Final answer:
A departure alternate is designated in a flight plan to be used if the departure airport becomes unavailable. The need for a departure alternate is determined by several factors, including weather conditions. CAT II minimums refer to the ability of an aircraft to land using specific ILS procedures with lower visibility and cloud ceilings.
Step-by-step explanation:
In aviation, a departure alternate is an alternate airport that is designated in a flight plan to be used if the departure airport becomes unavailable. The need for a departure alternate is determined by factors such as weather conditions, airport infrastructure, and airline operating policies.
It is generally required when the weather is below certain minimums, specified by the governing regulations for each airline or jurisdiction. When an aircraft is operating under CAT II minimums, it means that it is able to land using Category II Instrument Landing System (ILS) procedures, which require lower visibility and lower cloud ceilings compared to CAT I procedures.