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Growing Degree Days (hereafter GDD) is a metric to estimate crop maturation empirically based on observed temperatures.  Several GDD formulas and methods exist for various crops, such as corn.  LPWB uses the formula for corn via the "50/86 minus 50" method.

GDD = [(MinTemp + MaxTemp) /2 ] - 50

with any temperature below 50 set to 50 and any temperature above 86 set to 86.  Thus, after any adjustments to temperatures, the GDD count  for a day is the departure above 50 of the simple day average temperature.  Further, GDD are never negative, and GDD are not accrued during the months of January, February, and December.

Agricultural scientists have determined that corn growth is nearly linearly proportional to the ambient temperature.  However, growth rate becomes negligible at temperatures below 50, and temperatures above 86 enhance growth negligibly. 

King's Agriseeds, Inc. of Ronks, PA provides detailed agronomical data in text and graphics for Pennsylvania, but uses a slightly different method for GDD computations.

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Integrated Snow Depth (ISD) is a metric, formulated by LPWB, that quantitizes the extent of snow cover depth during a month, months, or a snow season running accumulation or entirety.  

ISD (Month) = Sigma [(1 to LastDay) {DayDepth + 1}] + MaxDepth + 1

where:

Sigma is the upper-case Greek letter which mathematically means the summation of a set from lower to upper bounds,

 DayDepth is a non-zero depth at obs time in inches, and

MaxDepth, any time, is non-zero.

Thus, each Trace contributes one (1) unit to the grand total. 

Examples:

For any month with a brief dusting (trace) on one day, ISD = 1

For a January with 1 inch depth at obs all 31 days, max 2, ISD = 65