The personal weather minimums in EZWxBrief include those for the departure airport, en route and destination airport. But where does one stop and the other one begin? In other words, what is the corridor size used when evaluating the personal minimums for the departure airport for ceiling and visibility versus the en route minimums for ceiling and visibility? The answer is a bit complex, but the following is close to the best summary.
For example, for the route above from Seattle to Spokane, EZWxBrief evaluates the weather against the departure personal weather minimums (other than crosswind) using an elongated terminal area. it's an elliptical region that is approximately 7 nautical miles to the left and right of the course and extending out approximately 5 miles along the route of flight as shown above.
It pulls the forecast data at a 2.5 to 10 km resolution for ceiling and surface visibility using forecasts from over a dozen forecast models within this oval and uses the harmonic mean to determine the representative value for the departure corridor. The harmonic mean is one of three of the Pythagorean means that also includes the arithmetic mean and the geometric mean. The harmonic mean is always the lowest of these three. This is a type of average that is calculated by dividing the number of values in the data series by the sum of reciprocals of each value in the series.
It's a bit of a complex set of calculations but creates a more representative sample of the conditions in the departure corridor for ceiling and visibility. The harmonic mean is also used to determine the ceiling for observations for an ASOS. Similarly, the goal is to create a representative value for the terminal area.
Once the flight is out of that corridor, the evaluation takes over for the ceiling and visibility for the en route personal weather minimums (the subject for a different EZTip). At times when the weather is fairly homogeneous, this will not make much of a difference, but when there's a substantial difference in the weather just a few miles from the airport, the departure and en route corridor evaluations can be fairly different as well.
If you want to learn more about the Skew-T diagram, you can order your copy of The Skew-T log (p) and Me book that is available in soft cover or eBook format.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
Founder, EZWxBrief™
CFI & former NWS meteorologist
