Sea Lion and the Royal Navy's maritime barrier

Could calm seas, fog, and the rapid capture of a port have given Sea Lion a chance?

The port would be defended, shipping would take time to assemble, and fog could hinder German aircraft while favouring British naval opposition.

The proposal assumes that German troops could quickly seize a port during calm, foggy weather. British experience of attacking ports is used to challenge that assumption: ports commonly contained substantial forces and prepared defences. Germany is also judged to have lacked the shipping to mount Sea Lion promptly after its Norwegian losses, while rebuilding and gathering craft would give Britain more time to rearm. British submarines stationed near German ports could report an outward movement and attack transports. Fog would restrict German air operations, weakening the assumption that poor visibility necessarily protected the invasion. Royal Navy crews trained for night and bad-weather fighting could exploit those conditions. A short spell of favourable sea conditions would leave the larger problem of moving and sustaining a landing force under naval attack.