Dew point is the temperature at which air must be cooled for water vapour to condense. When the overnight low approaches it, cut hay takes moisture back, and the drying you paid for in the afternoon is partly undone.
Dew point: the number that rewets cut hay overnight
window HAY WINDOWchecked 2026-09-16responsible Windrow Field Manual (a named masthead, not a person)
[['Low stays 5 °C or more above dew point', 'Little or no condensation', 'Leave the row out'], ['Low comes within 2–3 °C of dew point', 'Heavy dew, top inch rewets', 'Consider baling in the evening instead'], ['Low reaches dew point', 'Fog or dew forms; the row is wet by morning', 'Bale before the dew, or accept a late start']]
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Dew point and relative humidity are not the same number
Relative humidity tells you how close the air is to saturation at its current temperature. Dew point tells you the temperature at which it gets there. For the overnight question the dew point is the useful one, because it does not move much as the air cools: compare it with the forecast low and you have the answer directly.
| Air temperature | Relative humidity | Dew point | What the row does overnight |
|---|---|---|---|
| 24 °C | 50% | 13 °C | Nothing, if the low stays above 15 °C |
| 18 °C | 80% | 15 °C | Dew forms as soon as the air reaches 15 °C |
| 12 °C | 95% | 11 °C | Wet by midnight |
Why the top of the row rewets first
Condensation lands on whatever surface is coolest and most exposed, which is the top of the row. The rewetting is a surface effect, so it costs you the top inch or two rather than the whole cross-section. That is why a row can be dry enough to bale in the late evening and too wet at seven the next morning.
A row that was tedded into a fluffy shape has more surface, so it both dries faster in the day and rewets faster at night. The same property cuts both ways, and the overnight dew point is what decides which way it lands.
Boundary: dew point is not humidity, not wet-bulb temperature, and not the forecast low
Wet-bulb temperature is the temperature a wet thermometer reaches in moving air, and it sits between the dry-bulb temperature and the dew point. The forecast low is the minimum air temperature, which may or may not reach the dew point. Three different numbers, three different uses, and the overnight decision needs the dew point.
The misreading that costs a day: reading the afternoon relative humidity as if it were the overnight dew point. A 45% afternoon humidity sounds dry, and it is, but it says nothing about where the temperature will fall by four in the morning.
A claim to disagree with
The manual holds that in a dew-heavy climate the evening baling window is worth more than the morning one, and that most cutting schedules are built the wrong way round. Cutting so the crop is ready at six in the evening, when the dew point spread is widest, usually beats cutting for a nine o'clock start. The argument is arithmetic, and it is contestable, which is the point of putting it here.
Where the numbers come from
- S1Properly curing hay is crucial for forage quality, North Dakota State University Extensionsupports the curing requirement that overnight rewetting works against
- S2Establishment and Cultivation of Specialized Crops for Hay (Chapter IV), FAO Agricultural Services Bulletin 154supports the field conditions under which a cut crop dries
- S3Preventing and Extinguishing Hay Fires, Field Crop News, Ontariosupports the rewetting and heating sequence a damp crop follows
One link, one document. No home pages, no search results, no bare domains.
Put this page down Put this page down when you have compared tonight's forecast low with the dew point. That comparison is the whole decision.