Definition
The weather conditions that must be forecast at an airport for it to legally qualify as an alternate destination on an IFR flight plan. Under standard rules, an airport may be listed as an alternate only if the forecast weather at the estimated time of arrival meets or exceeds: for a precision approach, a 600-foot ceiling and 2 statute miles visibility; for a non-precision approach, an 800-foot ceiling and 2 statute miles visibility. Individual airports may publish higher (non-standard) alternate minimums, and some airports are not authorized for use as an alternate at all.
Plain English
The minimum forecast weather an airport must have for you to write it down as your backup airport on an IFR flight plan. The standard requirement is a 600-foot ceiling and 2 miles visibility if the airport has a precision approach, or 800-foot ceiling and 2 miles visibility if it only has a non-precision approach.
Context Anchor
Seen during IFR preflight planning when choosing an alternate airport, checking forecasts, and reading alternate minimums notes for instrument approach procedures.
Derivation
Alternate' comes from Latin alternatus meaning 'every other one' — here it means the backup airport you would go to if the destination becomes unusable. 'Minimums' refers to the lowest acceptable values; this term sets the floor for what counts as a usable alternate.
Why Pilots Care
Filing a legal alternate prevents a pilot from being forced to attempt a landing at an airport that has gone below minimums.
Intuition Check
Do not read “minimums” here as the lowest weather you may legally land in at the alternate. IFR alternate minimums are the weather required to choose that airport as the planned backup; the actual landing minimums for an approach are a separate set of numbers.
Example Sentence 1
The forecast at the destination was marginal, so the pilot picked an airport 40 miles away that comfortably exceeded the standard IFR alternate minimums.
Example Sentence 2
Because the destination forecast dropped below minimums, the chosen alternate had to meet the published IFR alternate minimums for ceiling and visibility.