The ‘other’ wind shears

Its gusts can cause rapid changes in airspeed, so rapid that a stall or overshoot can happen if pilots don’t anticipate them. Very often, a thunderstorm’s downbursts or outflow boundaries are to blame for these sudden events.

Vertical wind shear is a change in wind speed or direction with height. This is very common, as anyone climbing a few thousand feet can attest to the changes in groundspeed or required wind correction angle that so frequently occur with altitude. (Too bad it seems that so many of these changes result in encountering stronger headwinds!) On the other hand, vertical shear occurring within deep, moist, convective cloud masses can cause issues that go way beyond annoyance. If these winds strengthen and change direction with altitude, this sort of cloud-layer combination of vertical and directional shear can cause updrafts to strengthen, and the rotation of this wind shear field can set up supercell thunderstorms. Research has shown that if the shear occurring in the layer between the surface and 20,000 feet or so (about six kilometers) reaches a difference of 20 to 40 knots, then conditions are favorable for organized thunderstorms.

Add a change in wind direction with altitude—directional shear—and the stage is set for the cloud mass to begin rotating. When wind speeds increase and wind directions veer (turn in a clockwise direction, say, from 180 degrees at the surface to 270 degrees aloft) with altitude this allows warm air to enter the cloud mass, boosting upward movement and creating tornadic conditions. On the hodograph of a Skew-T Log-P thermodynamic diagram, you can see this turning of wind directions at altitude plotted as a circular path.

Speed shear happens when winds aloft change speed. Imagine a band of steady, fast winds suddenly encountering air moving at a slower speed. The faster winds “pile up” against the slower winds ahead of them, adding mass and causing convergence aloft. Or think of a line of cars moving at 70 mph, and then the cars at the end of the line suddenly slowing to 30 mph. This would cause a “gap” between the fast and slow cars—a gap we can visualize as divergence aloft.

What do convergence and divergence aloft have to do with weather? Plenty. It has to do with something called Dines compensation. Converging winds aloft produce subsidence, and the sinking air warms as it descends. Result? High pressure and few clouds. Diverging winds do just the opposite. The divergence creates a “vacuum” aloft, and low-altitude air rushes in to fill it, creating rising air that cools and condenses into clouds and less friendly weather.

So, shear’s a big deal, no matter if you’re on short final or in cruise. And yes, you can encounter all three types of shear at once, which would make for a very bumpy ride indeed. On winds aloft charts, you can get an idea of where turbulence might lurk by looking for areas where wind speeds and directions change abruptly. Pay special attention to the cores of highest jet stream winds at Flight Level 300—that’s where triple-digit wind speeds both change direction as well as speed up and slow down. That, or check out the Aviation Weather Center’s advisory pages for areas of both high- and low-altitude turbulence.

A barograph conundrum

What’s the difference between a weather geek and a weather dweeb? A geek has one barograph, a dweeb has two. That makes me a dweeb.

A barograph measures atmospheric pressure using a set of expandable aneroid wafers, a movable arm with a tip like a fountain or felt tip pen, and a rotating drum with a chart on it. The chart is labelled in inches of mercury or millibars. A small motor turns the drum, and the pen plots the pressure on the chart as it turns. You can watch the pen’s trace as it goes up and down as a week’s worth of pressure changes go by. This all assumes that the drum rotates, and the pen makes a visible trace.

I had an old Taylor Instrument barograph but gave it to my son. To replace it, I bought another barograph, this one a fancy, eight-aneroid model called the “Atlantic.” It even has a carrying handle in case you want to move it around.

This barograph uses a tiny felt tip pen to make its traces, but you can barely make them out. With the old Taylor you used an eyedropper full of red ink to fill the pen, and it makes a fat, red, easy-to-read trace of the week’s pressure antics.

So, I went on the hunt for an old Taylor barograph. (The company doesn’t make them anymore.) I finally found one online. It was from the 1960s but looked and ran OK—until the motor seized up. Now I have to figure out how to fix or replace an obsolete motor.

The decision tree is pretty straightforward from here. Pitch the Taylor if it’s junk and learn to live with squinting at the Atlantic’s nearly invisible traces. That, or pick up a National Weather Service-certified unit that probably costs thousands. In an upcoming “Wx Watch”: cautions on mounting an anemometer mast on your roof.

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