Randy,
You are correct about the barometric pressure. It directly relates to the boiling point of water. What I have for example..... my water boiling point is 210.7 My invert is say...50 off the glucose meter or 1% then I know I can make candy at 242.7, but if I take note that the barometric pressure is 30.0 and rising I know that it going to be a nice day and I can lower my boil temp by 1-1.5 degrees for a softer candy.....the opposite goes for if the pressure is lowering..... i'll have to boil a little harder to maintain the consistancy I'm looking for. You can make candy anytime by just boiling your syup at 242 (unless your invert is completely off the chart), but I've included the rising or falling of barometric pressure to be able to maintain the same consistancy, and quality of candy that is coming out of our molds. Our family is best know for melt in your mouth candy.....(best example I could give would be like a peanut butter cup....has that little snap on the outside, but is really creamy and smooth once it's in your mouth) To get this feature it only takes a small amount if variance to either have a batch that is too hard for our liking, or the opposite and it become too soft where you can smoosh it with your finger.
The syrup I have the most difficulty with is my lower invert sugar syrup.... .4-.6% What I have found is that I boil the syrup too hard and when make the candy it wants to set way too quick or after i take it out of the molds it has a light haze and begins to whiten. This is a tell sign to me that the candyhas been boiled too long for the level of invert sugar. See the lower the invert sugar the larger the crystals and this is what your seeing in the little haze on the candy or the light spots. Syrup I make my best candy with hovers around 1.4%....at times I believe I may slightly under boil because remember as your invert goes up your boiling temp should go up, but many times I'll finish at the same as lower invert and end up with a softer candy.
The last thing I have noticed is that (can't prove by scientific facts)but when the syrup is cooling before stirring if there is a be difference of temps through out the pan (infared therometer) I'll end of with white spots everytime. I can only guess that this is because the cooler syrup is crystallized more than the warmer syrup and during stirring you can't blend the different temps thus creating the spots. I'm no scientist but I have noticed this so take it as fact or crap just my opinion
I hope you find this somewhat helpful, but if you keep track of boiling point, barometric pressure, invert sugars, and pan cooling you find a direct corrolation between them to create a quality candy out of every batch. Some spots may still appear, but that's just the maple candy gods not letting us get to cocky!
Jake Moser
Moser's Maple
758 lbs molded maple candy last year (all by hand)
over 400 lbs sold on ebay (mosersmaple 1904)
Award Winning Maple Confections
2012 World Maple Festival Grand Champion Maple Syrup