Cup anemometers are within the most used wind speed sensors for the wind energy industry. Wind tunnel calibrations under controlled conditions are required, but during operation uncontrolled environmental conditions occur. This is accounted for in the IEC 61400-50-1:2022 international wind measurements standard, which specifies sensor classification based on their response to external conditions, due to the influence parameters turbulence, air temperature, density, and upflow angle. Temperature and density effects are not covered appropriately in the IEC 61400-50-1:2022, since it assumes that air temperature only influences the bearing friction of a cup anemometer. No guidance is provided on evaluating variations in density, which depends on temperature and pressure. To investigate this, two cup anemometers are measured in Deutsche WindGuard’s Climatic Wind Tunnel, where density is changed by varying pressure and temperature independently. The results show that the sensor’s response to temperature can have other effects than an increase in ball bearing friction. Using pressure or temperature to modify density can even cause opposing results. Hence, varying temperatures and pressures independently is crucial to characterize a sensor’s response. The results correspond to cup anemometers, but the methodology is applicable on all sensors.