David Megginson
Pattern Altitude
Today I learned. There was a double dashed purple line over Lake Michigan in the 0600z GFA for tomorrow morning, and I was curious what exactly it meant. A single purple dashed line is a trough, but there was nothing in the AIM or any of the Nav Canada weather legends/guides about a double dashed line, and I knew it couldn't be a TROWAL ("occluded front" to our American cousins), because that would be a hooked line.
In the end, I called the London (Ontario) FIC — we can still get a phone briefing with a weather expert who knows how to do more than just say "VFR not recommended" — and he explained that the double line means a higher-level/upper trough.
18 years since my instrument rating and I'm still learning. Here's the full 0600z Cloud/Weather GFA, for anyone interested (there's a separate one for icing and turbulence):
In the end, I called the London (Ontario) FIC — we can still get a phone briefing with a weather expert who knows how to do more than just say "VFR not recommended" — and he explained that the double line means a higher-level/upper trough.
18 years since my instrument rating and I'm still learning. Here's the full 0600z Cloud/Weather GFA, for anyone interested (there's a separate one for icing and turbulence):
