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The weight of the air that makes up our atmosphere
exerts a pressure on the surface of the earth. This
pressure is known as atmospheric pressure. Generally, the
more air above an area, the higher the atmospheric
pressure. This, in turn, means that atmospheric pressure
changes with altitude. For example, atmospheric pressure
is greater at sea-level than on a mountaintop. To
compensate for this difference in pressure at different
elevations, and to facilitate comparison between
locations with different altitudes, meteorologists adjust
atmospheric pressure so that it reflects what the
pressure would be if measured at sea-level. This adjusted
pressure is known as barometric pressure.
Barometric pressure changes with local weather
conditions, making barometric pressure an important and
useful weather forecasting tool. High pressure zones are
generally associated with fair weather, while low
pressure zones are generally associated with poor
weather. For forecasting purposes, the absolute
barometric pressure value is generally less important
than the change in barometric pressure. In general,
rising pressure indicates improving weather conditions,
while falling pressure indicates deteriorating weather
conditions.
Source: Davis Instruments
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Dew-point is the temperature to which air must be
cooled for saturation (100% relative humidity) to occur,
providing there is no change in water content. The
dew-point is an important measurement used to predict the
formation of dew, frost, and fog. If dew-point and
temperature are close together in the late afternoon when
the air begins to turn colder, fog is likely during the
night. Dew-point is also a good indicator of the
air’s actual water vapor content, unlike relative
humidity, which takes the air’s temperature into
account. High dew-point indicates high vapor content; low
dew-point indicates low vapor content. In addition a high
dew-point indicates a better chance of rain and severe
thunderstorms. You can even use dew-point to predict the
minimum overnight temperature. Provided no new fronts are
expected overnight and the afternoon Relative Humidity
50%, the afternoon’s dew-point gives you an idea of
what minimum temperature to expect overnight, since the
air is not likely to get colder than the dew-point
anytime during the night.
Source: Davis Instruments
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Because temperature plays an important part in the
rate of development of plants and many diseases and pests
(especially insects), a measurement including the
accumulation of heat with passing time is necessary to
predict maturation. Growing degree-days provide a measure
for calculating the effect of temperature on the
development of plants and pests. One growing degree-day
is the amount of heat that accumulates when the
temperature remains one degree above the base
developmental threshold for 24 hours. One growing
degree–day is also the amount of heat that
accumulates when the temperature remains 24º above the
base threshold for 1 hour. (What about Heating and
Cooling Degree-days?) Note that there are no negative
degree-days. If the temperature remains below the
threshold, there is no degree-day accumulation.
Unlike strict time predictions of plant or pest
development, growing degree-day predictions hold true
regardless of location or temperature fluctuations. As
long as you know the number of degree-days necessary for
plant/pest development, you may use degree-days as an
accurate predictor. For example, you may know that it
takes, in general, three weeks for a specific pest to
develop. What you will find, however, is that the pest
may take 4 weeks to develop in cooler weather and only 2
weeks to develop in warmer weather. The time prediction
can be off by up to a week by looking at time alone,
while the degree–day prediction should result in far
greater accuracy.
Source: Davis Instruments
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Although degree-days are most commonly used in
agriculture, they are also useful in building design and
construction, and in fuel use evaluation. The
construction industry uses heating degree-days to
calculate the amount of heat necessary to keep a
building, be it a house or a skyscraper, comfortable for
occupation. Likewise, cooling degree-days are used to
estimate the amount of heat that must be removed (through
air-conditioning) to keep a structure comfortable. Just
like growing degree-days, heating and cooling degree-days
are based on departures from a base temperature. 65º F
is almost always used as this base. It is assumed for
heating load calculations that the occupants, lighting,
equipment, appliances, cooking, bathing and other
activities will raise the temperature from 65º to 68º.
One heating degree–day is the amount of heat
required to keep a structure at 65ºF when the outside
temperature remains one degree below the 65ºF threshold
for 24 hours. One heating degree–day is also the
amount of heat required to keep that structure at 65ºF
when the temperature remains 24ºF below that 65º
threshold for 1 hour.
Likewise, one cooling degree–day is the amount of
cooling required to keep a structure at 65ºF when the
outside temperature remains one degree above the 65ºF
threshold for 24 hours. One cooling degree–day is
also the amount of cooling required to keep that
structure at 65ºF when the temperature remains 24ºF
above that 65º threshold for 1 hour.
Depending on the calculation method, both heating and
cooling degree-days can accumulate in the same day. Also,
note that there are no negative degree-days. If the
temperature remains below the threshold, there is no
degree-day accumulation.
Heating and Cooling degree-days may be calculated by
either the High/Low method or the Integration method.
High / Low method:
In the high/low method, the software uses the highest
temperature and the lowest temperature for a given day to
calculate the average temperature for that day. The
difference between the average temperature and the base
threshold are assumed to be the number of degree-days
accumulated on that day. For example, if the average of
the highest and lowest temperatures is 24º below the
base threshold, the software assumes 24 heating
degree–days for the entire day.
Integration method:
In the integration method, the software calculates
degree–days using the average temperature for an
interval and the interval time. For example, if the
average temperature during a 15 minute interval was 24º
below the base threshold, the software would calculate
0.25 heating degree-days during that interval (24 * 15
minutes in interval/1440 minutes per day). The number of
degree-days during each interval are added together to
arrive at a degree-day total. This method calculates
degree-day totals more accurately than the high/low
method.
The West Juneau Weather Station uses the Integration
method; except that for data prior to November 2002, the
High/Low method is used.
Below are some representative heating and cooling
degree-day totals from different parts of the United
States.
- Barrow, Alaska: Heating degree days 20,370;
Cooling degree days 0
- Juneau, Alaska (West Juneau): Heating Degree Days
8133; Cooling Degree Days 38 in Calendar Year
2003
- Kansas City, Mo.: Heating degree days 5,326;
Cooling degree days 1,388
- Bismarck, N.D.: Heating degree days 8,932;
Cooling degree days 499
- Key West, Fla.: Heating degree days 68; Cooling
degree days 4,820
- Hilo, Hawaii: Heating degree days 0; Cooling
degree days 3,134
- Yuma, Ariz.: Heating degree days 983; Cooling
degree days 4,244
Table data source: Williams, Jack. 1995. The USA TODAY
Weather Almanac and the West Juneau Weather Station.
Source: Davis Instruments with modifications and
additions by David Kent
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The Heat Index uses the temperature and the relative
humidity to determine how hot the air actually
"feels." When humidity is low, the apparent
temperature will be lower than the air temperature, since
perspiration evaporates rapidly to cool the body.
However, when humidity is high (i.e., the air is
saturated with water vapor) the apparent temperature
"feels" higher than the actual air temperature,
because perspiration evaporates more slowly.
Note: WeatherLink uses the Steadman (1979 & 1998)
formula to calculate Heat Index, which is more accurate
than the method used by the Vantage Pro console and is
calculated for all temperatures.
Source: Davis Instruments
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The High Rain Rate is the amount of rain that would
fall in one hour if the rain rate as calculated below
continued for the full hour.
The rain rate is calculated by measuring the time
interval between each rainfall increment. When there is
rainfall within the archive period, the highest measured
value is reported. When no rainfall occurs, the rain rate
will slowly decay based on the elapse time since the last
measured rainfall.
The West Juneau Weather Station uses a 5 minute archive
period.
Source: Davis Instruments with additions and
modifications by David Kent
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Storm rain displays the rain total of the last rain
event. The Davis Weather Instrument takes two
clicks (.02") to begin a rain event and 24 HOURS
WITHOUT RAIN to end a rain event.
Source: Davis Instruments
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Wind run is measurement of the "amount" of
wind passing the station during a given period of time,
expressed in either "miles of wind" or
"kilometers of wind". WeatherLink calculates
wind run by multiplying the average wind speed for each
archive record by the archive interval.
For Example:
Average Wind Speed = 5 mph
Archive Interval = 30 minutes (0.5 hours)
Wind Run = 5 mph x 0.5 hours = 2.5 miles of wind
The West Juneau Weather Station uses a 5 minute archive
interval and the 5 minute archive records are totaled for
the 24 hour period from midnight to midnight; and
continuously totaled for the graphs.
Source: Davis Instruments with additions by David Kent
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