What environmental factor should be closely monitored during landings?

The most critical environmental factor to monitor during landings is crosswinds that may affect landing alignment. Crosswinds can significantly impact the aircraft's trajectory as it approaches the runway. Proper alignment is crucial for a safe landing, as crosswinds can cause the aircraft to drift off the centerline of the runway during the final approach.

Pilots must be aware of the wind's direction and speed to apply the necessary corrections and techniques, such as crab or sideslip, to maintain control of the aircraft. Without careful monitoring and adjustment for crosswinds, pilots risk a compromised landing that can lead to runway excursions or loss of control.

While factors like air temperature at cruising altitude, cloud cover, and surface conditions such as rain or snow are all important for flight safety and operational decisions, they do not directly impact the immediate control and handling of the aircraft during the landing phase as much as crosswinds do. Hence, crosswinds take precedence as the environmental factor that should be closely monitored during landings.