Your location never leaves your phone

Daynostic can use weather data without ever learning where a person is.

Weather is one of the more interesting variables in life tracking. Temperature swings, pressure drops and long stretches of low light have been suspected for centuries of affecting mood and energy, and a daily mood log paired with daily weather data makes that testable at the individual level.

The obstacle is location. Knowing the weather requires knowing where someone is, and location is among the most sensitive signals a phone produces: a year of daily coordinates maps a home, a workplace, travel and routine. The aim was to obtain the weather data without ever assembling that map.

The lookup happens on the device

When Daynostic records weather for a day, the sequence is:

1. The device determines its own location, as any weather app does.
2. The device calls Open-Meteo, an open weather API, directly. The request travels from the phone to the weather service and never reaches Daynostic's servers.
3. The response (temperature, humidity, atmospheric pressure, wind, precipitation) is attached to the day locally.
4. With sync enabled, only those daily weather numbers reach the account. Coordinates are never part of the payload.

Daynostic's servers can therefore record that a given day was 18 °C with light rain, without knowing where on Earth that day took place. There is no location column in the database, because the data never arrives.

"Trust us" versus "we can't"

Most privacy policies are promises: location is collected, and the company commits to handling it carefully. That is a "trust us" design, and it depends on every future version of the company behaving well.

A "we can't" design is stronger. Data that is never transmitted cannot be misused, whatever anyone's intentions, and an architectural guarantee is much easier to verify than a stated one. The same approach appears elsewhere in Daynostic:

• Guest mode is fully local. The app works without an account, and everything stays on the device until the user chooses to sync.
• Data is exportable. Logged data belongs to the person who recorded it and can be taken out at any time in an open format.
• Deleting an account deletes its data. No copies are kept for analytics.

Why Open-Meteo

Open-Meteo is an open-source weather API that needs no API key or user account for the variables Daynostic uses. That is what makes the direct-from-device call possible: there is no secret credential that would force the request through a backend. It supplies the daily variables relevant to correlation analysis: temperature, humidity, pressure, wind and precipitation.

Because weather arrives automatically each day, the analysis receives a dense, objective data stream with no extra effort from the user. Once there is enough data, Daynostic can report, with the statistical backing described elsewhere, whether something like pressure-related headaches is real for a given person. Until then it reports nothing.