Tower controller experienced a loss of separation event when clearing an IFR aircraft for take off d...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1064436
Time
Date	201301
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Tower
State Reference	US
Aircraft 1
Make Model Name	Citation X (C750)
Flight Phase	Final Approach
Route In Use	Other VOR A
Flight Plan	IFR
Aircraft 2
Make Model Name	Gulfstream V / G500 / G550
Flight Phase	Initial Climb
Flight Plan	IFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

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.