This chapter presents the basic elements of the Commission for Instruments and Methods of Observation guide for the measurement of ambient temperature, relative humidity, wind speed and direction, and pressure. It discusses the advantages and disadvantages of typical practices using the ancillary instruments. All-weather sky imagers have been developed to provide regular measurements of sky conditions needed for improved solar resource modeling, including short-term forecasting, and ancillary information for characterizing solar and infrared measurements. Temperature is related to the average kinetic energy of molecules of a substance. Individual air molecules move at a variety of speeds and directions. Advanced technologies such as sonic anemometers are new research tools and are being incorporated in some monitoring systems for routine measurements. The propeller responds mainly to the airflow parallel to the axis of rotation. Sonic waves travel at different speeds depending on the direction of the path with respect to the flow of the wind.