The Winds of Filght - Local Circulations

Chapter 11 - Local Circulations

For me, the most memorable encounter with a mountain wave was on my flight test for my private. After putting me through my paces on the various maneuvers, my FAA designated examiner vectored me out into a curved valley about a thousand feet above the ridges. We went for a ride, the point was to determine if I could hold course and altitude. There was a brisk northwest wind flowing over the ridges and valleys of south central Pennsylvania. After I found it almost impossible to hold altitude within the required amount, it hit me. She had me going directly into some short wavelength mountain waves. After I pointed them out to her, she grinned, and we turned around for Frederick Airport. I passed the test.

The mountain wave is one of what meteorologists call local circulations or strongly forced circulations. They are strongly forced because the Earth's surface interacts with the airflow and causes relatively predictable things to happen with the airflow. These circulations affect what is going on at destination and alternates. Other local circulations are the sea/land breeze, mountain/valley breeze, chinooks , lake effect storms, mountain waves, city heat islands, and any other circulations which are related to the specific features on the surface of the Earth. Afternoon thunderstorms growing over the mountains in Arizona and New Mexico are prime examples.

Probably the sea breeze is the best known of these circulations, certainly it has been used by generations of fisherman probably before the advent of the written word. It is probable that birds and butterflies use it in their migrations. The sea breeze occurs along any beach. The Great Lakes, Lake Okeechobee, and other smaller bodies of water host these circulations. When winds are light, even rivers have this kind of circulation, sometimes called a river breeze. Even the Potomac sometimes shows up on satellite images because the air over the water and shoreline is clear and clouds are popping up a short ways from the river. Since the approach for at least one of Dulles' runways is over the Potomac, the ride down the glideslope sometimes requires adjustment for the river breeze.

Figure 11-1 shows a cross section from inland Florida, through a typical beach, and out over the ocean. At night, the land cools off but the water doesn't cool as much. The ground cools the air just above and this in turn cools the air through the first thousand feet or so. You can feel it during final in the late afternoon as you turn onto final. You've been in somewhat bumpy air on downwind and base legs. As you turn to final, the air suddenly smooths out as you settle into the inversion and get prepared to flare. The winds may be slightly different in the inversion, but usually not too much. The cool air in the inversion settles toward the beach.

Air in contact with the ocean doesn't cool. The more dense air over the land slides out to sea and forces the less dense air up. This part of the circulation is called the land breeze (we name winds land breezes from whence they came). There is a return circulation aloft which completes the flow. The land breeze starts to set in during late afternoon and early evening when the outgoing radiation from the land is greater than the incoming solar radiation. Since the land breeze operates all night until solar heating in the morning overpowers the outgoing radiation, the turning of the Earth (Coriolis effect ) does have an effect on the land breeze. It drives it to its right, so the wind which is coming from inland is turned to its right along the shore. On the East Coast, the land breeze flows from the north near the beach and from the South aloft. If you want some help coming back from Florida along the East Coast, fly at night, and eat and doze during the day like some birds do.

If conditions are right, off-shore thunderstorms can occur which provide a natural fireworks display unequaled by any Fourth of July display I've ever seen. The wife and I occasionally get to Florida and take walks on the beach at dusk. Once on a while, when the larger scale winds are calm, the offshore thunderstorms spawned by the newly formed land breeze seem to cover the horizon.

As the sun comes up, the heating by the sunlight overwhelms the cooling of the outgoing radiation. An hour or so after sunrise, the air over the land starts to be warmed by the ground. As the warming continues, the air above the ground becomes less dense than that out to sea. The land breeze slows to a stop and the circulation reverses itself. Warmed air over the land rises and is replaced by cool air from over the water. This part of the circulation is called the sea breeze. Cumuli form inland as the heated air is forced aloft. As with the land breeze, the Coriolis force comes into play forcing the sea breeze to turn to a southeast wind.

Along the east coast of Florida, on days when the general winds aloft are calm and when the sea breeze is the only circulation operating, the breeze from the ocean (coming from the east at Daytona Beach) is turned to its right, that is it is turned towards the north, becoming a south wind along the shore. At night, the land breeze from the west is turned to its right towards the south becoming a north wind. On the west coast of Florida, the circulations are the same but the compass directions are changed. On either coast, you find that living is more comfortable, and your air conditioning bills less, if you live within the Sea Breeze. In Daytona, the Sea Breeze extends inland to about Interstate 95, about 6 to 10 miles depending on the day.

Probably the most dramatic meteorological example of the effects of the sea breeze is the "morning glory" cloud which occurs daily along the southern coast of the Gulf of Carpentaria in Northern Australia. This is a particularly spectacular cloud band with only a wind squall and a sharp rise in pressure associated with it. It arrives soon after sunrise each day almost as regularly as the Old Faithful Geyser. It has been carefully studied and appears to be caused by the confluence of atmospheric waves (bores) initiated by the sea breezes from two coasts.

Another circulation common to most places is the mountain/valley breeze. This one is more subtle and is simply caused by the sun heating the topography. Early in the morning, we usually use runway 14 at Montgomery Airpark. Later on, in the middle of the morning pilots switch ends to 32. There are no bodies of water nearby and it is doubtful that the river breezes from the Chesapeake or Potomac affect the change in wind.

Figure 11-3 shows a typical valley in the Eastern U.S. with the river flowing to the right. Valleys in the Rockies are somewhat taller but the same concept holds there in spades. During the day, sunshine warms the air near the ground and it slides along the valley upward. In the Catskills, the smells from town waft up the valley in the daytime. It used to be most pronounced during fall when the smoke from the leaf fires burning during the day would flow up the valley. Since leaf burning has been outlawed, I miss that smell. I don't miss the smell of the burning dump that a neighbor had on his land. It was particularly bad when he got a load of tires in from the local repair shop.

At night, both the valleys and mountains cool off, but higher elevations cool off more as in Figure 11-4. The air slides down the hill towards the valley floor where the cool air pools and makes the smoke hang suspended in the air. Summer smells are trapped in the valley, and fog frequently forms along the river. I think the mountain breeze is more noticeable than the valley breeze probably because the nocturnal inversion which sets up later in the afternoon usually separates the wind aloft from that near the ground. The folk up the hill from the farm heat with seasoned hardwood and the smell of the wood smoke as it drifts down the valley is, to me, one of the pleasant aspects of an evening walk.

An old timer told me once that deer tend to move to the ridges during the day, probably because the smells of the houses drive them away from civilization along the riverbanks. He also said that the deer tend to come down to the streams at night when the air doesn't have as many warnings. Fortunately, the river breezes and valley breezes work in the same direction so the deer in narrow valleys with wide rivers don't become confused. I suspect that the difference in sound propagation between day and night may also have something to do with it, but he might be right.

Both the sea/land breezes and the mountain/valley breezes are driven by the solar cycle. The effects are very much tied to the nature of the slope of the land and the ability of water to store heat. Of course, when a storm is near, the large scale winds mask the breezes. They are still there adding to the wave generated large scale winds - speeding them up, changing their direction, or slowing it down.

As an exercise in studying these, a trip to the southern shores would be just right. The sea breezes operate all of the year, but observing them seems best in winter. Tropical islands are particularly good at showing the sea breeze as they sprout clouds which can be good navigation aids to safe harbor as long as you don't think the shadow of the cloud on the water is the island. Perhaps you can convince your significant other, the IRS, and your banker that the trip is primarily an aviation safety exercise. I've tried. I can't.

Neither of these local circulations normally cause dangerous weather but the concepts are handy when you are planning. A landing in a valley near a large river probably is a factor in determining the active runway. One would be tempted to build airports along rivers to take advantage of the river breeze if there weren't fog and potential flooding problems to contend with. Pilots normally take these into account when planning if they are familiar with the destinations. Not considering them leads to making the landing a little more difficult.

There is one variant of the mountain/valley breezes which can be dangerous, the low level jet in the Midwest. The low level jet occurs during nice weather over a large area. It gets especially vigorous around dawn and can cause some consternation or worse on final. Within a few hundred feet above ground, the winds will be affected by their local topography and the sock may very well be limp. But the Great Plains actually slope a little bit, from around 5,000 feet MSL at the base of the Rockies to a few feet near the Mississippi Delta. This is enough slope to get a dandy valley breeze started but it is above the local valley breeze. And once started it can really move. Wind speeds in excess of 100 knots at 1,000 feet above ground are not uncommon. At 4,000 feet, the winds will probably be light. Above this level, there will be a return circulation, but the return is apparently quite complicated involving a southerly component at one elevation and a northeasterly component at another. Neither of these circulations is as fast.

Imagine flying downwind in one of these, say a late night trip from the enver area to Oklahoma City. The winds are smooth but you are moving right along. You enter the pattern at 900 feet above ground level for runway 29 and by the time you've set your flaps you find you should be turning base. Slowing down on base, you find you're crabbing just to keep the runway in sight. The VASI says you're low. Turning final, the plane seems to struggle back to the airport, and you find you need to keep the rpm up there just to get to and maintain the glideslope . As you descend below 400 feet, there is a slight bumpiness, and, seconds later, you find you are over the threshold at 200 feet. Time to go around. Or, you can anticipate it and be ready with the flaps and throttle. The low level jet has been implicated in a few NTSB investigations of fatal accidents. So if you're into night flying, be aware of these. I've seen similar flows, not as fast of course, over the Piedmont in the East.

Mountain waves are different. They are not really circulations in the sense that they are not caused by local heating or cooling. However, there are often circulations within them near the ridges called rotors which have been known to pluck wings from aircraft or cause planes to descend at unusually high rates of speed and impacting the ground before the pilot and craft can recover. I can testify that between the peaks is not the most pleasant place to be during a session of mountain waves.

At 5,500 feet and higher over the Appalachians, most of the time all you feel is a gradual lifting and then sometime later a gentle settling of the aircraft. You feel you need to be constantly trimming to keep the altimeter pegged. If I'm not in a TCA, I usually let the plane ride up the hill and down the other side.

Once in a while with a stable west or northwest wind, the ride can be rough throughout the troposphere. In Class A air space, the ride can be rough enough to give flight attendants trouble. And, even if you're so equipped, you can't even run up into the stratosphere to hide from the clear air turbulence. Often it is amplified up there.

Figure 11-5 shows a cross section of the atmosphere above a long north-south ridge with winds blowing from the west. The situation is extremely similar to the way a rock below the surface of a flowing stream deforms the surface. The airflow will be forced up over the mountain, and the air above it forced up by the lower level flows. If one layer happens to have a high relative humidity a cumulus lenticularis cloud might form over the ridge. If that layer is very near the ground, the clouds may obscure the top of the hill and the usual radio and TV towers firmly attached to the crest.

The dotted line which I've drawn in from the crest of the mountain up and into the stratosphere connects the points where the amplitude of these waves is greatest. Usually this is a simply curved line. The line often bends upwind but sometimes bends downwind, meaning the crests of the waves above the mountain are sometimes upwind or downwind from the mountain. From extensive computer simulations, it appears that the curvature is not simple and depends on the vertical changes in the stability of the flow. Some of the most sophisticated model runs show that the mountain waves extend well into the stratosphere, even becoming more pronounced the higher you go into the stratosphere. Mountain wave activity and associated turbulence have been reported by pilots flying in the highest flight levels. And, I have heard rumors from the secure side of the military that they have noticed them too.

Of course, if you're a glider pilot, mountain waves are excitement time. Both the altitude records and the distance records have been set by sailplane pilots using mountain waves for lift. Finding the upwind edge of a solid mountain wave is almost as good as pushing the button for top floor in a glass hotel elevator. One of my former students held class spellbound as he described an encounter with a wave. He was clad in winter down clothes, had oxygen bottles and a special recording altimeter for his record attempts. I can’t recall how high he got but I know that sailplane flights over 40,000 feet are commonplace today.

A good deal of study of these waves has occurred, especially in the Rockies. Near enver, one of these waves apparently brought down air from the Stratosphere to near the old Stapleton Airport. Winds at ground level were blowing around 60 knots and had the same ozone content that measured stratospheric air had upwind of the mountain. Landing might be easy if there were no gusts but it might be interesting trying to taxi. The Stapleton experiences point up one documented major point about mountain waves, they can produce some strong downdrafts. At least one small plane has been unable to reach the ridges because they were climbing at 1,000 ft/min and the downdraft was 1,100 ft/min.

If you are climbing over the flat land to traverse the mountain ridge and you are aware of the possibilities of a mountain wave, a look at the clouds, a check of the instruments, and the feel of the aircraft should tell you if you need to back up to find a mountain-wave-induced updraft to gain the extra altitude you will need. Often there is an updraft-downdraft couplet of winds parallel to the ridges and some distance downwind of the ridge as in Figure 11-5. If you can find the updraft part of the couplet and use the additional lift to gain altitude, you've saved gas and gotten the critical altitude. Don't stop climbing when you reach the ridge altitude. Between you and the ridge is a region of downdraft which can cost some of the altitude you've gained. Just be sure to stay out of the rotor which will usually be directly above the base of the mountain. If the wind starts to be very bouncy as you edge into the mountain wave, ease back away from the mountain to find the updraft.

Often the wave effects from the mountains will be felt hundreds of miles downstream. I've seen the effects of the Appalachian Mountains while I was over the Chesapeake. It took a while for me, a new pilot, to catch on to the reasons the plane wouldn't maintain altitude for a while. A few minutes later I couldn't seem to keep it down. For a while, I thought it was me or a malfunction in the plane. Then, I caught on. Now I simply retrim , and observe the passing aluminum. There's lots of it.

Often in mountain wave situations you will see clouds which look like giant white eyeglass lenses on their side, called cumulus lenticularis. From a distance, lenticularis look like they are standing still, just remaining motionless over the peaks. They aren't. Lenticularis are constantly forming upwind of the peak and decaying downwind. They are to be avoided as they usually contain considerable turbulence. Because they are high in the very cold temperatures near the top of the troposphere, they form fast and decay fast. Droplets don't have time to form snowflakes so they are highly supercooled. In these conditions you are likely to pick up a load of rime ice fast.

The effect can be there in lower-level stratus, especially over the Appalachians during wintertime IFR. If you and yours are moving along through a layer of stratus over mountains, the ridges may force the stratus up, cooling the droplets in the cloud even more. The droplets, which weren't quite at a low enough temperature to stick to your prop and wings, with the additional cooling because they are moved upward, may start to stick. Forewarned is forearmed.

On the other hand, the stability may be just right for a return flow in a valley. As you're flying over undulating country, and the wind is reasonably strong, the circulation in the valleys below may be that shown in Figure 11-6. Here the friction of the air above the peaks drags the air in the valleys along but when it runs into the ridge it descends and flows backwards along the floor of the valley towards the other mountain. If it picks up a bit of moisture in the valley, clouds may form as the air rises. If you really have to land on a field down there, perhaps the engine has decided to take a nap, and you note a strong mountain wave situation, you can expect winds to be flowing in the same direction the wins upstairs. No waves, or minimal wave; don't be surprised by a return flow circulation as in Figure 11-6. Be prepared to make adjustments in your plans if the only field is a short one. Landing with the wind can be very hard on the aircraft and you. Fortunately, this is relatively rare and you can pick it up when you ask for the active runway. If the runway choice seems backward, suspect some return flow in the valley and prepare for some shear in the pattern.

Usually, clouds will be on the upwind side of the ridges where wind flows upward along the ground and is so moisture-laden that clouds form before the air gets to the top. Perhaps the best illustration involves the Rocky Mountains and the Coastal Ranges in Northern California and northward. Figure 11-7 shows what happens when humid air flows from the ocean and up over the ridges. Suppose the air temperature is 70 degrees Fahrenheit and the dew point is 60 degrees. Then the temperature of the ascending air in front of the mountain drops 5.5 degrees per thousand feet and the dew point of the same air drops half a degree for the same 1,000 feet. As the air approaches 2,000 feet above sea level, the temperature and dew point approach the same value, and condensation of dew on dust in the air begins - call it cloud. During the rest of the ascent up the mountain the temperature and the dew point are the same and they change only about 3 degrees F per 1,000 feet because the heat of condensation of water slows the cooling of the air. Cloud droplets impact the leaves and needles of the trees as well as collect on mosses and other flora. Some convert to rain and even snow if temperatures are cold enough. Much of the water, which only a short time ago was vapor, falls from the cloud to swell the rivers and promote growth of the plants which grow in abundance.

Once over the top of the mountain the temperature and dew point, in the example of Figure 11-7, are about 50 degrees F until the air starts downhill. Then the temperature increases at 5.5 degrees F while the dew point increases only 0.5 degrees F. They soon separate as the cloud evaporates in the downdraft. When the air arrives at the same elevation as the ocean, the air temperature is now 77 while the dew point has only increased to 52.

The best place to observe this phenomena is in Hawaii; however, tropical islands in the Caribbeans are also good. I've been trying for years to go to some of these islands to study them but so far I have been unsuccessful as the wife would rather do our holidays elsewhere.

There is another mountain effect we've been noticing for the last decade, the mountain jet. The term "jet stream" has evolved over the years. It once meant only the very high, 500 mb to tropopause , jet stream which had a minimum of 50 knots and usually found just to the west of a cold front. A similar jet stream was found over the tropics but wasn't as fast. So the term became more general. After the low level jet was identified and mapped, the term became more general since the low level jet had a top, at a few thousand feet above its core. Even the 50-knot wind criteria was dropped. The term "jet stream" now refers to any identifiable and mappable thin layer where the wind speed along the flow is significantly higher than that which surrounds it. I have heard the term used with the alongshore return flow in a well-developed sea breeze.

With the new radars, meteorologists identified a new type of jet stream associated with mountain ranges and cold air outbreaks. As a cold front comes over the mountains pushed by the movement of the upper level waves, the cold air spills over the lower mountains, and the Coriolis force turns the cold air to its right. The force pulls it southward along middle of the piedmont. This produces a dimple in the height of pressure surface at the elevation of the ridges. Figure 11-8 shows the situation.

The Appalachians form a series of ridges going from the Southwest to the Northeast. When the winds above them are from the Northwest, the flow is close to perpendicular to the ridge lines. If that air is stable, the flow will descend somewhat well east of the ridges. When the air descends, the pressure at the surface increases a little bit because of the added dynamic pressure. There will be a belt of relatively low pressure near the mountains, and the combination will cause the winds below the ridges to shift to a northeasterly flow. This tends to bring in cool air from up North. Since cold air is more dense than the warmer air along the base of the mountains, a pressure difference occurs at the elevation of the crest of the mountains. The pressure gradient tries to force air at 3,000 feet away from the mountains, but the Coriolis force turns it to its right, forcing it to flow parallel to the ridges south-westward. This type of jet produces wind speeds of 50 knots or so from the surface to about 3,000 feet for the Appalachians, higher for the Rockies. When this happens, the pressure differences balance out and the jet decays, but not without a good deal of turbulence and possibly a thunderstorm or two to the east of the jet.

The circumstances for this type of jet in the east are relatively rare; however, along the Rockies these jets are seen quite frequently. It will probably turn out that monarch butterflies have known about the mountain jet for millennia but haven't passed the knowledge to us.

To complicate matters, the mountains can enhance the flow of southwesterly winds. If the cool air is coming over the mountains slowly and there is warm air to the east, the pressure difference at the height of the mountain ridges will add to the speed and change the direction slightly of air already moving from the south-west. The importance of the mountains is yet to be seen. It will have to wait until the data are studied more or until we learn how to speak Canadian goose.

How big a hill will affect the winds? Any size hill will affect airflow, even the side of a garden slope. And speaking of gardens, I have had my students out to measure the temperature differences across a lawn on a nice spring day. There is considerable difference, often as much as 6 degrees between the measurements, where all measurements were at a standard height above the lawn, made with calibrated thermometers, and all were in shade. So how far do you carry this? Fortunately mathematics comes to the rescue again, this time from statistics.

Suppose each of the fields, hills, and water bodies has its wind field distributed in its region. A field on a hill would have a wind field distribution which is made of the sum of the effects of the hill and the effects of the field. If there were a pond in the field which was on a side hill (the pond would have to be on a flat part of the field), the distribution of the winds reflects the sum of the three effects. Now add the effects of the river at the bottom of the hill, the fields next to the river, and so forth.

As you fly over, your aircraft samples the results of the sum of all of the distributions of the wind fields. When you do all of the sums, a statistical theorem shows that you are measuring a mean (or average) wind and fluctuations which follow the normal (also called the Gaussian ) distribution.

Figure 11-9 shows the results of over 3,000 wind measurements made at 500 feet above ground. The measurements were made over a period of a few hours, and then the number of occurrences of winds within specific ranges (around 0.4 knots for the widths of the range) were plotted. Notice the bell-shaped curve which is characteristic of many different phenomena in nature. This is an experimental validation of the normal distribution. Most of the data of this type show the normal distribution except when they are taken very close to rough terrain. One wing length above a smooth runway, you should expect the turbulence, or variation of the wind speed to be normally (or Gaussian ) distributed. Notice there are no values on the graph in Figure 11-9. This is because the width of the curve will vary somewhat from day to day. When the winds are reported at 20, gusting to 28, you can judge the width of the curve somewhat by assuming the reported gust speed at anemometer height (about flare height) is close to the right edge of the curve. The slowest gust would be expected to come in at 12 knots.

I know, mathematical purists would say that one should expect an infinite speed gust. However, the statistics don't say how large in size a really fast one should be. Since we've never seen an infinitely large gust, it is safe to say that if one exists it should have a size smaller than the anemometers we measure winds with. I don't think the infinite large gusts will affect a much larger plane. This is probably a good time to say something about carrying a little extra speed on final on a gusty day, but I won't get into that argument. I'll let the FAA, your instructors, and everyone else in the hanger give you advice. Your choice.

One caution. There is always the possibility of some unusual wind gust even in the calmest days. I was once surprised on a beautiful calm summer evening by the wake turbulence from a Cessna 150. It almost upset me well into final. So don't be complacent even in the final seconds before the rubber meets the tarmac.