User`s guide
Virtual Weather Station User's Guide
-32- 04/10/07
threshold for one day. One degree day is also the amount
of heat which accumulates when the temperature is 24
degrees above the threshold for one hour.
6.17.1.1 Heating Threshold
The threshold for calculating degree-days heating.
6.17.1.2 Cooling Threshold
The threshold for calculating degree-days cooling.
6.17.2 Wind Run
Wind run is the distance wind has traveled. For example,
the wind run for a constant wind speed of 20 mph for 2
hours is 40 miles, or:
Wind Run = Wind Speed * Time
There may be wind run calculation problems with specific
computers due to the microprocessor and other hardware
devices. If you find the wind run values are out of range,
place the following line in your win.ini file:
wuseDefaultTimer=True
To reset these values, select Settings | Recall Daily,
Monthly and Yearly and edit the windrun for the day,
month and year.
6.17.3 Pressure Trend
Your station may not calculate the pressure trend. Virtual
Weather Station will calculate the pressure trend based on
your own specified threshold. If the rate of change of
pressure is above this threshold, Virtual Weather Station
will report rising pressure. If the rate of change of pressure
is below this threshold, Virtual Weather Station will report
falling pressure.
6.17.4 Wind Gust Calculation
Each weather station uses a different method for calculating
wind gust. You can override your station’s default method
and select the maximum within a specified number of
samples.
6.17.5 Filtered Average Time Constant
The time constant for the filtered average can be varied.
The filter is based on a low pass filter differential equation.
The output value reaches 63% of the input value after one
time constant (the default time constant is 60 minutes or
one hour), and will reach 95% of the input value after five
time constants (default = 300 minutes or five hours).
6.17.6 New Wind Chill Calculation
On November 1, 2001, the National Weather Service
(NWS) implemented a replacement Wind Chill
Temperature (WCT) index for the 2001/2002 winter
season. The reason for the change was to improve upon the
current WCT Index used by the NWS and the
Meteorological Services of Canada (MSC, the Canadian
equivalent of the NWS), which was based on the 1945
Siple and Passel Index.
The new formula made use of advances in science,
technology, and computer modeling to provide a more
accurate, understandable, and useful for calculating the
dangers from winter winds and freezing temperatures. In
addition, clinical trials have been conducted and the results
of those trials have been used to verify and improve the
accuracy of the new formula.
Standardization of the WCT Index among the
meteorological community is important, so that an accurate
and consistent measure is provided and public safety is
ensured. Our goal was to implement the new wind chill
formula in Canada and the United States during the same
time frame in order to have a consistent WCT Index for
North America.
Specifically, the new WCT index will:
9 use calculated wind speed at an average height of
five feet (typical height of an adult human face)
based on readings from the national standard
height of 33 feet (typical height of an
anemometer);
9 be based on a human face model;
9 incorporate modern heat transfer theory (heat loss
from the body to its surroundings, during cold
and breezy/windy days);
9 lower the calm wind threshold to 3 mph;
9
use a consistent standard for skin tissue
resistance; and
9 assume no impact from the sun (i.e. clear night
sky).
Virtual Weather Station allows you to use the old or new
wind chill models. For more information, please reference
the National Weather Service:
http://205.156.54.206/om/windchill/index.shtml
6.17.7 Use Channel 1 As Outdoor Temperature
Some weather stations (example, Texas Weather
Instruments, Columbia Weather Systems, Davis Vantage
Pro2, and Oregon Scientific) exchange Channel 1