I have often wondered, while standing in a gently breeze, what is the wind speed up several hundred feet, and, which way is it going on top of the trees? These two basic questions are probably asked by 99% of the pilots 100% of the time.
Other questions I ask myself are: Can I make it over the city?; Is there a shear up there?; Can I make the target from this spot?; If this is the right spot, will I get to the field before the hare takes off?
In a series of two articles, I hope to answer most of these questions for the rest of you who don't happen to have NASA engineers and retired NWS forecasters as neighbors. You might call this article, "Your Tax Money At Work!".
First of all, you must know how high the pibal is when you are trying to figure out what is going on up there. This is simple. All you need to know is the rate of climb for the pibal. The problem here is no one I talked to, up to now, really knew what that rate actually was. But, after talking to an old friend (and I mean old-he did his graduate work in '32) who retired from the National Weather Service, I was able to interpret several pages of math to the following simple equation:
Nest, go into a room with no moving air and set up your laboratory. For this experiment you will need a balance scale capable of reading 0.001 grams, a yardstick, a table, two carpenter's squares, a small Styrofoam cup, thread, helium, and some table salt. Now, follow these steps (for the pilots, I'll put numbers here so you don't get confused):
1. Weigh the entire count of balloons in grams and divide that weight by the number of balloons. This gives you the average weight of one balloon (W). Oh, by the way, don't forget to take them out of the bag.
2. Inflate one balloon with helium and measure the widest diameter (D). Do not inflate the balloon orally as the moist air from your lungs will condense on the inner walls of the rubber and add weight. Use a friend, or your spouse if you don't have any, and hold one of the squares so the inside of the sort edge is aligned at the zero inch mark on the yardstick with the short blade hanging over the table and against the balloon at its widest part. Move the other square so it is also against the balloon on the opposite side. You should have what looks like a big caliper around the balloon. Look where the second square meets the yardstick and read what the diameter of the balloon is. If you want to keep this diameter, and you will, mark it down so you can make a permanent caliper from wood or metal when you are out in the field.
3. Cut the Styrofoam cup down to about half of an inch. Carefully tie the cup, gondola style, to the balloon. Trim off any extra thread. Let the balloon rise with the empty cup attached. It must rise for this to work. If the cup is too heavy, cut more cup off.
4. Very slowly, pour salt into the cup until the balloon is at equilibrium. (If you don't understand that word, you better just follow some other pilot and hope for the best.) The air at this point must be absolutely still. Try to not get nervous at this point and get sweat on either the balloon or the salt. It doesn't take much to throw off this part.
5. After the balloon maintains equilibrium, remove the thread, cup and salt from the balloon. (The first time I did this, we let go of the balloon, it hit the ceiling and popped, and somebody dropped the cup.)
6. On the gram balance scale, find the combined weight of the cup, the thread and the salt. This is the Free Lift (L).
As an example, suppose each balloon weights 6.642 grams (w). Let's say the weight of the thread, salt, and cup was 33.21 grams (L). Using our formula, we now have
You now know that each pibal, inflated to the above diameter, will ascend at a rate of 443.7 feet per minute, providing you use only balloons from the same group that you weighed.
With this information, if you time your release, the pibal will be at 444 feet after one minute, 888 feet after two minutes, etc. Next time I'll give you some math for figuring out how fast the little sucker is moving away.
