home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: rec.models.rc
- Path: sparky!uunet!ftpbox!mothost!white!rtsg.mot.com!svoboda
- From: svoboda@rtsg.mot.com (David Svoboda)
- Subject: Re: Thermals don't travel at prevailing wind speed
- Message-ID: <1992Jul28.224446.24853@rtsg.mot.com>
- Sender: news@rtsg.mot.com
- Nntp-Posting-Host: guppie44
- Organization: Motorola Inc., Cellular Infrastructure Group
- References: <1992Jul23.221308.24156@bcrka451.bnr.ca> <1992Jul28.042041.926@nntpd.lkg.dec.com>
- Date: Tue, 28 Jul 1992 22:44:46 GMT
- Lines: 46
-
- In article <1992Jul28.042041.926@nntpd.lkg.dec.com> fisher@kay.enet.dec.com (kay::fisher) writes:
- |> <<< Note 3.4 by ::mkfeil@bcrki9 "mkfeil@bcrki9.bnr.ca ()" >>>
- |> -< Re: More R/C Gliding........ >-
- |>
- |>In article <1992Jul22.233354.6431@rtsg.mot.com>, svoboda@rtsg.mot.com
- |>(David Svoboda) writes:
- |>> forth across the region, always turning into the wind. The thermal moves
- |>> downwind, a little slower than the wind speed.
- |>
- |>Please explain. How can a thermal bubble move slower than the average
- |>speed of the surrounding air mass?
- |
- |The same way bubbles get to the surface of a river. If they only
- |went down stream they would never come up. If they come up then
- |obviously they aren't going down stream as fast as the rest of the
- |river.
-
- Uh, I think that a bubble *does* go as fast --horizontally-- as the
- rest of the river. It's just that the bubble also has a vertical
- component, so actually the resultant velocity vector has a larger
- magnitude than the actual speed of the water.
-
- Sorry, Max, that I didn't get back to your question; I will now:
-
- I said that it is helpful to *think of a thermal as a bubble*, I
- didn't say it *was* a bubble. A thermal is actually a near vertical
- shaft of rising air, which emanates from a particular region of
- earth. It is primarily fixed, at the bottom, althogh not in time
- (that is, it has a fairly short duration, and is cyclic). The
- prevailing wind blows the top of the cylinder downwind. In that
- case, for all intents and purposes any particular region inside the
- thermal (which can extend for tens of thousands of feet, and
- intensifies with altitude) can be thought of as a "bubble" of rising
- air, and the glider rides on "top" of the bubble (though we know that
- the glider is really *inside* a velocity vector field). It seems
- like there are warm air bubbles emanating from the ground periodically,
- but really it's only the velocity vector field intensifying and
- depleting, and only on the very bottom of the thermal. If you can
- get some serious altitude, thermals seem rock steady, and do not move
- around.
-
- Simply put, it is a lot easier (for me) to find thermals if I feel
- the wind, and the temperature, and picture big bubbles of air rising
- from the earth.
-
- Dave Svoboda, Palatine, IL
-