an advancing mass of cold air. When it
encounters a mass of less dense warm air, instability results, often triggering heavy rain. Represented on
weather maps by a line bearing triangles along one side.
Condensation: The formation of liquid water from water vapour; occurs when
moist air reaches its dew point and comes into contact with a solid surface or with condensation nuclei.
Conditions Colour: Feature of Weather Display software which outputs a
combination of Colour and Description designed to indicate how the current conditions feel. Colours are
triggered by the following various thresholds:
|
| Extreme Cold |
If the Wind Chill Temperature is less than -18°C |
| Very Cold |
If the Wind Chill Temperature is less than -9°C |
| Uncomfortably Cold |
If the Wind Chill Temperature is less than -1°C |
| Cold |
If the Wind Chill Temperature reaches 5°C |
| Cool |
If the Wind Chill Temperature reaches 12°C |
| Comfortable |
Temperature reaches 17°C |
| Warm |
Temperature reaches 20°C |
| Hot |
Temperature reaches 26°C |
| Uncomfortably Hot: |
Temperature greater than 20°C, Humidex > 38°C |
| Extremely Hot |
Humidex 27°C |
| Extreme Caution |
Humidex 32°C |
| Danger |
Humidex 40°C |
| Extreme Danger |
Humidex 54°C
|
D
Dew Point: All air contains water vapour of varying quantities. The dew
point indicates the amount of moisture in the air. The higher the dew point, the higher the moisture content
of the air at a given temperature. Conversely, the dew point of humid air will be higher than the dew point
of dry air.
Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure
and constant water vapour content) in order to reach saturation. A state of saturation exists when the air is
holding the maximum amount of water vapour possible at the existing temperature and pressure.
Condensation of water vapour begins when the temperature of air is lowered to its dew point and beyond. The
dew point, like other measures of humidity, can be calculated from readings taken by a hygrometer.
The dew point is an important measure used to predict the formation of dew, frost and fog. If the dew point
and the temperature are close together in the late afternoon when the air begins to turn colder then fog is
likely to form during the night.
H
Heat Index: The Heat Index (HI) gives a measure of how hot it actually
feels due to combined effect of the air temperature and the relative humidity. Hot, humid air actually feels
hotter than hot, dry air.
Humidex: The humidex is a measurement first used by Canadian meteorologists
to reflect the combined effect of heat and humidity. It differs from the Heat Index used in the United States
in using dew point rather than relative humidity.
When the temperature is 30°C (86°F) and the dew point is 15°C (59°F), the humidex is 34 (note
that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C).
If the temperature remains at 30°C (86°F) and the dew point rises to 25°C (77°F) the humidex
rises to 41.
The humidex tends to be higher than the Heat Index at equal temperature and relative humidity.
The current formula for determining the humidex was developed by J.M. Masterton and F.A. Richardson of
Canada's Atmospheric Environment Service in 1979.
According to the Meteorological Service of Canada, a humidex of at least 40 causes "great discomfort" and
above 54 is dangerous. When the humidex hits 54 heat stroke is imminent.
Humidity: Humidity is the amount of water vapour in the air. It is
measured in three ways: absolute humidity, relative humidity and specific humidity. Relative humidity is the
most frequently encountered measurement of humidity because it is regularly used in weather forecasts. It is
an important part of weather forecasts because it indicates the likelihood of precipitation, dew or fog.
Higher relative humidity also make it feel hotter outside in the summer because it reduces the effectiveness
of sweating to cool the body by preventing the evaporation of perspiration from the skin. This effect is
calculated in a heat index table. Warmer air has more thermal energy than cooler air, thus more water molecules
can evaporate and stay in the air in a vapour state rather than a liquid state.
Absolute humidity (AH) refers to the mass of water in a particular volume of air. Absolute humidity is
expressed as the number of kilograms of water vapour per cubic meter of air. Imagine a cube of air that is one
meter wide, one meter high, and one meter deep. If we could "squeeze" all the water out of that cube of air
into a container, we could weigh the container and see how many kilograms of water it contains. The amount
of vapour in that cube of air is the absolute humidity of that cubic meter of air.
Relative humidity (RH) is defined as the ratio of the partial pressure of water vapour in a gaseous
mixture of air and water vapour to the saturated vapour pressure of water at a given temperature. That is,
a ratio of how much energy has been used to free water from liquid to vapour from how much energy is left.
Relative humidity is expressed as a percentage.
Specific humidity (SH) is the ratio of water vapour to air (dry air plus water vapour) in a particular
volume of air. Specific humidity is expressed as a ratio of kilograms of water vapour.
O
Occluded Front: An amalgam of two fronts produced when a cold front catches
up with a warm front; usually associated with a low-pressure system.
P
Precipitation: The technical term for water in any liquid or solid form that
is deposited from the atmosphere, and which falls to the ground. It excludes cloud droplets, mist, fog, dew,
frost and rime, as well as virga (trails of rain, snow, or ice crystals that do not reach the ground).
W
Warm Front: The leading edge of an advancing mass of warm air. When it meets
a stationary cold air mass, the warm air rises and cools. Condensation may follow, forming clouds, and usually
producing widespread precipitation. Represented on weather maps by a line bearing hemispheres along on side.
Wet Bulb Temperature: A wet cloth is placed over the bulb of a thermometer
and air blown over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will
cool to a lower temperature than a thermometer with a dry bulb (normal thermometer) at the same time and place.
The depression in wet bulb temperature allows the humidity to be calculated.
If the air is fully saturated (100% relative humidity) the water cannot evaporate, so both the wet bulb and dry
bulb temperatures are the same.
Wind Chill Temperature: Wind chill is the apparent temperature felt on
exposed skin due to the combination of air temperature and wind speed. Except at higher temperatures, where
wind chill is considered less important, the wind chill temperature is always lower than the air temperature.
Any object at a temperature greater than the air around it loses heat to the surrounding air. The greater the
difference between the temperature of the air and the temperature of the object, the greater the rate at which
energy is transferred. Air, however, is a poor conductor; once the air in contact with the object is warmed by
the heat from the object, the rate of transfer slows. When the wind blows, it strips away the warmed air; the
warm object is surrounded by air at nearly-ambient temperature, and the rate of heat loss increases. This is
wind chill. Other factors affect the rate at which animals lose (or gain) energy from the ambient air,
including atmospheric pressure, the relative humidity of the air and perspiration, but these factors are
distinct from wind chill.
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