Diameter of thermals

[Waveguru]

Some days the thermals are big, and some days they are very small in diameter and it doesn’t seem to have anything to do with their strength or height. Do any of you know what determines their size? It seems that on the days they are small, they stay small all day, and vise versa.

Boggs

[Emir Sherbi]

El martes, 3 de abril de 2018, 18:34:39 (UTC-3), Waveguru escribió:
> Some days the thermals are big, and some days they are very small in diameter and it doesn’t seem to have anything to do with their strength or height. Do any of you know what determines their size? It seems that on the days they are small, they stay small all day, and vise versa.
>
> Boggs

Generally, when you take out all the other individual factors, is the atmospheric pressure

[Per Carlin]

Please explain that, is it a theory for size of the thermal as a function of pressure. Is it pressure at MSL or QFE that is relevant, or the pressure where you are at the moment when you catch the thermal?

[[email protected]]

Some days they are triagular shaped. Some know kung fu. :)
Its really the core size, thermals down low tend to have multiple small cores, which join together into larger smoother cores as the thermal gets higher.

[Emir Sherbi]

El miércoles, 4 de abril de 2018, 11:11:48 (UTC-3), Per Carlin escribió:
> Please explain that, is it a theory for size of the thermal as a function of pressure. Is it pressure at MSL or QFE that is relevant, or the pressure where you are at the moment when you catch the thermal?

The thermal goes up because there is a difference in the densities of the thermal and the air surrounding it. The air density depends mostly on humidity, temperature and pressure. With more pressure, the same quantity (mass) of air have less volume. That is in the assumption that the radiation of the sun gives the same energy to the same mass of air.

[Martin Gregorie[_6_]]

On Wed, 04 Apr 2018 07:13:13 -0700, davidlawley wrote:

> Some days they are triagular shaped. Some know kung fu. :)
> Its really the core size, thermals down low tend to have multiple small
> cores, which join together into larger smoother cores as the thermal
> gets higher.

.... and then stops when it hits the inversion.


--
Martin | martin at
Gregorie | gregorie dot org

[Duster[_2_]]

Assuming with his experience that he's quite aware of how thermals are generated, I'm not sure if we're addressing Mr. Guru's question about thermal diameter. Here's some references that address this issue on a more scientific basis (search "diameter" for each article if you don't want to wade thru).. Aside from those factors mentioned that affect density, thermal diameter will increase with height like an expanding baloon as the relative pressure decreases. Don't forget about the effect of inertia by all that upward-moving mass. Where's our Dr. Wx?

http://trace.tennessee.edu/cgi/viewcontent.cgi?article=1659&context=utk_gradthes

https://www.jstage.jst.go.jp/article/jmsj1965/54/6/54_6_427/_pdf

http://www.expandingknowledge.com/Jerome/PG/Aerology/PeterGray_ThermalMyths/Main.htm


https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1256/qj.03.180

file:///C:/Users/Michael/Downloads/1819-7038-1-PB.pdf

Now, how about a discussion on what causes sink surrounding lift on a blue day?

On Tuesday, April 3, 2018 at 4:34:39 PM UTC-5, Waveguru wrote:
> Some days the thermals are big, and some days they are very small in diameter and it doesn’t seem to have anything to do with their strength or height. Do any of you know what determines their size? It seems that on the days they are small, they stay small all day, and vise versa.
>
> Boggs

[Tom BravoMike]

On Wednesday, April 4, 2018 at 6:10:35 PM UTC-5, Duster wrote:
> Assuming with his experience that he's quite aware of how thermals are generated, I'm not sure if we're addressing Mr. Guru's question about thermal diameter. Here's some references that address this issue on a more scientific basis (search "diameter" for each article if you don't want to wade thru). Aside from those factors mentioned that affect density, thermal diameter will increase with height like an expanding baloon as the relative pressure decreases. Don't forget about the effect of inertia by all that upward-moving mass. Where's our Dr. Wx?
>
> http://trace.tennessee.edu/cgi/viewcontent.cgi?article=1659&context=utk_gradthes
>
> https://www.jstage.jst.go.jp/article/jmsj1965/54/6/54_6_427/_pdf
>
> http://www.expandingknowledge.com/Jerome/PG/Aerology/PeterGray_ThermalMyths/Main.htm
>
>
> https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1256/qj.03.180
>

I like the dedication in this (linked above) Master of Science thesis 'An Empirical Model of Thermal Updrafts Using Data Obtained From a Manned Glider":

"I would like to dedicate this thesis to my wonderful wife and children for putting up with my hectic schedule and allowing me to take the necessary time away from family time to finish this work".

I can imagine many glider pilots now willing to write a master or doctor thesis based on an empirical experience. In some cases it may take them years and years to gather the needed empirical material... :-)))

[[email protected]]

I can imagine many glider pilots now willing to write a master or doctor thesis based on an empirical experience. In some cases it may take them years and years to gather the needed empirical material... :-)))

And if you can secure funding for the project, think of all the joyous hours of soaring to be had on someone else's money.

[Roy B.]

its really the core size, thermals down low tend to have multiple small cores, which join together into larger smoother cores as the thermal gets higher.


David:
That certainly has been my experience where the thermals have a large fairly uniform fetch area (desert areas, large grain fields, etc) where you can often see multiple dust devils that eventually join together. But in irregular, forested or mountainous areas the thermals are usually single & isolated - but still they do vary in diameter, getting larger with altitude.
ROY

[Bruce Hoult]

On Thursday, April 5, 2018 at 2:08:02 AM UTC+12, Emir Sherbi wrote:
> El martes, 3 de abril de 2018, 18:34:39 (UTC-3), Waveguru escribió:
> > Some days the thermals are big, and some days they are very small in diameter and it doesn’t seem to have anything to do with their strength or height. Do any of you know what determines their size? It seems that on the days they are small, they stay small all day, and vise versa.
> >
> > Boggs
>
> Generally, when you take out all the other individual factors, is the atmospheric pressure

In what way?

In any given location and altitude, atmospheric pressure seldom varies by more than 3% from the average. Thermal sizes vary a lot more than that day to day.