Here's an explanation of how this works with the weather in OMSI
There are so-called ICAO codes from airfields that send weather information to pilots so that they can get an idea of the situation well before the landing. OMSI and BCS uses this technique predefined for every map and can theoretically access all worldwide ICAO's and download their information so that it is loaded in OMSI as a weather situation.
However, there are ICAO's that only send data at certain times, because the flight operation of the respective airport does not require it otherwise. Especially smaller places, which do not offer an instrument approach or do not send a beacon accordingly (only visual approaches possible), unfortunately do not bring us the right weather data within OMSI around the clock.
Often only a signal is sent when it is still bright or not exactly holiday, etc.
Landings outside this time are only possible against a previous paid PPR request (=Prior Permission Required, possibility of permission or airspace activation upon request in aviation, also known as pay-per-request ="payment per request").
OMSI (unfortunately *g) does not send this request.
Well, because it costs real fees
and you certainly don't want to pay for it every time OMSI is started.
Krefrath, for example, would have = EDKN
However, this airport has its regular flight operation only in summer Sundays and during holidays.
This ICAO sends:
In summer on Sundays and during holidays from 0800 to 1100 and from 1200 to 1700.
All other times, paid PPR must be requested.
So we prefer to take the nearby airfield EDDL (Düsseldorf). it broadcasts around the clock.
Therefore, the maps contained in OMSI should only access ICAO's, if possible, which send around the clock and are stationed as close as possible to the respective map.
In the case of fictitious maps, this information emerges from the description, or there are place names contained in the map, which happen to be represented somewhere on this planet.
Whenever there is no ICAO data in OMSI, you have the standard weather situation with 15°C, sun and free roads.