Chill hours
by ZIP code
How many hours below 45°F your area has banked since October 1, and how that compares with each winter since 2022–23. Built on NOAA GHCN-Daily observations for 33,000+ ZIP codes; the current season updates daily.
chill hours
| Season | Chill hours | Estimated days | Missing days |
|---|
What chill hours are
Most deciduous fruit trees need a period of winter cold before they break dormancy normally. A tree that gets too little chill leafs out late and unevenly and sets a poor crop, so nurseries rate varieties by the chill they need. Choose varieties whose requirement sits at or below the totals your area reliably gets, which is why several past winters are more useful than one.
This calculator uses the simplest and most common model: every hour below 45°F from October 1 counts as one chill hour, as described by Mississippi State University Extension. Past seasons run October 1 through March 31.
Typical chill requirements by fruit
Ranges span early- to late-chilling varieties; the variety tag or nursery listing gives the number for a specific tree.
| Fruit | Chill hours needed |
|---|---|
| Apple | 200–1,000 |
| Blackberry | 200–600 |
| Blueberry | 150–700 |
| Citrus | 0–100 |
| Fig | 100–200 |
| Grape | 100–600 |
| Peach | 200–800 |
| Pear | 400–900 |
| Pecan | 300–500 |
| Persimmon | 200–400 |
| Plum | 400–700 |
| Pomegranate | 100–200 |
| Strawberry | 200–400 |
Source: Mississippi State University Extension, Chilling Hour Requirements of Fruit Crops.
How the hours are estimated
Weather stations in this network report a daily high and low, not hourly readings. Each day's 24 hourly temperatures are reconstructed from that high and low with the curve published by Linvill (1990, HortScience 25:14–16): temperature rises along a sine curve from sunrise, then falls overnight toward the next morning's low. Sunrise and sunset come from the ZIP code's latitude and the date, as in the chillR hourly temperature method. Hours below 45°F are then counted.
Each ZIP code uses the nearest of its eight closest NOAA stations with usable data for that year, so the October–December and January–March halves of a season can come from different stations; the result lists both. When a station misses up to 5 days in a row, the missing highs and lows are filled by drawing a straight line between the days on either side, and the result says how many days were estimated. Longer outages are not filled, so the total is shown as “at least” that many hours.
Limits worth knowing: the below-45°F model also counts hours below freezing, which the 32–45°F and Utah models treat as giving little or no chill, so in cold-winter areas this total runs higher than those models. It does not calculate Utah chill units or Dynamic model chill portions; if a variety's requirement is published in one of those units, the numbers are not interchangeable. Station air temperature also differs from a specific orchard's microclimate, especially in valleys and on slopes.
Heat after chill
Once chill is satisfied, bloom timing depends on spring heat. For fruit-pest and bloom models that use growing degree days, the growing degree days calculator sums GDD for the same ZIP code with your own base temperature and dates.