Measuring the winds--Taking the first PIBAL of the season

PIBAL is short for Pilot Balloon. It is the simplest, easiest and most accurate way to measure wind speeds at short intervals below 1500 feet above the ground. Since our flight trains (balloon plus parachute plus suspension ladder plus payload) are as much as 1000 feet tall at launch this wind measurement capability is critical for us. The concept of a PIBAL is simple. You inflate a small latex rubber balloon until it just is buoyant with an inflation nozzle of a known weight attached. Now that you have a known amount of helium in the balloon you know how fast it will go up when released. In our case that is about 600 feet per minute. We use an instrument called a meteorological theodolite to visually track the balloon (we keep in the crosshairs through the scope). This is part of what is called an ARTS unit (Automatic Readout Theodolite System).

When we first setup the theodolite we orient the instrument so that it shows an azimuth (direction) of 0 degrees when pointed to north, and an elevation (tilt) of 0 when pointed straight out. After we release the balloon we crank the azimuth and elevation so that we keep the balloon in sight. The theodolite automatically reads the azimuth and elevation for us and sends the data down a cable buried under the ice to a laptop in the building. We have a piece of computer software that then reads the azimuth and elevation every 30 seconds and computes the winds. We can do this because we know how high the balloon is every 30 seconds because of the known ascent rate. We know how high it is, and how much the azimuth and elevation have changed since the previous reading (or since the start) so simple math will let us calculate the wind speed and direction. The results are valid at the midpoint between the two levels. For example, the first level as seen in the last picture is at 150 feet above the ground. That's because the first reading is at 0 feet, and the next reading 30 seconds later is at 300 feet, so the mid point is 150 feet.

The first two pictures are of our other meteorologist down here with me, Chris. I am training him on how we forecast for balloon launches at CSBF. The next picture is a picture of the data.