Diurnal winds are the name for local thermal-driven wind cycles that are driven by the heating and cooling of different types of terrain.
These patterns occur in many places, but usually refer to a reversal in wind direction on a 24-hour cycle. They are the result of uneven heating creating a temperature gradient, which creates a pressure differential.
During the evening, as the sun sets, the earth cools the surrounding air making it denser, creating a sinking motion. This sinking motion results in a down-slope flow.
Want to see mountain slope winds in action? Let's look at Leadville, Colorado (KLXV). Leadville airport sits on a large mountain slope. Looking at the winds reported over 24 hours we can see the local patterns.
In the afternoon, the winds flow upslope to the east as the air becomes less dense. Then in the evening hours, the wind direction reverses. The sun sets and the air cools, becoming denser, and causing a sinking motion. Now the winds blow downslope, to the west.
Wind flows up and down mountain valleys for the same reasons wind blows up and down mountain slopes. But mountain valley winds are often stronger since the terrain restricts their flow, and they primarily blow across 'more level' valley surfaces.
Looking at Eagle County Airport, an airport located in the Gore Valley of the Rockies, you'll see a daily wind pattern. Throughout a 24-hour period, the wind shifted roughly 140 degrees, with winds blowing up valley in the afternoon, and down valley at night.
Prevailing wind diurnality is not exclusive to mountainous regions. They can also be seen along coastlines. But rather than blowing upslope or downslope, these winds circulate onshore and offshore.
In the daytime hours, an onshore sea breeze develops as a result of more rapid heating of the land, compared to the ocean. This creates a convective current where air rises landside and sinks over the sea.
This temperature difference drives a pressure differential, where a local high-pressure center forms over the ocean as a result of the sinking motion. And a local low-pressure center forms over the land (air is rising). This high-pressure air seeks equilibrium, flowing towards the low pressure on the land creating an onshore flow.
Apart from predicting where the wind may come from, this diurnality drives regional weather patterns. This switch in onshore/offshore flow allows advection fog to linger for days, weeks, and even months in coastal regions.
The strongest diurnal wind patterns can be found in the mountains and coasts. Just like many other weather processes, they result from the uneven heating of the earth's surface, creating a temperature difference that drives a pressure gradient.
Knowing what drives this cycle will help you better anticipate the conditions you might arrive or depart into the next time you fly. This can be especially helpful if your airport doesn't have a TAF, or it's a "one-way in or out" type of field.
Where have you seen diurnal wind patterns? Tell us in the comments below.
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Nicolas is an Airline Pilot & flight instructor. He's worked on projects surrounding aviation safety and marketing. You can reach him at nicolas@boldmethod.com.
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