Heat Stress Degree Days & Damage Above the Threshold
Tracks wheat
Each day adds the amount the mean temperature sits above the crop's critical temperature — below it, nothing accrues. Get the total heat stress degree days over the period and whether the crop is stressed or safe.
Estimate accumulated heat stress
Next: relieve the crop — irrigate to cool the canopy, mulch, or use shade/anti-transpirant; 21 HSDD over 7 days can cut grain set and yield.
HSDD is a simple proxy: actual injury depends on growth stage (flowering is most sensitive), humidity, soil moisture and night-time recovery temperatures.
Heat stress degree days — key facts
- Daily HSDD
- max(0, mean − critical)
- Total HSDD
- sum of daily values
- Below threshold
- contributes zero
- Critical temp
- crop- and stage-specific
- More degree days
- more yield risk
- Weights by
- how far above the limit
- Use input
- daily mean temperature
- Privacy
- Runs in your browser; nothing uploaded
Not every hot day is equal — degree days say how bad
Heat damages a crop only once temperatures climb past a critical threshold, and how much it damages depends on both how far above the line and how long. Counting hot days misses that — a day 1° over is mild, a day 8° over can sterilise pollen. Heat stress degree days fix this by adding, each day, only the amount the mean sat above the threshold. Pile those up over a hot spell or a season and you get a single number that scales with the cumulative load on the crop.
This tool gives the total heat stress degree days, the daily figure and a stress-versus-safe verdict from your daily means and the crop's critical temperature. Use it to compare seasons, flag a damaging heatwave during flowering, or rank field risk. Pair it with the Vernalization, Yield Components and Monsoon Onset Sowing tools to read the season's climate.
Weight every hot day
Count how far above the limit, not just that it was hot.
Track cumulative load
Add up stress across a spell or a whole season.
Compare seasons and sites
Same threshold gives directly comparable totals.
Flag the danger windows
Spot stress that hits during flowering or grain fill.
Frequently Asked Questions
How are heat stress degree days calculated?+
For each day, the tool takes the daily mean temperature minus the crop's critical temperature and keeps only the positive part: daily HSDD = max(0, mean temp − critical temp). If the mean is at or below the threshold, that day contributes zero. Summing the daily values over the period gives the total heat stress degree days — a single number capturing how far above the danger line, and for how long, the crop sat.
What is the critical temperature?+
It's the threshold above which heat starts to damage the crop — pollen sterility, faster grain filling that shrinks yield, or wilting. The value is crop- and stage-specific: wheat is sensitive around grain fill, rice is sensitive at flowering, and many crops cite thresholds in the low-to-mid 30s °C. Set it to the figure for your crop and the stage you're assessing.
Why use degree days instead of just counting hot days?+
Counting hot days treats a day 1° over the threshold the same as a day 8° over — but the second is far more damaging. Degree days weight each day by how far it exceeded the limit, so a few extreme days and many mildly hot days are placed on the same scale. The accumulated total tracks cumulative stress, which lines up better with actual yield loss.
What does the tool output?+
The total heat stress degree days over the whole period, the average daily HSDD, and a simple stress-versus-safe verdict so you can see at a glance whether the crop has crossed into a damaging accumulation or stayed comfortable.
What counts as a stressed crop?+
The more degree days above threshold pile up — and the more of them fall during a sensitive stage like flowering or grain fill — the higher the yield risk. The safe-versus-stress flag gives a quick read, but the absolute total matters most: a small accumulation is generally tolerable, while a large one during a critical window signals real damage potential.
Which temperature should I enter — mean, max or min?+
This model uses the daily mean (average of the day's high and low) against the critical temperature, which is the simplest and most common form. Some heat-stress indices use the daily maximum instead, since brief peaks drive pollen damage. Stay consistent: if your threshold was defined against mean temperature, feed it mean temperature.
Does it work for any crop?+
Yes — wheat, rice, maize, tomato, grape and more all follow the same accumulation. Only the critical temperature changes between crops and growth stages; the max(0, mean − threshold) logic is universal. Use the right threshold and the total is meaningful for that crop.
How is this different from growing degree days?+
Growing degree days measure useful heat above a base temperature that drives development. Heat stress degree days measure harmful heat above an upper critical temperature. One tracks progress toward maturity; the other tracks damage. They use the same arithmetic shape but opposite intent and different thresholds.
Can I use this to compare seasons or locations?+
Yes. Run the same crop threshold against different periods or sites and the totals are directly comparable — a higher number means more cumulative heat load. It's a clean way to flag a heat-stressed season, rank field risk, or check whether a hot spell during flowering was as bad as it felt.
How accurate are the figures?+
They're a sound stress index, not a yield prediction. Real damage also depends on humidity, soil moisture, wind, night temperatures and exact growth stage. Use the total as a comparative risk signal and pair it with field observation. Garbage-in still applies — the result is only as good as the daily means and threshold you enter.