I haven't used a Chrony, does it have a diffuser above the eye? As DB and Roy said, bright clear sunlight makes it harder for the chronograph to "see" the bullet, and the Oehler skyscreen III's have a big translucent orange diffuser above each eye to stop the glare.
Unless the processor in the chrono is screwing up, I'd suspect extreme spread on the load. I don't know if the shock wave could rock the screens enough to cause that much deviation without just failing to read. Best I can recall, shock waves, when they read, show around 1300 fps. I set my 1st screen about 7-8 yds. Makes a smaller window to shoot through, but stops most errors. I've never thought about a clear or cloudy day making a difference in the readings. I just figured it would read if it had enough light and wouldn't if it didn't. Another addition to the huge list of stuff that I don't know.
I'd like to know what happens when you make the comparisons.
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time