Special Thunderstorms

A probably not so recent anymore AIM update identified two changes in the publication for which I wanted to look up the details.

The following airports have been identified for SPECI criteria for significant temperature changes between hourly reports: about half of Canadian airports

It then lists almost half of Canadian airports, the biggest ones. Normally an aerodrome observation, called a METAR is published once an hour, but if one of a number of specific significant changes occur, like precipitation starts or ends, or the ceiling or visibility changes past a specific limitation, they issue a special update observation called a SPECI. It's pretty clear from the descriptions that they are aimed at given a pilot the best chance of making the right decision about whether or not she can land there with her equipment and training.

It doesn't say why they have added temperature to the criteria. Temperature in and of itself doesn't offer the same impediment to landing as fog, hail, or thunderstorms, but a sudden change in temperature indicates the passage of a front, which could mean an abrupt change in weather, or sometimes freezing rain.

The second change is to Met 3.13 and I was curious when I looked at it what they had changed.

(a) active thunderstorms–the cumulonimbus (CB) symbol is used when thunderstorms occur, or are forecast to occur, over a widespread area, along a line, embedded in other cloud layers, or when concealed by a hazard. The amounts and the spatial coverage (in brackets) are indicated as:

ISOLD (isolated) – for individual CBs (less than 25%)
OCNL (occasional) – for well­separated CBs (25 – 50% inclusive)
NMRS (numerous) – for CBs with little or no separation (greater than 50%)

It looked the same as I remembered. As an instructor I used to teach students to match ISOLD to FEW (1-2 octas), OCNL to SCT (3-4 octas) and NMRS to BKN (>4 octas), where an octa, or maybe it's an okta--damn you really forget this stuff when you're not lecturing someone on it every day--is a one-eighth proportion of the sky. I houled out an older paper copy of the AIM and looked up what it used to be, and the old version omits the bits that are in parentheses in the lines quoted above. I wonder how I knew it, then. I guess it was in the old, old version, the AIP and somehow didn't get added to the new AIM until recently.

That's exactly how lore is created, but someone added it back in, so now it's information again. I love lore. Especially when it's incorrect but it can be traced back to a loopily logical origin.