Barometer Atlas
The single most useful instrument on board. Trend beats absolute value every time.
Trend rules of thumb
| 3-hr change | Implication |
|---|---|
| Steady or ± < 1 mb | Settled; little change in next 12 h. |
| Fall 1–3 mb / 3 h | Low approaching; expect wind to build + veer. |
| Fall > 3 mb / 3 h | Deepening system nearby. Gale plausible. |
| Fall > 6 mb / 3 h | Rapid cyclogenesis ("bomb" if 24 mb/24 h). Storm. |
| Rise 1–3 mb / 3 h after low | System passed; wind shifting (backing in N. hemi after cold front). |
| Rise > 3 mb / 3 h | Cold high pushing in fast. Squally on the leading edge. |
Always log at consistent intervals (1h on passage, more in weather). The rate of change is what matters.
Calibration
- In port or anchorage, compare to the nearest METAR/NDBC buoy sea-level pressure. Correct if off by more than 1 mb.
- Altitude correction: add ~1 mb per 8 m of barometer height above sea level (rarely matters on a boat; matters on a 40-ft aft cabin only by 0.5 mb).
- Mechanical aneroids drift. Digital sensors (BMP280/BME280-class) are better; log them automatically if you can.
Pressure + wind = Buys Ballot's law
N. Hemisphere: face the wind — low pressure is on your right (slightly behind). S. Hemisphere: low is on your left.
Combine with your falling barometer to locate the low roughly and decide whether to run left or right of the track.
Pressure-reduction formula (field use)
Most handheld barometers show station pressure. To compare with synoptic charts you want sea-level pressure (SLP).
SLP ≈ P_station × exp(h / (29.27 × T)) where h is height in m and T is temperature in K.
For small heights (< 10 m) the correction is tiny and usually within sensor noise.