Because the air is denser at sea level than at higher altitudes, you must also correct for the actual height above sea level.

Use the table “correction of millibar barometers to mean sea level” for the altitude correction.

This table is available in the Nautical Almanacs.

Let’s look at how to apply the correction. First, note your barometer reading, the air temperature and your actual height above sea level.

Let’s assume that the barometer reading is 1015.4 millibars, the air temperature is 20 degrees Celsius and your height above sea level is 5 meters.

By applying the air temperature and height above sea level to the chart, you see that the correction is 0.6 millibars.

Add this figure to your barometer reading.

Hence, the barometer reading corrected to mean sea level is 1015.4 millibars plus 0.6 millibars, which equals 1016 millibars.

Keep in mind that the greater the height above sea level and the greater the temperature, the greater your correction to mean sea level will be.