Transcript for the comic embedded in the link:
An XKCD comic about how to decode a METAR:
METAR - "Meter" (Usually misspelled)
KNYC - Station ID
251600Z - Time (25:16:002)
18035G45KT - Wind speed has been 18,035 knots for a good 45 minutes now.
6SM - Observer is a size 6 small.
VCFCFZVA - Sorry, the station cat walked on the keyboard.
+BLUP - Weird noise the sky made earlier.
NOSIG - The observer has no significant other :(
LTG OHD - We overheard someone saying there was lightning.
A3808 - Hey look, an Airbus A380-800!
RMK - Remarkable!
A02 - Fanfic archive equipped with a precipitation sensor.
SLP130= - Observer got sleepy around 1:30.
Image alt-text on the website: "In the aviation world, they don't use AM/PM times. Instead, all times are assumed to be AM unless they're labeled NOTAM."
That's not what's meant by "the alt text" in the context of XKCD comics, though, it's the transcript. The alt text is an additional joke in the title tag: "In the aviation world, they don't use AM/PM times. Instead, all times are assumed to be AM unless they're labeled NOTAM."