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Gisteren tijdens de landelijke bijeenkomst van het VWK in De Bilt is er uitleg gegevens over de nieuwe berekeningsmethodiek van de Windchill door het KNMI. Deze is vastgelegd in het document met de naam TR309. Nu heb ik nagekeken op de site van Davis of zij deze berekening ook hanteren voor de Windchill. En aan de hand van de gegevens die ik heb kunnen vinden is dit afhankelijk van de versie van de firmwire in de console en de gebruikte versie van WeatherLink.
Uit de annote28.pdf van Davis blijkft het volgende:
Older versions of software (Versions 5.0 and earlier) and firmware (Vantage firmware revisions before Sept. 7, 2001 and all non-VantagePro products including Echo) are based on the following formula (Siple and Passel, 1945):
0.0817 * (3.71V0.5 + 5.81 – 0.25V) * (T – 91.4) + 91.4
where V is the wind speed in mph and T is the outside air temperature in °F. Wind speeds above 55 mph are set to 55 mph. For wind speeds below 5 mph or temperatures above 91.4°F, the wind chill is set equal to the air temperature.
Newer product revisions (WeatherLink version 5.1 through 5.5.1 and Vantage Pro and Vantage Pro2 consoles with Sept 7, 2001 firmware or later and Vantage Pro2 consoles with firmware before May 2005) are based on the following formula:
35.74 + 0.6215T - 35.75 * (V 0.16 ) + 0.4275T * (V 0.16 )
As with the old formula, any place where the result yields a wind chill temperature greater than the air temperature, the wind chill is set equal to the air temperature. This always occurs at wind speeds of 0 mph or temperatures above 76°F. This also occurs at lower wind speeds with temperatures between 0°F and 76°F.
The new formula takes into account the fact that wind speeds are measured "officially" at 10 meters (33 feet) above the ground, but the human is typically only 5 to 6 feet (2 meters) above the ground. So, anemometers still need to be mounted as high as possible (e.g., rooftop mast) to register comparable wind speed readings and wind chill values.
An even newer version of this formula is available in WeatherLink version 5.6 or later andVantage Pro2 console firmware version and later. This newer version of the formula addresses the fact that the latest National Weather Service (NWS) formula was not designed for use above 40°F. The result of the straight NWS implementation was little or no chilling effect at mild temperatures. This updated version provides for reasonable chilling effect at mild temperatures based on the effects determined by Steadman (1979) (see THSW Index section), but as with the new NWS formula, no upper limit where chilling has no additional effect. The later version Rev A 5/11/06 28 - 3 for the console table only differs in that whole degrees and less resolution in the table are used for code and memory space conservation. As with previous versions of the wind chill formula, any place where the result yields a wind chill temperature greater than the air temperature, the wind chill is set equal to the air temperature. This always occurs at wind speeds of 0 mph or temperatures at or above 93.2°F (34°C). This also occurs at lower wind speeds with temperatures between 0°F (-18°C) and 93.2°F (34°C). As per Steadman (1979), 93.2 F (34°C) is the average temperature of skin at mild temperatures, thus temperatures above this value will actually create an apparent warming effect (see THSW Index section).
The Vantage Pro and Vantage Pro2 console uses the "10-minute average wind speed" to determine wind chill, which is updated once per minute. When 10-minute of wind speed data is unavailable, it uses a running average until 10-minutes worth of data is collected. The WeatherLink® software uses the 10-minute average wind speed also. If it is unavailable, it uses the current wind speed (which updates every 2.5 to 3 seconds). All other products use the current wind speed to determine wind chill.
The reason an average wind speed is employed in the Vantage Pro and Vantage Pro2 to calculate wind chill is as follows: The human body has a high heat capacity, thus high wind speeds have no effect on the body's thermal equilibrium. So, an average wind speed provides a more accurate representation of the body's response than an instantaneous reading. Also, "official" weather reports (from which wind chill is calculated) provide average wind speed, so using an average wind speed more closely matches the results that are seen in weather reports.
Dit betekent dat VP gebruikers, als zij WeatherLink regelmatig hebben geupdated, geen aanpassingen hoeven te doen.
Mocht iemand het anders lezen dan hoor of zie ik dat graag.
Met vriendelijke groet,
G. Molema - Meteo Delfzijl
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