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
than this time yesterday
A widget first weather app for iOS 27 and watchOS 27. No charts, no percentages, no ten day forecast. One sentence on your Home Screen telling you how today feels compared to 24 hours ago.
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
One question. Seven answers.
Every other weather app answers a question nobody asked. Dew point, barometric pressure, a wall of percentages, and you still stand at the door wondering whether to grab a jacket.
ThermoDiem takes temperature, wind and humidity, works out the apparent temperature, and compares it to the same moment yesterday. You already know how yesterday felt. That is the only reference point you need.
First built in 2013, and 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 in iOS 27, rides along in the Dynamic Island, and lives on the wrist in watchOS 27.
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
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
Those are the seven, cold to hot. 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
Every widget size
Small, medium, large, Lock Screen, and the new extra large 4×6 pane in iOS 27. Pick the one that fits your Home Screen.
03
Live Activity
Pin the answer to the Lock Screen and the Dynamic Island, then forget about it until it changes.
04
CarPlay
The verdict on the dashboard, as a widget or full screen. Built for a glance, not for reading.
05
watchOS 27
A full screen app, a Smart Stack widget and face complications. Answered before you drop your wrist.
06
Scrub the day
Drag across the hourly curve to see how the answer changes between now and bedtime.
07
Global support
Worldwide weather and multilingual, with dew point, relative humidity, wind and temperature all feeding the calculation.
08
Private by default
No account, no analytics, no tracking. Your location goes to Apple for a forecast and nowhere else.
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.
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 across the chart to scrub the whole day, hour by hour.
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 ThermoDiem 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.
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
Most apps hand you a single opaque number. ThermoDiem computes apparent temperature 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.
Download
the app
Free on the App Store. Built for iOS 27, watchOS 27 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.