Optical communication through clear atmosphere provides a means for high data rate communication (Gbps) over relatively short distances (∼km). However, turbulence in the atmosphere leads to fades of varying depths, some of which may lead to heavy loss of data. We consider spatial diversity at both the transmitter and receiver as a means to mitigate the short-term loss of signal strength. Using direct detection receivers and binary pulse position modulation as an example, we derive the outage probability of spatial diversity systems that use equal gain combining and optimal combining. The power gain of using these spatial diversity systems is found to be substantial. Hence, we feel diversity systems should be used as the key technique for clear atmospheric optical channels.