ThanYesterday started with a very small problem.
When my child began daycare, every morning I was standing in front of the closet asking the same question.
The Same Question Every Morning
Dress them the same as yesterday? Lighter? Warmer?
When my child was a newborn, I just went by feel. But daycare changed things. With friends, playtime, walks outside, the day's temperature really mattered. Suddenly I needed to know how different today was.
Every morning I'd open a weather app. 16°C, sunny. But was that similar to yesterday? Sharply colder? A sudden warm spell? I couldn't tell. The thing I needed wasn't numbers — it was change.
What Existing Apps Missed
I tried pretty much every weather app I could find.
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Apple Weather, Google Weather: today and the weekly forecast
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Carrot, Weather Underground: rich data but no "yesterday"
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Official meteorological agency apps: historical records, but the UX is for experts
Not a single one showed yesterday's temperature at this same hour next to today's. I genuinely couldn't understand why this didn't exist.
So I Built It
I decided to build it myself. The core rule was simple.
"When you open the app, a single sentence should be enough."
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Morning: "3° warmer than this time yesterday"
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Afternoon: "Today's high is 2° lower than yesterday's"
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Evening: "Tomorrow morning will be cooler than today"
The comparison you need changes automatically based on the time of day. That's the whole thing.
Design: WeatherShape
Most weather apps use photoreal sun-with-cloud illustrations. I went the other way. Inspired by Bauhaus and Swiss graphic design, I built an original set of geometric weather icons — one unique shape per condition.
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Clear: concentric circles for the sun (a gentle breathing animation)
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Cloudy: two overlapping cloud shapes
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Rain: falling lines beneath a cloud
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Night: a crescent or full circular moon
I call this set WeatherShape, and it became the app's visual identity.
Tech: Dual Data Sources
For reliability, I use WeatherKit (Apple) as the primary source, with Open-Meteo as a resilient fallback. If WeatherKit doesn't respond within 5 seconds, the app automatically falls back. The user never knows there was an outage.
For Korean users, air quality matters deeply, so I integrated directly with AirKorea, Korea's national air quality API. Most global weather apps show either stale or inaccurate PM data for Korea. It was worth building this piece manually.
And I still open this app every morning to dress my kid.
Search "ThanYesterday" on the App Store.
kkiruk studio · 2026
2026