Finding Clear Skies in the Data from the Telescope Array Observatory

The Telescope Array is located in the west desert of Utah and is the largest cosmic ray observatory in the Northern Hemisphere. The observatory employs two techniques to observe extensive air showers induced by ultra-high energy cosmic rays. The first technique is to sprinkle the desert's floor with scintillator detectors that sample the air shower's charge density when it reaches the Earth's surface. The second method utilizes fluorescence telescopes to observe the longitudinal development of the air shower via the nitrogen fluorescence light generated when the shower passes through the atmosphere. These telescopes have large mirrors which collect the shower light and focus it onto a camera made of photomultiplier tubes (PMTs). Thus, they are quite sensitive. They observe the skies over the array of scintillator detectors on clear, moonless nights. They are capable to see these showers tens of kilometers from the telescope, hence the density of clouds in the sky will affect the measurement of the shower development. Four Cloud Monitors were installed at the Middle Drum telescope station. These monitors measure the sky temperature in a 30-degree field of view using infrared sensors. In addition, telescope operators go outside and visually check the skies for clouds about once an hour. They report weather codes that quantify the direction and density of clouds in the sky. Here we analyze the correlation between the operator weather codes and the sky temperatures.