flywildcamel, there used to be a "suitable aerodrome" definition, as this is a 4 year old thread, but at least in EU-OPS the "suitable aerodrome" definition has been removed.
I believe the original poster was asking from a regulatory perspective not company regulations as many have posted in the past. So as here below from EU-OPS 1.192:
(a) Adequate Aerodrome. An aerodrome which the operator considers to be satisfactory, taking account of the applicable
performance requirements and runway characteristics; at the expected time of use, the aerodrome will be available and
equipped with necessary ancillary services such as ATS, sufficient lighting, communications, weather reporting, navaids
and emergency services.
No weather minima is required for the 60min distance adequate aerodrome, only the above.
Then for the enroute alternates also from EU-OPS 1.192:
(c) Adequate ETOPS en-route alternate aerodrome. An adequate aerodrome, which additionally, at the expected time of
use, has an ATS facility and at least one instrument approach procedure.
(d) En-route alternate (ERA) aerodrome. An adequate aerodrome along the route, which may be required at the planning
stage.
(e) 3 % ERA. An en-route alternate aerodrome selected for the purposes of reducing contingency fuel to 3 %.
So enroute alternate is a completely different thing then the adequate aerodrome. For all of these there is of course a weather minima requirement, e.g. the "one step up".
Then for the ETOPS adequate alternate weather planning minima is a bit different:
For a Precision approach CAT I, add a 200feet to authorized DA/DH (only from CAT I, CAT II/III not to be considered) and add 800m to the authorized visibility.
For a non-precision or a circling approach, add 400feet to the authorized MDA/MDH and add 1500m to the authorized visibility.