Have you ever noticed that the winds at your local airport seem to follow a particular pattern? Well, there’s actually a reason for this. It’s a fascinating phenomenon known as diurnal winds. The word “diurnal” means “during the day,” and these types of winds follow a set schedule that is influenced by the earth’s solar heating on a daily basis.
Understanding what causes this wind cycle can help you better anticipate the weather conditions you might encounter when flying in or out next time. This can be especially beneficial if your airport doesn’t have a Terminal Aerodrome Forecast (TAF) or if it’s a “one-way in or out” type of field.
What Causes Wind?
The short answer to how wind is created is a difference in air pressure. High-pressure air seeks out low-pressure air as it attempts to reach equilibrium, resulting in everything from a gentle breeze to powerful gusts.
You’ve likely heard that all weather phenomena are a consequence of the uneven heating of the earth’s surface, but how does this relate to the wind? Since the earth’s surface is not uniform, the sun heats various elements such as grass, sand, and water differently, which leads to different air temperatures. This temperature gradient in turn changes the density of the air, causing rising and sinking motions.
What Are Diurnal Winds?
Diurnal winds are local thermal-driven wind cycles that are driven by the heating and cooling of different types of terrain. These patterns can be observed in many places and typically refer to a reversal in wind direction within a 24-hour cycle. They are the result of uneven heating creating a temperature gradient, which in turn creates a pressure differential.
Where Do Diurnal Winds Happen?
1) Mountain Slopes
Generally, winds flow upslope during the day and downslope during the night. During the day, the sun heats the air, making it less dense and more buoyant. This causes the air to rise and flow up the slopes. In the evening, as the sun sets, the earth cools the surrounding air, making it denser, resulting in a sinking motion. This sinking motion leads to a down-slope flow.
To see mountain slope winds in action, let’s take a look at Leadville, Colorado (KLXV). The airport in Leadville is located on a large mountain slope. By examining the winds reported over a 24-hour period, we can observe 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.
2) Mountain Valleys
Wind flows up and down mountain valleys for the same reasons that wind blows up and down mountain slopes. However, mountain valley winds are often stronger because 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 notice a daily wind pattern. Throughout a 24-hour period, the wind shifted roughly 140 degrees, with winds blowing up the valley in the afternoon and down the valley at night.
3) Coastal Regions
Prevailing wind diurnality is not exclusive to mountainous regions. It can also be seen along coastlines. However, instead of blowing upslope or downslope, these winds circulate onshore and offshore.
In the daytime hours, an onshore sea breeze develops due to the more rapid heating of the land compared to the ocean. This creates a convective current where air rises on the land side 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 by flowing towards the low pressure on the land, creating an onshore flow.
In the evening hours, this flow reverses as the land cools.
Aside from predicting where the wind may come from, this diurnality also 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.
Where have you seen diurnal wind patterns? Tell us in the comments below.
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