Phenology is the study of seasonal timing — when leaves open, flowers bloom and swallows return. This lab models it with real growing degree-days (GDD): heat accumulated above a species-specific base temperature. Spy on how warming shifts oak leaf-out, bluebell bloom, lawn green-up and swallow arrival — and whether our seasons are getting longer.
Growing degree-days: GDD = Σ max(0, T̄_day − T_base), accumulated from 1 January. Each species has a measured T_base and threshold from the literature — oak leaf-out ≈ 150 GDD above 5 °C, native bluebell bloom ≈ 100 GDD above 5 °C, lawn green-up ≈ 90 GDD above 3 °C, swallow arrival is tied to prey availability (modelled with an 8 °C threshold), and orange-tip butterfly first flight ≈ 120 GDD above 4 °C. Temperature follows a sinusoid set by latitude and warming; UHI adds ~0.8 °C in spring. The warming shift you observe (~2 days per °C for oak) matches measured wood-phenology trends across Europe. Real GDD values come from thousands of station records; the model here is a simplified but realistic version for learning.