Climates and Microclimates – Part 1

Rocky Mountain Flying

By Rose Marie Kern

 

Throughout the US there are regional variations in terrain that modify the behavior of weather systems.  If the entire continent was flat it would be fairly easy to follow the progress of a front as it passed from west to east in summer or north to south in winter, but you add in high mountains, big lakes, and an ocean and things become more, well, interesting.

 

Over the next few months we will examine how regional terrain and weather create microclimates, and how this affects flying in the areas.  Some of our topics will include Mountain flying, the Great Plains, the Hill Country of Texas, the Southwestern High Desert, Pacific Marine activity, Great Lake effects, and Gales of the North Atlantic.

 

Rocky Mountain Flying

We have several mountain chains in the US, but the Rockies have the highest peaks.  When you fly over central Colorado they look like an unbroken line of grey teeth raking the sky.  The mountains create a barrier to fronts and moisture.

 

In general, weather systems move from west to east across the U.S.  When a moisture laden front blows into California and begins dancing its way east, the mountains of eastern California, Nevada and Utah tend to grab at it, slowing it down a little.  By the time it hits the big peaks in Colorado it is moving a lot slower than when it initially came on land. The mass fights its way out to the plains, but frequently leaves pockets of moisture trapped in the valleys.  The high pressure following the front may not be strong enough to force that moisture on its way, so for several days those valleys may contain low morning clouds and afternoon showers. 

 

As the main weather system moves east it gains strength again across the plains, then the Ozarks provide another, lower barrier.  Other systems from south and north will also affect that area which we will discuss in a future article.

 

In the summer, heat provides a lifting mechanism that insures those showers become thunderstorms. Unlike the plains, mountain thunderstorms are very spotty – you may be drenched, but your neighbor down the street doesn’t get a drop.  The storms always form over the mountains because the sunlight hitting the ground warms the air.  The warm air flows up the sides of the mountains striking the cooler, moisture laden air aloft, which creates the storms.  At that point the upper level winds take over and move the storms in different directions away from the mountains.   

 

The storms regenerate themselves until a few hours after sunset when the air has cooled, then they subside until the next day.  This pattern will repeat until a stronger weather system pushes away the moisture laden air.

 

Most pilots who live in the mountains know that in these circumstances it is best to aim for a late morning departure, and then try to get on the ground and tied down or hangared by about 3pm.  This is because the moisture can cause IFR in the valleys and mountain obscuration that is very shallow and will burn off by about 9am.  Radiation statistics show that solar radiation is strongest by about 2pm, so the storms are usually in full bloom by 3pm to 4pm daily.

The most significant factor associated with the usual west to east wind flow is an effect called “mountain wave”.  Whenever there is a strong flow striking the side of a mountain, the air is forced upwards, this causes the winds above the peaks to also push upwards.  If the push is strong enough, that upwards push can lift aircraft flying even as high as FL400. 

 

On the other side of the peak, the air drops down to the ground as fast as it rose, which pushes aircraft downward.  For those of you who studied physics, this is a live example of Bernoulli’s principle.

 

There are no AIRMETS or SIGMETS that predict Mountain Wave, though some of them will mentions that UDDFS (up and down drafts) are expected.  Mountain wave is present anytime there is a strong west-to-east wind flow, and it is strongest when a deep Jet Stream or upper level trough intensifies the winds at lower altitudes. 

 

The strength of mountain waves catches a lot of pilots by surprise, especially those who live in flatland country and rarely venture into higher terrain.  I have known several pilots who took a look at the winds aloft and seeing that they were very strong at higher altitudes told me that they wanted to save fuel by hugging the peaks.  Well, all that fuel they save can make a very spectacular fireball. 

 

Flying the valleys in these circumstances is not a good idea either.  The mountain walls create wind vortices in the canyons and passes that can toss your aircraft sideways.  Even if it causes you to have to deviate from a direct line, try to find a route that has generally lower elevations.

 

If you have to fly a small aircraft in these conditions, then keep as much space between you and the ground as your aircraft can safely handle.  Anticipate that with a westerly flow, the air on the west side of the mountains will push you upwards, then as you get to the east side (lee side) you will fight the downdraft to maintain altitude. 

 

On a day with a moderate westerly flow, the up and down drafts  will range from 500 to 1,000 feet per minute. These are usually stronger in the heat of the afternoon.   

 

Now let us look at winter in the Rockies.  You may have noticed on a map that the eastern face of the Rocky Mountains is a fairly straight line from Canada to Mexico.  The strongest winter cold fronts barrel down from the arctic towards the Dakotas and thence across the plains to Texas. 

 

This ground hugging front is stopped from advancing westward by those tall mountain peaks and in frustration the system lashes the eastern face of the mountains and the plains below with daggers of icy wind and snow. 

 

Frequently there will be several feet of snow dumped on the east side of a mountain, but the west side will receive a bare dusting – which is why the same cold front that buries Las Vegas, New Mexico may only brush Santa Fe. 

A front or low pressure system that lays against one side of a mountain range squirts wind through the passes towards the valleys and plains on the other side. Here in Albuquerque the wind funnels through Tijeras Canyon, which aims it directly at Albuquerque International Airport (ABQ) .  Because it is localized, ABQ may be getting 30 to 40 knot winds out of the east, while Double Eagle airport (AEG) just 20 miles west is getting little or no surface wind at all.

 

Eventually the prevailing westerly flow aloft will convince the cold front to push eastwards across the plains towards the Ozarks.  But that is another story.

 

Rose Marie Kern works at Lockheed Martin’s ABQ AFSS. If you’d like to ask Rose a question send her an email at author@rosemariekern.com.