Need for CYA on cloudy days

[Steve456]

When I removed my winter cover I found, as expected, green algae. My initial water test results were

FC = 0
pH = 7.2
TA = 70
CYA = 0

I decided last night to purchase CYA so that I could add some in the morning. I hit the pool hard with bleach last night starting at 5:00 pm. While I knew that I needed to add CYA I was pleased to be able to start the process without CYA since the Cl would be more effective without CYA.

After adding more bleach this morning my water test results are

FC = 14.5
CC = 0
pH = 7.2

However, this is a very overcast day. There are very dense clouds and no distinct shadows. If the sun were out most of the pool would be in the shade now due to nearby trees.

My question is the following: Is it the visible light that burns off Cl or ultraviolet light? Do you need CYA when the pool is in the shade or when there is heavy cloud cover? Specifically, if it remains overcast am I better off to add CYA or to shock without CYA until the sun comes out?

 

[JohnT]

It's UV light that causes your FC to drop. It probably isn't going to matter either way, since it takes several days to a week for CYA to dissolve.

 

[JasonLion]

You will lose FC to sunlight even on very cloudy days, just not as quickly. With CYA around zero it is probably best to shock a couple of times in the evening and leave things alone during the day. Maintaining shock level during the day will clear up the pool more quickly, but it will also use more chlorine than if you shock in the evening and overnight.