Pibals are small helium filled balloons that balloon pilots use to help assess the winds aloft. By knowing the empty weight of the balloon and its inflated diameter you can use the table below to estimate a pibals ascent rate.
To use this table:
- Determine the weight of an empty balloon. You could do this buy taking a bunch of them (10 – 50) to your local shop and have them weighed on the scales. Divide the total weight by the number of balloons to obtain the average weight in grams. If your scales measure in ounces then multiply by 28.35 to convert to grams.
- Inflate your balloon to a known diameter.
- Read off the climb rate from the table below.
Alternatively you can weigh the empty balloon and measure its free lift and calculate the ascent rate using the formula below.
Pibal Ascent Rates
|Diameter (mm)||Weight (gms)||Velocity (ft/min)|
Calculating pibal ascent rate
The ascent rate of a pibal can be approximated using the following formula. The actual pibal ascent rate will vary from this calculated figures due to a number of factors including density height and the aerodynamic form of the pibal. In general the ascent rate will tend to be slightly higher than the figure generated by these formula in the lower levels of the atmosphere and will reduce throughout the ascent.
- L = Free Light (grams)
- W = Balloon Weight (grams)
- Vvel = Balloon Ascent Rate (feet per minute)
This downloadable Pibal Ascent Rate Spreadsheet may also be useful.
Verifying your pibals ascent rate
We always verify the pibal comparing it to what the actual winds are early in a flight. You can do this by:
- Always timing your pibal
- Estimate the height that a distinct change in the wind direction is observed at.
- Compare your pibal observations to what actually occurs early in your flight In Brisbane, there is usually an inversion at about 600 feet above the ground. As the pibal passes through the inversion a change in speed and direction is evident.
After a bit of experimentation I figured out how big to inflate my pibals so that they pass through this change 2 minutes after release. 600 feet in 2 minutes = 300 feet per minute climb
Balloon Pilot Education – Pibal Ascent Rate – Technical data content credited to Mr Steve Griffin