Questions and Answers on using SkewT Diagrams

[Jim_Marconnet]

Daemon (Ian) has shared some info on SkewT diagrams in the past, mostly back in 2002.
One thorough post with several diagrams is: http://www.rcgroups.com/forums/showpost.php?p=545378&postcount=6

Here's a thread for asking and answering questions on the use of this data for R/C Thermal Soaring.

For the current or forecast SkewT diagram for your location, use this link http://rucsoundings.noaa.gov/ and type into the site box the three letter abbreviation of your nearest airport, HSV in my case, or your lat,long then click the java-based plots button.

[dephela]

How to use and understand?
http://www.rcgroups.com/forums/showpost.php?p=545378&postcount=6

[Jim_Marconnet]

How to use and understand?
http://www.rcgroups.com/forums/showpost.php?p=545378&postcount=6
dephela:
Yep, that's one of his good ones. A Lot of good stuff in there.

My hope in this thread is that we can take some simpler, smaller questions and give answers to them concisely - something that pilots could digest more easily than that whole writeup in one post.

As for me, I'm contemplating my first question. In the meantime, hoping that some folks with experience in using SkewTs for R/C soaring are around and will be willing to share what they know.

Jim Marconnet

[Jim_Marconnet]

(1) What on the (forecast) SkewT diagrams would tell we whether it's likely to be blue sky, some cumulous clouds, or very cloudy such that the ground might not get enough sun light for thermals to form? And why?

I already understand that if there is a sufficient inversion, that the warm air rising from the ground will be trapped at that inversion altitude and can not rise high enough for the air to cool to the dew point and saturate, forming cumulous clouds.

[Jim_Marconnet]

(2) How do you determine cloud base from a SkewT?

Clearly if the blue (dew point) line and the red (air temperature) line touch, then that's a cloud base. But if the two lines don't ever touch, where is cloud base?

From The Front: "When these lines are three degrees C or less apart, expect clouds and IMC. Dry air is present when the temperature and dew point lines are farther apart. The greater the distance, the drier the air. Hazy conditions are likely when the difference is between 3 and 5 degrees C."

[Jim_Marconnet]

(3) Determining an inversion

How sharply does the red line have to veer to the right (warmer) to call it an inversion?

Then given that there is an inversion, is there a "strength" to an inversion based on how far to the right (warmer) it gets, how steep it slopes, or is an inversion an inversion, all or nothing?

[Jim_Marconnet]

(4) What on the SkewT diagram would tell me to expect a time of all or nearly all sink?

Ian said in 2002: (get) The red temperature sounding line, plus some local surface temp readings (taken from wunderground.com or outside your door), and a click on the chart at the appropriate altitude and temperature and you'll quickly find out if you can expect any convection. If the new parcel trajectory line starts and stays to the left of the temp line, the sky will be all sink. If it starts and stays to the right, it'll be all lift (and will probably cloud over quickly), and if it starts right and hits an inversion, it'll be lifty to that altitude.

Can someone explain this?

Thanks,
Jim

[Jim_Marconnet]

(2) How do you determine cloud base from a SkewT?

Clearly if the blue (dew point) line and the red (air temperature) line touch, then that's a cloud base. But if the two lines don't ever cross, where is cloud base?
If you determine a starting air temperature and altitude and click on the SkewT plot at that temp/altitude, then the Java-generaged Black line where it crosses the Red line is the altitude for an inversion for that particular air, I think. Now how does that relate to cloud base?

[Jim_Marconnet]

(5) Given it's a sunny day, how can I use a series of SkewT diagrams prepared for today, an hour apart, to figure out when the first thermal of the day should occur?

[Jim_Marconnet]

I ran into some interesting detailed briefings on SkewTs and interpretation at: http://www.sailplane-racing.org/weather.htm
Click on any of the three weather articles of your choice, then step thru the pages and explanatory text using the < and > arrows at the bottom of the page. You may also need to drag the text block border up vertically to make it larger and thereby easier to read.

[Phoebusflyer]

Jim, unless your just enjoying learning something new, you can get accurate, easy to understand info about thermal strength, height, cloud base from this good gentleman:
http://www.drjack.info/BLIP/ETA This mans website has a free service if you just register that will give thermal strength for free. If you pay his small yearly fee, you can get it all for one year. Those who fly full scale REALLY appreciate his website!!
Allen

[Jim_Marconnet]

Jim, unless your just enjoying learning something new, you can get accurate, easy to understand info about thermal strength, height, cloud base from this good gentleman:
http://www.drjack.info/BLIP/ETA This mans website has a free service if you just register that will give thermal strength for free. If you pay his small yearly fee, you can get it all for one year. Those who fly full scale REALLY appreciate his website!!
Allen
Allen:

Thanks for the suggestion.

I had spent some time at drjack site a few months ago looking in the free stuff. It looked really neat for cross-country full-scale planning, but we fly R/C at one or two specific sites with specific trees, hills, fields surrounding, etc.

So beyond "Neat!" I've not really figured out in any depth what is available there, what it means, and how to use it for our purposes.

They have a nice forum there, but I did not manage to benefit much from that for R/C purposes.

Knowing a little more now, I might get a better hang of it today if I revisit his site.

The local full-scale sailplane club bought a membership and a specific location blipmap several years ago for their airport and surroundings, but they never really did much with it and let it go.

Can you or someone else here point me to a specific part of his site and share what it is and what you actually do with it? That might help me a lot.

Thanks,
Jim Marconnet

[Phoebusflyer]

Jim, I use it to plan flights in my full scale, but I also check it and the NWS to see what it means for r/c flying as well. If the surface winds aren't higher than about 12 or 15 mph, then I check the thermal updraft velocity and the cumulus potential to see if it's going to be "one of those days"! If it is and I'm not flying full scale, I go out and fly. I have not been disappointed in the results. Sometimes I have to wait till later in the pm for the full potential of the day to develop, but most times I am NOT disappointed.
Sorry to hear the full scale club was not able to take advantage of Dr. Jack. I made at least 5 flights last year over 120 miles based on Dr. Jacks forecasts. This year I want to stretch it a little and go more than 600. We'll see! NWS and Dr. Jack are going to have to agree on a great day all the way into Nebraska.
Anyway, I have used his website in conjunction with NWS to decide if I want to go r/c flying and am pleased with the results.
Allen

[Jim_Marconnet]

I've been playing around a little using the SkewT to try to see when the overnight ground inversion (prediction) lifts. Using the default settings and models and HSV, all I can tell is that the ground inversion clearly exists at one hour and is gone the next hour. Perhaps there's a way to request predictions more often??

In the process, I've noticed recently that the Launch Code Cloud Base often does not compare with what I'd predict using the Java features of the SkewT. LC often predicts 12K feet when SkewT shows just a few thousand feet. Has anyone done any significant comparisons to have any insight when and why they would compare favorably, and when not. Earlier I remember seeing them compare favorably. But not lately.

Cloud base is not generally all that important to us R/C pilots, unless it's real low.

An aside, I'm beginning to think that very few R/C sailplane pilots here use SkewT much, if at all, based on responses here.

[Jim_Marconnet]

For the US, I just ran into a rather nice source of short- and long-range forecasts of Highs, Lows, and Fronts at: http://www.hpc.ncep.noaa.gov/ Just click on the appropriate map thumbnail for the forecast of your choice.

What this is theoretically useful for to us is looking for forecast Highs (H) or Lows (L) near your location. With a High overhead or nearby, then you may expect overall generalized sink (with the possibility of thermals, of course). With a Low overhead or nearby, you may expect some generalized rising air (with the possiblilty of thermals, of course).

[Phoebusflyer]

[QUOTE=Jim_

Cloud base is not generally all that important to us R/C pilots, unless it's real low.

An aside, I'm beginning to think that very few R/C sailplane pilots here use SkewT much, if at all, based on responses here.[/QUOTE]




Jim, I imagine most r/c'ers are a lot like fisherman. It's good to go fish/fly anytime you can, regardless of what small things the weather may be doing. Unless of course, it's raining cats and dogs. Especially if it's decent and not too cold or windy.

[Jim_Marconnet]

Jim, I imagine most r/c'ers are a lot like fisherman. It's good to go fish/fly anytime you can, regardless of what small things the weather may be doing. Unless of course, it's raining cats and dogs. Especially if it's decent and not too cold or windy.
I'm hearing that from several sources and I definitely see the merits of that approach. It's the old, A rotten day fishing beats a day at work saying. Go for it!

But with not all that much effort, I'm learning how to let the computer crunch the forecast data (for free!) so I can easily get an idea of what to expect today before I drive an hour to the nearest contest. I see today, for instance, a large ground inversion that goes away between 9 and 10. Were I there and ready to fly at 8, I would not bother flying expecting early-bird thermals. Of course, the contest starts at 9:30, so somebody is obviously planning well or something.

I can also see it's going to be dry. That relates to my comfort, plus probably no cumulus clouds to watch or to block the sun today. So wear sunscreen!

And with a little more help from my friends, I hope someday to know what to make of the way the dew point swerves right a ways and then rockets to the left.

[Jim_Marconnet]

Looked at today's skewT just before going flying.

Clicked on my guess of the current air temperature and see the line go up, but it shows two cross-hatched areas, one to each side of the line. What do these mean?

Thanks,
Jim

[ciurpita]

i learned about skew-t plots from lee murray articles in RC soaring digest. i wrote an article in the Feb 2001 issue that you may find interesting: http://www.rcsoaringdigest.com/pdfs/RCSD-2001/RCSD-2001-02.pdf

my understanding was confirmed when i read the FAA Glider handbook (chap 9): http://www.faa.gov/library/manuals/aircraft/glider_handbook/media/faa-h-8083-13.pdf

the jist of it is to predict how high a parcel of air at ground level and at ground temperature will rise as it cools. i continue to use a script that leans the skew-t plot to determine expected maximum altitude for any ground temperature. i then determine the expected thermal altitude given the predicted temperature.

there are two things to consider:
1) there are hot spots on the ground, bare earth, bridges, pavement, the exceed the average gound temperature that are significant sources for thermals before and after ground termperatures reach the "trigger" termperatures.
2) since rising air is associated w/ sinking air, i've wondered if when the above indicates a strong day for thermals that local and low-altitude condition may suffer, meaning that it might be better for models when the conditions indicate lower altitude thermals (< 2-3 thousand ft), there may be more frequent and smaller thermals at the field instead of long periods of lift and sink?

[Phoebusflyer]

2) since rising air is associated w/ sinking air, i've wondered if when the above indicates a strong day for thermals that local and low-altitude condition may suffer, meaning that it might be better for models when the conditions indicate lower altitude thermals (< 2-3 thousand ft), there may be more frequent and smaller thermals at the field instead of long periods of lift and sink?[/QUOTE]


In flying full scale gliders, you will find that usually humid days will produce more thermals with lower cloudbases. Dryer days tend to produce much higher cloud bases with the thermals further apart. However, any full scale pilot will tell you, there are lots of small thermals between the big boomers that are too small for the full scale flyers to take seriously. For R/C, though, they will be perfect. Unfortunately, I don't know if the lower cloudbases produce more r/c size thermals, or the higher ones.
Again, go fly when you can, and build when you can't!!

[Libelle201B]

Jim, I imagine most r/c'ers are a lot like fisherman. It's good to go fish/fly anytime you can, regardless of what small things the weather may be doing. Unless of course, it's raining cats and dogs. Especially if it's decent and not too cold or windy.
Phoebusflyer, I think you pretty much put it in perspective. The technical atmospheric info is great to know and understand, but really not practical for the rc soaring pilot. The SkewT definately helps the full scale crowd in that it gives us some idea of the conditions (instability, lift potential) we will have in our normal operating range of altitudes (2K and above), but really isn't much help when soaring at release points of 300' or so as in rc soaring. Wind and topography take on a whole new meaning at these lower (mixing) altitudes. If you are skilled enough in rc, you can catch a thermal from a hand launch. Based on the transmitting range of rc transmitters, you only have a few thousand feet or so, maybe one mile from where you are standing, either up or horizontaly. Much after that you have either lost radio control or eyesight of your sailplane. Unless you are going rc Xcountry, you are limited to that area. There is a thermal distance formula based on cloud base heights, but again, unless you are going rc Xcountry, that formula isn't practical for local rc soaring. When I was very involved in rc soaring, I just looked out the window.:)

[Phoebusflyer]

Can I get an AMEN to that. That's what I'm talking about, Libelle201. If it looks like a good day, and you can tolerate the wind, GO FLY! It might just surprise you! And you know what, that works for the full scale guys too! Many days I've been surprised by 2 hour flights that never got more than a thousand feet above release altitude when I'm flying full scale. It was a ho-hum day, but I managed to wring a 2 hour flight out of it by just staying with it. If it looks like a decent day, go fly, no matter what. You will learn something on the weak, weird days and you will have the time of your life on the days that end up being something you never expected!
Enjoy, no matter what. Just don't bend your plane doing it!!!!

[ejett]

In my particular situation, I don't get a lot of time off for flying, so if the weather is anywhere reasonable, I fly. If it's not I build. However, at some time in the future I will (theoretically) have more time. I might try to use the Skew T then, but it would be of limited use to me now.

EJ