Laundry Drying Forecast

Which days dry laundry outside: rain, humidity, wind and temperature from MET Norway.

Try an example: Berlin, light load, 8:00–19:00 · Manchester, towels, 9:00–17:00

How it works

Rates the next days for drying laundry outdoors at your place. It reads the MET Norway forecast hour by hour, finds the longest dry stretch in your drying hours and estimates how fast washing dries from temperature, humidity and wind (Penman evaporation), with a verdict per day.

Fetches the MET Norway Locationforecast 2.0 for the place rounded to 0.01° (about 1 km) and caches it until the time in MET’s Expires header; stale copies are revalidated with If-Modified-Since, as MET’s terms of service ask.
Uses hourly steps where MET provides them (the first two to three days) and splits later 6-hour steps into hours with linearly interpolated temperature, humidity and wind.
An hour is wet with at least 0.1 mm of rain in a 1-hour step, at least 0.2 mm in a 6-hour step, or a stated rain chance of 40 % or more. The longest run of dry hours inside your drying hours is the day’s drying window.
Drying speed is the Penman (1948) open-water evaporation from wind and vapour-pressure deficit: E = 2.6 (1 + 0.536 u2) (es − ea) mm/day, with es from FAO-56 eq. 11, ea = RH · es and the 10 m wind reduced to 2 m (FAO-56 eq. 47). Sunshine is not added, so the estimate is for drying in the shade.
A light load counts as dry after 1.2 mm of evaporation potential, a heavy load after 2.4 mm (rules of thumb: a warm, breezy afternoon dries shirts in about three hours). Verdict: great ≥ 1.5 × the threshold, good ≥ 1 ×, partly ≥ 0.6 × (finish indoors), else dry indoors.

Forecasts can change quickly, especially showers. The drying times are estimates for shade and average fabrics; sunshine dries faster, thick seams slower.

Sources

MET Norway Locationforecast 2.0 (api.met.no) (Norwegian Meteorological Institute (MET Norway))
FAO Irrigation and Drainage Paper 56: Crop evapotranspiration (Allen et al., 1998), eq. 11, 19, 47
Shuttleworth, W.J. (1993): Evaporation. In: Maidment (ed.), Handbook of Hydrology, ch. 4 (Penman wind function) (McGraw-Hill)

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How to use the Laundry Drying Forecast

Find the days that dry washing outside at your place, read the drying window and hours-to-dry, and know when the forecast is too coarse to trust.

What it is for

Hanging washing outside saves the dryer's electricity, but only on a day that actually dries it. This tool reads the MET Norway forecast for your coordinates and rates each day of the next week: it looks for the longest stretch without rain inside the hours you can hang washing out, and estimates how fast it dries from temperature, humidity and wind.

It answers "which day this week?", not "dry by 17:00 exactly". Sunshine is left out, so the estimate is for drying in the shade. It knows nothing about your fabrics or how well your machine spins.

Worked example

A household in Sydney wants to dry towels. Inputs: place -33.87, 151.21, time zone Australia/Sydney, load Heavy (towels, jeans, jumpers), hang out from 8 to 19, 6 days ahead. The run on 7 November 2026 returned "Good drying days ahead: 4; the best is 2026-11-09":

Worked example
Date	Verdict	Dry window	Hours to dry	Rain (mm)	Wind (m/s)
2026-11-08	good drying day	15:00–19:00	3.8	2.8	15.2
2026-11-09	great drying day	08:00–19:00	5.5	0	11.7
2026-11-10	great drying day	08:00–19:00	7.6	0	5.8
2026-11-11	dry indoors	17:00–19:00	—	2	2.1

The 8th has rain in the morning, but a windy, fairly dry afternoon still dries towels in under four hours — if they go out at 15:00. The 9th and 10th are dry all day; the 9th wins because the stronger wind dries faster than the warmer, calmer 10th. On the 11th only two dry hours are left after the showers, so the washing should stay inside.

For comparison, the same week in Berlin (2 to 8 °C, humidity around 85 %) gave "dry indoors" on every day, even on days without rain: cold, damp air takes up very little water.

How to read the result

Verdict compares the drying power of the dry window with what your load needs: great means it dries one and a half times over, good means it just dries, partly means bring it in damp and finish indoors.
Dry window is the longest run of hours without rain between your start and end hour. Hang out at its start.
Hours to dry counts from the start of the window. A dash means the window is too short or too damp.
Rain is the total for the drying hours of that day; a rain chance of 40 % or more also counts as a wet hour.
The drying score bars give a quick ranking (100 = dries 1.5 times over).

Tips and common mistakes

Enter coordinates, not a town name; any map shows them when you right-click a place.
Use the correct time zone, otherwise the drying hours shift.
From about the third day the forecast comes in 6-hour steps: the window edges become rough, and short showers can hide inside a step.
A frost warning means wet washing can freeze on the line; it dries, but slowly.
Choose Heavy for towels and jeans — they need about twice the drying power of shirts.

FAQ

Why does it say "dry indoors" on a day without rain?

Without wind and with humid air close to 90 %, the air simply cannot take up enough water in a day. The estimate follows the vapour-pressure deficit, which is small on cold, damp days.

Where does the weather data come from?

From MET Norway's Locationforecast (Norwegian Meteorological Institute), licensed under CC BY 4.0. The place is rounded to about one kilometre and the forecast is cached until MET says a new one is due.

Does sunshine count?

No. It estimates drying in the shade, so it is on the safe side for a sunny garden.

Can I use it in Hearth?

Yes. In Hearth's Good days section the same tool runs for your household's place, and good drying days appear in the agenda and the calendar export.

Guide last updated: November 7, 2026

For scripts and AI agents

JSON API: POST https://hearthdryingforecast.usefulweb.app/api/v1/calculate (same calculation as this page)
Interface description: OpenAPI 3.1 · AI guide: llms.txt
MCP server (Streamable HTTP): https://hearthdryingforecast.usefulweb.app/mcp · tool name hearth_drying_forecast

Example request

curl -s -X POST https://hearthdryingforecast.usefulweb.app/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"place":"52.52, 13.40","timeZone":"Europe/Berlin","load":"light","fromHour":8,"toHour":19,"days":7}'

Feedback · ideas & bug reports welcome