Cumulus versus clear blue sky days - RC Groups
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Oct 17, 2002, 06:56 AM
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fprintf's Avatar

Cumulus versus clear blue sky days

Between periods of rain we have had some beautiful clear blue sunny sky days with light winds, what would seem perfect for thermaling. I didn't have my RC glider with me but did a few trials with my free flight glider (wrampager) during those days and noticed very few thermals. Admittedly I am just learning some thermal finding abilities, so my question is are clear blue sky days typically weak thermal days? Do there need to be cumulus clouds as indicators of strong thermals?
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Oct 17, 2002, 07:49 AM
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stephen.s1's Avatar
Hi Stuart,

Clear sky thermals are known as "Blue Thermals"

I find 'em (occasionally) to be as strong as the other kind..

Are you heading up to Brattleboro on the 27th?

Off to the field.. Ain't retirement grand!

Oct 17, 2002, 08:27 AM
Registered User
I hear cumulus clouds form when moist air is lifted to an altitude where the temperature is less than the condensation temp (dew point). So if there are cumulus clouds are above, there must be moist air and lift. The air doesn't have to be very moist if the lift is great enough. New Mexico develops great clouds daily in summer and the air at the gruond is pretty dry. In the north Calif central valley where I fly we almost never get cumulus clouds but have plenty of thermals. The air is dry and the thermals don't loft high enough to form condesation but fine for flying sailplanes.
Oct 17, 2002, 11:51 AM
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fprintf's Avatar
Originally posted by stephen.s1
Hi Stuart,

Clear sky thermals are known as "Blue Thermals"

I find 'em (occasionally) to be as strong as the other kind..

Are you heading up to Brattleboro on the 27th?

Off to the field.. Ain't retirement grand!

Thanks guys for the answers. Next nice day I just might take off work a little early for the field regardless if there are clouds or not.


Retirement is grand, certainly, and I'd love to be able to go flying today as it is beautiful and cool outside! I am not going to wish my life away waiting for it, though, as age 34 has its own advantages. As for the 27th, I am certainly thinking of going up for it and observing/timing again.
Oct 18, 2002, 05:34 PM
Registered User
fledge's Avatar

blue sky thermals

i would always fly regardless if there any clouds or not,if the sun is shining at all there will be thermals,the only differance with blue sky thermals is that there is no moisture to condense,

if the sun is out,it is creating heat,what you need to ask yourself is:where is the heat going?

blue thermals usually go a lot higher also and less energy is spent supporting moisture

Oct 18, 2002, 10:57 PM
Registered User
Daemon's Avatar

Stable/Unstable atmosphere and SkewT diagrams.. (long)

This is long, but there's a purpose. If you *really* want to know when/where/why to fly on any given day, keep reading. This isn't theory of small scale thermals. This is about atmospheric stability and clouds, but it'll tell you a lot about whether there are/will be thermals during the course of the day.

Whether air rises or not depends on the lapse rate which defines how fast a parcel of air cools with decreasing pressure/increasing altitude, and the actual temperature of the air at various altitudes. As long as there is warmed/warming air (sun heats ground, ground heats air) and cooler air above it, the warm air will rise. It'll continue to rise, cooling as it goes (how fast it cools depends on how much moisture it's carrying and the dry and saturated lapse rates), until it encounters air above it that is warmer at which point it stops.

If you can find out what temperature the air is right now at various altitudes (or close to now), then you can use the lapse rate to determine whether the air is rising, how fast it's rising and how far it'll rise before it hits an inversion. Luckily for us, the computer forecast models take virtual soundings, spaced every 20km-40km spanning the entire US every hour during the day, as well as seeding the computer models regularly with real balloon soundings at various population centers and interesting areas (such as tornado alley) and satellite soundings. The soundings show the predicted air temperature, wind speed, wind direction, and dewpoint temperature, at about 50-60 different altitudes from ground level up to 70 thousand feet. (links at the bottom)

Also, as the pressure and temperature of the air drops, the quantity of moisture it can hold at 100% saturation decreases (essentially "100% humidity" represents a smaller and smaller amount of water molecules per unit volume, at lower temperatures and pressures). If a parcel of air manages to rise high enough without first running into an inversion cap, it'll find an altitude at which the the moisture it's carrying can no longer be contained (it reaches 100% saturation for that temperature), and the moisture precipitates out and forms clouds as the warm air continues to rise and cool. The altitude at which that can occur is defined as cloudbase regardless of whether there are visible clouds. Knowing the air temperature
near the ground, and the relative humidity and lapse rate will tell you where cloudbase should be.

Soo.. If there are cumulous clouds in the sky, then there *is* moisture, and there is rising air. And when there are no clouds chances are the warm air rising from the ground is hitting an inversion (a layer of air where the temperature increases instead of drops with increased altitude) somewhere below the predicted cloudbase. The rising warm air hits warmer air above it and stops, and never gets to the temperature/altitude where it is forced to release it's moisture.

If puffy clouds form early in the day it probably means that there is little to no inversion, or high humidity, so air rises early.. hits cloudbase altitude, forms clouds.. etc, and does that as long as it can. Problem with days like that is that the rapidly forming clouds will eventually block the sun and cut off the heating at the ground.. No more heating at the ground, no more rising air. The atmosphere starts out unstable and works its way into a stable configuration (warmer air above cooler air).

If clouds form only later in the day and over develop into thunderstorms, there's a fair chance that there was a good sized inversion early in the day, (lots of cooler air trapped under warm air which is conditionally stable). As the day progresses the sun keeps heating the air near the ground.. it rises.. until it hits the inversion cap, and the inversion fills up with warm air from the top down. At some point the entire inversion area under the cap finally fills with warm air, and the cap simply disappears and all of a sudden all that built up warm air ends up just a bit warmer than the air above it, and it all starts to rise at once. It also builds up some momentum, so even a little air slipping past the cap, can start a process that can't be stopped. Large quantities of rising air under those conditions is what causes thunderstorms.

So.. What can you do? How can you know? Is the atmosphere stable or unstable today. How do you answer the question "Should I fly or not?"
Same way the weather forecasters do. Learn to read Skew-T diagrams.
Start with at least one of the following tutorials:
and another couple

Ok.. you don't need to look at all of those in one sitting. At some point you'll want to play.
Go to and click on the link on the left under the "Soundings" label for either Interactive (Java) version or Non-interactive version. I use the Java one.
Enter a nearby airport code, or lat,lon in the Site box.. Pick a start time (It's Zulu time.. so for instance here in Colorado it's local time plus 6 hours.. noon = 1800 Zulu etc) and click on Java based plots button.

What you'll see is a Skew-T diagram with two jagged lines rising across it. The red one on the right is the actual sounding plot showing the temperature of the air at increasing altitude.
The blue one on the left is the dewpoint temperature plot. And the multiple diagonal red lines (rising up and to the right) are the temperature lines (which is why they call it a Skew T (temperature) diagram).
Click your mouse anywhere, and it'll show a violet air parcel trajectory line.
Example: 11am about 3 miles south of my house today.

Click the mouse on the very bottom end of the temperature sounding plot. The Violet line appears rising up and to the left. In this case it shows how a parcel of air, released at 5390ft altitude, at 63degrees F *wants* to rise given its initial temperature, the lapse rate and the moisture content. However, notice here that the entire temperature sounding plot is to the right and above the parcel trajectory line. That means the atmosphere is fundamentally stable. Air at the surface temperature won't rise on its own because all the air above it is warmer. It will rise if you heat it up a bunch more (click further to the right) but it crosses over pretty quickly.

Let's jump forward to 3pm.

Ok, now I clicked at the base of the temperature plot and you can see the Violet line starts out on the right/above th temperature line and stays there for a while before it crosses over. That means a parcel of air released at the surface will stay warmer than the air above it all the way up to about 12 thousand feet, where it hits a major cap (a very stable high pressure ridge is keeping the upper atmosphere very boring). Notice the short horizontal black line up at 17k feet or so? That's cloudbase. Since the rising air can only climb to 12k before it stops, it'll never hit cloudbase, thus the sky stays clear.

What's all this mean?
If I looked at the first Skew-T, I'd know that it would be a pretty hopeless time to fly, unless I know that the actual ground temperture is quite a bit warmer than what it's showing as the base of the temperture plot.. BUT, ground warming throughout the day (and I know there aren't going to be any clouds all day) will push the bottom of the temperature plot right (warmer) as the day goes on, and eventually it'll do what it did at 3pm. The paragliders simply call it the "lapse". "The sky lapsed". The ground warming finally reaches a point where a lot of warm surface air will start to rise across the entire sky. And that is infact what happened today. At about 3:30pm there were over a dozen paragliders all cruising around the sky at altitudes between 6k and 12k feet and I was flying my gliders at about 6800 feet. Also as the sun goes behind the mountains to the west, the air in the mountain's shadow, cools quickly and warm air right at the shadow boundary is suddenly induced to rise as the air above it cools. So the paragliders and I both had great broad lift until about 4:30 or so and then had to work further out tot he east to catch the thermals rising from the shadow line as the sun went down.
Also notice how the wind at low altitudes (the funny wind barbs on the right) also stabilized between 11am where it was light from the NW, to 3pm later the day where it was stronger and stable from the NE up to 10k feet, which is optimal conditions at my local slopes.
BTW, the upper air was doing something really weird. A day ago it was blowing 100knots up over 30k feet.. today at 11am, it was as dead as I've ever seen it.

Anyway, you can waste as little or as much time on Skew-Ts as you want, but in general they've been quite useful. I forgot to mention that they project the soundings out into the future which is what you'll probably find the most useful, but of course being predictions they won't be 100% accurate. On the one hand, the forecasts will often give you a good idea of whether the atmosphere will become more or less stable, but the wind direction and speed predictions can't always be trusted. I like to look at the last plot plus 3 or 4 hours into the future, and then go back at the end of the day and see how right it was, as the actual sounding data is filled in.

Hope at least one person finds this information useful.

Last edited by Daemon; Dec 21, 2011 at 01:42 AM. Reason: some corrections
Oct 22, 2002, 03:40 PM
Registered User
Thanks Daemon for the very interesting post and links.
Oct 24, 2002, 06:37 PM
Registered User
The info you posted is great. I have a few questions. I went thru the first tutorial and got much of it - will go over it again tomorrow and see if I get more out of it. When I tried to use the site ,
I could not figure out how to enter the area I wanted to look at. I found reference to Chico. Ca which is about 50 miles from me. I live at 122 deg 33' W , 40 deg 00' N. How do I enter this info?
I am really interested in learning how to get this working.
Oct 24, 2002, 08:27 PM
Registered User
Daemon's Avatar
Ok, here's the URL I've bookmarked for my own area to load the most current sounding plus 3 hours into the future..,-105.221&start=latest
&n_hrs=3.0&startSecs=1031461200&endSecs=1031472000 &data_source=MAPS
I load this one up just before I go out to fly.

See the part that says airport=... ?
That takes either an airport code, or lat/long in the format as you see it there. So for you.
(You really on 40.000? Might want to narrow it down more)

Anyway, here's the modified URL for your coords.
Click Here
Modify as appropriate and then bookmark it.

To bring up soundings for different times, click on the Load Soundings button, back it up to some time earlier in the day (you'll get used to Zulu time after a while) and set "Number of hours to load:" to like 4.0 (That'll give you 5 plots.. one at the specified time plus 4 more.. Also note that field must be a decimal number.. eg. 4.0 not 4). Then click on the MAPS button to set it going.

To clear soundings, hold down the control key and click on the buttons representing the soundings you want to remove.

Let me know if you have any other questions about actually reading them. Looks to me like you were totally socked in early in the day today, and probably cloudy the rest of the day, very stable atmosphere, winds south at 3-5mph. Not a fly day.

Last edited by Daemon; Oct 24, 2002 at 08:38 PM.
Oct 25, 2002, 08:32 AM
Registered User
The skew-t site works fine for me now. Thanks. You are right, yesterday we were socked in, with very light mist and no wind all day.
Yes I do live at 40.000. Actually 40 deg 00' 48".
This is an unusual weather pattern right now. We rarely see clouds at all in the warm months and don't see cumulous clouds any time except over the mountains 20-50 miles away.
I will have questions when I try to use the skew-t charts to see what is going on in the air above.
Mar 15, 2004, 11:49 AM
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fprintf's Avatar
Interesting to read this post from over a year ago when I was just starting out. This weekend we had some great thermals and it was cold, windy and occasionally quite cloudy. Quite the opposite of what I questioned earlier, but nevertheless goes to show that we need a bumper sticker like the cheezy fishermen have: any day of flying is better than a day of work.

In any event, it was so odd to catch these really strong thermals when all indications were that the lift would not be strongly organized. Similarly there have been blue sky, strong sun days where I haven't been able to find anything sustained. You *can't* go by the weather forecast in the micrometereology environment we have to cope with (e.g. the Soar forecast is probably too broad for our use).
Mar 15, 2004, 12:23 PM
B for Bruce
BMatthews's Avatar
Having flown both RC and Free Flight over the years I have found that there's only one really true way to determine if there's lift to be found. If the wind is variable in strength and direction there are thermals. If it's steady in strength and direction there are none to be had or they are very light in relation to the wind strength. The variations in direction and strength are signs that the thermals are pulling the air about by causing local variations in the otherwise steady wind. Granted on blustery days it can be hard to feel them and on calm light wind days they may be very feathery light but if the wind has variations there is something causing them. Of course standing downwind of a line of trees or buildings will mess this up fer sure....

The only exception I've seen to all this is very early dew laden mornings. In these conditions the dew of the ground seems to form a lift "pillow" up to a 100 feet or so that will take a light model up and hold it there at the balance point of dew rise and model sink. I've seen this so many times with my free flight models. I even got a 20 some odd minute flight out of a Gentle Lady by putting it over a very wet field on a foggy, dead calm morning one time. It hit about 80 feet and circled there in a trimmed in turn for the whole time without me touching the TX more than once or twice a minute. So if someone tells you that they have a 3, 5, 7 or whatever minute "dead air" model because they tested it on an early morning and you know they live in a place that has dew just nod your head to avoid any arguments. The French and British were famous a few years back for claiming they had developed many a 4 minute capable Coupe d'Hiver because they tested it in the early morning. Does anyone know where there would be MORE DEW than in Britain or France? I sure don't....
Mar 15, 2004, 06:57 PM
Hot Dawg Glider Pilot
schrederman's Avatar
Thanks Ian,

Saved us from a lot of typing...excellent post.

Jack Womack

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