Pilot Weather Wisdom

Nearly 90% of Canadians live within 100 miles of the American border so there is a good chance a pilot will head south of the border or visit Alaska. We know weather has no political boundaries, but you’ll find METARs have inherent differences when comparing observations vis-à-vis (face to face).

***Sadly, NAV CANADA’s archaic website (AWWS) only provides Canadian weather. A dangerous proposition for those crossing the border unless you know about the tab (U.S WX ADDS) that sends you State side. Yes, they have a new improved weather website coupled with flight planning, but why don’t they take down their old site? It’s like buying a new car, but letting the old one rust in the driveway.” ***

Right away, Canadian METARs start with the ICAO letter “C” whereas America implements a “K” except over Alaska where you’ll see “P” for Pacific. Now come the issue times. In America, the actual time the observation is taken is used, but in Canada you will always see it on the hour as that is when the barometer must be read. After that, wind, visibility, weather, cloud, temperature and dewpoint are the same format. However, unlimited visibility is 10 statute miles in the States, but generally it’s 15 miles for Canada. And when visibility gets low, RVR values are a tad different. A ½ and ¼ mile in Canada is 2,600 feet and 1,200 feet respectively, but in the States, it’s 2,400-2,600 feet and 1,400-1,600 feet. Americans observe in increments of a 1/16^th of a statute mile in low visibility, but Canada only goes to an1/8^th.

Surprisingly, Celsius rules in temperature for American METARs. In 1996, surface actuals morphed into present day METARs. Because of it, Celsius replaced Fahrenheit, but to make things interesting, a code is appended on every American observation to fine tune temperature and dewpoint in tenths. Great to know when precipitation is flirting at zero degrees Celsius, i.e. is it rain or freezing rain?

When we venture into the RMK (Remarks) section things are noticeably different. South of the border, you’ll notice letters A02 denoting a “precipitation discriminator” i.e. the equipment can detect the difference between snow or rain. If A01, there is no precipitation discriminator. Canada doesn’t have this. SLP (sea level pressure) is the same in both countries, but in America the units are called millibars, but Canada labels pressure in hectopascals yet they are equivalent units. In Canada, we denote cloud types and opacity amounts, but Americans omit this except if there are TCUs or CBs.

You will find density altitude in Canada when the equivalent airport elevation pressure exceeds 300 feet. And we love adding remarks like FROPA (frontal passage), FROIN (frost on indicator) and PRESSFR (pressure falling rapidly).

***A uniquely American addition is timing the onset and ending of precipitation. For example, RAB15E40 means “rain began” 15 minutes after the hour and “ended” 40 minutes past the hour. If you know this stuff, then you are nearing the expert level in METAR interpolation. ***

Another very distinct unit sometimes found at the extreme end of an American METAR is the dollar sign (it’s an American dollar not a weak Canadian one J), denoting there is a degradation in the weather observing equipment i.e. maintenance is needed. If a hurricane wreaks havoc along the coast of Florida you will see dollar signs pop up on the METARs as the equipment is destroyed.

A METAR face-off between the two capitals: CYOW (Ottawa) versus KIAD (Washington, Dulles)

CYOW 120000Z 26006KT 15SM FEW006 FEW040TCU SCT070 BKN140 22/21 A2981 RMK CF1TCU1AC4AC1 SLP095 DENSITY ALT 1500FT

For Ottawa, the time is on the hour of midnight Zulu, visibility is 15 miles, with cloud types given and cloud cover depicted in Oktas (1/8ths). Density altitude, strictly a Canadian thing, is given when 300 feet above airport elevation. The airport sits 377’ ASL so a density altitude of 1,500 feet is getting significant.

KIAD 112352Z 18004KT 10SM FEW050TCU BKN150 BKN250 29/22 A2993 RMK AO2 SLP132 T02940217 10333 20294 53003

This observation was taken at 2352Z, 8 minutes before the hour. You will not see 0000 Zulu unless the observation is late. There are no cloud types given except for TCU or CBs and cloud amounts are never depicted. The temperature is 29° C with a dewpoint of 22° C (it’s muggy), but really the numbers T02940217 fine tunes things to 29.4° C and 21.7° C respectively.

Those last three groups are truly numbers a pilot does not need to know and you’ll see them every 6 hours. They are synoptic codes for weather gurus which includes maximum and minimum temperatures and pressure tendency.

Doug Morris flies the B787 and is a certified meteorologist. He has written two weather books, Canadian Aviation Weather and Pilot Weather: From Solo to the Airlines. Both books cater to Canada/U.S. differences.