Here is all the info gathered in one place.
I used the Wunderground site to get my data for the high temperatures for each day. https://www.wunderground.com/history/
Finished size is approx 42" x 70". The edging is Knit Picks Brava Sport Marina.
I used a G hook and Knit Picks Brava Sport yarn in colors solstice heather, marina, cornflower, grass, orange, canary, paprika and red.
The foundation chain is 226 for a total of 225 single crochet stitches. You're working (sc, ch 1, sk 1) so that requires 2 stitches, plus you need a final sc at the end. That means you need to end up with an odd number to work with so you need an even number for your foundation chain. You lose one chain for turning. That chain does not count as a stitch - you go into the very first stitch of the row when working single crochet.
The one thing I regret is that I used the same hook for Row 1 and it is wider, wrinklier, than the rest. For my tension I should have used an F for that row. (Most people need to go up a hook. I'm a weird duck.)
The afghan was crocheted using the seed stitch, also called linen, moss, woven and granite stitch. The pattern I used is a repeat of rows 3 and 4.
In all the months that begin with Row 3, the odd numbered days begin with sc, sc at the beginning and end of the rows, the even numbered days begin with (sc, ch 1, sk 1).
In the months that begin with Row 4 it is reversed. Odd numbered days begin with the (sc, ch 1, sk1) and the even numbered days begin with the sc, sc.
If you are working your afghan with this stitch in a year where February has 28 days, the months of Jan, Apr, May, Aug, Nov, and Dec start with Row 3. The months of Feb, Mar, Jun, July, Sept and Oct start with Row 4.
I put a stitch marker at the end of each month for counting purposes.
The number of rows in each color for my afghan for 2013 in my zip code are:
40s solstice heather (7)
50s cornflower (34)
60s marina (54)
70-74 grass (42)
75-79 canary (93)
80-85 orange (49)
86-89 paprika (54)
90s red (74)
There are 177 days with temperatures between 86-97 in that year. That's almost half the year!
Each month seemed to take just over 6".
I worked over as many tails as I could, but that still left a good number of them to do when I was finished crocheting. I also sewed the tails in for the first few months as I finished each month. I intended to do that throughout but got lazy and left them to the end. The former method is much better because then when you get to the end you're just about done.
Here are pics of each month to show the progress.