Thread Tools
Sep 06, 2015, 02:51 PM
Registered User
Thread OP
Discussion

clear sky thermals


i have had my planes into thermals many times, be free flight towline gliders or rubber or engine powered, or r/c sailplanes.
so far i am under the impression that thermals rise from the ground in the shape of bubbles, although i could buy the idea that there are also columns. these bubbles or columns reach the level of condensation and form clouds. i have seen my planes reaching a cloud and being swallowed, so i believe this.
but what about a plane climbing into clear air? no clouds in sight?
this happens to me some times.
is there a source where we can learn about this?
meteorological data?
thanks
Sign up now
to remove ads between posts
Sep 06, 2015, 03:55 PM
An itch?. Scratch build.
eflightray's Avatar
My logic says, -- warm air rises, it doesn't have to include moisture ?, (to start off a cloud).

I could imagine a large desert creating many thermals with clear blue skies above.
Sep 06, 2015, 04:01 PM
Registered User
Ray, here is the same thing you wrote in (too) many more words <

As a soarhead who has flown RC sailplanes way more than 300+ hrs. in thermals, I must say that I have never noticed a clear correlation between clouds and thermals below 1000'. That is, depending on when and where one flies, the terrain and immediate meteorology is most significant and there may or may not be clouds associated with these low-level thermals. Wind, temp. and humidity, and how they vary with altitude have different effects on water vapor and models. It is easy to expect to find super lift heading into a culmulus, but we have all witnessed birds and models in lift with no connection to a cloud above. An easy example is a standing thermal vortex over a school heating efflux outlet. Or the bull-bat/nighthawks in Nashville continuously soaring nonstop over the pizza place at night under clear skies. Of raptors and flocks of gulls in nice LARGE bubbles soaring over Lake Zürich, the flock showing and defining the bubble as they fill it. Most of those bubbles dissipate, but when the raptors climb out, those thermals often head into the clouds. And the raptors leave then well before they go into the clouds [good eyes but no cloud-piercing radar to spot prey].

To put it another way, the bubbles may join, form columns or disappear as they rise. And they may or may not form clouds.
Sep 07, 2015, 06:37 AM
greg
ciurpita's Avatar
Quote:
Originally Posted by phil alvirez
i
but what about a plane climbing into clear air? no clouds in sight?
this happens to me some times.
is there a source where we can learn about this?
meteorological data?
clouds indicate the presences of thermals when the lifted air cools to the dewpoint. Balloon data measure temperature and dew point altitude.

there's no reason to believe there aren't thermals if there are no clouds. It just means the dew point at altitude is very low or gets lower with altitude. (look at balloon data).

there's also no reason to believe a cloud is formed from just a single column or bubble of air. I've read that many smaller columns combine into larger columns that combine under a single cloud. I've flown full size in what I thought was the center of a thermal with another glider several hundred yards away doing the same; neutral air between us.
Sep 07, 2015, 02:55 PM
B for Bruce
BMatthews's Avatar
It's not that the dew point is lower. It's that the dew point occurs at a higher altitude than what the lift is rising to. On such days, which are generally cloudless or may have some high altitude cirrus whisps the ground thermals are simply not rising as high as the altitude where the dew point is located.

Not all thermals are strong enough or rise high enough before dissipating that they'll reach even a lower altitude dew point either. I've ridden in a lot which lift me away and then just spread out and die by the time they hit a few hundred feet. Such thermals never get high enough to reach the dew point altitude.

I've seen it suggested that thermals only grow in size and energy while they are attached to the ground. Once they break away their life ("lift"?) span is limited to the time that it can hold the heat within itself. But with the air flowing around and within the thermal in the commonly seen toroidal format it exposes the warm air to the cooler surrounding air very generously. If the bubble isn't large enough and warm enough when it breaks away from the ground it may only last a short time and rise a limited amount.
Sep 07, 2015, 03:45 PM
Registered User
Thread OP

post 5


thank you for the great input.
i couldn't agree more.
it explains very well what sometimes i have experienced.
Last edited by phil alvirez; Sep 08, 2015 at 03:50 AM.
Sep 07, 2015, 04:37 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by ciurpita
c......there's also no reason to believe a cloud is formed from just a single column or bubble of air. I've read that many smaller columns combine into larger columns that combine under a single cloud......
I've seen that myself when the dew point was at a low enough altitude that ground based thermals were forming clouds. At such times you can see the swirling and growing crowns of the forming clouds grow and link together. At some lower altitude the lift tended to be under the one end or the other of the cloud.

At such times as the dew point is only about 400 to 600 feet off the ground and there's good thermal activity the clouds can be seen to form then grow. After a little time the cloud stops swirling and lays calm and finally tends to dissipate. Been there and watched it happen while trying to find lift under the "calm" clouds then flying over to get under those with an outward rolling growth to the cloud. The low altitude of the dew point on such days makes this study easy to do.
Sep 08, 2015, 07:43 AM
greg
ciurpita's Avatar
Quote:
Originally Posted by BMatthews
I've seen that myself when the dew point was at a low enough altitude that ground based thermals were forming clouds.
not sure what you mean by the dew point is at a different altitude.

the dew point varies with altitude (see left most line in plot) and depending on the starting point of the air on the ground that cools as it rises (due to expansion), it may not reach the dew point. (it cools to the surrounding air temperature).

the plot illustrates that the dew point may be significantly lower at specific altitudes before rising again.



of course a thermal only continues to rise as long as it is warmer than the surrounding air (see rightmost curve in plot). But if it cools to the dew point and the water vapor condenses, the heat from condensation raises the air temperature and it will continue to rise higher than if there was no condensation. (I would expect stronger thermals when clouds form and strong lift above the clouds).
Sep 08, 2015, 08:10 AM
Registered User
Thread OP

follow here


thanks for all your input.
Last edited by phil alvirez; Sep 22, 2016 at 05:57 PM.


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Clear sky thermals 2400RDR Thermal 9 Sep 12, 2009 04:45 AM