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Saturday, October 26, 2013

Making Sense of Aviation Weather 2: "T" is for "Terminal"

Last time, I promised you a warning about TAFs, so this post will be terminal to you.

Terminal Aerodrome Forecasts aren't as lethal as they may sound, as long as you keep in mind that "terminal" means just that: the terminal area of the airport the forecast is for. That's logical enough, but it just leads to another question: what does "terminal area" mean?

The answer is simple but important. The terminal area is a ring 5 nautical miles around the airport. Five miles isn't much at all! In fact, it's such a small area that at a place like Cleveland-Hopkins (KCLE), the runways stretch across almost 30% of the forecast circle:

The inner ring of Cleveland's Class B airspace matches up quite nicely with the area contained in a TAF. The second ring around it also gives a good idea of the second area that is sometimes contained in a TAF: the "vicinity".

The VC (or "vicinity") area is another ring that goes out from 5 miles to 10 miles. It doesn't line up as perfectly in this excerpt because KCLE's second ring of CLass B only extends out 8.5 miles instead of 10, but it's close enough to bring home the main point: there's not much area contained in that circle.

So what? How much different can the weather actually be from 10 to 15 miles? A lot different, actually. Take a look at this picture I took standing on the ramp at Lorain County Regional Airport (visible as the magenta strip at the center-left of the chart above) one day:

The camera is pointed almost directly toward KCLE, and that is a thunderstorm parked almost directly over Cleveland-Hopkins while I'm standing underneath calm blue skies only 14 miles away.

(In case you're wondering why I took a picture of a rather weak thunderstorm, it's because I took this on December 21, 2011, only four days before Christmas. This is so rare in Cleveland that it qualifies as a freak storm; usually by this time of the year the ground is covered in snow, not thunder.)

This isn't some fluke occurrence that I'm trying to blow up into a bigger thing than it actually is. Many places have their own peculiar weather patterns that local pilots are used to but pilots just passing through may not know about. Around Cleveland, it's extremely common for it to be nice on the west side and IFR on the east side due to the effects of Lake Erie.

In fact, as luck would have it, while I've been writing this post, a good example of this phenomenon has popped up. The window of my study faces almost due west, and the sun has been slowly sliding down the sky as it settles in for a beautiful orange and blue sunset framed by a few fringes of thin clouds. While I waited for the above image to upload, I went into the kitchen to top off my coffee. The kitchen windows face almost due east (toward Cleveland) and the scene through them was the polar opposite: angry gray, moisture-laden clouds choke the sky completely, suffused with an oddly-contrasting orange light shining on them from the sun settling peacefully on the other side of the same sky.

The radar shows why there's such a huge difference between east and west. Unsurprisingly, it's a textbook example of Lake Effect and shows the pattern of weather that takes place time after time around here:

The west side is clear (the circular glob of yellow and green in the center can be ignored since it's just ground clutter interfering with the radar site) and the east is about to get another visit from Old Man Lake Effect.

So keep in mind when you're doing your flight planning that a TAF is only a snapshot forecast at a very specific point in space and time. It only covers that airport, not that region or state as a whole. Nonetheless, TAFs are an extremely useful part of your weather picture and will serve you well as long as you use them well, so don't let this discourage you from using them. Just like Lucky Charms are an important part of a balanced breakfast, TAFs are an important part of the balanced weather picture you should consume before you hop on that magic carpet.

I'd love to hear about any weather quirks your own home area may have that might make you take your local forecast with a grain of salt. Leave a comment below and share them with me!

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