How Muggy works

Two readings, one verdict

Muggy looks at the air in two ways and gives you one answer.

• What kind of air it is. Dry, muggy, oppressive. That is how much water the air holds, which decides how well your sweat can dry.
• How heavily it sits on you. The heat, the damp, the sun and the breeze added together. This part decides whether to slow down.

The second one leads the headline and the first describes it. They are not allowed to argue. Damp air on a mild morning is common, and the honest way to say it is "Muggy but mild". An older version of this app once said "It's muggy, slow down" right above "nothing will slow you down". That can't happen now.

Not every pairing can happen. This is every combination the engine can produce, from a sweep of more than 150,000 weather situations.

Air that wet always carries some heat, so oppressive or miserable air is never easy. Muggy air is often mild, though. Think of a morning or evening only a few degrees above its dew point.

The six air bands

Your body cools itself by sweating, and sweat only cools you if it can evaporate. The dew point measures how much water the air already carries. It is the temperature the air would have to cool to before that water starts condensing. The higher it is, the worse sweat works. Relative humidity can't tell you this on its own. Fog on a cold morning is 100% humidity, and nobody calls fog muggy.

Muggy sorts the dew point into the bands WeatherSpark made familiar, converted from Fahrenheit. You see the band, never the number.

Sweat only matters once it's warm. Below 18° Muggy says whether the air feels damp or dry instead, because a foggy morning and a crisp one land in the same band. Cold air holds so little water that its dew point is always low. So in fog the Air tile says damp, even though the band underneath is dry. In real heat, comfortable air shows as not sticky, since nothing is comfortable at 35°.

How heavily the air sits on you

The band can't tell you how hard the air is working against you. Muggy air at 24° and the same air at 34° in full sun are different afternoons. For that Muggy uses WBGT, the wet-bulb globe temperature. Sports bodies, armies and Japan's heat-stroke alerts all use it.

Think of it as three thermometers blended into one reading.

• A wet thermometer (70%) shows how well sweat can still cool you. Damp air makes it read high.
• A black globe (20%) shows how much the sun and warm surroundings heat you up. It is why full sun can move you a whole level.
• The air itself (10%).

Muggy works this out twice. In the shade means under a solid awning or a tree. In the sun means standing in direct sunlight. When the two land on different levels the headline gives both, as in "Easy in the shade, noticeable in the sun". At night they are the same and Muggy says nothing about the sun.

What each level means

The ranges and advice follow the Japanese Society of Biometeorology's daily-life guideline and the Japan Ministry of the Environment's WBGT table. Muggy splits their lowest band in two so that a mild day and a noticeable one are not lumped together.

A reading that sits right on a line does not flip the verdict back and forth. Muggy keeps the earlier level until the reading clearly crosses.

Try it

Pick the weather and see what Muggy would say. It runs the same engine and the same words as the app.

How we know it's right

A model that sounds precise and is wrong is worse than no model. So Muggy's heat load was checked against the real thing. The Ministry of the Environment measures WBGT with an actual black globe at 47 Japanese weather stations. The check used the same kind of forecast data the app uses, over 434,405 measured summer hours.

Each station was predicted with settings chosen without that station, so the model never marks its own work.

• The typical miss is 0.79 °C of WBGT, with an average lean of -0.16 °C.
• It lands on the measured level, or the one next to it, in 99.8% of hours.
• It called a Hard or Dangerous hour Noticeable or lower 0.13% of the time.

That matches Japan's own official estimator on the same inputs. Muggy adds what an estimator can't, which is shade versus sun and what the load is made of. The full results, including runs that failed along the way, are published in the repository.

"Is this normal?"

A band alone doesn't tell you whether to be surprised by it. So Muggy compares the air right now with ten years of history from the same place. It takes every hour within a week of today's date and two hours of the current time of day, about 750 of them. A morning is compared with mornings, never with whole days. You get one number, like "stickier than 78% of mornings", and the bar shows how often each band turns up at this time of day.

"When will it get better?"

"Better" means what the headline means. Less heat first, then drier air. So sunset counts as relief, not only a change of air, as in "Easy from 21:00, once the sun is down." Hours between midnight and six can extend a stretch of relief but never start one, because nobody plans around 03:00. When nothing better is coming, it says so. When the air is already dry or fresh and the heat is easy, there is nothing to wait for, so the card stays away.

The technical bits

WBGT = 0.7 · Tw + 0.2 · Tg + 0.1 · Ta

• Definition. Japan's Ministry of the Environment computes its measured WBGT with a psychrometric wet bulb Tw (Iribarne & Godson) and a 6-inch black globe Tg. Muggy's levels and alert marks are MOE's, so the index is too.
• Globe. The energy balance of Liljegren et al. (2008), ported line by line from Argonne National Laboratory's source and checked against the original to 0.02 °C. Convection follows ISO 7726 for 150 mm globes.
• Sun. The forecast's own direct and diffuse radiation, with the sun's angle averaged over the sunlit part of each forecast interval (Hogan & Hirahara 2016). It stays within 0.4 °C of minute-by-minute physics even in the hours around sunrise and sunset.
• Wind. 10 m wind brought down to 2 m with town stability classes, and one wind for shade and sun.
• Calibration. Two physical settings, surface albedo (0.2) and a minimum 2 m wind (0.13 m/s), chosen by a fixed rule in leave-one-station-out cross-validation.
• What it's made of. The breakdown into damp, sun and breeze is an exact Shapley attribution against the same air temperature in dry, shaded air with a 2 m/s breeze. The shares always add up.

Earlier versions of Muggy used a different heat index that couldn't tell shade from sun, and the page could show two verdicts that disagreed. That index and its wording are gone.

Where the data comes from

Weather, history and historical forecasts come from Open-Meteo (CC-BY 4.0). Measured WBGT for validation comes from Japan's Ministry of the Environment, Heat Illness Prevention Information. The air bands follow WeatherSpark's thresholds. The WBGT model is by James C. Liljegren, Argonne National Laboratory.

This product includes software produced by UChicago Argonne, LLC under Contract No. DE-AC02-06CH11357 with the Department of Energy.

Who made this

Muggy is by Okan Erturan, a product engineer in Tirana, a city that spends its summers muggy or worse. That's where the idea came from. The forecast kept saying 27° and the air kept saying something else.

The whole thing is open source. If a number looks wrong or your city misbehaves, open an issue.