Saturday, January 28, 2017

Tides and Barometer



A while ago I wrote a bit about the barometric pressure impact on the actual tides.  Yesterday, Mrs. SkunkBayWeather and I took a walk on the beach with the dogs.  We started walking with about a 7ft. predicted tide level.  When we got on the beach I couldn’t believe how low the tide was.  We had lots of sandy beach, rather than having to worry about climbing over and around downed trees and walking in the water.  There are two things going on here.  One, the strong north wind storms we had a while ago carved away the sand right up to the edge of the banks.  There are places with a 2 ft. drop.  All that sand is spread evenly out onto the beach, which in turn raises the level of the beach relative to the water.  We have a way more sandy beach now.  But there is another reason.  Yesterday the barometer was the highest so far this year at 30.65” / 1038mb.  This lowered the tide level by 10”.  This helped contribute to the beautiful long, sandy beach.



During the big North winds we had recently, the barometer bottomed out at 29.07” / 984mb (the lowest of the year).  This raised the sea level 11” above the prediction.  Tides are predicted at the average barometric pressure of 29.91” or 1013mb.  This brings up a pretty amazing fact.  Between these two events the actual swing (low to high) from prediction caused by atmospheric pressure is 21”.  This is just a good reminder that the predicted tide levels are just that.  They are predicted on an average barometric pressure.  Actual tides fluctuate tremendously due to the wide swing in atmospheric pressure.



A picture tells the story….  Normally anything above 7.5’ and we are climbing over downed trees or walking in the water.  Here is a picture taken this morning on a 7.1’ tide and a 30.57” / 1035mb barometer.  This lowered the tide from prediction by 9"….  No climbing over trees this morning….


I provide this adjustment number on my site as shown here:


There is also a link to a conversion chart I put together.  Here is the chart:










2 comments:

  1. This is very helpful and informative. As kitesurfers we are always monitoring tide levels for areas to launch. Could you please comment if the rate of tidal change is altered by pressure.

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  2. Hi Goiner, Very good question. I am not an expert on this, but I would assume from the laws of physics that it would be proportional to the rate of rise/fall of the barometer. Example: If the barometer were to rise by 3mb in one hour, a very loose calculation would suggest an impact of 1”. So, on an outgoing tide, the rate would be impacted by one inch in that hour….. To take it further, let’s use today’s current tide. The high here today was 11.45’ at 6:48am and the low is going to be 5.18’ at 12:38pm. That is a 6.27ft run off in 350 minutes…. Or 1.07’ per hour. So, if the barometer rose 3mb in one hour, the tide would fall approximate 2.07” in that hour…. A very extreme (and approximate) example, but it demonstrates how this could be calculated. There are many other factors that affect the tide level, but this component can be calculated…. Currents, back eddies, storm surge and heavy rainfall runoff are just some more examples of what can impact the tides.

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