The usual perimeter for airport surveillance should be set 50 kilometers out.
This way, when enemy planes approach at a speed of 150 kilometers per hour, the airport will have around 20 minutes to react.
(Note: At this time, many fighter planes can reach a maximum speed of about 190 kilometers per hour, but it's difficult to maintain this speed consistently; they mostly cruise and only go full throttle during combat.)
During these 20 minutes, the airport can scramble fighters and prepare for battle.
Therefore, even in peacetime, the airport periodically dispatches fighters to patrol, ready to return and fire signal flares for alert upon detecting an enemy presence.
However, if all patrolling fighters are shot down without exception, a fatal window of vulnerability occurs at the airport.
Therefore, on rainy days with poor visibility, the airport usually intensifies fighter patrols to guard against possible surprise attacks from the enemy.
The problem is...
