Barometric Pressure
The weight of the air in the atmosphere exerts a pressure on the
surface of the earth known as atmospheric pressure . Since the
atmospheric pressure varies with altitude its equivalent value at sea level is
calculated, called barometric pressure. Barometric Pressure is the
single most important reading used to forecast the weather. Low pressure zones
(known as cyclones or depressions) usually bring clouds,
precipitation, high winds and unsettled weather. In high pressure zones (known
as anticyclones) winds are lighter and the skies are clearer.
Anticyclones usually bring fine weather in the summer, cold weather in winter
and cold nights but quickly warming up during the day in spring and autumn. The
pressure reading alone is generally not as good an indicator as its recent
trend: Rising pressure usually means improving weather while falling pressure
is associated with deteriorating conditions.
Heat Index
The Heat Index uses the temperature and humidity to determine how
hot the air actually "feels". If humidity is low, the apparent temperature will
be lower than the air temperature because perspiration evaporates rapidly to
cool the body. But when humidity is high it feels hotter because perspiration
evaporates more slowly. The Heat Index is the equivalent temperature in the
shade based on normal humidity levels that would have the same heating effect
on the human body. For example, if the air temperature is 30° C with 70%
relative humidity, the effect of these conditions on the body is the same as
35° C with normal humidity (about 30%). Below 14° C, humidity has a negligible
effect on apparent temperature and so the Heat Index will be the same as the
air temperature.
Rain Rate
The Vantage Pro has a rain collector bucket that tips up and
empties its contents whenever 0.254mm of rain has fallen into it. The rate of
rainfall is calculated by measuring the amount of time between each tip of the
bucket. The Rain Rate figure displayed in the Current Data tables and Weekly
Data graphs is actually the maximum rate attained in the period between
readings (currently 15 minutes). This is to ensure that if a short burst of
rain were to fall in this period it would not go unrecorded.
Relative Humidity
The amount of water vapour that the air can contain
varies with air temperature and pressure. Relative Humidity takes these factors
into account and offers a reading that reflects the amount of water vapour in
the air as a percentage of the amount the air is capable of holding. Note that
where the word "humidity" is used on the data pages of this web site it is
actually referring to the Relative Humidity.
Wind Chill
Wind Chill takes into account how the speed of the
wind affects our perception of temperature. Our bodies warm the surrounding air
molecules by transferring heat from the skin. If there is no air movement, this
insulating layer of warm air molecules stays next to the body and offers some
protection from cooler air molecules. But if there is a wind, the warm air
molecules are swept away from the body. The faster the wind blows, the faster
heat is carried away and the colder it feels. Above 33o C, wind has
a negligible effect on apparent temperature so the Wind Chill will be the same
as the outside temperature. Note: Wind Chill is calculated using the average
wind speed in the past 10 minutes.
Sunlight Times
Astronomical Dawn - The
time in the morning when the sun is 18 degrees below the horizon. At this time
the sun starts to lighten the sky. Prior to this time the sky is completely
dark.
Nautical Dawn - The time in the morning when the
sun is 12 degrees below the horizon. At this time there is just enough light
for objects to be distinguishable.
Civil Dawn - The time in the morning when the sun is 6
degrees below the horizon. At this time there is just enough light for objects
to be distinguishable and for outdoor activities to commence.
Sunrise -
The time in the morning when the first part of the sun
appears above the horizon.
Sunset -
The time in the evening when last part of the sun disappears below the horizon.
Civil Dusk - The time in the
evening when the sun is 6 degrees below the horizon. At this time objects are
distinguishable but there is no longer enough light to perform outdoor
activities.
Nautical Dusk - The time in the evening when the
sun is 12 degrees below the horizon. At this time objects are no longer
distinguishable.
Astronomical Dusk - The time in the evening when
the sun is 18 degrees below the horizon. At this time the sun no longer
illuminates the sky.
Moon Phases
New Moon - The Moon's un-illuminated side is facing the
Earth. The Moon is not visible (except during a solar eclipse).
Waxing Crescent - The Moon's disc is partly but less than
half illuminated. The fraction of the Moon that is illuminated is increasing.
First Quarter - Half of the Moon's disc is illuminated. The
fraction of the Moon that is illuminated is increasing.
Waxing Gibbous - More than half but not all of the Moon's
disc is illuminated. The fraction of the Moon that is illuminated is
increasing.
Full Moon - The Moon's illuminated side is facing the Earth.
The Moon is completely visible.
Waning Gibbous - More than half but not all of the Moon's
disc is illuminated. The fraction of the Moon that is illuminated is
decreasing.
Last Quarter - Half of the Moon's disc is illuminated. The
fraction of the Moon that is illuminated is decreasing.
Waning Crescent - The Moon's disc is partly but less than
half illuminated. The fraction of the Moon's disk that is illuminated is
decreasing.
Links
The UK Met Office web site can be found at
http://www.meto.gov.uk/.
The Davis Instruments web site can be found at
http://www.davisnet.com/home_flash.asp.
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