Something like the following should do it efficiently:

my %keys = (
     'skc'  => "clear-day",
     'few'  => "partly-cloudy-day",
     'sct'  => "partly-cloudy-day",
     'bkn'  => "cloudy",
     'wind' => "wind",
);

my $keys = join '|', map quotemeta, sort keys %keys;
my $pattern = qr/(....)($keys)(...)/;

while (<DATA>) {
    s/$pattern/$1$keys{$2}$3/;
    print;
}
[download]

Dave.