Wednesday, October 7, 2015

Using a Portable Barometer to Set Ship's Barometer to SLP

Ship captains often prefer their barometers set to read sea level pressure (SLP) to simplify weather reports and comparisons with weather maps. 

Recalling our jingle "point four-four per floor" as a reminder that a 12-ft change in elevation corresponds to a pressure change of 0.44 mb, we see that a large ship could easily have a correction of 3 mb or more, depending on the instrument height above the water. Also it is not always easy to  determine the actual barometer height above water level when it is 5 or 6 levels above the main deck. Sometimes the value historically assumed on board could be more an anecdotal value than one rooted in an actual measurement or taken from ship's drawings.

This question and be answered—or even by passed—with an accurate, portable barometer. First, such a barometer could be used as an accurate altimeter to measure this height difference in feet or meters, or as we note here you can use it to circumvent this question, and set the ship's barometer directly without the intermediate step or knowing or measuring its height above sea level or a specific water line, such as the Summer Load Line.

In short, we just measure the pressure at sea level with an accurate portable barometer then proceed to the ship's barometer and set it to that value. This involves writing down the pressure at sea level because it will change (decrease) as soon as we move up the ship. And since this process must often be done in a short time with numerous distractions, this writing it down offers a chance for error that could have long term implications to the weather work of the vessel.

Some electronic barometers offer a special option to record a specific pressure at a time of your choice, and most all accurate aneroid barometers have a set needle to mark a specific pressure.
Step 1.  Hold the barometer at or near sea level for a minute or so, tapping the dial of aneroids as needed, then log the pressure or set the needle.

 Step 2a.  If you just want to set the ship's barometer to SLP, then go directly to the ship's barometer and set it to the logged pressure. (See Note 3 below on the possible influence of tide height on this adjustment.)

Step 2b. If you want to know or check the elevation of the barometer above sea level or a specific water line, then carry the barometer directly to the height of the ship's barometer and record the new pressure. It will be somewhat lower than it was at sea level. Then find the Pressure Difference:

Pressure Difference = [portable baro pressure at sea level] -
[portable baro pressure at ship's barometer height]

Find the ship's barometer elevation from:

Ship's barometer elevation above sea level = Pressure Difference x (12 ft / 0.44mb)

See Note 2 on the use of different reference levels. 


1. We need to hold the portable barometer near sea level (or at any reference level we are using) for a few minutes because electronic instruments  might be averaging over some time period (check the manual) and aneroid barometers take some time to adjust, especially if you have brought it there fairly quickly from a notably higher location

2. We have shown Step 2b schematically using the waterline as a reference for the height measurement, but it could be done using the summer load line level for this reference. Our jingle for the size of the correction keeps this difference in perspective.

3. The height measurement in Step 2b, regardless of reference level selected, does not depend on tide height.  But the Step 2a procedure of setting the ship's barometer to read sea level pressure does in principle depend on the tide height.  In many cases this will not matter, but in harbors with large tidal ranges it could well be that the height of the water does not represent mean sea level. We have discussed this issue in an earlier note called Mean Sea Level, Tides, and Barometers.

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