Freefall Drift – Part 1 2


weather balloonWe are starting a 3-part series today on how to calculate freefall drift.  Winter time here can bring some crazy high winds at jump run altitude and this has a huge impact on spotting.  So, if you want to land on the airport, it’s time to learn about how to calculate freefall drift.

To begin with, you’ll have to get comfortable reading the winds aloft forecast.  

Notice the word forecast – this is not what is actually happening in your area.  The winds aloft report is a forecast created four times a day by supercomputers running mathematical models to make predictions.  Luckily, these predictions are fairly close to what you can expect, as the computers compile data from pilot reports and weather balloons (scientific details about balloons, their launches, and data gathering here and a less complicated version on how weather balloons work here).  However, there are times when they are quite a bit off…so beware.

There are several apps out there, and the good old Aviation Weather Center, that can give you the winds aloft report.  I’ve taken a screenshot of the report that was available while I was working on this section of the blogpost.  As you can see, it was 1957 UTC (translates to 2:57 pm EST) and this data was taken on the 4th day of the month at 1200Z, and could be used from 1400 to 2100Z (translation: 9 am-4 pm EST).  For those that want to start using UTC (coordinated universal time, also known as Zulu time) here is a handy conversion chart for UTC to any US time zone.

winds aloft screenshot

Since this screenshot is on the fuzzy side I have typed out the data for JAX (Jacksonville airport) since that is the airport closest to my location.  I also decided to include only the data that pertains to skydivers (3000′ to 12000′ MSL), but you can see that there is data here up to 39,000 feet.  The altitudes are shown in parantheses, so that I didn’t have to construct the whole table.

3417 (3000′) 3118+06 (6000′) 2921+02 (9000′) 3124+04 (12000′)

To read this forecast you need to know that the first two numbers correspond to the wind direction (remember to add a zero to the end of those first two numbers) and the next two numbers indicate the wind speed.  For example, at 3000′ MSL the wind forecast shows that the winds are from 340 degrees at 17 knots.  At 6000′, the winds are coming from 310 degrees at 18 knots, and the temperature is +6 degrees Celsius.  9000′: winds from 290 degrees at 21 knots, temperature is 2 degrees Celsius.  12000′: 310 at 24 knots, 4 degrees Celsius.

And now you know how to read the winds aloft report.  Next up, what to do with all of these numbers.

Go to Part 2: the Calculations.

Go to Part 3: Putting it all together and determining the Exit Spot.


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