HOW TO ‘READ' A WINDROSE
It is most common to display windroses using the direction the wind came from. This is because it is often more important to know where the air has been rather than where it is going. For example, cold winds come from the North and Northwest while warm winds come from the Southwest and South. The graphics below explain information available in the windrose itself as well as the accompanying documentation.
First let's view the directions. While everyone is aware of North, South, East & West, there are other directions between those you may not be familiar with. The names won't be used on the windrose itself but you may want to use them when you are discussing a windrose. When dealing with 16 direction windroses the directions are named in the graphic below.
Since MEDEP is presenting 36 direction wind roses the names listed above would give you a general sense of the direction. To be more specific you could use the degrees each spoke of the 36 direction windrose represents (10, 20 . . . 340, 350, and 360). Please use the image below to orient yourself to the degrees around a windrose.
In the next graphic, the types of information provided along with the windrose are pointed out. The length of each spoke on a wind rose indicates how often the wind comes from this direction. Longer spokes mean the wind comes from this direction more often. To give you a rough idea of the percentage there are dotted rings labeled in percent. Additionally, the colors on each spoke display how often the wind speed from this direction falls within a given range.
In the example above, we chose a site in the Kennebec River Valley . At this location the river runs from the north toward the south. One will see that the wind comes from the South more often than any other direction because that spoke is the longest. Yet the strongest winds come from the West-northwest and the Northwest more often than other directions because you see the colors from the higher speeds at the end of the spokes. You can even see from the windrose how the hills that make up the sides of the river valley direct the winds in that area.
