Interpreting Wind Roses Found on Pilot Charts
Each Wind Rose on a Pilot Chart shows the distribution of the prevailing winds in the area and month. These have been recorded over a long period of time. In general the lengths of the arrows indicate how often the winds came from that direction. The longer the arrow the more prevalent were the winds coming from that direction. The length of the shaft is generally to a scale to indicate the percentage of the winds from that direction—not as its printed, but the proportions are correct and as a general indicator; the diameter of the circle is about 15 percent. So—if the length of an arrow is about equal to the diameter of the circle, it would indicate that about 15 percent of the winds are coming from that direction.
The wind blows in the direction the arrows fly. When the arrow is too long to be printed in a practical manner, a number is indicated. In the sample here, that would be the number 34, which means that 34 percent of the time the wind blew from the east. The number of feathers on the arrow (each arrow in this example has four feathers) indicates the strength of the wind on the Beaufort scale (one feather is Force 1, two is Force 2, etc. See below). The number in the center of the circle shows the percentage of the time that the winds were calm. They all add up to 100 percent. In the windrose shown here, 34 percent came from the east and 2 percent were calms (a total of 36 per cent), so the remaining arrows add up to 64 percent. These symbols are used here to generally show where the winds came from during that month in that 5-degree square, how strong and how often.
In this example, moving around the circle clockwise, we can see that about a third (34 percent) of the time the winds came from the east; about 15 percent of the time (the length of the arrow is roughly about the diameter of the circle) from the SE, Force 4; about 10 percent from the south, Force 4; rarely from the SW, West, and NW (about 3 percent each x 3 = 9 percent), Force 4; about 10 percent from the north, Force 4; about 20 percent of the time from the NE, Force 4. It was calm 2 percent of the time. That is a total of 100 percent.
Beaufort Scale (in knots): Force 1(1-3); Force 2(4-6); Force 3(7-10); Force 4(11-16); Force 5(17-21); Force 6(22-27); Force 7(28-33); Force 8(34-40); Force 9(41-47); Force 10(48-55); Force 11(56-63); Force 12(64-71 Hurricane)
Click Here to see a hi res version of a pilot chart