Will the washing dry outside today? When a T-shirt, a towel, jeans and a sheet dry on the line

What dries washing: dry air, wind and sun matter more than heat#

Wet washing dries as the water in it passes into the air. Three things decide how fast: how much more water the air can take, how often the wind brings fresh air past the fabric, and how much energy the fabric gets from the sun.

Heat matters less than it seems. The humidity in a forecast is a percentage of what the air can hold at that temperature, so the same “60%” means different things in winter and in summer. A better guide is a thermometer wrapped in a wet cloth: the evaporating water cools it. The drier the air, the bigger the drop; it says more than the humidity percentage about how fast washing will dry. Cool, dry air can beat warm, humid air.

Estimates from the same model as the forecast above, for a cotton T-shirt after a 1,000 rpm spin, under an overcast sky, with a 2.7 m/s wind at 10 m (about three quarters of that at the height of a washing line) unless stated. The first figure is the middle estimate; in brackets, the range given by the model’s assumptions.

A 10 °C day with dry air dries washing at least as fast as a muggy 30 °C day. Wind can halve the drying time, and sun can cut it to a third or less. Washing dries below freezing too: the ice in the fabric turns straight into vapour without melting, only more slowly.

The sunniest day of the week is often not the best day for washing#

For each of the 44 Romanian towns and every week from 2016–2025, the sunniest day was compared with the day the weather dried the most, among days without rain between 9 a.m. and sunset.

49%of weeks, the best washing day was not the sunniest one (median of the 44 towns; between 33% and 58% depending on the town and the assumptions)

In those weeks, the winning day had nearly twice the wind of the sunniest day: 3.5 m/s against 1.9 m/s, at the median. Sun matters a lot, but a windy day with dry air can beat it. The warmest day is no better guide: it was also the best washing day in 47% of weeks.

An example from Bucharest. 2 October 2017 had the most sun of the week, but the wind was light (1.7 m/s). On 29 September 2017, with less sun and a 6.2 m/s wind, the weather dried about 30% more.

A February day dries 2.5 times as much washing as a December day#

December and February are almost equally cold: the 24-hour mean, median of the 44 towns, is 2.9 °C in December and 3.5 °C in February. For washing, they are very different months. With the washing out at 9 a.m. both times, a February day dries, at the median, 2.5 times as much as a December day, between 2.0 and 3.2 times depending on the town and on the model’s assumptions.

In Constanța, on the Black Sea coast, February is even slightly colder than December (4.4 °C against 4.6 °C, 24-hour mean), and still dries 2.1 times as much. Part of the difference is light: there, from 9 a.m. to the last hour of usable light there are 6.9 hours in December and 8.1 in February, and the sun climbs higher and warms the washing.

The same gap appears in autumn: an October day dries 2.3 times as much as a November day. For washing on the line, November, December and January are the hard months; February has more in common with March than with December.

Frozen washing dries too

Below zero, the water in the fabric freezes and the washing goes stiff. The ice still turns straight into vapour (sublimation), more slowly than water: ice gives up vapour less readily (the vapour pressure over ice is lower than over water at the same temperature), and sublimation needs more energy than evaporation. Across the 44 towns, ten years held 4,221 rain-free days that stayed below freezing from 9 a.m. to sunset. A T-shirt would have dried outside on 49% of them, by the middle estimate. The range is wide, 4% to 87%, because on short, cold days the result leans hardest on the model’s assumptions.

Ten years of washing days, day by day#

Each square is one day from 2016–2025: the four-piece load (T-shirt, towel, jeans, sheet) hung out at 9 a.m. and taken in at sunset. The colour shows how long it took for everything to dry, or why it didn’t. The calendar follows the place chosen at the top.

Where washing still dries outside in winter, in Romania#

In winter (December–February), the four-piece load hung out at 9 a.m. dries by sunset most often in Buzău, on the plain north-east of Bucharest (38% of days), and least often in Bistrița, in the hills of northern Transylvania (11%). Towns on the southern and eastern plains and on the coast lead the ranking; those in the mountain basins and the north trail it. The percentages depend heavily on the model’s assumptions, the order of the towns much less: Bistrița and Baia Mare (both in the north, below the Carpathians) are in the bottom three under every set of values, and Buzău comes first under two of them. Click the map to choose a place for the forecast at the top.

The spin speed matters most in winter#

After the spin, washing keeps some of its water: the faster the spin, the less. The EU energy label sorts washing machines into classes by the water left: class A leaves under 45% of the dry weight of the washing, class B between 45% and 54%. A laboratory test on one machine measured 58% water left at 1,150 rpm and 52% at 1,350. For 800 rpm the page assumes 75%, by extrapolation.

In summer the difference is small: jeans dry in 2.3 hours after an 800 rpm spin and in 1.8 after 1,400 (median of the towns). In winter, when the day is short, the extra water decides whether the washing dries before sunset at all: by the middle estimate, the four-piece load dries outside on 18% of winter days after 800 rpm and on 32% after 1,400. Under each of the three sets of values the faster spin gives 1.4 to 4.4 times as many winter drying days. The spin buttons at the top change the forecast.

When the washing stays indoors#

Washing that can’t dry outside dries inside, and its water goes into the room. A design guide from the Glasgow School of Art, working from a laboratory experiment, puts the water a load of washing adds to the air when dried indoors at 2–2.5 litres; citing an American study, it puts the daily output of a home with a family of two children at 5–10 litres. In the Glasgow homes the authors monitored, drying washing passively indoors went with more mould spores in the air.

That water has to leave the house. Cold outside air holds little water, even when it feels damp: once it comes in and warms up, it can take up water from the room and carry it out again.

How it is calculated, and how sure it is#

The forecast. For the chosen place, the page asks Open-Meteo for the hourly forecast: temperature, humidity, wind at 10 m, sunshine and rain. Each hour goes through a Penman-type evaporation equation, used for decades for water evaporating from a wet surface: energy from the sun plus dry air brought by the wind, divided by the heat water needs to evaporate. The washing loses water from both faces. Near the end, when the water is left only inside the fibres, drying slows down.

The washing. A 160 g cotton T-shirt, a 500 g towel, 650 g jeans and a 600 g bed sheet, in partial shade. After a 1,000 rpm spin they hold 62% water relative to their dry weight. “Dry” means the extra water has gone.

How sure it is. No published measurement could be found that ties the drying time of washing on a line to the weather at the time. The equation is tested on open water, not on fabric, so three constants remain assumptions: how much light a hanging garment absorbs, how well the wind ventilates it, and how slowly the last water leaves. The page uses three sets of values. The times at the top are the middle one, with the range given by the other two. The comparisons on the page and the order of the towns hold under all three; the exact percentages do not, so they come with their range.

Ten years of weather. The statistics come from the ERA5 reanalysis (Copernicus/ECMWF, through the Open-Meteo archive), hour by hour, 2016–2025, for 44 Romanian towns: Bucharest, every county seat, Sulina on the Danube Delta and Petroșani in the mountains. The washing goes out at 9 a.m. and comes in at the last sunny hour. An hour with at least 0.2 mm of rain counts as the washing getting wet again.

The check against weather stations. ERA5 is a grid about 25 km across. For 8 towns with an airport or weather station in the NOAA register, the same statistics were rerun with the temperature, humidity and wind measured at the station, for 2016–2024. The share of days the washing dries in December, in February, in winter and over the year differs by at most 7.0 points (median difference: 2.1). The share of weeks in which the sunniest day is not the best comes out between 37% and 58% with the station data; the biggest gap is in Sibiu, in southern Transylvania (41% with ERA5, 58% with the station).

A check indoors. In still room air (20 °C, 55% humidity, no sun), the model gives a 4.5 kg load a starting rate of about 460 grams of water an hour, and after a 1,000 rpm spin the washing holds 2.8 litres of water. A 2025 experiment in a 40 m³ room, with about 2 litres to evaporate, measured more than 100 g/h for the first two hours and at most 360 g/h (only the abstract is public), and the Glasgow guide gives 2–2.5 litres per load and about 1 litre in the first two hours of its experiment (roughly 500 g/h). The model falls between the two measurements; the exact rate outdoors remains untested.

Sources#

• Open-Meteo, prognoza orară și arhiva ERA5 (CC BY 4.0) / hourly forecast and ERA5 archive open-meteo.com
• Hersbach et al. (2020), The ERA5 global reanalysis, Q. J. R. Meteorol. Soc. doi.org
• FAO Irrigation and Drainage Paper 56: Penman–Monteith, presiunea vaporilor, vântul la 2 m / vapour pressure, wind at 2 m www.fao.org
• NOAA, Integrated Surface Database (ISD-Lite), stații din România / Romanian stations www.ncei.noaa.gov
• Regulamentul (UE) 2019/2014, Anexa II, clasele de eficiență a centrifugării / spin-drying efficiency classes eur-lex.europa.eu
• Stamminger et al., test de laborator al apei rămase după centrifugă / laboratory test of remaining moisture dsp.tecnalia.com
• Menon & Porteous (2011), Design Guide: Healthy Low Energy Home Laundering, MEARU, Glasgow School of Art radar.gsa.ac.uk
• Porteous et al. (2014), Domestic laundering: environmental audit in Glasgow, Indoor and Built Environment doi.org
• Indoor Environments (2025), emisia de umiditate a rufelor uscate în cameră / moisture emission from laundry dried indoors (rezumat / abstract) www.sciencedirect.com
• Stull (2011), Wet-bulb temperature from relative humidity and air temperature, J. Appl. Meteor. Climatol. doi.org
• Haurwitz (1945), Insolation in relation to cloudiness and cloud density, J. Meteorology doi.org