Why patrol-aircraft density mattered in the Atlantic

How did Coastal Command aircraft make the Atlantic harder for submarines to operate in?

Regular patrols made surfacing, recharging, and traveling on the surface risky, but they worked only when enough aircraft could maintain continuous coverage.

Patrol aircraft created a corridor in which submarines risked detection whenever they surfaced to recharge batteries, ventilate, or travel quickly. Airborne radar and intelligence made that threat more effective, but sustained coverage remained the decisive constraint. A submarine could surface once a lone aircraft had passed unless another patrol followed close behind. The method required enough aircraft to keep cycling through the search area.