These experiment will be unique in their quality and quantity of data and thus in their potential for physics discovery. The HiRes experiment is a new and significantly upgraded version of the original Fly's Eye experiment.
Cosmic rays make extensive air showers in the atmosphere.
Thus the air fluorescence signal provides a calorimetric measurement directly proportional to the initial cosmic ray energy. HiRes uses the resulting air fluorescence to measure the properties of the initial cosmic rays. Showers are observed by 2 fluorescence eyes, i.e. in stereo , to obtain a precision measurement. The fluorescence detectors require cloudless moonless nights with little aerosol light scattering. The experiment, with construction almost completed at this time, is already taking data.