A frequency wind rose shows how often wind came from each direction; speed needs a separate key.
Wind direction tells you where the air comes from. A north-east wind comes from the north-east and moves broadly towards the south-west.
A wind rose groups observations by compass direction. Read the scale and legend: the length of a spoke may show a count or percentage. In this practice rose it shows a count, not wind speed.
There are 30 invented observations, all with some wind. The counts are N 3, NE 12, E 5, SE 1, S 2, SW 4, W 2 and NW 1. North-east is most frequent: 12 / 30 x 100 = 40%. That does not mean every observation had a north-east wind.
Some wind roses use colours within each spoke for speed groups, or show calm conditions separately. This one has no speed data and excludes calm observations. It cannot tell you the maximum wind speed or the share of all observations that were calm.
Step by step
Read the scale
Check whether length shows counts or percentages, and whether colours show speed.
Name where it came from
Use the compass direction from which the air arrived.
Calculate the share
Divide that direction's count by the stated total. Do not infer speed if no speed data are given.
Worked example: A useful limit
You can state that the north-east direction was most frequent in this sample. You cannot conclude that it is the prevailing wind for the entire year without a representative longer record.
Watch out for this
The longest spoke shows the fastest wind.
On a frequency rose, length shows how often that direction occurred. Speed must be encoded separately and explained by a key.
Check your understanding
What does the north-east spoke of 12 mean in this practice rose?
- Wind speed reached 12 km/h towards the north-east.
- Twelve observations recorded wind coming from the north-east.
- Every day of the year has a north-east wind.