Weather Measure
Finding True North
Hints about orienting a wind vane to TRUE NORTH for accurate wind direction measurementsNo matter how precise your electronic wind vane is, if the zero point of the wind vane's base is not oriented to north the readings will be in error.
There are some easy ways to find north. Once it gets dark look up into the sky and find the north star. When you have done that you are looking in the direction of true north.
Another aid can be our streets and roads. Many city streets are laid out in a north-south grid pattern. Just line the zero point of the anemometer so it points parallel to the north-south running streets.
The use of a compass to locate true north requires a little extra work. Magnet compass needles point to the magnetic north pole which is not at the same place as the geographic north pole. A correction must be applied to convert the magnetic north direction to the true north direction. The magnitude of the correction depends on where you are located. The following URL will take you to a U.S. government site that takes you through some very simple steps to convert magnetic compass readings to true direction readings for your location:
http://www.ngdc.noaa.gov/seg/geomag/jsp/IGRF.jspOnce you have successfully aligned the anemometer to true north, your wind direction readings will report true wind directions within the limit of the errors of the anemometer.
Labels: technical support, wind
Why is it so windy in Reno?
Here is an interesting illustration from the Reno Gazette-Journal that explains why very high winds frequently blow in the valleys just to the east of the Sierra Nevada. The predominant wind blows from west to east over the Sierra Nevada and high winds aloft, associated with storms, spill down the lee side of the mountain range following the terrain. These winds of 50 mph or greater spill out onto the valley floors. Winds in the Washoe Valley south of Reno are frequently of sufficient strength to over turn large semi trucks and trailers. The drawing shows how this effect works. This kind of down slope gusting is not unique to the Sierra Nevada. Those of you who live along the Front Range of the Rocky Mountains are familiar with the same effect.
Labels: wind
Wind Direction Potentiometer Configuration
360° Format: For years a 360° potentiometer has been the standard for measuring wind direction. Because the resistance element of the potentiometer is roughly 358°, there is a "dead area" or "slot" of approximately 2°. This opening in the pot is customarily oriented to the north. The 360° format is extremely well suited to applications where a display indicator is used, either alone or with analog recording instrumentation. Its disadvantage is that when the wind is from the north readings will vary between maximum to minimum output as the wiper in the pot goes back and forth across the slot, thereby causing a pen trace across the full width of an E-A recorder or presenting numerous other complications for computer data logging.
540° Format: In recent years, a new configuration has been used quite successfully. Two potentiometers are joined together with wipers in the pots 180° from each other, thus providing a 540° uninterrupted display. This convention of dual potentiometers has greatly minimized the problems encountered in 360° displays. The signal conditioning provides automatic switching between the two pots.
Labels: technical support, wind
What Is a Wind Rose?
The wind rose is the time honored method of graphically presenting the wind conditions, direction and speed, over a period of time at a specific location. To create a wind rose, average wind direction and wind speed values are logged at a site, at short intervals, over a period of time, e.g. 1 week, 1 month, or longer. The collected wind data is then sorted by wind direction so that the percentage of time that the wind was blowing from each direction can be determined. Typically the wind direction data is sorted into twelve equal arc segments, 30° each segment, in preparation for plotting a circular graph in which the radius of each of the twelve segments represents the percentage of time that the wind blew from each of the twelve 30° direction segments. Wind speed data can be superimposed on each direction segment to indicate, for example, the average wind speed when the wind was blowing from that segment's direction and the maximum wind speed during the logging period. A good example of a wind rose application is shown at the following URL
http://www.wcc.nrcs.usda.gov/climate/windrose.html. Another
site that has some interesting wind rose information and a simple program for generating a wind rose is
http://www.windpower.org/en/tour/wres/rose.htm.
The information provided by the wind rose can be applied to many and varied situations. Sailors use wind rose information taken from the "Pilot Charts" by the U.S. Naval Oceanographic Office to get average winds for each ocean for each month of the year to help the create optimal sailing routes between ports. Architects do, or should, use wind rose information for the siting of buildings and stadiums. If wind rose data had been collected and used prior to the construction of San Francisco's famous windy Candlestick Park, the stadium could have been placed a few hundred yards to the north where it would have been protected from the prevailing westerly wind. Windpower "farms" do extensive wind rose type studies prior to erecting their wind turbines. Thus the wind rose is a simple information display technique that has a multitude of uses.
More wind rose linksLabels: wind
Apparent Temperature
Does it feel colder outside than the thermometer says it should? Maybe it's the
wind chill effect. But on a hot summer day, you'd better check the
heat index before you head outdoors.
Labels: relative humidity, temperature, wind
