The variable physical properties of the atmosphere that affect helicopter performance are:

• Temperature: As ambient temperature increases, the air density reduces. This causes the thrust and therefore the output power or performance to decrease at all phases of flight.
• Pressure: Atmospheric pressure also influences air density. Lower pressures, which occur at higher altitudes, result in lower air density, negatively impacting lift and engine power.
• Humidity: Water vapour is important because it is invisible, and affects the humidity ratio of the air. Adding water vapour reduces air density because its molecular weight is lower. So, as humidity increases, the air density reduces. This causes the thrust and therefore the power output or performance to decrease at all phases of flight.

The above factors are crucial, as they directly impact the lift generated by the rotor blades and the overall performance of the engine, determining the efficiency, capability, and operational safety of the helicopter.