Katabatic Winds

What is wind?

The question seems simple, but I cannot recall learning about this topic in science classes in school. In simplest terms, wind is the movement of air. But why does air move? What causes wind? The simple concept bleeds into complexity rather quickly, as new questions emerge.

Wind occurs because of pressure gradients in the atmosphere. Specifically, wind blows from high-pressure to low-pressure areas. Imagine a balloon. The air inside the balloon is under more pressure than the air outside. If you open the balloon’s nozzle, the contents quickly spill outward, moving from high to low pressure. This phenomenon is what happens on a larger scale with wind, though we cannot perceive pressure with our eyes.

Why does air pressure vary from place to place? The answer is a combination of the Sun and our planet’s spin. Our star is a light that never turns off, but the Earth spends half its time in the dark. The Sun’s rays heat the planet, but the heating is not uniform because we’re turning. So, the parts of Earth that are in daylight warm up, while the shaded portions cool. The density of gases is directly affected by temperature. Warmer air is less dense than colder air. Further, pressure and density are directly related. So, colder air is under more pressure than warmer air. Because it’s less dense, warm air rises, leaving behind an area of low pressure. Physical and chemical systems really want to be in equilibrium, so this low-pressure area “attracts” air from areas of higher pressure.

Voila: wind.

Though some of the specifics vary because the Earth is a massive system, this basic building block of wind applies over the entire planet and causes a fairly predictable general wind pattern. On large scales, the Sun’s uneven heating creates temperature and pressure imbalances, leading to wind.

The Sun isn’t the only gusty wizard, however.

On small scales, other factors can contribute to wind. One such magus is gravity!

The gorgeous photo above demonstrates katabatic wind.

In Greek mythology, a katabasis was a trip to the underworld, as the word translates direction to “descent.” In meteorology, katabatic wind is the descent of a wind over a slope under the influence of gravity.

In the preceding image, we can see the effect thanks to the fog, which is carried down the slope by the katabatic wind. What differentiates katabatic from normal wind? The underlying mechanism is the same: temperature and density gradients. Most katabatic wind forms thanks to radiative cooling. Overnight, air closer to the ground cools more than higher layers. They can also transpire when encountering cold land masses, such as glaciers. When a slope is involved, colder, denser air can develop close to air above a valley, which remains warmer. This condition creates a pressure gradient, which pulls the cooler air toward the warmer air. So far, normal wind. However, when these pockets hit the slope, gravity joins the party, pulling the denser air toward the bottoms of the slope.

The result can be ferocious, localized winds.

In Greenland, katabatic winds have been measured at over 180 miles per hour, which is well over the threshold for a category 5 hurricane! Gravity-assisted gusts in Alaska and Tierra del Fuego have topped 140 miles per hour. Gravity rides everything.

Continuing a long-term trend, I first encountered katabatic winds as a child, but not in school. Joining other terms we’ve investigated in the past – jökulhlaups, will-o’-the-wisp, ball lightning – I first learned about this phenomenon through Magic: the Gathering.

The names of these terms are so intriguing that they beg exploration. The world of science is full of fantastic nomenclature, but the area of wind seems to occupy the upper echelon of incredible phrasing.

Just within the specific purview of katabatic winds are a slew of doozies. Greenland’s version is piteraq, which translates to “that which attacks you.” In Alaska and the Straits of Magellan, sailors called them “williwaws.” In the coastal area of the Adriatic Sea, locals call the katabatic wind “bora,” which shares a root word with Boreas, the Greek god of the cold north wind. The Japanese have “Oroshi.”

And, as we’ll investigate in an upcoming article in this series, southern California has a version named after Santa Ana.

Since these winds are out of the ordinary and bring destruction, it makes sense that people would develop individualized classifications and attribute anthropomorphized causes. Without a traditional storm, sudden winds moving at hurricane force would likely occupy the imaginations of humans living near mountains.

The confluence of a gravitational descent and a mythological descent seems rather fitting for these winds, elemental examples of the power and grandeur of the natural universe.