Cloud cover lies. Real drying comes from dry, thirsty air plus wind plus sun. DryLine reads the full forecast and tells you which days will actually dry your washing outside, and when to hang it out.
Drying is evaporation, and evaporation is driven by how much more moisture the air can still absorb, not by whether the sky looks grey. The core number is the vapour pressure deficit (VPD): the gap between how much water the air could hold at the current temperature and how much it already holds.
Per hour it computes saturation vapour pressure es(T) = 0.6108·e^(17.27T/(T+237.3)) kPa. The air actually holds ea = es(T)·RH/100. Sun heats the wet fabric above air temperature, up to about 8°C in strong sun (600 W/m²), so the fabric surface sits at es(T + 8·sun) and the drying gap is es(T + 8·sun) − ea. A warm day at 40% humidity has roughly 10× the gap of a cold 93% morning, so it dries about 10× faster even under the same cloud, and a sunny day still dries somewhat even when the air is humid.
That gap is multiplied by a wind factor (1 + 0.3·u) with u in m/s (moving air sweeps away the saturated layer next to the fabric), then halved to keep the rate scale. The result is an hourly drying rate. The forecast's rain, sun and rain-probability values, which describe the hour before each timestamp, are shifted so every hour is judged on its own weather.
Each day it finds the longest rain-free daylight window, adds up the drying rate across it, and compares the total to a load budget (light 6, normal 10, heavy 16 rate-hours). If the total clears the budget the load dries; the verdict scales with how much it clears it by. The recommended hang-out time is when drying first becomes productive, and the dry-by time is when the running total reaches the budget.
Hours where rain is likely (per your caution setting) are blocked entirely, because rained-on washing is the whole problem you are trying to avoid. On the Cautious setting the hour before likely rain is blocked too, so there is time to bring everything in.