Originally Posted by
Nerull
The inlet ports for any barometric pressure reading at speed must be carefully designed to minimize airflow effects on pressure. I doubt the Garmin's design takes that into account. I'm not sure it even has enough surface area to do so. Airflow must be smooth and uniform around the inlet, and airflow direction at speed needs to be perpendicular to the inlet - which might be why windy days have more effect than descents.
I suspect that Garmin essentially measures stagnation pressure (much easier to measure than static pressure) and then uses Bernoulli's equation to correct for speed. Which is why it gives correct readings during fast descents (Bernoulli's equation will work as long as there's no wind) but produces weird results in high wind.
If that's the case, 30 mph wind would reduce apparent elevation by ~60 feet if you're standing still. It's more complicated if you're moving.