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During a normal flight at cruise speed and altitude, amid the delicious airline meal and the enjoying a latest movie, all of a sudden the seat belt sign turns on.

Then a few seconds later we enter turbulence, grab a hold of that half-finished soda cup and wait patiently for the bumps to be over while looking out the window.

So, how exactly do the pilots know that the airplane is about to enter in an air pocket which will cause turbulence?

... or does it happen like this?

I have already gone through the excellent answers here and here.

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There are a few ways that pilots are aware of potentially turbulent areas.

Eyesight The most obvious way is just by looking outside and observing the sky. Large billowing clouds, called cumulus clouds, indicated pockets of unstable air (the clouds are rising because the air under them is as well). If the pilots must fly through these clouds then its a safe bet that there will be some turbulence.

Weather Radar Just like using your eyes, except the radar can see further through haze and other clouds. Typically this is useful for finding embedded thunderstorms, but it can also be useful to find areas of potential turbulence.

Communication Pilots talk. Both to each other and to air traffic controllers. En route controllers frequently ask pilots for "PIREPS" (pilot reports), to build an accurate picture of the flight conditions at different altitudes. Often commercial aircraft will request to change altitudes or deviate around weather/ turbulence, so it is in the best interest of the controller to know ahead of time where the bad flight conditions are, and have a game plan of how to route traffic. This makes it much easier for the controller to route traffic, rather than getting request after request from individual aircraft.

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    $\begingroup$ All very true, however, at cruise altitude (of jets) most turbulence would be caused by clear air turbulence (CAT). CAT is largely invisible to eyesight and weather radar, so pilots would depend on communication to avoid it as much as possible. $\endgroup$
    – Ben
    Nov 12, 2014 at 10:35
  • $\begingroup$ @Ben: Thunderstorms usually do reach all the way to jet cruise levels and are responsible for significant portion of turbulence there. But yes, clear air turbulence is significant and not detectable in advance. Sometimes it can be predicted, i.e. when you know where the jet stream is. $\endgroup$
    – Jan Hudec
    Nov 12, 2014 at 12:24
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    $\begingroup$ @reirab I can confirm I've been on the inside. Its not pleasant and it isn't always on purpose, but it happens. The things you really want to avoid in big convection are the updraft and near the updraft where the big hail is falling, while keeping in mind small hail can be lofted in the anvil. $\endgroup$
    – casey
    Nov 12, 2014 at 14:48
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    $\begingroup$ @vsz: Cumulus clouds probably not, but cumulonimbus clouds often extend well above 40,000 ft. And when there is a squall line across your path, you use the radar to avoid the worst parts but there is some turbulence wherever you fly. $\endgroup$
    – Jan Hudec
    Nov 12, 2014 at 17:59
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    $\begingroup$ I have literally had to deviate off course because the heads of cumulonimbus clouds were towering above our service ceiling of 40k. $\endgroup$
    – Rhino Driver
    Nov 13, 2014 at 0:14
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At night time and with a broken radar....

The above mentioned methods are good but the ability to plan ahead and perhaps even give you that brief whilst we are on the ground is due to the fact that we always carry the latest Significant Weather (SIGWX) charts that cover our route.

Here's what one looks like (view in new tab):

SIGWX chart

You can find a legend for such a chart online but basically the thick bold lines indicate strong upper level winds (also known as jet streams). Crossing one of these at right angles is what brings the most amount of turbulence.

Second to that, the areas contained within the cloudy bubble like shapes (making it very easy for you to understand here ;) ) are areas in which we can expect to find CB clouds or other storm related activity. Of course, flying through a big lumpy CB cloud will almost always have turbulence associated with it. ISOL EMBD means the CB clouds are isolated (relatively speaking few and far in between) and may also be embedded inside other types of clouds that are in that area. Most of the time and especially in winter we fly above these areas anyway (for cruise at least).

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    $\begingroup$ Does the volcano signs mean that those volcanoes are expected to be active? $\endgroup$
    – orique
    Nov 20, 2014 at 7:56
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TL;DR clouds and a Doppler radar in the nose registering uneven air.

If the plane flies into clouds then there will get some turbulence because at the boundary the air will change, especially in thunderclouds.

Clear air turbulence is more devious as it happen in clear air (like it says in the name), these are detected by instruments on the plane like a Doppler radar that can detect the uneven air ahead.

Besides those methods, planes ahead of them will be able to report that they experiences turbulence so the pilots behind them can anticipate.

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    $\begingroup$ Note that radar can only see things like rain, hail, bugs and birds. Radar wont show you clouds or clear air turbulence. $\endgroup$
    – casey
    Nov 12, 2014 at 2:34
  • $\begingroup$ Yeah, casey is correct. Radar doesn't show CAT. You just have to get reports from other pilots for that. $\endgroup$
    – reirab
    Nov 12, 2014 at 14:28
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Pireps is how I get most of my info. And you should pass them along on handoffs between controllers. When I get a handoff I say something like "Denver Center, N12345, level 250, light chop". They will respond back with "N12345, Denver Center, it should smooth out in 50 miles or so".

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According to the FAA the official Way is still ask a Flight Service Specialists in preflight briefing who the weather forecasts are and if turbulence can occur. But additional in the preflight briefing the Pilots can see like on this Site the turbulences. It is also stated in this FAA Document:

For pilot reports (PIREPs), the ADDS Java tool can zoom in on a specific part of the country and specify the type of hazard reported (icing, turbulence, sky and weather). The tool also allows you to limit data to specified altitudes and time periods. Map overlays including counties, highways, VORs, and Air Route Traffic Control Boundaries are available.

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  • $\begingroup$ Is there any pilots typed rated on 707 who can answer the following;in Aug 1989 I was travelling aboard a domestic flight on a PIA 707-320_ $\endgroup$
    – RAYHAN ahmed
    Feb 2, 2015 at 23:10
  • $\begingroup$ C. I was travelling on a domestic flight from Karachi to Lahore. During the flight the aircraft started to drop while level with engines roaring . The aircraft seemed in a downward wash looking outside it was pitch dark . I was so alarmed that me my mother uncle and aunti started to hold each others hands. The aircraft then settled off and calm fell to me that my prayers were answered. It there any ex707 pilots or flight engineers out there who can explain to me what this was? $\endgroup$
    – RAYHAN ahmed
    Feb 2, 2015 at 23:24
  • $\begingroup$ there had been no landing instructions by the cabin crew and can't remember the seat belt indications lights coming on $\endgroup$
    – RAYHAN ahmed
    Feb 2, 2015 at 23:33
  • $\begingroup$ On my return flight in Nov 1989 from Lahore to Karachi I travelled on a PIA A300B4 without any Scarry momments $\endgroup$
    – RAYHAN ahmed
    Feb 3, 2015 at 1:03
  • $\begingroup$ I was travelling back home to London were I live. $\endgroup$
    – RAYHAN ahmed
    Feb 3, 2015 at 1:06
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The most common means in level flight is actually the least technological: pilots ahead of them report the turbulence in that area.

If you are listening to the pilot chatter, and you hear "there is light chop", you can guess that the seatbelt light is about to come on.

(This is an interesting answer to a related question illustrating this point - the pilot refers to "ride reports" and PIREPS.)

Generally, if weather conditions are bad enough to show up on radar, you don't fly into them at all.

It's true that flying into clouds is another time when they know there will be turbulence, but this is most often on ascent or descent, and seatbelts will be on anyhow.

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