250 seamanship secrets
Also note the speed of the trend—rapid, slow, or steady. An hourly rise or fall of
1 millibar (0.03 inch) is considered a slow rate of change. Th is usually indicates
good weather; in some cases, it signals the approach of a weak frontal system. On
the other hand, an hourly change of 2 millibars (0.06 inch) foretells strong wind
and possible precipitation. To make accurate forecasts, you need to log at least
3 hours of observations. A series of readings gives a clear indication of what the
future holds (see table).
Barometric Trends
Time Reading Trend
1700 1010 mb —
1800 1010 mb steady
1900 1009 mb falling slowly
2000 1007 mb falling rapidlyMany mariners prefer to log inches instead of millibars. Indeed, many barometers
read in inches. Yet most weather maps show pressures in millibars. Just remember
that 1 millibar equals 0.03 inch.
Diurnal Pressure Variations in the Tropics and U.S. Waters
During the day, temperatures rise and the atmosphere becomes less dense. Dur-
ing the evening, temperatures fall and the atmosphere becomes more dense. Th is
causes a subtle shift in barometric pressure, called the diurnal (daily) variation. Th e
amount of variation depends on the time of day and your location relative to the
equator. At 0400 and 1600, readings are slightly lower than normal. At 1000 and
2200, readings are slightly higher than normal. From the equator to a latitude of 23^1 / 2
degrees north or south (between the Tropics of Capricorn and Cancer), the diurnal
variation averages about 3 millibars (or 0.09 inch). Th e farther north or south you
travel from the equator, the less the diurnal variation. Over most of the U.S., the
diurnal variation averages 1 to 2 millibars. In Northern Europe you might fi nd only
1 millibar of diurnal variation. Always take diurnal variation into consideration
when tracking the trend of your barometer. Here’s an example:
You are cruising in the Caribbean. Because you are near the equator, use
3 millibars (mb) of diurnal variation. You take barometric readings between
1600 and 2200. What is the actual rise or fall in the barometric readings
shown below? What is the trend?
1600: 1015 mb
2200: 1012 mbTh e actual fall was 6 mb. Th ink of the 1600 reading as a datum line. Because of the
diurnal change, you would start your calculation 3 mb above this datum line. Call