[thorn bird]

IFR Alternate Airports (1-2-3 Rule).


When filing an IFR flight plan, you must include an alternate destination airport when the weather forecast at your original destination predicts conditions below those specified in CFR 91.169. To help remember those conditions, you can use the 1-2-3 rule.

1 - During the time 1 hour before to 1 hour after the estimated arrival time
2 - Ceiling less than 2,000 feet
3 - Visibility less than 3 miles

If the above conditions exist, an alternate airport must be filed.

Simplicity itself, has worked well and stood the course of time, and the US of A still outclasses us in the safety stakes by a wide margin.

Meanwhile over on UP, a robust debate is occurring on what our confusing convoluted bullshit means or doesn't mean.
The real world has CAT111 approaches, CAsA gives us CAT1.5