Atmosphere, pressure and the onboard weather log

Chapter 1 / 12 · 5 min reading + practice

Read pressure, temperature and moisture together; use a barometer as evidence rather than an automatic storm alarm.

A useful weather log answers three questions: what is changing, whether it matches the forecast, and which decision needs reviewing. Pressure is especially valuable offshore because it remains observable without an internet connection. But a single reading cannot tell you the wind tomorrow. Combine the barometer with temperature, moisture, cloud, wind and a chart covering the surrounding weather system. The aim is to recognise a changing situation early, not to replace the official marine forecast with a rule of thumb.

Pressure: a column of air, not a weather verdict

Atmospheric pressure is force per unit area exerted by the air above a location. Weather charts normally show mean sea-level pressure so readings from different elevations can be compared. One hectopascal, hPa, equals one millibar; 1 hPa is 100 pascals. The often-quoted standard sea-level value of approximately 1013 hPa is a reference, not a dividing line between safe and unsafe weather. A pressure of 1020 hPa can coexist with strong wind if the pressure changes rapidly across the surrounding area.

Separate the spatial gradient from the time tendency. The gradient is how pressure varies from place to place and helps explain wind. The tendency is how pressure at your moving boat changes over time. Your track across isobars contributes to that tendency, alongside the weather system’s own evolution. Falling pressure is therefore a reason to investigate, not proof that a particular depression will pass directly over you.

Temperature, moisture and rising air

Relative humidity expresses how close air is to saturation at its current temperature. Cooling can raise relative humidity without adding moisture. Dewpoint is the temperature at which that air would become saturated if cooled at approximately unchanged pressure and moisture content. A shrinking temperature–dewpoint gap is relevant to visibility, but does not by itself predict fog: mixing, sea temperature and the incoming air mass matter. Saturation also does not automatically mean rain.

Rising air expands and generally cools; condensation can then produce droplets or ice. Sinking air tends to warm and suppress deep cloud development. This helps explain the usual association between lows and unsettled conditions, and highs and more settled conditions. It is not a guarantee. Persistent low cloud or fog may sit beneath a high, while an intense pressure gradient on its flank can produce demanding sailing.

Circulation and the hemisphere check

Large-scale wind is shaped by the pressure gradient, Earth’s rotation and friction. Around a surface low, flow is broadly anticlockwise and inward in the Northern Hemisphere, clockwise and inward in the Southern Hemisphere; around highs, the pattern reverses. Near the equator, the rotational constraint weakens. Coastal terrain and small convective outflows need not follow a neat chart-scale spiral. Treat the hemisphere rule as a framework for checking a chart, not a compass for escaping a storm.

Global circulation helps explain trade-wind belts, subtropical highs and mid-latitude westerlies. These are climatological patterns: seasonal displacement, monsoons and individual weather systems alter what happens on a particular passage. An ocean planning chart is useful background; it is not today’s forecast.

Make the instrument and the record trustworthy

Check the barometer against a recent nearby official sea-level pressure observation, allowing for distance, time and height differences. Follow its calibration instructions; do not keep adjusting it simply to match each new app. Protect the sensor from unusual airflow and cabin pressure effects. Record UTC, position or route leg, pressure, observed wind, cloud, visibility and temperature if available. Write whether a value is an observation, forecast or estimate. A handover should include the last readings and the next planned check.

Worked scenario: a falling barometer in the Aegean

Illustrative exercise, not a current forecast. A yacht records 1018.2 hPa at 06:00 UTC, 1016.7 at 09:00 and 1013.9 at 12:00. The total fall is 4.3 hPa in six hours; the last three-hour fall is 2.8 hPa. The crew also notices increasing cloud and a stronger, less steady wind. These are meaningful observations, but they do not produce a reliable departure instruction on their own.

Check the log and sensor: were all readings taken with the same instrument, and did a setting change?
Look at the track: did the yacht move towards lower pressure on the latest chart?
Compare the forecast sequence and the official marine bulletin. Does either show tightening isobars, a frontal feature or an earlier deterioration?
Check the route consequence: could a changed wind make the planned anchorage exposed, the return leg harder or the exit from a narrow channel demanding?
Write the unresolved question and the next source update. If the observations diverge from the expected pattern, reduce dependence on the original plan and seek a fresh assessment.

The defensible conclusion is ‘the observed change warrants comparing the latest forecast and reassessing the exposed leg and alternatives’, not ‘4.3 hPa means a storm at a specified hour’. Even if pressure subsequently rises, swell and residual wind may remain. The journal preserves why a decision changed and supports later learning.

Common mistakes

Reading the word ‘fair’ printed on an aneroid dial as a forecast.
Comparing station pressure on a hill with sea-level pressure on a chart.
Ignoring the yacht’s movement when interpreting the pressure tendency.
Calling a high automatically calm, or copying Northern Hemisphere circulation into Southern Hemisphere planning.
Using a universal pressure-fall threshold instead of the relevant official warning and the route context.

Before the next watch

Confirm units, time reference and instrument settings.
State what changed, over which interval, and what else changed with it.
Check the pressure pattern beyond the single forecast point.
Keep observed facts separate from explanations and uncertainties.
Agree who checks the next bulletin and what would trigger a passage-plan review.

Quick chapter check

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1. While your yacht is moving, pressure falls from 1018.2 to 1013.9 hPa in six hours. What is the most complete interpretation?

The fall gives the exact time a front will reach the yacht. The fall merits a forecast and observation recheck; both weather evolution and movement across the pressure field may contribute. Because the yacht is moving, the pressure trend contains no useful weather information.

2. Air cools toward its dew point while its water-vapour content changes little. What should happen to relative humidity, and what does that prove?

Relative humidity rises toward saturation, but that alone does not prove that rain will occur. Relative humidity must remain constant because the amount of vapour has not changed. Reaching saturation guarantees rain at the yacht's location.

3. Which broad near-surface circulation is associated with a low-pressure system in the Northern Hemisphere, away from the equator?

Clockwise, with an inward component. Anticlockwise, with an outward component. Anticlockwise, with an inward component; local terrain may still modify the wind at the yacht.

Check your understanding

First explain your answer in your own words, then open the explanation. These are original Sailing Weather exercises, not official exam questions.

1. Does 1020 hPa guarantee light wind?

No. Wind depends strongly on the surrounding pressure gradient and local effects, not only the absolute reading.

2. Why can the barometer fall even without a deepening low?

The yacht can move across isobars into lower pressure; system movement also changes the reading. Separate track effects from system evolution.

3. Does 100% relative humidity mean rain?

No. It describes saturation at that level; precipitation also requires suitable cloud processes and growth of particles.

4. What changes around a surface low in the Southern Hemisphere?

Broad circulation is clockwise and inward rather than anticlockwise and inward. Local terrain and convective winds still need separate assessment.

Sources and verification scope

Met Office: high and low pressurePressure patterns and the usual association with rising or sinking air.
Met Office: measuring atmospheric pressureSea-level reduction, exposure effects and pressure tendency.
NWS: humidity and dewpointRelative humidity, dewpoint and why saturation alone does not imply rain.
Met Office: wind flowPressure-gradient forcing and different circulation in the two hemispheres.
Met Office: global circulationTrade winds, subtropical highs and mid-latitude circulation as background patterns.