Citation X (C750)
Air Traffic Control Fully Certified
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Human Factors; Procedure; Weather
The weather at [the airport] was 1 SM visibility and 600 FT ceiling. Aircraft X was on a VOR A approach. I had an IFR release on Aircraft Y; as soon as Aircraft X checked in and said he had the airfield insight and I had him in sight I cleared Aircraft Y for take off and he conducted the SID as published; at this time Aircraft X stated he was executing the missed approach. Aircraft X executing the missed approach was unexpected and put him and Aircraft Y in unsafe proximity; less than 3 SM. This was unintentional and I was sure that Aircraft X was going to land because he reported the field in sight and I saw his landing lights. I will use a more conservative approach when working in IFR conditions such as 'One in One out' or not issuing a take off clearance until the inbound IFR aircraft has landed and is exiting the runway.
Tower Controller experienced a loss of separation event when clearing an IFR aircraft for take off during marginal weather when an arrival aircraft unexpectedly executed a missed approach.