Cloud cover
In the early days of World War II, bombers were expected to strike by daylight and deliver accurately in order to avoid civilian casualties. Cloud cover and industrial haze frequently obscured targets so bomb release was made by dead reckoning from the last navigational "fix"—the bombers dropping their loads according to the ETA for the target. Some airforces soon found that daylight bombing resulted in heavy losses since fighter interception became easy and switched to night bombing. This allowed the bombers a better chance of survival, but made it much harder to even find the general area of the target, let alone drop bombs precisely.