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In their METAR, CYPK reported ... 25SM FEW037 SCT050 OVC250 21/15 A3016 RMK FU1FU2FU5 DENSITY ALT 400FT.

Since it's quite smokey out, I would have expected a simple FU in the weather section after visibility.

I'm wondering what FU1FU2FU5 is about. According to the remark format at http://meteocentre.com/doc/metar.html it should be "layer type and opacity in oktas". Is this just the way to say "sky covered by smoke"?

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It's indeed the layer type and okta, but in the remark the layer type is repeated for each reported cloud (smoke) layer. From your example:

FEW037  →  FU1
SCT050  →  FU2  →  1+2=3
OVC250  →  FU5  →  3+5=8

The oktas of the layers are added, that's how OVC is shown as 5 and not 8, by adding the previous oktas it does become 8.

FEW = 1 to 2
SCT = 3 to 4
BKN = 5 to 7
OVC = 8

Similarly for CYCD:

METAR CYCD 012200Z 34010KT 12SM BKN038 BKN051 30/11 A3012
RMK FU5FU2 VIS LWR N-E SLP201 DENSITY ALT 1600FT=

BKN038  →  FU5
BKN051  →  FU2

When no remark is needed for the smoke (French: Fumée), the FU code is placed before the remark:

METAR CYBD 012200Z 29007KT 8SM FU FEW240 28/13 A3019
RMK CC1 SLP225 DENSITY ALT 1400FT=

According to Wikipedia, this is how Canada handles the remark section. Here is what navcanada.ca says:

Where observed, the obscuration and cloud type amount for each reported layer is included in remarks. It may exceed 8 oktas if a higher layer is observed through transparencies in a lower layer

(Emphasis mine.)

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