Helicopter's performance in all phases of flight depends on Air density and more specifically on Temperature and Altitude, which both affect the air density.

1. As ambient temperature increases, the air density decreases. This causes the thrust and, therefore, the output power or performance to decrease at all phases of flight. Thus, the less dense the air, the higher the aircraft "thinks" it is and the less efficiently the engine and rotors will perform.

2. Density Altitude is the pressure altitude corrected for non-standard temperature, the ISA deviation. In other words, it is the altitude in ISA, which gives the same air density as the prevailing non-ISA combination of temperature - pressure altitude and is used to establish helicopter's performance, as it is a figure that expresses where your engine "thinks" it is, as opposed to where it actually is. As density altitude increases, the air density reduces. This causes the thrust and therefore the output power or performance to decrease at all phases of flight.

Thus, on a hot day, it is critical to observe the outside air temperature and pressure altitude because they significantly affect an aircraft's performance.