A plan to start the night's observing session shortly after dusk (and after a rain shower had passed through) were thwarted as CloudSensor Clarity reading were only 15-20, which is normally taken to represent Cloud with potential for rain,  and opening of the Observatory Dome was prevented by the Observatory Control/Protection System.  This was despite the fact that the sky was most definately clear and shown by the AllSky Camera view, which showed absence of cloud and plenty of stars.

 It was initially thought that a bird might have come to rest on the Cloud Sensor (as this was likely explanation of some previous sudden changes in Clarity value), but checking outside with a torch didn't reveal any bird or transient object on the CloudSensor Unit.

It had been very recently raining/sleeting which had frozen on contact with the ground, and this might have formed a covering on the Cloud Sensor or otherwise temporarily affected the CloudSensor unit.  Clarity values gradually increased over course of 15 mins, but only began to return to expected values a point some 25-30 minutes after rain/sleet stopped.  After that cloudSensor readings responded normally to lthe evel of cloud cover for the remainder of the night.  

Examining CloudSensor data records shows that the CloudSensor's internal temperature cooled from 14.2°C before the freezing rain event to 11.3°C when rain stopped (a 3°C decrease in temperature).  The Sensor's interval temperature remained at around 11.3 to 11.2°C until around 17:40 when temperature increased to 13.0°C over a 5 minute period before climbing to 13.8 over a further 10 minute period. The sharp temperature change at 17:40 to 17:45 corresponds with the end of anomalous clarity values, suggest that the anomaly is driven by an unsual ambient/sensor temperatures.    The extent to which this also corresponded with an atmospheric temperature effect is unknown.

Given that the issue was of only short duration and evidently rare and one that increases safety marigins by keeping the shutter closed it was decided to close the issue after documenting it for future awareness & reference.

Graphs and an examples AllSky Image for the period of anomalous cloud sensor readings are shown below.

Anomaly in Cloud Sensor Data between 17:15 and 17:40 Clarity Values from Cloud Sensor 15-20 indicate cloud, but AllSky Star Count and AllSky Image indicate clear sky The time period was proceeded a short interval of freezing rain or sleet at 16:55 to 17:05

AllSky Image at 17:32 (taken during period with Anomalous Cloud Sensor Data (17:15 to 17:40) The Cloud Sensor indicates Cloud , but the sky is definitively Clear (see the abundance of stars across the whole sky)

Graph showing that CloudSensor Internal Temperature being affected by Freezing Rain Event for period of some 40 minutes after rain had stopped.

Following an issue with shutter opening in Session S847 (2021-01-08)  (opening failed after 10-27s with ShutterStatus Error=5  and opening stutters/jumps awkwardly when chain bolt goes over guide wheels) and some advice from Pulsar Observations, a couple for adjustments were made to the Observatory's Shutter Chain today (2021-01-15).

•  Spacing on Chain Bolt has been reduced in order to provide more clearance when head of chain bolt passes over guide wheels.  One of the two spacer nuts that were present on the 6mm chain bolt, has been taken out and replaced with a couple of washers, so that there is effectively 1.1/2” nuts worth of spacing instead of 2. With the chain attached closer to the shutter, the head of the chain bolt is now ‘lifted’ over the guide wheels as the shutter opens/closes, but is not too close that the chain is too far distorted when the shutter is at its limits and the chain is drawn against either the upper roof line and lower bottom line.
  
  With just 1 nut worth of spacing the chain gets too distorted (too bent) when it is at either limit and I was previously worried about it potentially causing the chain to delink. With 3 nuts worth of spacing there is potential for the shutter to bind on the guide wheel brackets.
   
• Chain has been tightened in order to remove the previous slack.  This was accomplished by taking out one link in the chain.
  
  When I tried with one less link during dome's original installation I found things were too tight and had to put link back in.  And since then have lived with the chain being on the slack side (slack even with the shutter drive positioned to apply maximum tension on the chain). That I can now go with one less link makes me think that the chain has either slightly ‘lengthened’ with 2.1/2 years use or my realignment of the shutter drive unit in late 2018 reduced the required chain path length just enough. Certainly the slack in chain has now all gone.
  

Photo showing previous chain bolt spacing (2 nuts + 1 washer) (photo taken 2021-01-09)		Photo showing new reduced chain bolt spacing (1 nut + 3 washers) (photo taken 2021-01-15)
Photo showing previous slack  in shutter chain (photo taken 2021-01-09)		Photo showing shutter chain post-tightening (slack removed by taking out one link in chain) (photo taken 2021-01-15)
Photo showing upper pulley wheel with tightened chain and reduced chain bolt spacing (photo taken 2021-01-15)		Photo showing shutter chain after tightening (photo taken 2021-01-15)