Jim Logajan writes:

I believe the transmission of weather information by use of abbreviated
METAR codes originated in the days of 10 character-per-second Teletypes (or
possibly even earlier). The ASR 33 Teletype (the model I'm personally
familiar with) could only print uppercase characters. At that slow rate of
printing and usingg such a limited character set, brevity was an asset.

Brevity is still an advantage today, albeit a less important one.

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