Marine Glossary

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diurnal pressure variation

The daily variation in atmospheric pressure due to a tidal action in the atmosphere. The swelling and relaxing of the air mass causes the pressure to swing through a daily cycle of almost 3 mb. There are typically two highs at 10 am and pm and two lows at 4 am and pm, local time of about +/- 1.5 mb. A typical cycle is illustrated in figure 9.3-1 of Modern Marine Weather... or just find any NDBC site in the tropics and look at their pressure plots.

You will also see this with a model forecast of the pressure (ie GFS) when you look at at meteogram of the pressure in the tropics, and sometimes even in the subtropics.

It is important to recognize this behavior in the tropics, since any approaching pressure pattern must be discerned within this daily variation. The effect takes place at all latitudes, but is more often noticeable in the tropics. At higher latitudes the pressure changes from day to day and throughout the day on a more erratic basis because there are more pressure patterns moving around there.

Note that many textbooks, as well as the US Navy and USCG exams call this the "diurnal" pressure variation, and there is indeed a diurnal variation in the pattern (ie one high and one low), but the dominant component by far is the semidiurnal (two highs and two lows) part, if we are to use an analogy with tide cycles. Thus this is either misnamed or they are using the word diurnal in a more general sense to mean "daily."