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    29th March 2011

    Orographic Lift and Mountain Clouds

    I’m taking a Nature Writing class at CSU right now with the fabulous Dr. SueEllen Campbell, a many-times published and still publishing nature writer. In the class we read nature writing, but we also have to write some of our own essays. Here is an essay that we just got back from Professor Campbell. There are some things I might change about it, but for now I’ll leave it as is, with the exception that I’m going to link some pictures of the phenomena into the article.

    I took my undergraduate degree from Calvin College in Grand Rapids, Michigan, a notoriously cloudy spot that gets buried in lake-effect snow every year from the start of January until the end of March. After graduating, I was happy to move back to the Colorado Front Range where most of my family was still living. There were and still are many reasons I love Northern Colorado: the mountains are always there to the west, looking beautiful; the people are laid back and friendly; there are a whole bunch of great local breweries, like New Belgium and Odell; and the roads are cyclist friendly so I feel safe getting out on my handcycle.

    But when my Michigan friends ask for the number one reason living in Fort Collins beats living in Grand Rapids, I’ve never have to stop and think. The answer is easy. It’s the sunshine. Most Front Range communities brag about getting over 300 days of sunshine each year. That statistic is a little deceiving, however, because that counts every day that we get at least one hour of sun. According to the Colorado Climate Center at Colorado State University, the Front Range actually averages close to equal parts clear days (no significant clouds), cloudy days (at least 80% cloud cover), and partly cloudy days (everything in between). Many, if not most, of those partly cloudy and cloudy days also receive at least some sunshine, but it would be dishonest to portray Colorado as some sort of cloudless bubble.

    And why would anyone want to? Over the years I have come to believe that Colorado just might have some of the most uniquely beautiful clouds in the world. “Aren’t all clouds the same?!” you might ask. Well, in some ways, yes, but in other ways, no. Nearly everyone who would read this article has learned something about clouds, either in grade school, a college introductory science course, or from watching the weather channel. Here in Colorado we see all of the usual suspects: There are the cotton-ball, fair-weather cumulus clouds; heavy, rain-bringing, blanket clouds like the nimbostratus; the high, thin and wispy cirrus clouds that indicate changing weather; and there are the intimidating cumulonimbus clouds that balloon into towering thunderheads before they pour over the mountains down onto the plains below, often putting on a startling sound and light show as they travel. However, because of the extensive range of mountains to our west, those of us who live on the Colorado Front Range also are witness to some clouds that generally only occur under specific conditions. These conditions also help explain why it is that the Front Range has more sunny days than the average.

    Most clouds are formed by some variant of a convection cycle, a circular system whereby heat is distributed throughout a liquid or gas. When you sit mesmerized by a lava lamp you are watching a convection cycle: solid wax is more dense than water so it stays at the bottom; as the wax at the bottom is melted and warmed by the light, it becomes liquid and less dense than the water around it; the wax then rises to the top where it will cool and fall back down to the bottom. Repeat ad infinitum. Likewise, basic cloud formation happens through convection: Radiation from the sun warms the ground and causes water to evaporate. The warmed ground works with the sun and together they warm up low-lying layers of air. Warmer gasses are less dense than cooler gasses so the warm air rises, taking water vapor with it. As the air rises, it cools off; once it has cooled off enough, the water vapor will condense back into liquid droplets or freeze into tiny ice pellets. If there is enough water vapor in the air mass, clouds form and the mass of droplets scatter the visible light rays from the sun, causing areas dense in droplets to appear white or grey. If the droplets become large enough or the air is so saturated with droplets that it cannot hold any more of them, they fall to the earth as rain – or, if it’s cold enough, as snow. After the air has cooled and released enough energy, the cooler air again sinks lower and the process repeats itself.

    In areas with mountains or other obstacles, clouds are also formed by a process called orographic lift. The Greek word for mountain is oros and the Latin word for rise is oriri. Either etymology is appropriate as orographic lift describes the rising movement of air over mountains. When air moves perpendicularly across a mountain range, that air is forced to rise so that it can make it over the peaks. As in normal cloud formation, as the air rises, it cools. If there is enough water vapor in the rising air, condensation occurs, forming clouds of condensed or frozen droplets. The side of the mountains that the wind generally hits first is called the windward side while the far side of the range is called the leeward side. Fort Collins, where I live, and the rest of the Front Range are on the leeward side of the Rockies because most Colorado weather systems move across the mountains from west to east.

    To paint a picture of orographic lift, picture yourself out for a walk when all of the sudden the wind picks up and soon you’re battling a gusty gale. There is dust and trash flying everywhere and you are five miles from home. The area you are in flat and has almost no trees, however, there is a large brick wall marking the edge of someone’s property. What do you do? You get on the leeward side of the brick wall, and sit with your back up against it, knowing that the brick wall will disrupt the flow of the wind. You can’t see the wind, but you can hear it whistling and can occasionally see dust and trash flying over and around the wall. Or consider a large rock stuck in the middle of a river. The water is flowing steadily downstream when it encounters the rock. Where the water had previously been flowing in a relatively straight line, the rock causes the flow of the stream to bend around and over itself, usually creating a ripple effect of some sort downstream from the rock. If enough water is flowing quickly over big enough rocks, whitewater rapids will even form, and the water appears to become wavelike. Air masses function much like that river, but the Rockies are much bigger than the average stone in a stream, and likewise the magnitude of their impact is multiplied.

    Once the moving air has been pushed up, cooled off, and water has condensed, orographic clouds will often stay over the high country until they are relieved of their water content through precipitation. By the time the clouds reach the far side of the mountains, they have usually lost a fair amount of their moisture. When this dry air mass descends the leeward slopes, the temperature rises and any remaining condensation in the air will usually evaporate. This can cause it to appear as though there is a wall of clouds rising up at the edge of a mountain range, as though an army of storm clouds marched across the peaks to attack only to be stopped in their tracks before descending. This is called a foehn wall and in the Alps it is associated with foehn winds – warm, dry wind that is infamous among mountaineers for causing rapid snow melt. This wind can be quite a bit warmer than the air was at the same elevation on the windward side of the range because the air has absorbed the latent heat – or potential energy – from the water phase changes.

    In Colorado we know this balmy current as the Chinook wind, though the name and myths associated with the Chinook comes from the Pacific Northwest. One of these stories tells of a young girl named Chinook-Wind who was married to Glacier, but couldn’t help missing her much warmer home country by the sea. To signal her people to come rescue her, she sent a burst of warm air down the mountain and her family members came to the rescue, destroying Glacier in the process. In Fort Collins, the Chinook wind will come unexpectedly: you’ll wear a heavy jacket and have to scrape ice off your car in the morning, but in the afternoon or evening a warm and gentle breeze will coax you into a t-shirt as the temperature rises to 50 degrees F or higher. As mentioned above, orographic lift can create a low-lying foehn wall rising from the mountains, but it can also create a Chinook arch, smooth bands of stratus clouds that stay in place, appearing to hang over the mountains. A Chinook arch can truly appear arch like or else it can be flat and will often extend out beyond the mountains themselves.

    Orographic lift also sometimes leads into another meteorological phenomenon caused by mountains. When air masses creates a wave like pattern by rising over a mountain and then falling again, it will sometimes continue repeating that wave pattern beyond the edge of the mountain range – sometimes for hundreds of miles – layering moist and dry streams of air one on top of the other. The air traveling through these waves acts as though it is still crossing a mountain range. As the moist layers of air rise, the water vapor condenses at the top of the wave, and then as the air drops down to the next trough the droplets evaporate. This creates altocumulus standing lenticular clouds, also known as standing wave clouds – but also called simply lenticular clouds for short. These clouds look like upside-down bowls and when there are many layers of air moving in waves, the clouds can stack up and look like a flotilla of flying saucers. Sometimes, especially with tall stand alone mountains, like Fuji or Rainier, a single lenticular cloud will form directly over a peak so that it appears as though the mountain is wearing a conical Asian coolie hat of sorts. Stand alone lenticular clouds are less common on the Front Range, but we do frequently have waves of lenticular clouds, stacked up like heavenly pancakes over the spike of Longs Peak and bathed in the heavenly golden syrup of the sun.

    The clouds that form from orographic lift on the windward side of a mountain range are often full of moisture and so the windward side usually receives more precipitation. This is why the Colorado mountains are able to support a booming ski industry during the winter, and why the coastal mountains of the Pacific Northwest are temperate rainforests overgrowing with green things. On the other hand, orographic clouds that travel across the range usually don’t have much water content by the time they make it to the leeward side. Clouds that are formed on the leeward side, such as lenticular clouds, also don’t typically produce much precipitation. The dry air that does come down onto the Front Range through the Chinook wind also discourages the development of rain-bearing convective cloud formation in the low country. Translation: More sunny days – or at least more days with sun.

    ***

    Fort Collins, Colorado; Thursday, February 10, 2011; 5:17 pm: I leave my class on the campus of Colorado State University and head west, strolling to the parking lot where my car is parked. My mind is moving a million miles a minute, going over all of the things I need to accomplish this evening and I don’t pay attention until I come out from behind a hedge of bushes and into the open parking lot. That’s when I realize that a scene of extraordinary beauty is unfolding above and in front of me. The sky is on fire; clouds burn with pink and red and purple and all their varieties: warm coral, heavy maroon, soft salmon, and rich Tyrian purple. The clouds themselves are oil, a foehn wall painted as puffs climbing up from the Front Range and foot hills and then pulled across the azure canvas directly above, turning into lenticular streaks and waves radiating from the spot where the sun is just now dipping below Horsetooth Mountain. The air is freezing and during my years here in Colorado I have seen hundreds of sunsets much like this one – but I am still ambushed by the raw beauty and find myself unable to continue moving. I stop and bathe myself in air and light, plugging into the great battery charger of souls. After ten minutes the light fades and I get into my car. I do love Colorado for the sunshine – but I’m starting to think that maybe I love it for the clouds, too.?

    End note: I found much of my information in Meteorology Today: An Introduction to Weather, Climate, and the Environment, by C. Donald Ahrens (West Publishing Company, 1988). I also looked at the Colorado Climate Center’s website online for some information about Colorado weather. The Wikipedia entry on “Orographic Lift” also gave me some ideas of what information to look for.

    Copyright 2005 by Daryl Holmlund - All rights reserved.