How to read a wind rose

What does a wind rose show?

A wind rose is a circular chart that summarises many hours of wind observations at one place. The compass directions run around the circle, often in 16 sectors (N, NNE, NE and so on), sometimes in 8. From the centre, a petal or bar extends towards each sector: the longer it is, the more often the wind blew from that direction.

By meteorological convention, wind direction is the direction the wind comes from, measured clockwise from true north. A petal pointing east-north-east means wind arriving from the east-north-east. Reading it the other way round is a common mistake, and it reverses every design conclusion drawn from the chart.

How to read petals, rings, speed bands and calms

• Rings: the concentric circles are a frequency scale, for example 5%, 10% and 15% of all hours. Read each petal's length against the rings, not against the size of the chart.
• Speed bands: each petal is split into coloured segments, one per wind-speed range. A segment's length is the share of hours with wind from that direction in that range. The legend gives the ranges, in m/s, km/h, knots or Beaufort force.
• Calms: hours with almost no wind have no meaningful direction, so they are reported separately, as a percentage in the legend or as a circle at the centre whose size grows with the share of calm hours. The World Meteorological Organization (WMO) reports calm when the average wind speed is 0.2 m/s or less; tools use different thresholds, so check the legend.
• Period: the legend or caption should state which years, months and hours were used. A rose built from every hour of the year says little about a summer afternoon.
• Scale: two roses can use different ring values. Check that the rings match before comparing sites or seasons.

Prevailing winds, strongest winds and seasons

The prevailing wind is the direction with the longest petal: the wind a site gets most often. It matters for natural ventilation and for where smoke, smells and noise are carried. The strongest winds are the petals with the longest high-speed segments, and they may come from another sector entirely, for example frequent light winds from one side and rarer but strong winter winds from the other.

An annual rose averages very different situations. Split by season, the same site can show a cold winter wind to shelter from and a summer breeze worth catching. Split by time of day, coastal sites often show a daytime sea breeze and a night-time land breeze, because temperatures swing much more over land than over water between day and night. Read wind together with temperature: a breeze on a hot afternoon helps, the same wind on a winter morning adds to heat loss and discomfort.

Where the wind data comes from, and its limits

Most wind roses are built from weather-station records or typical-year weather files such as EPW files, which store wind speed in m/s and direction in degrees from north. WMO guidance places anemometers 10 m above open, level terrain, with obstacles at least 10 times their own height away, because wind speed increases with height and changes over rough ground.

Buildings, trees and hills alter both the speed and the direction of the wind; WMO notes that the wake of an obstacle can extend 12 to 15 obstacle heights downwind. A station rose therefore describes the regional wind arriving at a site. On a dense urban site, pedestrian-level wind can differ a lot, and wind-tunnel or computational studies are the tools for that question.

Using a wind rose in building design

• Natural ventilation: place openings to catch the frequent warm-season winds, and check how often those winds are strong enough to help rather than calm.
• Entrances and outdoor spaces: keep doors, terraces and play areas away from the strongest and coldest winds, or plan screening and planting on that side.
• Pedestrian comfort: in its Beaufort table for estimating wind speed, WMO describes force 5 (8.0 to 10.7 m/s at 10 m) as the 'limit of agreeable wind on land'. It is a descriptive cue rather than a comfort standard, but frequent winds in or above that band may call for shelter.
• Air quality: locate fresh-air intakes upwind of exhausts, flues and car parks relative to the prevailing wind.
• Pair with other analyses: the sun path and temperature data tell you whether a given wind is welcome or not at the time it blows.

How C4B shows wind roses

C4B draws an annual wind rose for any location with 16 sectors and six speed bands grouped from the Beaufort scale, and links each band to the outdoor activities it remains comfortable for. A daily wind rose shows the wind at four times of day for each season. Both are available on the Free plan with weather-station data. Projects on paid plans add directional outdoor comfort with windbreak guidance, and future climate overlays on the wind rose.