Interprets the NARWC VISIBLTY variable, which carries two different
encodings in the same column, and reports whether each record met a
visibility threshold.
Why this is not a simple comparison
Handbook 8.A.38 explains that VISIBLTY was originally a one-digit code
recording only whether visibility reached the 2-nautical-mile CETAP standard
and, if not, the weather responsible. In 2004 the field was redefined to hold
the actual estimated clear visibility in nautical miles. During the 2021
archive update the old codes were folded back into VISIBLTY as negative
numbers:
-1 | clear visibility for at least 2 nautical miles |
-2 | less than 2 miles, fog |
-3 | less than 2 miles, haze |
-4 | less than 2 miles, rain |
-5 | less than 2 miles, snow |
So a plain VISIBLTY >= 2 test — as used by the original processing code —
marks every legacy record as unacceptable, including -1, which actually
records good visibility. On a multi-year dataset that silently discards all
pre-2004 effort.
A -1 record asserts only that visibility reached 2 nmi, so it cannot
satisfy a threshold stricter than 2 and returns NA in that case rather than
a false TRUE.
References
Kenney, R.D. (2023) The North Atlantic Right Whale Consortium Database: A Guide for Users and Contributors, Version 8, section 8.A.38. NARWC Reference Document 2023-01.
Examples
visibility_ok(c(5, 1.5, -1, -2, NA))
#> [1] TRUE FALSE TRUE FALSE NA
# A legacy "clear" code cannot support a stricter threshold
visibility_ok(-1, min_nmi = 3)
#> [1] NA