First Frost Date Calculator

Enter station data, elevation, and microclimate factors quickly. Get a date range for your area. Save results, compare scenarios, and share reports easily online.

Calculator Inputs
Use your local normal date, then refine with microclimate adjustments.
Build a date for the year you’re planning.
Use a local normal from your nearest station.
Example: Oct 20 → month 10, day 20.
Higher means a wider likely window.
Higher confidence gives a wider range.
Hard frost often arrives later than light frost.
Applied only when threshold is 28°F.
Elevation of the normal-data station.
Higher sites usually frost earlier.
Tune this factor for your region.
Warmer nights can delay first frost.
Water can buffer temperature swings.
Use negative for colder pockets, positive for warmth.
Positive means later frost over time.
Typical: 2015 for 2011–2020 normals.
Saved into exported reports.
Tip: If you don’t know variability, start with 10–14 days.
Example Data
These scenarios show how adjustments can shift the date.
Scenario Normal (MM/DD) Elevation diff (m) Urban Water Custom (days) Result (approx)
Baseline 10/20 0 None None 0 ~Oct 20
Hill garden 10/20 +300 None None -2 ~Oct 13–16
Urban near water 10/20 0 Moderate Near +1 ~Oct 27–30
Hard frost threshold 10/20 0 Light None 0 ~Oct 29
Example results assume a 28°F shift of +7 days and a typical elevation factor.
Formula Used
A practical, adjustable model for planning and comparisons.

The calculator starts from a local normal first frost date and applies day-based adjustments:

Adjusted Date = Normal Date + Round(Total Adjustment Days)
Total Adjustment = Elevation + Urban + Water + Custom + Trend + Threshold Shift
  • Elevation adjustment: -(GardenElev − StationElev) / 100 × DaysPer100m
  • Trend adjustment: TrendDaysPerDecade × (PlanningYear − NormalsCenterYear) / 10
  • Threshold shift: added only for the 28°F option.

The likely window uses a normal-approximation interval:

Half-Width = ceil(z × VariabilityDays)
Window = Adjusted Date ± Half-Width
z is chosen from the selected confidence level (two‑sided interval).
This tool provides planning estimates. Local cold air drainage, cloud cover, and wind can shift frost by days to weeks.
How to Use This Calculator
A simple workflow that improves decisions in autumn.
  1. Find a normal first frost month/day from a nearby weather station or local guide.
  2. Set the threshold: 32°F for light frost, 28°F for harder frost risk.
  3. Enter station and garden elevations, then adjust the elevation factor if needed.
  4. Choose urban and water settings to reflect your site’s nighttime warmth.
  5. Add a custom adjustment for known microclimate effects in your garden.
  6. Pick variability and confidence to get a realistic date window.
  7. Calculate, then export to CSV or PDF to share or compare scenarios.

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