Wx Watch: Calm up high, roaring down low

It’s a question that often pops up in the minds of high-time airline captains and beginning students alike: Why do winds increase with altitude? Are there any exceptions to this informal rule? Sometimes I get e-mails from pilots wondering about strange, weaker-than-usual winds aloft, many times with descriptions of increasing winds during the descent phase of the same flight. Yes, this sort of phenomenon happens from time to time. Explaining this flip-flop in wind speeds poses a challenge, though. If you e-mail me about this or that weather situation, be sure to attach weather charts, METARs, winds and temperatures aloft data, and any other pertinent information to help me figure out the probable reasons for any weird weather.

Friction effects

We all take it for granted that wind speeds increase with altitude, but never give a thought about the reason. Maybe it’s because a full-blown explanation would take up too much space in textbooks. Maybe it’s because we just accept it, use the wind information from our briefings to plan our flight, and move on. But knowing more is better than knowing less, and you never know when someone will pitch you the question—on the assumption that pilots must certainly know everything about the medium they fly in.

The first reason for increasing winds at altitude is easy to grasp. It has to do with the planetary boundary layer (PBL). This is the layer of atmosphere that lies just above the Earth’s surface. Depending on elevation, the PBL can extend to 5,000 feet agl or so. This layer is mainly influenced by solar heating of the surface during the day, radiational cooling of the surface at night, mechanical turbulence created by mountains and valleys, and even snow cover and vegetation. Generally speaking the PBL is thinner at night and thicker during the daytime—especially in the warmer months of the year. How can you tell where the PBL ends? It’s marked by a temperature inversion. A look at a nearby 12Z atmospheric sounding can give you the answer, if there’s a radiosonde station near you. Where the temperature trace goes from warmer to colder, that’s the top of the boundary layer.

When wind blows in the PBL, it encounters friction from interacting with the surface. That’s a big reason for slower speeds at lower altitudes. It also explains low-level turbulence, as anyone who has taken off after cold frontal passage can affirm.

Because this friction slows down the air moving in the PBL, it has an effect on Coriolis force. (Hey, wake up! This won’t take long. The word Coriolis has a soporific effect on all pilots.) Slower-moving air near the surface is air that spirals more directly toward a center of low pressure. That’s because friction reduces Coriolis force. But the pressure gradient force retains its strength. The net result is that wind in the PBL crosses isobars at about a 30-degree angle.

Above the PBL, friction is less of a factor. Also, PBL air isn’t mixed with the air above it—thanks to that inversion “barrier” to vertical motion—so diurnal temperature fluctuations don’t occur. It may be 90 degrees Fahrenheit/32 degrees Celsius at the surface, but at 10,000 feet look for more like 32 degrees F/0 degrees C! But when heated air in the PBL forces its way through the inversion, look out! When rising parcels of heated air hit that colder air aloft, expect thunderstorms.

Decoupling and the PBL

The inversion atop the PBL plays a big part in the Midwest in the warmer months of the year. In the summer, there is an interaction between the inversion and the air above it, with a lot of mixing. So convective eddies interact, and slow down, the free-moving air above the PBL during the day. But at night, the surface cooldown intensifies the inversion, convection is suppressed, and the air above the colder, denser PBL “decouples” from the surface air and is free to fly over it unimpeded by turbulent convective eddies.

Unimpeded indeed. Many times, this air moves at speeds up to 60 knots or more, beginning at altitudes as low as 1,500 feet agl. For this reason, these winds are called low-level jet streams (LLJs). They’re a major force in feeding warm, moist air from the Gulf of Mexico northward. LLJs feed convection in the Midwest, and can spawn mesoscale convective complexes (MCCs). MCCs are large clusters of thunderstorms that can cover many counties—and even entire states—and are primarily a nocturnal phenomenon.

Anyone flying in the center of the nation may experience LLJs during descents from higher altitudes, which explains this sort of reversal of the “stronger winds aloft” preconception. So a word to the wise: A strong, low-altitude wind out of the south is a bad sign in the heartland.

Thermal winds

In previous articles, we’ve talked about the relationship between temperature, pressure, pressure surfaces, and wind speed. This is called the thermal-wind relationship. Here’s how it goes, in a super-simple nutshell:

• decreases with altitude.
• pressure decreases with altitude.
• height of a pressure surface varies according to temperature. (Just remember the old, “high to low, look out below” warning about changing your altimeter setting.)
• temperatures mean higher pressure surfaces (for example, the height contours on constant pressure charts—and ditto on the “high to low” warning.)
• temperatures mean lower surface pressures. (Again, the origin of the “high to low” warning.)
• colder and warmer pressure surfaces closely abut each other, a pressure gradient is created. This is visible on a constant pressure chart by observing closely spaced height contours. So there’s a height gradient as well.
• steeper the pressure gradient, and the more radical the temperature gradient, the faster the winds aloft. In this, winds aloft follow the same rules applied when there are closely spaced isobars on a surface analysis chart. But in the flight levels, where the air is thin, a temperature gradient of five to 10 degrees C over a 100-nm cross-section can produce a 50-knot difference in wind speed. It’s these high-altitude temperature differences that create jet streams.
• where temperature gradients are less steep, wind speeds are lower.

Where would you find lower-than-normal wind speeds at high altitude? Along the axis of a trough aloft is one good place to look. So is a big ridge of high pressure aloft. These are places where temperature contrasts are minimal. In other words, any place where weak temperature/pressure/height contour gradients occur, that’s where weak winds aloft are most likely.

Meanwhile, closer to the surface, stronger winds aloft can at the same time be created by surface-based fronts. So someone descending from a tranquil temperature/pressure setup at high altitude can certainly encounter strong frontal winds associated with the tighter temperature and pressure gradients of surface weather—as shown on a surface analysis chart.

“I’ve always found that there is a difference between what the met books teach and what you find whilst flying the line,” wrote Cormac Brady, an airline captain who flies trans-Atlantic routes. Prior to a September, 5, 2008, descent into Cork, Ireland, Brady encountered mere five-knot winds at Flight Level 390, but ran into 30- to 54-knot winds below 6,000 feet—and right down to the blustery landing. A look at the charts for that day showed Brady smack-dab in a trough aloft west of Ireland. But at the surface, a compact low with tight pressure gradients and a wrap-around cold- and occluded-front setup was hard at work. Brady saved the charts for the day in question, which made an explanation for the wind situation a lot easier. He also sent me some Irish good luck in one of his e-mails. Now I’m passing it along to you.

E-mail the author at [email protected].