As far as I understand glider pilots like to look out for thermals. I assume that when you are in one, you can perceive that and you can make roughly sure to stay in it.

But what if you're not in a thermal? How do you look for one? Are these methods (like finding a cumulus?) significantly/statistically more efficient than just randomly browsing? Or is it mostly a question of luck?

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    $\begingroup$ you look for sunlit slopes and other geographic markers $\endgroup$ – ratchet freak Mar 19 '14 at 19:17
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    $\begingroup$ In addition to cloud formations (cumulus) you can look for ground vegetation (open fields have different characteristics w.r.t. woods, for example) $\endgroup$ – Federico Mar 19 '14 at 19:32
  • $\begingroup$ ratchetfreak and @federico. Yes, but my point is that these seem somewhat like "conventional wisdom" and theorising. Is there hard evidence or really strong theory for these methods to work (and, rather importantly, randomly looking not to work as good)? $\endgroup$ – Řídící Mar 19 '14 at 19:37
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    $\begingroup$ The only real way to guarantee the lift is if there is something/somebody else climbing in it... Hence why often glider pilots don't make it back and end up in a farmers field... we know where the lift "should" be, and I could tell you, 99 times out of 100, if you have air blowing against a hill it will go up. However, due to the nature of the weather, there is no guarantee that the lift will be there, mountain wave in the right position will often negate ridge lift (which is a bit of a pain if you're not close to the site, personal experience!) Source: I'm a glider pilot. $\endgroup$ – Joe Harper Mar 24 '14 at 10:25

There are a few different things that you can look for, and to some degree it is trial and error as well. Gliders will have an instrument that measures vertical speed so that they can tell "how good" a thermal is as well (a variometer or a Vertical Speed Indicator(VSI)).

Some of the things to look for:

  • Color variation on the ground because the difference in the amount of heat absorbed by the ground varies based on the color/composition. The hotter area will cause the air to rise (hot air rises) and will create a thermal. I.e. A dark colored area surrounded by a light colored area or vice-versa such as a parking lot with fields around it.
  • Birds circling in an area because they tend to circle in thermals.
  • Certain types of cloud formations that can indicate lift.
  • Other gliders! :-)
  • If the wind is blowing across a mountain ridge or other large structure, the wind has nowhere to go but up so is also a good place to look for lift as well.

Here is an example showing how some of the thermals form and how the wind affects them. Notice also how once the air has been lifted to the point where it cools below the dewpoint that a cloud will form:

Thermal Formation

Here is another showing how terrain can cause lift:

Thermal Formation 2

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  • $\begingroup$ +1 for birds and other gliders. I would like to know if the other bullets have been shown to work better than not following them. (Are these definitely beyond merely theoretical?) $\endgroup$ – Řídící Mar 19 '14 at 19:42
  • $\begingroup$ @GlenTheUdderboat Absolutely. It's pretty simple physics actually. Think of walking barefoot on a concrete sidewalk compared to an asphalt road right beside it. The road will always be hotter (when it is sunny) because it is black and absorbs more heat, and will therefore heat the air immediately above it which will then rise because it is hotter than the surrounding air. Also, certain clouds are only formed by thermals so they are a clear indication of lift. $\endgroup$ – Lnafziger Mar 19 '14 at 19:46
  • $\begingroup$ That is precisely what I mean by 'theoretical'. :) I'm not looking for the explanation, I'm rather hoping for 'experimental evidence'. But because it is 'conventional knowledge', perhaps this is too much to ask for. :( $\endgroup$ – Řídící Mar 19 '14 at 19:54
  • $\begingroup$ That isn't theoretical, it's a pretty simple physical process. Fact: Dark colors absorb more heat than light colors. Fact: A hotter area will heat the air above it. Fact: Hot air rises. It isn't my opinion. :) $\endgroup$ – Lnafziger Mar 19 '14 at 19:57
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    $\begingroup$ I'm not sure how to respond to that. Are you implying that the air may actually remaining motionless and stay in one spot even though there is wind? A thermal moves along with the airmass that it is in just like a cloud does or a boat moves with the water in a river that it is in.... $\endgroup$ – Lnafziger Mar 19 '14 at 20:30

the wiki article you linked say the following

Thermals are often indicated by the presence of visible cumulus clouds at the apex of the thermal. When a steady wind is present, thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by soaring and glider pilots.

enter image description here(source the next linked article)

in absence of humid air (a blue sky) you can still find thermals by the techniques in this article

If the air is very dry or the lift doesn't go high enough, lift does not generate clouds to act as markers. On such "blue days" (i.e., the sky is only blue, no white), you use other techniques:

  • "House thermals" are locations that frequently produce thermals. Most local pilots know the location of these faithful life savers, use them, and will share the knowledge with visitors or new comers.
  • Soaring birds have an uncanny ability to find thermals. If you see a soaring bird, following it will usually lead you to lift. If the bird leaves a thermal, follow it. It probably senses an even stronger thermal nearby. We don't know how birds find thermals, but they are better at it than any human or instrument we have yet devised.
  • Look for other sailplanes that are climbing. I emphasize the last part because several glider pilots often land in the same farmer's field. When they discuss what went wrong, each will often say, "Well, I saw you circling so I figured there must be lift."
  • Look for dust devils [mini sand tornado]. The lift can extend well above the top of the visible portion.
  • Look for topographic features that are likely to trigger thermals. In addition to those enumerated earlier, rocky slopes facing the sun, steep canyons, mountain peaks or passes, dark cirques facing the sun, and airports (lots of asphalt to soak up the sun's energy) are good examples.
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  • $\begingroup$ +1 for birds and other sailplanes. Are the others more than merely theoretical? $\endgroup$ – Řídící Mar 19 '14 at 19:44
  • $\begingroup$ @GlenTheUdderboat According to the glider-folks I've listened to "cloud highways" are definitely more than "merely theoretical" (though not being a glider person myself I can't attest to it firsthand :) $\endgroup$ – voretaq7 Mar 19 '14 at 20:16
  • $\begingroup$ @voretaq7 I don't deny their existence. I only wonder how sure we can be that they can be spotted (other than by birds or other planes using them). $\endgroup$ – Řídící Mar 19 '14 at 20:20
  • $\begingroup$ Because they have a line of cumulus clouds topping them. They really are very obvious when you know what to look for. $\endgroup$ – testerab Sep 5 '14 at 22:22

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