Day length (photoperiod)
The Sun's declination is δ = 23.44° sin(2π(d − 80)/365). At latitude φ the day length is 24/π · arccos(−tanφ tanδ) hours, with polar day and night when the argument leaves ±1. Move the latitude slider: day length at the solstices diverges, and that is the signal many animals and trees use because it is not changed by warming.
Temperature
Annual mean and half-amplitude are interpolated from typical continental-interior values for each latitude, then T(d) = T̄ + A cos(2π(d − 205)/365) plus smooth weather noise. The warming slider simply adds an offset. This is a teaching climate, not a forecast.
Snow, ice and soil
Precipitation below 1 °C adds snow; melt follows a degree-day rule of about 1.7 cm per °C per day. Lake ice thickens when it is colder than −1 °C and thins above 0 °C. Soil temperature is a ~22-day running mean of air temperature.
Leaves and plants
Leaf-out needs both warmth (a ~10-day mean above 5 °C for temperate trees, lower for taiga and tundra shrubs) and day length above ~11.5 h. Autumn colour starts when the day shortens below ~13 h and runs faster in cold spells; leaves drop after colouring. Flowering follows accumulated growing degree-days. Food availability combines grass, browse and autumn fruit, reduced under deep snow.
Hare coat and mismatch
Snowshoe hares moult to white when day length falls below ~12 h and back to brown when it rises above ~13 h, over roughly 70 days. Because the trigger is day length, a warmer climate leaves white hares on bare ground. Foxes in this model notice a hare from 6 + 20 · m metres, where m = |coat whiteness − snow cover|, so mismatch directly raises predation. Real hares can partly adjust, and some populations are evolving earlier brown moults.
Hibernation and migration
Bears den when food is scarce and the cold sets in, and emerge when the soil thaws and food returns. In deep sleep their heart rate falls from ~50 to ~10 beats a minute and they live off fat without eating or drinking. Geese leave when the ~10-day mean temperature falls below a latitude-dependent threshold and return as that temperature rises through it, so spring arrives with the thaw front. Wood frogs freeze solid and thaw out when the soil warms above about 2 °C.
What is simplified
Animals follow rule-based steering, not real energetics. Deer and caribou coat changes follow the calendar. Frogs are drawn about 3× life size. Fawns, leverets and population change are simplified and fox pounces never injure anyone. Every number in the species cards comes from field guides, but individual animals vary a lot.