Often, it’s listed as dates, such as March 19, 2018 to March 25, 2018. It can also cover much longer periods, such as 30 years or more. Sometimes, it may just cover a 24-hour period.
In some cases, the key will give you extra information that you can use. For instance, it may tell you what percentage of time the winds are calm without you having to calculate it. It will also give you any units for the diagram you may be missing. [3] X Research source
Typically, the circles themselves are marked with the percentage or number of hours.
For example, a spoke may extend upward toward the north, though it will be shaped like an arrow pointing toward the middle of the circle. In that case, the wind is blowing from the north toward the south. Some wheels just use 8 directions, north, south, east, west, northeast, northwest, southeast, and southwest. However, many use 16, adding north-northeast, east-northeast, and so on.
For instance, you may notice one spoke is green at the center and red at the end. After checking the key, you discover the green means the wind blew from 0 to 10 miles (16 km) and red means the wind gusted up to 40 to 45 miles (64 to 72 km) per hour. That means that within the given time period, the wind blew at speeds of 0 to 45 miles (72 km) per hour from that specific direction.
For instance, if the color is purple, indicating on your particular map that it blew from the north at 15 to 25 miles (24 to 40 km) per hour, see where the color begins and ends along the rings within the spoke. If the purple area along the spoke starts at a smaller ring (10 percent according to the label) and ends at a larger ring (24 percent according to the label), then subtract 10 from 24, as that tells you what percentage of time the wind blew at that speed and direction, 14 percent. Usually, the central circle indicates calm conditions. [8] X Research source
For instance, say the study was over 16 days, from April 1st to April 16th. You noticed that the wind blew from the north at 15 to 25 miles (24 to 40 km) 14 percent of the time. Multiple the number of days by the percentage and then divide by 100: 16 x 14 = 224, 224 / 100 = 2. 24 days. Some wind roses will have the circles already marked in days or hours so you don’t have to calculate this number. Instead, just look at the numbers on the wheel and subtract them as you did with the percentage. For instance, maybe the largest circle is 30 hours and the next one is 20. If the color representing 30 to 35 miles (48 to 56 km) per hour moves from about halfway between the 2 circles (indicating 25 hours) to the 30 circle, then the wind spent 5 hours of time at this speed from that particular direction, as you subtract 25 from 30 hours. [9] X Research source
