Pressure difference as primary driving force in confined local waters

Resources on the use of Atmospheric Pressure in Navigation


As a rough rule of thumb wind speed in the Strait of Juan de Fuca and Puget Sound is proportional to the pressure difference between their end points, blowing from high pressure end to lower pressure end.

With wind speed in knots and pressure differences in mb,

Approximate wind speed in kts
 Pressure difference in mb
mid Puget Sound
~ 10 x
 (Port Townsend - Tacoma)
mid Strait of Juan de Fuca
~ 10 x
 (Tatoosh Island - Smith Island)

* These estimates can be used to anticipate wind strengths that might develop based on recent pressure observations, or this reasoning can tell you which direction calm or light air will fill in from based on recent pressure changes. We need more data, but for now is seems a factor of 10±2 covers most cases.

Again, this is rough guide [see note below], just to keep the pressure effect in perspective. If we see 1 mb difference over about half these distances then it would create ~20 kts per mb difference, and so on.

In the tables below, wind is in m/s. Multiply by about 2 to get kts (or 1.93 is closer).


Pressure differences in Puget Sound (For past 6 hr, every 6 min.)
Port Townsend, PTWW1   ( See all ) Tacoma, TCNW1   ( See all )
Pressure differences in Strait of Juan de Fuca (For past 24 hr, every hr.)       See neat pressure diference plot from BigWaveDave.ca
Buoy JA, 46087   ( See all ) Smith Island, SISW1  ( See all )

Note on pressures over local waters
The above estimates generally apply when there are isobars crossing the waterways. In other cases, there is a long ridge over the waterways, with the pressure on either side of the waterway about the same. This can be seen in the UW WRF data.  When that happens, the pressure in the middle has to be somewhat higher than on either side, and the ridge height can also be somewhat different at the two ends. These differences, however, should be less than about ± 0.5 mb else there would be another isobar, and the pressures within these limits could be different at both ends. That is, you could have a light S wind at West Point with the pressure higher a bit at Port Angeles than at Tacoma, which would normally call for a light N wind in the Sound. This situation would indicate some form of a saddle point near West point with, say, 1014.0 at West Point with 1014.3 at Tacoma (giving rise to a light southerly at WP) and 1014.5 at Port Townsend. In these conditions one might sail from a light southerly to light northerly when headed north up the Sound, having to punch though a zone of calm or fluky air.

Pressure tendency (PTDY)
The tendency is listed every hour. This is the change in pressure in mb over the past 3 hr. For a quick look at trends we can just look at the trend of the PDTY over the past few hours. If the pressures are now the same at the end points of a waterway, but the tendency is higher at one end, then we can guess which end is going to win... ie be the source of the wind as soon as the pressure difference itself gets to a mb or so.