Arrival alternate weather criteria is also much more complex than the basic part 91 criteria. The standard to determine if an alternate airport is required is the same (within 1 hour of ETA, weather forecast better than 3SM/2000' cig).

However determining if an airport is a legal alternate is much more complex. It's based on a series of criteria that include the forecast weather, types of approaches available, number of runways and approaches available. This was called the "method 1/method 2" standard at my previous employer.

Method 1 (basic rundown):

For airports that have at least one approach to a suitable runway (based on winds/weather/notams/notes), the weather must be forecast (at the time of arrival) to be at least 400' and 1sm greater than the published CAT1 minimums for the approach to be flown.

Method 2:

For airports that have at least two different suitable approaches (using different navaids) to two different suitable runways (based on winds/weather/notams/notes), the weather must be forecast (at the time of arrival) to be at least 200' and 1/2sm greater than the published straight in CAT1 minimums for the approaches to be flown.

Confused yet? Probably. But don't worry too much about it. Just remember that if the weather is forecast lower than 3sm and 2000' within one hour of the ETA at the primary destination, then an alternate airport is required.