Wind Shear in Temperature Inversions

[Trever Oakes]

I'm having trouble understanding why wind shear occurs in areas of temperature inversion.

I've been out on calm nights where I could see a layer of fog/smog which indicated a possible inversion.It is very calm aloft but when coming in for landing, it gets pretty bumpy sometimes.

Can someone explain how this works? I always thought of inversions as a calm atmospheric occurrence.

 

[dmspilot]

It occurs at the border between the inversion layer and layer above it. They're different layers of air and may be moving at different speeds and/or directions. Crossing the layer border, the wind changes...definition of windshear.

I have noticed that frequently on clear calm nights (when inversions most often happen), at least in the eastern part of the country, wind may be dead calm at ground level, but 25 knots or more at 2,000 feet.

 

[Clip4]

I suppose there are different inversions in different part of the country, but think of being in moving air at a altitude and losing the headwind when descending into the calm air. Windshear is a change of velocity or direction. As DMS described, the warm air is moving on top the more dense still air.

 

[Capt. Geoffrey Thorpe]

Wind above can lack vertical movement because it is isolated from surface effects - makes it feel smooth. Air below can be pretty much just sitting there. In betwixt you have mixing which can be bumpy. Or, the inversion could be breaking up, that can be bumpy.

I recall one early morning when the wind was close to calm on the ground, but as I was climbing out, I noticed that I really wasn't making any forward progress due to the strong headwind above the inversion (I got passed by a golf cart - really). In that case, the transition between the air masses was smooth. But weird.

 

[SloRoam]

I've also had really bumpy air in the mountains above the inversion due to wind and dead calm and smooth a hundred feet below just inside the inversion.

Remember that an inversion is air getting warmer as we ascend, then cooling again at some point. This can make for some strange air currents in valleys as daily heating and cooling takes place, and also as the "lid" moves up and down during the day.

I almost always expect to feel bumps at the interface.

You can learn more by studying the skew charts btw.

 

[Dan Thomas]

On the prairies I often encountered shear in the morning. As the day warmed, the shear layer would drop until you might find it at 100 feet. One learned to pay attention on final if your groundspeed was low due to strong headwind, but the windsock was dead. Speed up some to mitigate the airspeed and altitude loss as you pass through the shear.

 

[AKBill]

Not why but here in SE AK wind shear is normally around 2000 feet. With the hills around here mechanical turbulence is often present

 

[Martin Pauly]

A temperature inversion acts like a vertical barrier for air. Air below the inversion stays below; air above it stays above. This stability is disrupted, of course, when the air below is heated significantly.

Without this barrier, there will be some vertical movement or air parcels, e.g. induced by thermals. These air parcels going up and down exchange energy between lower and higher areas in the atmosphere, and contribute to evening out different wind speeds and direction across altitude. When an inversion exists, that exchange of air parcels between low and high altitude doesn't take place. The inversion is like a boundary layer between air below and air above. Wind direction and wind speed can change significantly over very little altitude.

That's why on many days the surface wind picks up late morning, when the sun has heated up the ground and thermals start, while at night the thermals stop and friction slows down the surface wind - but only below the inversion. Above it, the wind doesn't change at night.

Here's a picture I took a few days ago, when the temperature inversion was very visible. All the haze and humidity was below; above it was dry air and great visibility.

- Martin

 

[FlightInsight]

During the day, ground heating causes thermals which have the effect of mixing surface air with winds further aloft. Typically, surface winds are a bit weaker and veer inwards toward low pressure due to a weakened Coriolis Effect. After the sun sets and thermals dissipate, there is less mixing of surface air and air further up, so a sharp boundary layer exists just above the surface where dangerous wind shear can occur.

The Coriolis Effect is what turns winds parallel to isobars (or lines on a chart of equal pressure) As pressure gradients cause winds to flow, the Coriolis causes them to turn (to the right in the Northern Hemisphere).



On a typical night after a day of heavy winds. You can expect to see winds aloft remaining strong, but experience wind shear close to the surface as you descend below this boundary layer into calmer air. Now is the season for night time wind shear as days are dominated by heavy winds.

Dan, CFI