Yesterday · High
Today's high feels a little warmer than yesterday's high.
Peak against peak, which is the fairer question if you are out all day.
Right now feels A LITTLEWARMER than this time yesterday
Thermo Diem (stylized as ThermoDiem) is a widget-first weather app for iPhone, Mac and Apple Watch. One sentence on your Home Screen, desktop or menu bar tells you how today feels compared with 24 hours ago.
Coming soon: Thermo Diem for iPhone Duo
Tap a color to preview a state
Philadelphia, PA
Last updated 9:30 AM
Right now feels
than this time yesterday
Compare today to
YesterdayTomorrow
78°
+3.5°F
Partly cloudy
Same timeHigh
Numbers are not context.
Most weather apps give you numbers. Thermo Diem gives you context. It is a feels-like weather app, and that is the whole idea.
A 72° day after a week of freezing cold feels completely different from 72° after a heat wave. Comfort is relative, because your body adapts to what it lived through yesterday. So Thermo Diem works out the feels like temperature and compares it to yesterday, tomorrow and the days ahead, and answers the only question that matters at the door: how does today feel compared to yesterday?
First built in 2013. The idea has not needed changing since. What has changed is the engine underneath it and every place it can now appear.
Widget first
Most people will never open the app. That is the point. It sits on the Home Screen in every size the system offers, including the new extra large 4×6 pane, rides along in the Dynamic Island, and lives on the wrist.
Right now feels
than this time yesterday
79°now, 76° yesterday
Right now feels
than this time yesterday
79°now, 76° yesterday
Right now feels
than this time yesterday
79°
Right now feels
79°
Right now feels
a little warmer
79° now · 76° yesterday
9:32
Right now feels
than this time yesterday
78°
+3.5°F
Same timeHigh
9:32
Right now feels
than this time yesterday
In the car
Real screens, not mockups. One glance at a red light and you know whether the jacket on the back seat was a mistake.
A car screen is the hardest place to read anything, which suits an app whose whole output is three words. No charts to parse, no numbers to compare, no scrolling. The color does most of the work before you have read any of it.
Seven answers
Seven shifts, cold to hot: much cooler, cooler, a little cooler, about the same, a little warmer, warmer and much warmer. The color shifts with the verdict, so you usually have your answer before you have read the word.
Most excellent features
Less is more
01
Minimal design
The only thing you actually care about and nothing else. How it will feel outside compared to 24 hours ago.
02
Poster widgets
Bold multi-line posters, split overview cards and spectrum slider gauges, plus a circular Lock Screen widget rebuilt edge to edge for far larger type. Long press any widget to change what it compares against.
03
Trend curves
Today's feels-like trajectory drawn straight over yesterday's, so you can see where the two days part company.
04
Future Shift
A seven day dial. Scrub ahead to any upcoming hour or day and compare it against right now, or against yesterday's high.
05
CarPlay and commute
Relative temperature and a color-synchronized spectrum gauge on the CarPlay dashboard, the Lock Screen and the Dynamic Island, so you know before the commute starts. Set up a hands-free weather briefing.
06
Apple Watch app
Native and standalone. Rapid launch, corner, circular, rectangular and inline complications, and its own background updates.
07
Nationwide movers
See where the biggest 24 hour swings are happening across the major climate regions.
08
Four languages
English, Spanish, French and German, fully native. Fahrenheit or Celsius throughout. The app has no account, ads or tracking.
09
Published science
A multi regime apparent temperature engine built on NOAA, MSC, BOM and UTCI formulas, not on an opaque vendor number. See the maths.
Four ways to ask
Yesterday or tomorrow, at the same time or at the day's high. Two switches, four sentences. The Future Shift dial in the app extends the same comparison to any hour or day of the coming week.
Yesterday · Same time
Right now feels a little warmer than this time yesterday.
The default. Clock hour against the same clock hour, 24 hours back.
Opens here
Tomorrow · Same time
Tomorrow will feel a little warmer than right now.
The same comparison pointed forwards, so you can dress for the commute home.
Tomorrow · High
Tomorrow's high will feel a little warmer than today's high.
Worth checking the night before, when it is still worth doing something about.
12 PM
warmer
82° today · 78° yesterday · +5°F
12 AM6 AM12 PM6 PM11 PM
Drag to scrub the day hour by hour. In the app the same curve runs seven days ahead.
And the app itself
For when you want to change city, flip to Celsius, or read the maths behind the verdict.
Philadelphia, PA
Last updated 9:30 AM
Right now feels
than this time yesterday
Compare today to
YesterdayTomorrow
78°
+3.5°F
Partly cloudy
Same timeHigh
The app is the settings screen and the explanation. The widget is the product. If you open Thermo Diem more than once a week, something has gone wrong with the widget.
Tap through from any widget to land on the reading it was showing, with the model breakdown underneath.
Now on your Mac
The same feels-like clarity, right at your desk. Check the menu bar before you step outside, or keep the verdict on your desktop.
A glance at the menu bar
See the verdict and temperature shift while you work. Click for the full reading.
Today, yesterday, tomorrow
Switch between the same time and yesterday's high, preview tomorrow, or explore the comparison curve.
Widgets at your desk
Small, medium and large widgets for your desktop and Notification Center. Save cities or follow your current location.
Free on the Mac App Store · macOS 14 or later.
Coming soon
The day,
The day,
unfolded.
Thermo Diem for iPhone Duo
Closed, understand the shift. Open, understand the comparison.
We're building a Duo experience that puts the reference and target side by side, gives Future Shift room to breathe, and keeps your chosen day and hour as you fold or unfold.
Planned for the Duo release. In development.
-
See both sides
The reference and target together, with clear times, feels-like temperatures and one shared answer.
-
Explore without losing your place
A larger Future Shift workspace for choosing a day and hour. Your comparison stays selected as the layout changes.
-
Set it down
A planned tabletop layout puts the answer above the chart and controls, where supported.
Trailer
Ten seconds, which is about nine more than the app needs.
From the archive
Period footage · 2014
The 2014 film
Watch on YouTube
Yes, we kept the old one. Every pixel in it has since been thrown out. Different typeface, different colors, different logo, different app icon, and iOS has gained nineteen version numbers in the meantime.
What has not changed is the sentence in the middle of the screen. Twelve years, a rebuilt interface, a weather engine written from scratch, and it still says the same thing it said in 2014. That felt worth leaving up.
The engine
Not a
black box
Air temperature tells half the story. The feels like temperature, properly called apparent temperature, is the heat balance between your skin and the air around it: vapor pressure and humidity resisting evaporative cooling, wind stripping heat by convection, and direct sunlight adding it back. It is also the one number most apps hand you without explanation. Thermo Diem computes it with a multi regime biometeorological model drawing on NOAA, the Meteorological Service of Canada, the Australian Bureau of Meteorology, the Universal Thermal Climate Index and Bufkit psychrometrics.
Every formula below is published, attributable and reproducible. Move the controls and watch the model hand off between regimes in real time. This is the same code path the app runs.
46°F
54°F
76°F
84°F
Cold regime
Blend
Temperate regime
Blend
Hot regime
Apparent temperature
95°F
7.0°F above the thermometer
Cold regime · T ≤ 46°F
Wind chill
NOAA / MSC joint standard, Osczevski & Bluestein (2001)
Twc = 35.74 + 0.6215·T
− 35.75·V^0.16
+ 0.4275·T·V^0.16
− 35.75·V^0.16
+ 0.4275·T·V^0.16
Derived from facial skin and clothing heat flux rather than the old still-water model. Suppressed below 3 mph, since calm cold air produces no chill.
Temperate regime · 54–76°F
Steadman AT
Australian BOM psychrometric model, vapour pressure via Tetens
e = 6.1078·exp(17.27·Td / (Td+237.3))
AT = T + 0.33·e − 0.70·v − 4.00
AT = T + 0.33·e − 0.70·v − 4.00
A sensible and latent heat balance rather than a lookup. It captures the light humidity load and the cooling breeze that both heat index and wind chill ignore entirely.
Hot regime · T ≥ 84°F
Heat index
Rothfusz nine term regression, with NWS humidity corrections
HI = c₁ + c₂T + c₃R + c₄TR
+ c₅T² + c₆R² + c₇T²R
+ c₈TR² + c₉T²R²
+ c₅T² + c₆R² + c₇T²R
+ c₈TR² + c₉T²R²
Gated on dew point. Below 54°F dew point the regression produces false heat spikes in dry air, so apparent temperature stays capped at ambient where sweat evaporates freely.
Radiation
Solar flux
UTCI / BOM shortwave absorption
ΔTsolar = UV × 0.55
× (1 − 0.70·cloud)
capped at +6.0°F
× (1 − 0.70·cloud)
capped at +6.0°F
Standing in direct sun is measurably warmer than standing in shade at the same air temperature. Driven by live UV index, cloud cover and daylight state.
Continuity
Cosine blend
C¹ continuous across both handoffs
w = ½ − ½·cos(π·t)
t = (T − lo) / (hi − lo)
AT = A·(1−w) + B·w
t = (T − lo) / (hi − lo)
AT = A·(1−w) + B·w
Naive implementations cliff at their boundaries, where 49°F computes wind chill and 51°F abruptly does not. Blending across 46–54 and 76–84 keeps hourly curves and sparklines smooth.
Inputs
What feeds it
Apple WeatherKit, worldwide
temperature dewPoint
windSpeed uvIndex
cloudCover isDaylight
windSpeed uvIndex
cloudCover isDaylight
Six live fields, one deterministic pipeline. The seven word verdict is the difference between today's apparent temperature and the same clock hour yesterday.
Sources: Osczevski & Bluestein (2001) wind chill · Steadman (1984) and the Australian Bureau of Meteorology · Rothfusz (1990) NWS Technical Attachment SR 90-23 · Tetens (1930) saturation vapour pressure · UTCI (Bröde et al., 2012) · Bufkit apparent temperature notes and the NOAA NDFD definitions.
Questions
What people ask before they download it.
What does “feels like” temperature mean?
It is what the air does to a body rather than what the thermometer reads. Below about 46°F moving air strips heat from skin faster than still air, so wind chill takes over. Above about 84°F humidity stops sweat evaporating, so heat index takes over. In between, the two are close together.
Its formal name is apparent temperature, and Thermo Diem computes it from temperature, wind, humidity, dew point and sunlight rather than taking a single number from a weather service.
Why compare to yesterday instead of showing a forecast?
Because 72°F means nothing on its own. You have no memory of what 72 felt like, but you have a perfect memory of what yesterday felt like, because you were outside in it.
Comparing to the last day you actually experienced turns an abstract number into a decision about a jacket.
Is this a full weather app?
Deliberately not. There is no radar, no ten day forecast and no hourly grid of percentages. It answers one question in one sentence, and it is built to live on the Home Screen, the Lock Screen, the Dynamic Island, Apple Watch and CarPlay so you rarely open it.
Where does the weather data come from?
Apple Weather, through WeatherKit, worldwide. Thermo Diem reads six live fields from it and runs its own apparent temperature model on top, rather than displaying Apple's feels like number directly.
No account is needed and no personal data is collected. The privacy policy has the detail.
What's new
Now on Mac, plus iOS version 3.5 and everything since the 3.0 redesign.
3.5Latest iOS
- Now on Mac. A menu bar app, a one-click dashboard, and desktop and Notification Center widgets.
- Home Screen Quick Actions. Jump to tomorrow's forecast, nationwide movers or your commute Live Activity from the iPhone app icon.
- Widget and outlook refinements. Improved refresh scheduling, smoother tomorrow previews, and performance improvements.
3.4
- A bigger, bolder circular Lock Screen widget. Rebuilt edge to edge, so the type is dramatically larger and readable at a glance.
- A synchronized spectrum slider. The needle sits exactly on your feels like tier and matches the headline color, across widgets, Live Activities and the app.
- Lock Screen and CarPlay polish. Higher contrast, refined delta badges, and smoother live commute updates.
3.3
Four languages. Spanish, French and German alongside English, fully native, with editorial grammar written for each.
3.2
CarPlay commute. A live temperature comparison banner right on the dashboard.
3.1
Nationwide movers. A live tracker of the cities with the biggest 24 hour temperature swings.
3.0
The complete redesign. NOAA biometeorological modeling with wind chill, heat index and solar radiation, a 24 hour interactive comparison, Dynamic Island Live Activities, and the Apple Watch app with complications.
Download
the app
Free on the App Store for iPhone and Mac. Requires iOS 26 or later, or macOS 14 or later. Also works with Apple Watch and CarPlay. No account, no subscription, no notifications you did not ask for.
Press kit
- Fact sheetOne page PDF, the facts and the seven states
- App icon1024, 512, 180 and 120px PNG
- LogoMark as SVG and PNG, lockup light and dark
- ScreenshotsSeven iPhone screens, three Apple Watch
- Widget tilesAll seven states, plus medium and extra large
- FilmsThe trailer cuts, with poster frames
Contact
Questions, bug reports, or a review copy. Anything at all, really.
Replies usually within a day. If something is broken, tell me your city and the time of day and I can usually reproduce it.