Simply put, monarchs require a fixed amount of heat in order to complete development from egg to egg-laying adult. There are two developmental threshold temperatures for monarchs - 52.7F and 91.4F. Between these temps monarch development occurs and outside this temp range no development occurs.
The units of "developmental heat" may be measured in "degree days" - if the temperature remains one degree above the minimum threshold temperature for a 24 hour period this is defined as one degree day. Similarly, if the temperature is two degrees above the minimum for 12 hours, one degree day is accumulated. As you can see, when temperatures are several degrees above the minimum the number of degree days can accumulate quickly - a single warm summer day may contribute 30 or more degree days.
The total number of degree days (DD) required for monarchs to develop from the time an egg is laid to the time at which an adult female lays her first egg is 720 Fahrenheit degree days.
The running total allows us to calculate the potential number of generations of monarchs in any given area - typically based on airport temperature readings. We already know that observations don't seem to agree with these readings so one goal of the temperature monitoring program is to clearly show any temperature differences between a monarch habitat at the height monarchs are developing and the local recorded temperature.
This is just a brief explanation and we are working on a more comprehensive yet easily understandable text - something that will evolve as we receive your feedback in the beta testing phase of this program.