Precipitable Water Vapor

PWV products of MODIS contain two different algorithms: a) near infrared (daytime with 1 km spatial resolution); b) infrared (both day and nighttime with 5×5 km2 spatial resolution). Since astronomical site selection concerns night-time sky conditions, infrared algorithms are preferred. Note that MODIS atmospheric profile product (MOD07 L2 & MYD07 L2) consists also other atmospheric datasets such as ozone, temperature profile, atmospheric stability and moisture profile. The dataset contains 5 minutes swath images at 6.1 version (C6.1) in HDF4 format with 16–bit integer data type. The storage size is 288 files per day. Some days are missing in the dataset, which amounts to a maximum of 3 days per year. PWV dataset has three subsets for the whole atmosphere: low, high, total. In this study, we only use the last one. Values of dataset are in cm unit and spatial resolution is 5 km. As with the other MODIS products, PWV is converted to WGS84 datum first, and then a mosaic is created later. They are stored in GEOTIFF format for further analyses in GIS. Using both satellites (Aqua and Terra), a mean PWV image was created for only nighttime. Note that MODIS generates PWV values only for unclouded pixels.