WELCOME [ Log In · Register ]        SITE [ Search · Page Index · Recent Changes ]    RSS

Thermal

Air that rises faster than our gliders sink is what turns a mere descent into soaring flight. Whether knocking around the airport or stretching out on a cross country flight, the ability to find rising air and make the most of it is essential to mastery in the sky.

Warm air rising from the ground in a thermal is the most ubiquitous form of lift and the glider pilot’s most direct use of solar energy.

 

Contents

Understanding thermals

    Thermal revealed (photo)

    Bubbles and plumes (graphics)

    Articles on thermal science

        Structure and Behavior - Wayne M. Angevine

        Getting to the core - content and author needed

Centering and climbing

    Tighten up - thermals are small

 

 

 

 

thermal.jpg

 
 

 

 

 

 

 

 

 

THERMAL REVEALED

 

Conical in shape, note the regions of reverse flow at various points on the cone surface – depending on the height at which you encounter this thermal there will indeed be a significant downdraft on entry, but not at all heights or from all directions.  The thermal does indeed get bigger as it rises, and its quite clearly not homogeneous. (The photo and caption come from Doug Jacobs' presentation on climbing. Is he the photographer?)

 

 

 

 

 

 

  BUBBLES AND PLUMES

  thermal looklike.jpg

 

 

thermal cycle.jpg

 

Articles on Thermal Science

    STRUCTURE AND BEHAVIOR

Angevine1.jpg

 Angevine2.jpg

angevine3-1.jpg
       

  angevine4.jpg

 angevine5.jpg

  angevine6-1.jpg

   angevine7.jpg

  angevine8.jpg

 angevine9.jpg

 

Getting to the Core

Wayne Angevine tells us, based on observations, that the normal thermal is from several hundred meters wide to two kilometers wide. We know for our flight experience that thermals are large. The clouds are often large and the bumping and gusting associated with them is spread over a pretty large area. But we also know that the core, let's define that as the strong lift that is usable for climb, is small. Maybe less than 500 feet in diameter in the average Eastern thermal. Because these cores are what we're really interested in we usually call them thermals.) We like to post an article that bridges the gap between Angevine's large thermals and the small cores we use. How and why does the core form?

 

 

Centering and Climbing

 

 

Tighten up

 

Doug Jacobs does a little exercise on thermal size in his advanced cross country classes. He asks: How big do you think is the diameter of the average thermal. 200 feet? 500? 1,000? 2,000? The guesses can be all over. But then he says: “A speed of 60 knots is about the same as 100 feet per second. So let’s see what a 1,000 foot diameter thermal would sound like on the audio. Somebody time me for 10 seconds.” He then does a pretty good impersonation of the  sound a variometer makes to signal climb.

Before the 5th second has passed the exercise has already become laughable. The thermals we’re used to in the East are clearly smaller than 500 feet across.

 

Now that you know how small a thermal is, how big is the circle you fly in your glider?

Attached Files   

Angevine1.jpg Robin Feb 18, 2008 140 KB
angevine1
Angevine2.jpg Robin Feb 18, 2008 115 KB
angevine2
angevine3-1.jpg Robin Feb 18, 2008 114 KB
angevine3-1
angevine4.jpg Robin Feb 18, 2008 67 KB
angevine4
angevine5.jpg Robin Feb 18, 2008 137 KB
angevine5
angevine6-1.jpg Robin Feb 18, 2008 113 KB
angevine6-1
angevine6.jpg Robin Feb 18, 2008 107 KB
angevine6
angevine7.jpg Robin Feb 18, 2008 141 KB
angevine7
angevine8.jpg Robin Feb 18, 2008 100 KB
angevine8
angevine9.jpg Robin Feb 18, 2008 57 KB
angevine9
page8a-1.jpg Robin Feb 18, 2008 86 KB
angevine8-1
thermal cycle.jpg Robin Feb 18, 2008 42 KB
thermal cycle
thermal looklike.jpg Robin Feb 11, 2008 62 KB
what a thermal looks like graphic
thermal.jpg Robin Feb 10, 2008 16 KB
thermal revealed

Comments    [ Add a Comment ]