Since October 1999, NOAA polar-orbiting High-Resolution Infrared Radiation Sounder (HIRS) multispectral data have been archived from Kwajalein, Republic of the Marshall Islands. Cloud occurrence, height, and effective emissivity are determined through CO₂ slicing techniques for the area between 5°S and 30°N, 155°E and the International Date Line. This tropical study uses data for NOAA-12, 14 and 16. This study will further the understanding of High Altitude Tropical (HAT) Cirrus.

Data is divided into morning and evening cycles which is further divided into different layers for the heights of the calculated cloud tops. Low clouds extend from 1000 to 700 hPa, medium clouds from 700 to 400 hPa, and high clouds exist above 400 hPa. Each cycle also has a grid of data that has been calculated as clear. Each layer is sub-divided into thickness categories Thin, Thick and Opaque. Calculated emissivity values range between 0.0 (fully clear) and 1.0 (fully opaque). Thin clouds are defined as those with emissivity values less than 0.5, while Thick clouds have emissivity values between 0.5 and 0.95. Opaque clouds are defined as those with emissivity values greater than 0.95. These data are stored in 1/4 degree latitude-longitude grid squares. Total counts of clouds are stored at each grid point. Since the CO₂ algorithms detect only the top layer cloud top heights, the data set is biased toward the higher clouds.