How Balloon Track Produces Flight Predictions

Balloon Track for Windows uses the winds aloft data to predict where a balloon will land after a flight. This data is obtained by the National Weather Service from a multitude of sites across the US twice a day. Balloon Track can also use predictive data that will attempt to define the winds aloft parameters for a specific moment in time several days into the future.

The University of Wyoming has an excellent meteorology program and also is a well respected provider of real-time and historical weather data.

An excellent source of future wind predicted data is "Real-time Environmental Applications and Display sYstem" or READY

So, you download this data ... but, how does a Balloon Track prediction program convert it into a projected track for your balloon?

The concept is quite simple. The atmosphere is layered with winds moving in different directions at different speeds. For a simple example let�s suppose the following:

In our imaginary atmosphere there are only 4 layers of wind.

The blue layer extends from the surface to 20,000 feet above sea level (ASL). In this layer the wind is coming from the west moving at 10 knots.

The red layer extends from 20,000 to 40,000 feet. The winds are coming from the south at 10 knots.

The green layer winds extend from 40,000 to 60,000 feet with the winds coming from the east at 10 knots.

And finally, the yellow layer extends from 60,000 to 80,000 feet with the winds coming from the north at 10 knots.

Balloon Track for Windows requires you input an ascent rate and a descent rate. For this example, we�ll use an ascent rate of 1000 feet per minute and a descent rate of 1000 feet per minute. With this information combined with knowledge of the winds in those 4 layers it�s a simple process to calculate where the balloon will land.

Here is the data Balloon Track for Windows generates:

Bear Rng  EL. Vertical                      Dist. Bearing Range VOR
Time      Alt   Deg  Mi. Deg   FPM   Latitude  Longitude  to LOS   Mag    NM   ID
-----------------------------------------------------------------------------------------
13:00:03     51   90    1  45   1000  40.4737   -104.9633    10     253    19   GLL
13:20:00  20000   90    4  45   1000  40.4736   -104.8905   190     251    16   GLL
13:40:00  40000   45    5  54   1000  40.5292   -104.8904   269     264    16   GLL
14:00:00  60000  360    4  71   1000  40.5292   -104.9635   330     263    19   GLL
14:20:00  80000  270    1  90   1000  40.4737   -104.9635   380     253    19   GLL
14:25:21  60000  180    1  85   3743  40.4588   -104.9635   330     250    19   GLL
14:33:58  40000  238    2  76   2320  40.4588   -104.9950   269     251    21   GLL
14:47:32  20000  314    2  59   1474  40.4965   -104.9950   190     257    20   GLL
15:07:32      1   54    3  -1   1000  40.4965   -104.9219     1     257    17   GLL

As you can see in the plot above, the balloon takes off from the center of the graph and heads east. Since the balloon is ascending at a rate of 1000 feet per minute, it takes 20 minutes to ascend to 20,000 feet. For the first 20 minutes the balloon is traveling east at a rate of 10 Knots. This means that the balloon will have traveled 3.333 nautical miles or 3.835 statute miles. The program rounds this off (in this printout only) to show the balloon at a bearing of 90� from the launch site at a distance of 4 statute miles. Then it travels 3.835 statute miles north, then the same distance west and the same distance south to arrive back at the launch point. But now it is at an altitude of 80,000 feet and the angle of elevation from the launch site to the balloon is 90� or looking straight up.

But, why isn�t the descent path the same square shape and why doesn�t the balloon return directly to the launch point.

Take a look at the descent rates in the data above. Although the payload system is estimated to descend at a rate of 1000 feet per minute at sea level, the actual descent rate in the rarefied upper atmosphere is much faster. With little atmospheric density for the parachute to work against, the payload descends at a very fast initial velocity. As the payload system descends to lower altitudes the atmospheric density increases and the descent rate decreases.

The payload system descends the first 20,000 feet from 80,000 feet to 60,000 feet and only travels 1 mile south of the launch site. When the payload turns west at 60,000 feet its descent rate has slowed somewhat and it travels further on this leg. Likewise with each subsequent layer of the atmosphere, the descent rate decreases and thus taking a longer amount of time to traverse the layer which gives the winds more time to move the payload a greater distance.

In reality things are much more complex. There are no discrete boundaries in the atmosphere. The winds in each layer not only move laterally they also have a vertical movement component. Because of this, no prediction made with Balloon Track for Windows can be really exact. However, a reasonably accurate prediction is possible with the program.