Understanding the term “prevailing winds”

What are Prevailing Winds?

Many people believe the phrase “prevailing winds” implies that the wind blows from a certain direction, or a limited set of directions, most of the time. While this interpretation has become commonplace, it can be wildly inaccurate and is often misinterpreted by building design professionals. In reality a “prevailing wind” rarely occurs the majority of the time and can actually have a relatively low frequency of occurrence. If this sounds confusing, or seems counterintuitive, read on…

When assessing wind conditions at a site, it is important to understand that the term “prevailing” does not equate to “most of the time”.

While it is true that winds from the prevailing direction may occur more frequently than winds from any other direction, it is rare that winds from the prevailing direction actually occur more frequently than the resulting sum of all remaining wind conditions at the site.

In other words, prevailing winds likely account for a much smaller portion of time than winds from all other directions.

For example, consider wind conditions occurring at LaGuardia International Airport in New York between 1981 and 2011. Figure 1 is the wind rose plot constructed using the meteorological data at this location.

Figure 1: Wind Rose for LaGuardia International Airport in New York between 1981 and 2011

At first glance it appears that the prevailing winds at the site are from the northwest direction. However, from the frequency data we see that northwest winds only occur 12% of the time. This is a reasonably large percentage of time, but is far below the 50% mark that would indicate the “majority” of winds. In fact, this means that wind conditions at the site occur from non-prevailing directions 88% of the time!

To further highlight the importance of this concept, consider that even if the three most predominant wind directions (i.e., northwest – 12%, northeast – 10%, and south – 8%) are summed together, these still only equate to a little over 30%. Once again, this means that winds from other directions are occurring at the site almost 70% of the time, or a true “majority” of the time.

How prevailing winds might affect your project?

The distinction between the term prevailing winds and the amount of time that winds actually occur at a site is important for assessing wind-related issues such as pedestrian comfort, air quality, natural ventilation and snow drifting. Consider the following wind-related phenomena and how they might impact your design decisions and the on-going performance of a building:

• different prevailing wind directions may be experienced during different seasons;
• strong wind events may not coincide with prevailing wind directions;
• the design and performance of natural ventilation systems and how they are affected by local wind conditions;
• unexpectedly frequent re-entrainment of diesel odors into building’s intake system; and,
• wind-driven rain events (and snow drifting events in cold climates), may be associated with only a few wind directions.

A better understanding of the wind conditions actually experienced at a site will assist in improving building design and performance. If we were to only rely on the largest petal of the wind rose for a specific period to make critical design decisions, we might miss the mark completely.

Don’t let yourself or your design team be misled by terminology. The next time you hear or use the term “prevailing winds”, try and avoid the association with “most of the time” or the thought that it means greater than 50% of the time. In many cases, winds from the prevailing wind direction occur less than about 10-15% of the time – which may be counterintuitive for many building designers.