Why Care About Rain Shadows?

You’ve stumbled across this blog all about what makes rain shadows and how they are linked to atmospheric rivers. Here, we explain what a rain shadow is, and at the end of the post, you might even be able to answer the question “why care?”  Any geographical region will have prevailing wind patterns, which are the direction(s) the wind blows most often. A rain shadow is the dry region on the opposite side of the prevailing wind. This weather phenomenon can be seen all over the world.

Remember that big L on the meteorologist’s screen? It stands for low pressure system, and they tend to carry a lot of moisture. Imagine a low pressure system, full of water, creeping across a flat plain or valley until it hits the windward side of a mountain chain. As the air rises up the slope (a natural process called orographic lift), it cools down and begins to condense – just like your breath on a car window on a cold day. As the air continues to condense and reaches the top of the mountains, it will fall as precipitation. The air is now drier as it continues to pass over the summit, leaving the other side, the leeward side, without as much precipitation. While leeward areas do receive precipitation, it does not happen as frequently or as heavily as on the windward side.

The ultimate question left is why would we care about rain shadows? This naturally occurring process is a major influence on snowpack (how much snow falls over the course of the winter), stream flow, and local vegetation. You can easily observe that local flora is different in a rain shadow compared to the windward of the mountain. The types of trees and bushes in the Sierra, for example, vary depending on the side of the mountain you are looking at: we tend to find more California Foothill Pine, manzanita, and chamise on the windward side, and more sagebrush, mountain mahogany, and Russian olive on the leeward side. The types of plants that thrive in the rain shadow are able to survive on less water. Unexpected heavy precipitation can lead to flooding and mudslides if the soil is not prepared to absorb that much water. Further, a low water year overall will show a decreased water table and snowpack in a mountain, which some areas rely on as their main source of water.

The city of Reno, Nevada is commonly affected by the severe rain shadow off of the Sierra Nevada Mountains. While Lake Tahoe will receive several feet of precipitation in a typical year, Reno will receive mere inches. Water managers thus use reservoirs to prepare for dry years, and residents are accustomed to conserving water and dealing with the cold, dry conditions.

So why care about rain shadows? When you know the pattern, you can be awestruck when nature doesn’t follow her typical routine. The 2017 winter season has seen a unique amount of precipitation (A LOT), being unusually rainy very early in the season. Several atmospheric river events have taken place this year, pushing concentrated moist air from the Pacific Ocean up and over the mountains. Almost disregarding the usual behavior of orographic lift, this dense, moist plume brought unprecedented precipitation to the city of Reno. We will discuss atmospheric rivers in a future post, but it is interesting to see that while places like Reno, Nevada are in for a surprise when Mother Nature gets a little deviant and diverges from the normal climatological rain shadow patterns!

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One thought on “Why Care About Rain Shadows?

  1. Pingback: Lift. Fall. Move. | In the shadow of the rain

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