Fig. 3.23   This is a sample of how I would have derived the first day of spring based on predetermined criteria. In this case I looked for the first straight 5 days of temperatures 70°F or higher and circled the first temperature in that chain. Next I searched for the first straight 5 days of 45°F or warmer minimum temperatures and circled the first temperature in that chain. Next, I searched for the first 5 days of 5 or less heating degree days and circled the first in that chain. Next, I assigned numbers to circled items in order of occurrence and then took the sum total of those numbers and divided by 3 to arrive at 5. I counted 5 days down from the first criterion occurrence and that date would represent spring. Below the sample spreadsheet is the criteria assigned to define each season.