Wednesday, 4 July 2012

Aeolus Rules the Waves - Proof from Portland, Australia

Poseidon may be the god of the ocean, but Aeolus, god of the winds, has a strong hand in ruling it. While working on something else entirely, I plotted the hourly tide data for Portland, Victoria for January/February this year, hoping to be able to correlate wind direction with tide height.

The red line is a 25-hour moving average, 25 being appropriate for two reasons; a moving average should span an odd number of periods or data-points (so it has a centre), and the tidal cycle is much closer to 25 hours than 24, so it should be largely filtered out. While wind direction must have an effect on tide height, higher when wind is blowing onshore, lower when offshore, the effect was being overwhelmed by something else; something I was well aware of, but had thought (hoped) to have a lesser effect. It's barometric pressure - a change of one millibar (hPa if you're insistent) affects sea level by about 1 centimetre; lower if the pressure's higher, and vice versa. Higher pressure "pushes" the surface down. Because the effect is magnified 10 times, plotting the pressure on the same chart reveals nothing; the pressure plot barely wiggles. By converting the tide height to millimetres, turning the pressure values into anomalies (deducting 1000), and multiplying by 10, adding 1000 (to position the resulting plot), I got the following:

That looks to be a fairly convincing correlation, but because the relationship is inverse, the effect's spoiled. Here's the result of inverting the anomaly and dropping the line a little:

IMHO that's a convincing illustration of Aeolian power. Atmospheric pressure is altering sea level by as much as 50% of the lunar tidal range (difference between low and high water).

Now let me see - if I deducted the pressure effect, allowed for the predicted tidal cycle.... but that's for another day, another post (maybe).

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