Indoor line-drying time tracks the gap between room temperature and dew point more closely than relative humidity alone

Hypothesis

Relative humidity says how close the air is to saturation at its current temperature. The National Weather Service page on dew point versus humidity gives the dew point as the temperature the air must be cooled to for 100 % relative humidity, states that the higher the dew point rises the greater the amount of moisture in the air, and notes that relative humidity can be misleading, with an example in which 100 % at a low temperature carries less moisture than 50 % at a high one. Laundry dries by evaporation into the room's air, which depends both on how warm the air is and on how much vapour it already holds. The hypothesis is that, for one household drying similar loads on the same indoor rack, the time until the load's mass stops falling is ordered better across days by the gap between room temperature and dew point (the dew-point depression, read or computed from the same instrument) than by relative humidity alone, and that days with equal humidity but different temperatures fall in the order the gap predicts.

Prediction

Over twenty or more loads logged on different days, ranking the loads by drying time will agree more closely with ranking by dew-point depression than with ranking by relative humidity, and the spread in drying time among loads with similar depression will be smaller than among loads with similar humidity. Where a fan runs or a window is open for part of the time, the prediction weakens and those loads are expected to sit off both orderings.

Proposed test

1. Fix the rack, its room, the instrument's position at the rack, the load type (for example the same set of towels) and the spin setting; weigh the load immediately after spinning.
2. Weigh at fixed intervals (for example every 30 min) until two consecutive masses are equal at the scale's resolution; the drying time is the elapsed time to the first of the two.
3. Beside each weighing log temperature, relative humidity and dew point at the rack, and log ventilation and heating events with times.
4. After twenty loads, compute a rank correlation between drying time and the session mean of each variable, and report both with the number of loads and the sessions excluded.
5. Publish the raw weighing series so that another household can recompute with a different "stable mass" rule.

Status

No result is claimed. The hypothesis may fail where air movement rather than vapour content dominates drying, and a consumer hygrometer's humidity error passes into the computed dew point. Two households with different rooms would help distinguish a property of one room from the proposed effect.

Alternative predictor: vapour pressure deficit

Evaporation into still air is driven by the vapour pressure deficit, the gap between the saturation vapour pressure at the rack's temperature and the actual vapour pressure of the air, not by the dew-point depression itself. Because saturation vapour pressure rises roughly exponentially with temperature, the same depression means a larger deficit on a warm day than on a cold one. The test therefore computes a third variable from the same rows (e_s(T) by the Magnus formula, e = RH · e_s(T), deficit = e_s(T) − e) and ranks loads by it as well. If the deficit orders drying time better than the depression, the hypothesis as stated is false while its mechanism is supported; if the depression orders as well or better, the simpler variable stands.