Hygrometer
You watched the forecast all week. Three clear days, no rain, warm enough. You cut on the Monday.
By Wednesday afternoon the swath still feels heavier than it should, and you are watching the light go with a decision to make about whether to bale tonight or leave it another day.
The forecast gave you sunshine. It did not give you the number that decides whether each night undoes the day.
The daily cycle nobody forecasts
A cut swath lying in a field is a near-perfect dew collector — a large, thermally isolated surface with a clear view of the sky. It behaves exactly like a lawn, and it radiates heat overnight until it falls below the dew point.
So every clear night puts water back in. Each day starts by drying off what the night deposited before making any real progress on what is still in the stems.
On a night with a wide dew point spread, that penalty is small. On a night where the temperature falls to within a couple of degrees of the dew point, the swath is soaked by dawn and the first three hours of the following day are spent recovering.
Three "clear days" can therefore mean very different things:
| Wide spread nights | Narrow spread nights | |
|---|---|---|
| Overnight dew point | Well below the low | Within 2–4°F of the low |
| Dew on the swath | Light or none | Heavy, every night |
| Morning spent | Drying the crop | Drying the dew |
| Day three | Fit to bale | Still marginal |
The moisture targets
The reason this matters is that the tolerance is narrow and the failure mode is severe.
| Package | Target moisture |
|---|---|
| Small square bales | Up to about 20% |
| Large round bales | Up to about 18% |
| Large square bales | 12–16% |
Above those figures, microbes in the crop keep respiring after baling and generate heat. In a dense bale that heat cannot escape, so the temperature rises, which speeds the respiration further. The mild outcome is mould and lost feed value. The serious one is a barn fire, and it is not rare enough to dismiss.
Reading the week properly
For each night between cutting and baling, look at the gap between the forecast overnight low and the dew point:
- Over 15°F — negligible dew. The crop keeps what it dried.
- 8–15°F — light dew, off by mid-morning.
- Under 5°F — heavy dew. Expect to lose most of the morning.
The hourly forecast shows both lines, so a week of "clear" weather can be read for what it actually offers.
Timing the bale
Mid to late afternoon, generally — after the day's drying has done its work, and before the evening dew starts. Baling too early leaves moisture in the stems; leaving it until after dark means baling crop that has already started taking water back on.
The exception is leafy legumes, where a slight overnight dampening reduces leaf shatter and is sometimes courted deliberately. That is a narrow, knowing choice rather than the general case.
The measurement that settles it
None of this replaces a moisture meter in the bale. The dew point tells you what the week will allow; the meter tells you what you actually have. Use the forecast to plan the days and the meter to make the call.