As simple as.......
Even includes a little memory jogger to help you out.No wonder things are safer over there.
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.