I just found a calulator at Weather.com for "degree days".
http://www.weather.com/outdoor...
You can give it your ZIP code, and a base temperature, and a range of dates, and it will calculate your "degree days" or "heat units" for you
You can even enter future dates, and it will predict how many heat units you should expect by that date.
Does anyone know where to look up how many degree days different crops "prefer"? Or even better, how many degree days different tomato varieties require to ripen?
I think this is how degree days are calculated:
For each day, find the average temperature by averaging the daily high with the daily low. It doesn't matter how many hours were high or how many hours were low.
Subtract the "Base Temperature" from that average. If you are comparing your results to some published result for other regions or a particular crop, "50 degrees F" seems to be the usual choice.
I think that negative numbers are "rounded up" to zero, as if cold days just prevent growth and maturation, they don't put the plant into reverse.
Then the result for each day is added together into a grand total.
For example, 3 days each averging 60F would count as 30 heat units or 30 degree days. One more day averaging 70F would bring the total to 50 heat units.
Several weeks of days eeach averging 50F or less would leave the degree days unchanged at 50 heat units.
U Washington says that the Seattle area averages 2156 degree days between last and first frost, using "50F" as the base temp.
