  EUSO-SPB1 was a balloon-borne pathfinder mission of the JEM-EUSO (Joint
Experiment Missions for the Extreme Universe Space Observatory) program. A
12-day long flight started from New Zealand on April 25th, 2017 on-board the
NASA's Super Pressure Balloon. With capability of detecting EeV energy air
showers, the data acquisition was performed using a 1 m^2 two-Fresnel-lens
UV-sensitive telescope with fast readout electronics in the air shower
detection mode over ~30 hours at ~16--30 km above South Pacific. Using a
variety of approaches, we searched for air shower events. Up to now, no air
shower events have been identified. The effective exposure, regarding the role
of the clouds in particular, was estimated based on the air shower and detector
simulations together with a numerical weather forecast model. Compared with the
case assuming the fully clear atmosphere conditions, more than ~60% of showers
are detectable regardless the presence of the clouds. The studies in the
present work will be applied in the follow-up pathfinders and in the future
full-scale missions in the JEM-EUSO program.
