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The above link takes you to an ftp server
that houses the wind rose plot images. The directory is
organized by State and Climate Station name. Within each
Climate Station directory, there are images in both .emf and .gif
formats for each of the twelve months. The .emf images can be
viewed using Internet Explorer 6.x or by using the "insert picture"
option in MS-Word. The .emf images are larger than the .gif
images as shown in the example below, and show more detail.
Note: Wind speeds shown are in meters per second. To convert meters
per second into miles per hour just multiply by 2.237. Thus, a 5
m/sec wind is an 11.19 mph wind, and a 10 m/sec wind converts to
22.37 mph.
Additional note: Anemometer heights were not adjusted to a common
height in the SAMSON database (see below). For user's information, a
file containing the anemometer height history at each of these
stations is available
HERE.
Two files are shown: one in meters, and the other in feet. Only data
from 1961 to 1990 were used in SAMSON, and in the development of
these wind roses.
A wind rose gives a very succinct but information-laden
view of how wind speed and direction are typically distributed at a
particular location. Presented in a circular format, the wind rose
shows the frequency of winds blowing FROM particular directions. The
length of each "spoke" around the circle is related to the frequency
of time that the wind blows from a particular direction. Each
concentric circle represents a different frequency, emanating from
zero at the center to increasing frequencies at the outer circles.
The wind roses shown here contain additional information, in that
each spoke is broken down into discrete frequency categories that
show the percentage of time that winds blow from a particular
direction and at certain speed ranges. All wind roses shown
here use 16 cardinal directions, such as north (N), NNE, NE, etc.
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An example is shown here. It is the April wind
rose for Fresno, California, based on 30 years of hourly wind
data (all hours of the day). This rose shows that the winds at
Fresno in April blow from the northwest much of the time. In
fact, the 3 spokes around the northwest direction (WNW, W and
NNW) comprise 50% of all hourly wind directions. This is quickly
calculated by taking the sum of the frequencies of each of these
directions (16+25+9=50%). This also shows that the wind rarely
blows from the northeast or the southwest. These wind roses also
provide details on speeds from different directions. Examining
winds from the northwest (the longest spoke) one can determine
that approximately 8% of the time in April at Fresno the wind
blows from the northwest at speeds between 1.8 and 3.34 meters
per second. Similarly, on this spoke it can be calculated that
winds blow from the northwest at speeds between 3.34 and 5.4
m/sec about 10% of the time (18% - 8%), at speeds between 5.4
and 8.49 m/sec about 6% of the time (24-18), between 8.49 and
11.06 m/sec about 1% of the time (25-24), and less than 0.5% of
the time at speeds greater than 11.06 m/sec. Please note the
legend at the bottom of the wind rose that gives the speed
categories and their associated colors. |
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Select the above example for full GIF Image |
The legend at the bottom gives additional information such as the
unit (m/sec), the average wind speed for the month over all hours
(in this case 3.61 m/sec), and percentage of time that the winds are
calm (7.53%), and the years and month and hours of data on which
each rose was constructed. Note: Even though it says 1961 as the
year, these data are for 30 years (1961-1990). The software
only prints the beginning year. All hours of the day (24
readings per day) are used to construct these wind roses. All wind
roses available here are for the period 1961-1990.
The software used to generate these high-quality wind roses is
courtesy of Lakes Environmental Software and is called
WR-PLOT.
To calculate the typical amount of time that the wind blows from
a particular direction and certain speeds just multiply the
respective frequency by the appropriate amount of time. In our
example with Fresno in April, there are 30 days x 24 hours/day in
April, or 720 hours. From the wind rose we calculated that winds
blow from the northwest at speeds between 5.4 and 8.49 m/sec 6% of
the time. This represents 0.06 x 720 = 43.2, or about 43 hours
typically have winds from the northwest at these speeds at Fresno in
April.
A note on the wind database: These wind roses are based on hourly
data from the Solar and Meteorological Surface Observation Network
(SAMSON) CDROM, available from the National Climatic Data Center (http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwAW~MP~CD).
The period of record is for 1961-1990. SAMSON is a 3-volume CD-ROM
set is divided geographically into regions: Eastern, Central, and
Western U.S. It contains hourly solar radiation data along with
selected meteorological elements for the period 1961-1990. It
encompasses 237 NWS stations in the United States, plus offices in
Guam and Puerto Rico. The dataset includes both observational and
modeled data. The hourly solar elements are: Extraterrestrial
horizontal and extraterrestrial direct normal radiation; global,
diffuse, and direct normal radiation. Meteorological elements are:
Total and opaque sky cover, temperature and dew point, relative
humidity, pressure, wind direction (true north) and speed,
visibility, ceiling height, present weather, precipitable water,
aerosol optical depth, snow depth, days since last snowfall, and
hourly precipitation. Joint NCDC and NREL product. DOS only. SAMSON
can be ordered from NCDC by clicking on the above link.
The hourly winds measured at airports are normally 2 or 3 minute
averages of 3 or 5 second samples at the top of every hour. These
are not gusts. Newer equipment, particularly the Automated Surface
Observation System (ASOS) used at most locations since 1996 or so,
automatically records these values from cup anemometer values. Older
observations (generally prior to 1996) represent data recorded by
personnel working at weather stations who manually observed wind
speed and direction at the top of every hour, and made an estimation
of hourly winds over some time period, typically 2 to 5 minutes in
length.
For more information, contact
Jan Curtis
(jan.curtis@por.usda.gov) |
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