Can casting in cold weather effect bullet weight?

This one got me stumped.
I cast about 300 of my favorite .45 cal 330 gr bullets this afternoon and they were averaging about 10 grains lighter than normal with the same metal I always use but the finish looks kinda granular. The only thing different that I can think of is that it was 24 degrees outside while I was casting. In warmer weather they average from 328 to 332 grains and today they were mostly 320 to 324 grains with the whole range going from 318 to 326 grains. I'm kinda ocd when it comes to procedure. So did I miss something or is it temperature related?

In cold weather you need to adjust temps of alloy some, also a quicker pace to keep the blocks up to temp. With a bottom pour pot the nozzle and stream cool more quickly as do the blocks and sprue plate on the mould. Try casting at a quicker pace and add 25*-50* to the alloy temp

I got it figured out I think. I always cast pretty fast but when the bullets started to look frosty I turned down the heat and it still took a lot of time for the lead to cool down on the sprue plate so I turned down the heat again. Also I noticed by the end of my casting the lead was having difficulty flowing from the pots spout so I drained the pot and cleaned out the spout which was obstructed. I'm thinking it might have had to do with the speed and force of the metal flowing as well as the temp outside. we've got the same weather here today so I'm gonna run another batch and compare them. This has also made me decide to get a thermometer for my pot. Thanks for the help

Possibility of some small voids forming in them inside as well, causing the loss in weight?

Depending on the size of the cast, now that I have a PID I like to run 720~750*F. I can get the sprue to freeze in the 3~5 second range by putting the mould down flat on the aluminum base of my trusty 'drip-O-matic', that base will suck the heat out of a rifle mould PDQ! Allows a faster pace using hotter melt.

Heres something to ponder, One of the differences between a casting shop and home casters. The casting shops machines have a "shot" tube that fills the moulds. These can from 6" bigger depending on the casting. When the shot tube cycles the molten metal is "shot" into the mould under pressure and very quickly and held there for a short time. This pressure makes a stronger better grain structure and more consistant casting. Where as we pour the molten metal into the mould at a much slower rate compared to the above. We also rely on the sprues volumne to fill in voids and pockets before it cools.

The thermometer will be a good addition to your tools. make a small mount so it can set in the pot and show temps as you cast, some pots temps increase as volumne get lower. Another thing to help is use clean ingots only in the casting pot as this keeps a lot of dirt and crud out helping prevent leaks and obstructions.

Once I've gotten everything up to temp I don't notice a difference but I cast better, faster and longer in the colder months. The cooler temperatures make it more enjoyable for me.

Let the frustration continue. I ran a 100 of those 330 gr bullets again using the same metal in the same weather conditions and they are still averaging 6 to 7 grains light. I even got a lyman digital thermometer to check the temp with (that was a waist of 31.00 bucks) according to it molten lead is 200 degrees F. But wait it gets better I also tried another 2 molds as controls and they were both in there respective weight ranges all though they are both lighter weight bullets 180 & 210 grs. I'm glad I've got plenty of good 330's to hold me till spring time. Its gonna be in the 50's here next weekend so I'm gonna try again.

Sounds like your alloy has changed.

It could be but I used the first batch of bullets to make the second batch.

The alloy in the first batch of bullets, remelted, could still be a different alloy than the older bullets that weighed in properly?