[JohnF]

Remembering the last time I was on a plane I've thought of a question about an encounter we had with turbulence...

We're flying almost at cruise altitude, I'm guessing we're at 25,000 to 30,000ft with low level cloud(overcast) at around 2,000 to 5,000ft.
Looking out the window I spy below us an area of bumpy cloud(an area where the cloud rises up in lumps!) in the otherwise flat cloud and correctly guess thatmeans we're going to encounter turbulence as we pass over it.
Which as it happened turned out to be quite bumpy, but nothing serious.

My question is what meteorological effect can cause such a relativly small area of low level cloud to be "bumpy" and then have enough force to force an effect 20,000ft above it?

Would the bumpiness be caused by something under the cloud? And why would it be so localised? As the rest of the flight, before and after was completely smoooth.
I'm presuming it was a thermal of some kind but, again, why so localised?

 

JohnF

 

 

[Paul Domaille]

Hi John,
I can think of a number of reasons why clouds would have a "lump" in them, I have seen it myself a number of times. Orographic lift, forcing air higher. Isolated convection, lift due to power station outlets, possibly UHI effect etc etc. I would have to guess at the upper air turbulence, unless it was pureley coincindental, when air is lifted or forced up at lower levels it would push against the air above in effect lifting a whole column, I don't know how long thi effect would take to reach higher altitudes though as there would bound to be some compression effect on the way up. Interesting quetsion though.
Cheers

[cromergoer]

Would this be a similar effect when crossing the coast? I've noticed, on a couple of occasions, flying above the cloud, a bit of turbulence (well, alright, a bit of a twitch, really) and on checking the route follower up front, we're crossing the coast.

 

When travelling with a nervous co-passenger, a cheery "we're crossing the coast" works wonders, but is it the same, or am I imagining it ? [dunno]

[Paul Domaille]

Hmm, it could be Malcolm. So many factors can come into play, at higher altitudes Clear Air Turbulence (CAT) is the common cause. When parcels of air are moving in relation to each other, remember, at 500mph you don't actually need a huge variation in wind speed/direction to cause some quite nasty turbulence.
Turbulence over coastlines is quite common on cloudless days especially at lower altitudes where the water/land boundary has different insolation properties...the basic cause of diurnal winds. There are some far more knowledgeable folks on here who could I'm sure give a better answer.
Cheres

[Dan G]

I would say this is almost certainly "wave", a phenomenon that allows sailplanes to regularly reach over 20,000' in Britain (I will one day!).

It's basically a train of ever-larger standing waves through the troposphere downwind of anything that deflects the prevailing wind upwards - hills, mountains, even cumulus streets will do it. For a more detailed answer, take a look at this article on waves, particulary figure 7 which shows how bands of turbulent wave reach higher and higher into the atmosphere downwind.

I'd bet the lumps were where the clouds were moving hills, and you flew through the wave from those hills.

[Falconjet]

Listening to aircraft on the scanner for many years I've noticed that high level traffic flying just ahead of the CS cloud shield of warm front always encounter turbulence much of it continuous moderate turbulence. Pilots are forever changing their cruising levels in an attempt to circumvent the worst of the bumpy ride for their passengers resorting to asking the controller if other aircraft flying in the vicinity are experiencing a smoother ride at a different level. I'd imagine the turbulence in this case is caused by the high level jet with associated jet streaks.

Cheers,

[Matt D]

If anyone has access to it, there is a very informative section (3.2.2.3) on turbulence associated with jet streams (and how to indentify regions of potential turbulence on satellite images) in the superb book by Bader et. al (1995), Images In Weather Forecasting.

Cirrus/upper level moisture boundaries and the shear created by jet streaks are a breeding ground for medium to upper level turbulence.