Clouds and Thermals?

[2motheus]

I had my longest-ever thermal flight yesterday - an hour and 10 minutes with a less-than-pristine 2m Aspire ARF.
I was going between clouds whenever drift or altitude made visibility difficult. :D

I am now questioning an assumption I've had about how clouds and thermals interact. There were some high whispy clouds with smaller puffy clouds forming underneath. The puffy clouds weren't forming thunderstorms and they covered about 15% of the sky. I always thought that warm moist air forms clouds as it rises and hits the cool air higher up, and I envision a thermal as an angled column bent over with the wind, with the cloud marking the top.

What I saw yesterday didn't totally agree with that. There was usually lift somewhere under a "target" cloud, and it drifted with the breeze I could feel on the ground, but I didn't always encounter lift on the upwind side of the cloud. And the clouds seemed to be drifting in a different direction than the plane. The plane would drift south while the clouds moved southwest.

Does this simply mean that the air was moving in different directions at different altitudes? I couldn't let the plane go any higher to see if the drift direction changed or if the lift would eventually take me right to the base of the cloud.

I'm just curious about others' experiences and if my thinking needs to be adjusted a little.

And I certainly don't want to ruin a great flight with too much analysis!

[rogerflies]

The base of most clouds this time of year is probably higher than you expect, and probably higher than you could see your Aspire. I flew a Spacewalker with a wingspan of 135" and a chord of 24" to an altitude of 3840' above ground level (measured with a Casio recording altimeter watch), and I wasn't into the base of the "fluffy" cumulus clouds on a moderately warm day (80*) in SC. I stopped climbing because I could no longer tell which way the plane was pointing, and that's a LOT larger than your Aspire.

Wind direction does change with altitude, and the thermal bends with the wind and gets larger as it gets higher. That is, if the wind isn't strong enough to tear it apart. The thermal you were flying in may not have gotten high enough to start forming a cloud, it may have been getting torn apart, or it may have been going to a different cloud than you were connecting it to.

On cooler, more humid days, I could get the Spacewalker into the base of the clouds, but it's not a really good idea. The plane would sometimes come out in a totally unexpected direction in an unusual attitude. Since it was a VERY stable plane (I could easily let it go hand's off for several minutes), I'm pretty sure the turbulence at the base of the cloud was pretty bad. That agrees with my experience in full-size light planes.

Roger

[schrederman]

This all truly depends on what part of the country you're in. In Houston, the cloudbase was sometimes 2,000 feet or lower. Here in New Mexico, where it's so dry, the cloudbase is often above 20,000. I'm talking about the base of cumulus clouds formed at the tops of thermals. I've been at 17,900 in my sailplane and flying fast to stay out of illegal airspace for VFR gliders. It's hard to fly R/C sailplanes in relation to cloudbase because the cloudbase is so high. Out here under normal conditions, it's actually just about impossible. The more humid the airmass, the lower cloudbase will be.

enjoy...

Jack

[Curare]

I've seen a 144" Slingsby Skylark (how old skool are you?) get sucked up into the fluff.

That's a loooong way up for any sailplane, big or not, and while it's cool, it is VERY frightening, because after that, you might as well look away and fly, I remember we all had our hearts in our mouths till it came back out of the cloud.

[schrederman]

That Skylark was from an RCM plan? Designed by Jack Headley or something like that? That model got me interested in R/C Soaring.

Jack Womack

[2motheus]

Thanks for the replies. Some good insights.

In high school I had a 100" CraftAir Golden Eagle get sucked into a cloud. After an eternity it emerged, but without any tail feathers. Scary. I rebuilt it as a T-tail.

Does anyone know a web site where I can find out what the cloudbase was around here on a given day?

[darrell_f]

This site is good for windspeed and it has cloud base.

http://usairnet.com/cgi-bin/launch/code.cgi?sta=KMLB&model=avn&state=FL&Submit=Get+Forecast

[dephela]

A really good explanation, to what I think you might be looking for, was posted in another thread, here's the message:http://www.rcgroups.com/forums/showpost.php?p=545378&postcount=6

[bluefork]

I have noticed most of the time when I'm thermalling in my paraglider the thermal drift isn't the same way the wind is blowing. The wind will be blowing at 320(NW) and the thermal will be drifting to 240(SW). The thermal will move side to side and do sharp turns or just disapate before it reaches cloud base. Every day is different and so is every thermal. The temperature of the air, wind velocity at certain alt. dew point, humidity in the air, are all factors. Were I live we get really bad inversion and trying to break threw that layer is very hard sometimes. Check and see what the thermal index is before you go out flying. This will help you get a good grasp on how strong the thermals will be that day.

[kak8]

I wonder how big the temperature difference is inside a thermal and the surrounding air?

[schrederman]

The thermal will usually strengthen slightly at cloudbase. This is caused by a little heat energy that gets released as the water vapor returns to liquid form. Sometimes the thermal is much hotter than the surrounding air. It depends on what it was over when it broke loose from the ground, what the ambient air temperature was to begin with. Sometimes the thermal draws lots of air up with it. Often under a dying cloud, what lift is there is air moving under the inertia of the column of air that passed a little earlier, and won't be any warmer at all. In West Texas and Eastern New Mexico, we often get thermals over feed lots for cattle. The lift is strong, but so's the odor. Over a steel mill just north of Midlothian, Texas, I was rocked by a thermal that pegged the variometer at 10 knots up, and the outside temperature was 22 degrees Fahrenheit. That was the high for the day.

Fun and games...

Jack Womack

[bluefork]

I can relate to Jack. One of our soaring sites has a cattle feed lot infront of it. You can literally smell the thermal. But what a sweet smell it is when you are going up.

[LJH]

I use to fly my Sky Sergio at a field that was right next to the Thomas's English Muffin plant. If the breeze was from the south you could hook into a thermal produced by the plant at around 200' and take it all the way too 1200' (where I would start to get scared) according to my RAMII and boy did those thermals smell great especially when they were making cinnamon raisin muffin :p


Cheers,
Jim

[2motheus]

Thanks for everyone's interest. I have learned a lot. Bringing the thread back to the original topic, I have two remaining questions.

1. If the clouds are drifting in about the same direction as the wind, at hi-start levels where I begin my search is it worth seeking a thermal upwind or under a specific cloud? Or should I just use the puffy clouds as a sign that there is probably good lift somewhere in the sky.

2. Does anyone know how to use the weather databases to review what cloudbase and other parameters were at a specific time. I.e. historical vs. predicted.

Thanks,
Tim

[TMorita]

...
Does this simply mean that the air was moving in different directions at different altitudes? I couldn't let the plane go any higher to see if the drift direction changed or if the lift would eventually take me right to the base of the cloud.

I'm just curious about others' experiences and if my thinking needs to be adjusted a little.

And I certainly don't want to ruin a great flight with too much analysis!

It's called "wind shear" - it's when you have winds at different altitudes going different directions at different velocities.

The phenomenon where lift increase just under a cloud is usually called "cloud suck". I've been stuck in cloud suck in a paraglider...was at 7800' and pulling big ears to increase my sink rate, but I'm still climbing up...

Toshi

[John Gallagher]

When I ran out of money and switched from full size to RC sailplanes, I thought that I could just judge wind shift and find the thermal source of a cloud upwind on the ground. You have to remember the cloudbase altitudes on a decent day is anywhere from about 3000' to 20,000' depending on what part of the country you live. The thermal source of the cloud could be miles upwind, and the wind changes direction with altitude. It would be extremely difficult to guess the spot on the ground that generated the thermal for any cloud --- for an RC pilot on the ground.
I've flown rc gliders on calm days at altitudes above ~ 1200', and it looked like the position of the clouds and the thermals matched. There's unusual ground thermal generators - the feed stahl and baking plant mentioned above, smokestakes, nuclear power plant cooling towers (glider comes back with a nice glow to it) that might be associated with specific clouds. But for the most part you are better off just ignoring the clouds and looking for likely thermal ground generators, wind shifts, and birds and other gliders marking the lift.

[John Gallagher]

I remember many days while flying full size sailplanes, when there would be an inversion level at a certain altitude. The lift would dissappear below a certain altitude. Above that altitude the lift was great all the way up to cloudbase. A novice in our club, decided to try her first cross country on a particular day. She did what the books tell you to do. She climbed to cloudbase over 7000' (from a 3000 foot tow) and then used the altitude to glide a lot of distance towards her goal. Then your supposed to look for lift under your comfort altitude. A highly experienced cross country pilot told her that 2000' was a good comfort altitude. Unfortunately, she found no lift down at that altitude and was forced to land out. After they retrieved the Schweizer 1-34 sailplane (took about two hours), I set out on the same course and just flew from cloudbase to cloudbase, stopping to climb back up (never letting the altitude get below 5500'). I travelled nearly the same distance (took a bit longer) and still had about 6500' of altitude before I turned back towards the airport. I used the same cloud bumping method on the way back. Over the airport I burned off the altitude and was surprised to discover that I couldn't find any lift below 2700'.
Some other days this dead zone topped out at anywhere from 1200 to 1800 feet. Of course on most days the lift went from the ground to cloudbase.

The only explanation that I could come up with, was that the cloud suck extended that far down - unlikely but nothing else seemed to fit.

[jrgospod]

2motheus

This is the link I use for cloud base. You will have to adjust the location to your local. I have not seen any history but I have not looked.

http://usairnet.com/cgi-bin/launch/code.cgi?sta=KFWA&model=avn&state=IN&Submit=Get+Forecast


John

[TMorita]

When I ran out of money and switched from full size to RC sailplanes, I thought that I could just judge wind shift and find the thermal source of a cloud upwind on the ground. You have to remember the cloudbase altitudes on a decent day is anywhere from about 3000' to 20,000' depending on what part of the country you live. The thermal source of the cloud could be miles upwind, and the wind changes direction with altitude. It would be extremely difficult to guess the spot on the ground that generated the thermal for any cloud.
...


Maybe it's because you're on the ground? From the air, it's fairly easy to determine which features are generating the thermals, in my experience.

Toshi

[John Gallagher]

Maybe it's because you're on the ground? From the air, it's fairly easy to determine which features are generating the thermals, in my experience.

Toshi

Thanks. I did leave out something from that statement.
I've corrected it.

[will_newton]

I was told that sometimes you could spot lift under low clouds by looking for a hazy mist or fog underneath the cloud. I assume this is from water vapor being drawn up and then beginning to condense into cloud mass.

I do not see this phenomenon often, but I have watched the same fellow, who is a full scale glider pilot, spec out his r/c glider under this condition. This is in eastern North Carolina where high humidity is the norm.

Any opinions on this phenomena?

Will

[TMorita]

I was told that sometimes you could spot lift under low clouds by looking for a hazy mist or fog underneath the cloud. I assume this is from water vapor being drawn up and then beginning to condense into cloud mass.

I do not see this phenomenon often, but I have watched the same fellow, who is a full scale glider pilot, spec out his r/c glider under this condition. This is in eastern North Carolina where high humidity is the norm.

Any opinions on this phenomena?

Will

This is called "cloud suck" by paraglider pilots. When we fly under clouds, we tend to get sucked into them, which is bad, because you go into a condition called "whiteout" where you can't tell which direction you're going, and you can wind up endlessly circling in a cloud...

In fact, there's an article on the USHGA (US Hanggliding Assocation, now US Hanggliding/Paragliding Association) website about avoiding cloud suck:

http://www.ushga.org/article19.asp

Also, there's some articles on how to spot thermal generators by Will Gadd, who has the current world record for paragliding cross-country flights (263 miles/423 km):

http://www.ushga.org/article26.asp
http://www.ushga.org/article27.asp

Article on the Will Gadd record flight:

http://www.ojovolador.com/eng/read/stories/record_will_gadd/record_will_gadd.htm

Toshi

[will_newton]

toshi-good info!

That last article about Will Gadd's record flight was amazing!

Will

[bluefork]

Will Gadd is a paraglider skygod!

[John Gallagher]

What's the hang gliding world distance record?

[TMorita]

What's the hang gliding world distance record?

Looks like 700.6 km:

http://records.fai.org/hang_gliding/history.asp?id1=o-1&id2=1&id3=12

Hang gliders fly faster and have a better glide ratio than paragliders (paragliders are 7:1 to 10:1, hang gliders are about 12:1 to 18:1).

Toshi

[sleipnir]

And the Glider WR is over 2000KM!