  Space-based ultra-high-energy cosmic ray detectors observe fluorescence light
from extensive air showers produced by these particles in the troposphere.
Clouds can scatter and absorb this light and produce systematic errors in
energy determination and spectrum normalization. We study the possibility of
using IR remote sensing data from MODIS and GOES satellites to delimit clear
areas of the atmosphere. The efficiency for detecting ultra-high-energy cosmic
rays whose showers do not intersect clouds is determined for real, night-time
cloud scenes. We use the MODIS SST cloud mask product to define clear pixels
for cloud scenes along the equator and use the OWL Monte Carlo to generate
showers in the cloud scenes. We find the efficiency for cloud-free showers with
closest approach of three pixels to a cloudy pixel is 6.5%, exclusive of other
factors. We conclude that defining a totally cloud-free aperture reduces the
sensitivity of space-based fluorescence detectors to unacceptably small levels.
