Why the instruments work
Theory
The useful physics behind the Barometer, Altimeter, Pressure Trend and Tide Clock.
Station pressure and sea-level pressure
The pressure sensor measures the air pressure where the phone is sitting. That is station pressure. Because pressure decreases with height, weather observations are commonly reduced to a sea-level equivalent so values from different elevations can be compared. Marine Barometer uses the calibrated station pressure and reference elevation to produce the Barometer reading.
Why the Altimeter needs a reference setting
An altimeter converts pressure into indicated altitude. The same atmospheric pressure can correspond to different indicated altitudes depending on the reference setting. QNH, QFE and QNE make that reference explicit rather than hiding it inside the calculation.
What pressure trend tells you
A rising barometer is often associated with improving or more settled weather; a falling barometer is often associated with deteriorating or unsettled weather. Rate and size of the change matter. Local weather, topography and movement of the phone still matter, so the pressure trend is one useful observation rather than a complete forecast.
Why altitude movement can fool a weather log
Climbing to a higher elevation lowers station pressure even if the weather has not changed. Descending raises it. That is why a phone carried up and down hills can produce apparent pressure “events” in a stationary weather log.
Why a Tide Clock is approximate
The principal lunar semidiurnal tide has a period of about 12 hours 25 minutes, which is why traditional tide clocks turn slightly more slowly than an ordinary 12-hour clock. Real local tides combine several astronomical constituents and are shaped by the coastline, seabed and weather. A simple tide clock therefore represents phase, not an exact harmonic prediction.