Frost dates can be downscaled by applying a standard lapse rate to elevation.
Well established · fatal if ignored → block
This is one the planner refuses to enforce. The claim is recorded here with what is actually known, rather than dropped — a rule that was checked and failed is worth more to you than one that was never checked.
What is actually known
On clear still nights, which are exactly the nights that produce radiative frost, cold dense air drains downhill and pools. The valley floor is colder than the slope above it. A standard lapse rate predicts the opposite and makes frost dates worse in the terrain where accuracy matters most. PRISM exists because of this.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
Penn State Extension, 'Frost, Critical Temperatures, and Frost Protection', extension.psu.edu/frost-critical-temperatures-and-frost-protection (read 2026-07-11): 'Radiation frosts occur when the heat that has been stored in the upper soil surfaces is radiated back into the atmosphere at night. Wind speeds are generally low, and an inversion usually develops with temperatures near the ground surface falling to, or below' freezing; 'Cold air is heavier than warm air; therefore, it will naturally flow down to lower elevations. Orchards or orchard pockets located at lower elevations will see more damage due to the collection of this cold air. This is the reason that site selection is critical to avoid low-lying land.'
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- promote to verified — no lapse-rate frost downscaling
- cite Penn State frost for cold-air-drainage refutation of lapse rate (confirmed)
Cite this page
Milpa Gardens. "Frost dates can be downscaled by applying a standard lapse rate to elevation." Evidence grade: Well established. https://milpa.garden/rules/frost-dates-can-be-downscaled-applying-standard/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.