By Michael L. Shakman

This outline focuses on basic areas of information needed to begin to use the weather for cross-country soaring.

This discussion is divided into three parts.

The first part deals with the minimum information needed to prepare for a cross country flight.

The second part deals with how to find out what the weather will be.

The third part deals with the sort of thinking you have to do in the glider to find and use lift, and what you need to do when the weather conditions aren't what you expected, or have changed during the flight. Part three includes examples based on experience in contest flying.

At a minimum we need to know three things:

First, will the sun shine?

If the sun is not shining, cross-country soaring in thermals is not very likely.

Second, will the wind blow, how strongly and from what direction?

If strong winds are forecast, cross-country soaring is made much more difficult, and streets of lift and sink may occur. In northeastern Illinois, if the winds are from the northeast or east they threaten to generate lake effect and poor soaring conditions.

Third, we need to know if there will be thermals.

If air isn't likely to be lifted high enough and rapidly enough by heating, there will be no useful thermals, and cross-country soaring will be impossible.

Therefore, we need to know how to determine if the sun will shine, the wind will blow and the air mass will generate thermals. How do we find out?

There has never been a better time to get weather information quickly and at no cost, than today, thanks to the Internet and other readily available media.

Sun. The first of our three questions can be answered by the nightly TV weather program or by newspaper weather forecasts. They are usually focused on whether the sun will shine the next day. If you are planning to fly on the weekend, you should watch the weather forecasts starting in mid-week, to see how the forecast trends develop and turn out.

We want to know if there will be clouds, how the cloud cover may change during the day, and when and why. If it starts sunny and becomes overcast soaring may end suddenly. If we are far away from home that won't be good. If it starts overcast but opens up, we would like to know when that will occur to determine if it will permit soaring during part of the day.

For this purpose a terminal area forecast [TAF] is useful. I suggest getting the TAF for DuPage [DPA], Midway [MDW] or Rockford [RFD]. All three are reasonably near. You can ask the Flight Service station for the TAF or you can get it on line at the following website:

http://weather.noaa.gov/weather/taf.shtml See Exhibit A.

Wind. TV and newspaper weather reports will give a general idea of the anticipated surface wind. But we also want to know what the winds aloft will be, and how they change with altitude. Winds aloft forecasts can be obtained from these sources:

Duats -- you need to open a free account and learn how to log on and use this service. Duats is oriented to general aviation power pilots more than glider pilots, but it has lots of useful information, including winds aloft forecasts.

When are winds aloft forecast produced and how often are they updated?

The NOAA web page has more information on this.

There are several winds aloft websites. Here is one that provides the basic data:

http://www.awc-kc.noaa.gov/awc/awc-fd.html

Another is: http://aviationweather.com (See Exhibit B.)

Here is one that produces maps of winds aloft:

http://adds.awc-kc.noaa.gov

(See sample, Exhibit C.)

Flight Service Stations (1-800-WXBRIEF):

The same information can be obtained from the FSS by calling and asking. If you do so, be sure to ask for the soaring forecast and one or more TAFs as well.

Will there be thermals, and how high will they go?

The Soaring Forecast will tell you this by indicating the top of the lift and the height of the three-degree spread.

You can also get the same information from a very valuable website maintained by Kevin Ford.

Its address is http://csrp.tamu.edu/soar/for.html. What you will see if you fill out the data request is shown in Exhibit D.

Or you can go to one of several sites that produces a lapse rate plot. The NOAA site is a good one. http://raob.fsl.noaa.gov/

What you will see from this site is shown in Exhibit E. Another example is Exhibit F, from a plotting program I wrote for the Mac. If anyone wants a copy you are welcome to it.

Any of these sources will give you the information you want. I like to use a combination of the plotted lapse rate and the Kevin Ford computation of the altitude of the three degree spread. Kevin Ford's program includes a simplified plot of the lapse rate, but its not as easy to read and understand as the NOAA plot, for example.

What is the three degree spread and why do we care about it? First what spread are we talking about?

It is the difference in temperature between the ambient air at any altitude and the temperature of a parcel of air heated at ground level and then cooled at the standard adiabatic rate for dry air as it climbs as a thermal. Unless the thermal is warmer than the ambient air, it won't go up. The bigger the difference, the more rapidly it will go up.

That is, the better the thermal. Look at the lapse rate plot in Exhibits D, E or F. See the spread. See how it changes with altitude.

Why is a three degree spread of interest? Because experience teaches that a glider will be able to climb at least to the altitude where the spread is three degrees. It may well climb higher. The altitude at which the three degree spread occurs tells us how high we can expect to go based on a given ground temperature that heats up a thermals and sends it up.

There is something else we would like to know about thermals, that wasn't on my list of the big three -- sun, wind and thermals. That is whether there will be convective clouds.

Why do we car if there will be convective clouds? Because they help us find thermals. Whether they will appear depends on the amount of moisture in the air.

There is a formula that lets us estimate rather accurately if the amount of moisture in the air the moves up from the surface will be sufficient to generate clouds, and at what height. It comes from the rule that the wet and dry lapse rates decrease at different rates as air rises, with the result that the dry lapse rate at some point may equal the wet lapse rate and create clouds. The rule of thumb is that dry air cools at 5.4 degrees fahrenheit or 3 degrees centigrade per thousand feet. Wet air cools at 1 degree fahrenheit or 5/9 degree Centigrade per thousand feet. They converge at the rate of 4.4 degrees F or 2.5 degrees C per thousand feet. If you know how far apart they are when the sounding is taken, you can see whether they will make clouds.

The formula is divide the temperature-dewpoint spread by 4. If its 20 degrees F, the convective clouds should form at 5,000 feet, if the thermals get that high. The centigrade formula divides the spread in degrees centigrade by 2.2.

If you have mastered what I have described so far, you know the three critical pieces of information that you need in order to make a pretty good estimate of whether cross country soaring will be possible on a given day, and how good conditions will be.

We also want to know when conditions will become soarable. How do we determine that?

It is a question of how hot the temperature must get to push a thermal through the typical morning inversion. Look at any chart of the lapse rate taken at 12 Zulu or 7 a.m., and you can tell right away. Its the temperature at the surface that when traced upward at the dry adiabatic lapse rate gets past the blocking point in the lapse rate due to the inversion. Look at Exhibits D or E for this information. There is even a website that will predict for you when the ground temperature will reach that point and how high it will go at various times later in the day. That site, which Greg Chisholm found, is http://www.arl.noaa.gov/ready/cmet.html.

Remember that this sort of prediction depends on a bunch of factors including how moist the ground is, whether there are clouds that cut down on solar heating, and whether the winds are strong enough to disturb surface heating.

Practice getting the relevant information on a regular basis. It will take you fifteen minutes, maximum, with a computer, to find the information and analyze it -- once you have set up your browser and become accustomed to the sort of information you have to ask for on line. And you will come to the glider field knowing more than 90% of the pilots who come out to fly.

Compare how the actual weather in flight differs from what was forecast, and you will begin to learn how subtle factors impact soaring weather.

Now lets imagine we have obtained the weather forecast and it looks like the sun will shine all day, the winds will blow out of the west at speeds between 10 knots at ground level and 20 knots at 6,000 feet, the three degree spread occurs at 5,000 MSL, and there will be clouds, with bases at 5,500 MSL. We should be able to determine all of this in advance. Its a day in June or July and we start out on our cross-country flight. I'll skip questions of how early we start and how far we try to go, since others will cover those issue.

I want to talk, instead, about in-flight weather issues. How they arise and how to deal with them. All are from actual experience.

This topic divides into three parts: How to use clouds. What to do if there are no clouds and what to do if the weather is not as predicted.

i. How to use clouds.

The clouds that we can use to help find lift are conventional fair weather cumulus clouds. If you know how to thermal you probably have already learned how to use clouds. There are several basic rule.

First, watch clouds form and dissipate. You want to find clouds that have just formed or are in the process of forming. The lift will be better than those that are dissipating. A flat or concave bottom is the best indicator of strong lift. Remember that identical clouds will look different if you are viewing the cloud up sun or down sun.

Second, the closer you are to the cloud base, the closer the thermal will be to the cloud. The darkest part of the cloud usually has the most water vapor in it. That means that the lift is probably concentrated there. As you know, wind causes thermals to tilt downwind from their ground source. If you are relatively close to the ground and the wind is strong, the clouds will be of little use in finding lift. Ground sources are of maximum importance. If there is little or no wind, this rule doesn't apply. You may find lift directly under the cloud even though you are several thousand feet below cloud base.

Third, once you have found a relationship between clouds and lift on a given day, it is likely to be true for most clouds that day. If the good lift is on the southwest corner of the cloud in one case, look there first at the next cloud. If the good lift is found between 3,500 feet and 5,000 feet in one thermal the same is likely to be true in the next thermal.

If there are no clouds, what are your options?

First, it is still possible to find lift by reference to ground sources. Look for smoke rising. It will give you a good idea of the relationship of ground sources to thermals, how they slope and how high they go. Try to visualize the relationship and apply it to towns, plowed fields, refineries, trailer parks, utility plants and any other likely source of heat.

Second, look for haze domes. Even though there is not enough moisture to produce well defined cumulus clouds, there may be enough to produce barely visible haze domes. If they exist, they are an important indicator of lift.

Third, remember that lift is generally organized by the wind. If you aren't finding lift, in one direction, fly at right angles to the winds aloft. You will improve your chances of contacting lift.

Fourth, thermalling sailplanes are the best indicators of lift. On some days several gliders flying within sight of one another will have a far greater chance of encountering a thermal between them than will any one of the gliders flying alone. All the gliders have to be within gliding distance of the glider that finds the lift.

Sometimes the day will be just fine. The forecast will come true. You will be able to climb as high as the three degree spread or higher. Puffy cumulus clouds will mark the thermals. You will work a band of lift from 4,000 to 6,000 MSL, and the winds will be as forecast.

Last season we had successive days in June when Nigel Cripps and Don Kroesch did their 300 km flights with Bob Macys, and all went great. I followed along behind them and had two back-to-back 300 km flights.

But very often you encounter days when things start out one way and become very different. Sometimes you never know why.

This leads to the most important rule of cross-country weather: What matters in the air is what is actually happening. However good the forecast, if you encounter something different, you have to recognize that immediately and adjust to what is really going on.

If its better than forecast that is usually not a problem, although you may want to upgrade your task. If its worst than forecast, however, you may not be able to complete the flight if you don't adjust.

To figure out what is going on is often one of the hardest things for me to do. I start out with a mental picture of what to expect and if it isn't quite right, I don't catch on as fast as others, often with very bad results. This is one area in which I need to improve.

Some examples may put this into perspective and give you a feel for what really can happen.

In the Region 7 contest we held at CGC and Hinkley last year, we had one contest day flown from Hinkley. The task was Hinckley-Illinois Valley (Peru)-Whiteside County-Hinckley. I got off tow and immediately found a strong thermal to over 4,000 feet MSL. It was the best thermal of the day, but turned out to be the worst thing that could have happened to me. I assumed, once I had experienced that thermal, that there would be more like it. There weren't.

As result, however, I hung around the start gate looking for the good lift while others started. I was among the last to start. I got to Peru without any difficulty, and got to 4,000 MSL plus over the town. Then I headed out on course for Whiteside County. I worked successive thermals that were weaker and did not go as high. Being impatient, and expecting better, I moved from one to the next, never getting above 3,000 msl. Then I couldn't find anything. I ended up in a bean field about half way to Whiteside county, and landed about 4 p.m.

I later learned that one pilot, John Cochrane, had made it all the way around. Another, Nigel, had stayed with John and had gotten half way to Hinckley from the last turn point. John attributed his success to having Nigel's company in the difficult legs from Peru on. Two gliders flying within sight of one another can do things that one glider cannot alone. The chance that one will hit lift that the other can use is much higher than for a single glider.

Neal Ridenour and Mike Greenwald, if I remember right, reached Whiteside County and landed there. Significantly, all landed long after I did. Why didn't I do better?

I think the answer is that I made a series of weather mistakes.

First, I assumed the day would get better and kept looking for thermals to match the first one I hit. I was too slow in recognizing that the one good thermal was a fluke.

Second, I left late and lost the chance to fly within sight of other gliders that might serve as thermal markers. Leaving late and alone is not usually a good idea on a weak day since as the sun heads west the thermals rarely get better.

Third, I failed to shift to survival mode after the first turnpoint at Peru. I might have landed out, nonetheless. But in retrospect I should have climbed to the highest possible altitude in each of those weak thermals I encountered.

Let me take another example from contest flying. This time of doing something right with the weather. It was the first day of a regional contest at Ionia several years ago. The task was a 125 mile out and return to the northeast. As we were griding I could see high cirrus beginning to come in from the southeast. I decided to start at the earliest possible moment after the start gate was open. Other pilots hung around the start gate. I raced the cirrus to the turnpoint, some 60 miles out, and headed back. Large parts of the area were now overcast with high cirrus reducing the lift to little or none. In the overcast more or less on course back to Ionia there was an airport that most of the standard class pilots headed for. I was tempted, since it would permit an aero retrieve.

But off course to the west were occasional breaks in the overcast where the sun was falling on plowed fields. I headed that way, found weak lift that I worked for all the altitude it would provide, and had just enough altitude to make a rolling finish at Ionia. If I recall correctly only three of the standard class made it back that day, and all just barely did so.

The lesson is simple. Adjust for conditions at the earliest possible moment. Applying the rule is much harder.

Let me take a third example from contest flying. Several years ago we had an unsuccessful contest at Whiteside County. We flew only one day. That day was windy with no clouds. The task was Whiteside-Sandwich-Peru and back to Whiteside. Only one pilot in the standard or 15 meter classes made it around. That was Gene Hammond. The rest of us ended up at various locations. I made it to Sandwich and on to Mendota, where I found four or five others on the ground, and Duane Eisenbeiss working weak lift at about 2,000 AGL when I got there. The day died, I landed and so did Duane.

Gene later explained his success: He thought the day would be difficult and weak; therefore, he left as soon as he could. He also said that he had seen haze domes suggesting the location of thermals, and worked from dome to dome. I never saw domes, and had a much less successful flight. Much of my thermal finding was from ground sources that I picked up at low altitudes. These were dry fields (many fields were flooded from recent rains) and towns.

Earlier, I mentioned streets of lift and sink that often occur when the winds are strong. This was a windy day and some pilots reported that lift was arranged in streets. I never found a street. Pilots who did find the streets reported pretty good conditions moving upwind from Sandwich to Peru. In my case, however, I was blown all the way back from a low save near Sandwich airport to a point between Hinckley and Aurora airport. That tells you something about the wind. That thermal gave me enough altitude to get upwind to Earlville. Working lift off the town, I got to Mendota.

Streets or haze domes were the way to go that day, but I didn't see or find either. I wasn't looking for them, which may be why I didn't see or find them.

There is another point to be made from these examples of difficult days. It is important not to limit your cross-country flying to the easy days. Once you have developed a bit of cross-country experience, then leap at the chance to fly cross-country when its windy, or the thermals don't go above 4,000 MSL. That's the only way you will develop the skills needed to deal with difficult conditions. When you complete a flight in those conditions you will find it more satisfying that a long 300 K flight on an easy day.

Wallington, Meteorology for Glider Pilots John Murray, 1974. The best single book on the subject.

SSA Soaring Flight Manual, Ch. 4. (1998). A short summary of the major ideas.

Reichmann, Cross-Country Soaring (1978), pp. 82-92.

Lindsay, Handbook of Soaring Meteorology, (1988). A thorough discussion of soaring forecasting.

OSTIV Forecasters Manual for Soaring Flight (1976). A similar publication.

FAA Aviation Weather, ch. 16. A good general description of soaring weather.

May 10, 2000.