Turbulence, shear and mountain waves

The atmosphere is a fluid in motion. Everything here follows from that one idea — a stream develops waves and eddies as it passes an obstruction, and so does the air.

Weather Information — FI.III.C · CA.I.C · PA.I.C

Where it comes from

Mechanical turbulence is the air tripping over things — terrain, buildings, tree lines. It is why the last two hundred feet on a windy day are rough at one airport and smooth at another with the same wind.

Convective turbulence is rising and sinking air on a heated afternoon. It has a daily rhythm, which is the useful part: the same route flown at eight in the morning and three in the afternoon is two different flights.

Frontal turbulence comes from the lifting at the boundary between air masses, and from the shear across it.

Wake turbulence is made by other aircraft — a by-product of lift itself, so it exists whenever a wing is working, and it sinks and drifts.

Clear-air turbulence happens at altitude in cloud-free air, associated with wind shear and particularly with the boundary between a jet stream core and the air around it. The name is the trap: nothing visible marks it, and the smooth air you are looking at is not evidence.

Mountain waves

Treat the atmosphere as a fluid and this becomes obvious. Air meets a ridge with sufficient wind and a stable atmosphere, and a wave forms — just as a stream does over a rock. On the upwind side the air rises in a smooth, strong updraft. Past the crest it becomes turbulent, and the wave propagates downwind for a long way.

Two things follow that pilots consistently underestimate. The wave can extend far downwind of the ridge, so being clear of the terrain is not being clear of the wave. And the downdraft on the lee side can exceed the climb performance of a light aircraft — the aeroplane is not underpowered, it is in air going down faster than it can go up.

Low-level wind shear

Shear is a change in wind speed or direction over a short distance, and low down it is an airspeed problem before it is a comfort problem. It is not only a thunderstorm phenomenon: a temperature inversion with a strong wind above it, a frontal passage, or terrain funnelling can all produce it on an otherwise benign day.

The severe case — the microburst — is covered under thunderstorms, because the sequence it puts you through deserves its own treatment.

How it is reported

AIRMET Tango carries moderate turbulence, sustained surface winds of 30 knots or more, and low-level wind shear. Severe turbulence is a SIGMET, not an AIRMET — the step up in product is itself information.

And as with icing, PIREPs are the real currency. Turbulence intensity is defined by what it does to an aircraft and its occupants, so a report is worth more when you know the type that filed it: what is light chop in a jet is not light chop in a trainer.

What is in force right now

Keep going

• Thunderstorms and microbursts
• Structural icing and the freezing level
• PIREPs, AIRMETs, SIGMETs and TFRs
• Winds aloft over the US

Sources

Mountain wave behaviour follows the AIM 7‑6‑7; turbulence classification and reporting are in the FAA Aviation Weather Handbook (FAA‑H‑8083‑28).