PILOTS AND Students will all know that an aerodrome with weather reporting publishes terminal conditions called a METAR every hour, giving information including wind speed, ground visibility, cloud ceiling, temperature and dewpoint and altimeter setting. But the weather can change significantly over the course of an hour and if it does a “SPECI” is a weather observation issued for an airport in between the hourly intervals. The criteria for issuing a SPECI are tightly defined – like everything official in aviation. It isn’t left up to the discretion of a forecaster or weather observer.

Today, I was watching the weather at CYTZ and a view of the NavCanada page with the CYTZ (Billy Bishop) METAR showed the following observations between 1800Z and 1900Z (1pm and 2pm local time):

METAR CYTZ 051900Z AUTO 22017G22KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051859Z AUTO 22017KT 9SM BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051853Z AUTO 22016G21KT 9SM -RA FEW019 BKN024 OVC030 04/02 A2947 RMK SLP984=
SPECI CYTZ 051851Z AUTO 22012G21KT 9SM -RA SCT019 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051850Z AUTO 22012G21KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051849Z AUTO 23013G22KT 9SM -RA SCT021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051848Z AUTO 23011G22KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051847Z AUTO 23009G22KT 9SM -RA SCT021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051846Z AUTO 23010G22KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP985=
SPECI CYTZ 051844Z AUTO 23014G22KT 9SM -RA SCT019 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051826Z AUTO 23016G23KT 9SM -RA BKN024 OVC030 04/01 A2947 RMK SLP985=
SPECI CYTZ 051823Z AUTO 22016G23KT 9SM BKN024 OVC030 04/01 A2948 RMK SLP986=
SPECI CYTZ 051819Z AUTO 22015G23KT 9SM -RA BKN026 OVC033 04/02 A2948 RMK SLP986=
SPECI CYTZ 051812Z AUTO 22016G23KT 9SM -RA BKN024 OVC031 04/02 A2948 RMK SLP986=
SPECI CYTZ 051811Z AUTO 22014G23KT 9SM -RA SCT024 OVC031 04/02 A2948 RMK SLP987=
SPECI CYTZ 051809Z AUTO 23012G23KT 9SM -RA BKN024 BKN029 OVC033 04/02 A2948 RMK SLP987=
METAR CYTZ 051800Z AUTO 22014G24KT 9SM -RA BKN027 OVC033 04/01 A2948 RMK SLP987=


That’s a total of 15 different updates through the hour. What a lot of SPECI’s!

What’s going on here? Why so many updates on a moderqtely blowy and cloudy but quite warm January day?

The answer appears to lie in the AIM, section MET 8.4.1 “Criteria for Taking Aerodrome SpecialMeteorological Reports (SPECI)” which says that “Special observations will be taken promptly to report changes that occur between scheduled transmission times whenever one or more of the following elements have changed in the amount specified.” It then goes on to list the precise changes that require the issue of an weather update. Sufficiently large changes in wind, precipitation, clouds, visibility, a tornado is sighted, or a thunderstorm or a volcanic eruption begins. Yes, per the AIM, “A SPECI shall be issued when a volcano erupts.” As I said, nothing is left to chance.

If you look through the list of SPECIs you’ll see that mostly what’s changing is the ceiling (apart from at 1823Z and 1859Z when the precipitation stops and 1826Z when it starts again). Here’s the list repeated with the ceiling and precipitation changes in bold:


METAR CYTZ 051900Z AUTO 22017G22KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051859Z AUTO 22017KT 9SM BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051853Z AUTO 22016G21KT 9SM -RA FEW019 BKN024 OVC030 04/02 A2947 RMK SLP984=
SPECI CYTZ 051851Z AUTO 22012G21KT 9SM -RA SCT019 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051850Z AUTO 22012G21KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051849Z AUTO 23013G22KT 9SM -RA SCT021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051848Z AUTO 23011G22KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051847Z AUTO 23009G22KT 9SM -RA SCT021 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051846Z AUTO 23010G22KT 9SM -RA BKN021 OVC027 04/02 A2947 RMK SLP985=
SPECI CYTZ 051844Z AUTO 23014G22KT 9SM -RA SCT019 OVC027 04/02 A2947 RMK SLP984=
SPECI CYTZ 051826Z AUTO 23016G23KT 9SM -RA BKN024 OVC030 04/01 A2947 RMK SLP985=
SPECI CYTZ 051823Z AUTO 22016G23KT 9SM BKN024 OVC030 04/01 A2948 RMK SLP986=
SPECI CYTZ 051819Z AUTO 22015G23KT 9SM -RA BKN026 OVC033 04/02 A2948 RMK SLP986=
SPECI CYTZ 051812Z AUTO 22016G23KT 9SM -RA BKN024 OVC031 04/02 A2948 RMK SLP986=
SPECI CYTZ 051811Z AUTO 22014G23KT 9SM -RA SCT024 OVC031 04/02 A2948 RMK SLP987=
SPECI CYTZ 051809Z AUTO 23012G23KT 9SM -RA BKN024 BKN029 OVC033 04/02 A2948 RMK SLP987=
METAR CYTZ 051800Z AUTO 22014G24KT 9SM -RA BKN027 OVC033 04/01 A2948 RMK SLP987=


It looks like the automated ATIS equipment spits out a SPECI every time the ceiling crosses the threshold of 2500 feet – going either above, or below.

Normally you wouldn’t expect a ceiling change to trigger a SPECI unti the ceiling crosses 1500, as described in AIM MET 8.4.1. But there is a get-out clause in 8.4.2 that says “Additional criteria [to issue a SPECI] may be established to meet local requirements” – and perhaps that’s what’s happening here: because of the tall buildings in Toronto. Or perhaps it’s a glitch with the equipment. If you work for NavCanada and you have inside information, please get in touch!