Weather and tides

Many mariners set their schedules based on the expected state of the tide. In some cases, this is necessary, and in other cases simply convenient. The forces that affect the tides are fairly well known, and this has allowed for very accurate predictions to be made for the time of high and low tide for many different locations, as well as for the height of the high and low tides. In fact, many people treat the tide tables (which are published in many different forms) as absolute truth, not realizing that they are, in fact, just predictions which can be in error, although usually such errors are not that significant. Sometimes, though, the state of the tide can be different enough from the data shown in tide tables that it can have a significant effect on sailors, and usually the reasons for the differences are related to the weather.

The definition of tide is: “The alternate rising and falling of the surface of the ocean.” The forces, which lead to this periodic motion, are dominated by the gravitational pull of the sun and the moon (mostly the moon), and this portion of the tide is called the astronomical tide. This can be calculated fairly precisely, and this is used to produce the tide prediction tables which are widely available. There is also a contribution to the level of the ocean’s surface from meteorological conditions, and when this is combined with the astronomical tide, the result is the total tide, and this is what is observed in a given location at a given time. Under “normal” weather conditions, the contribution from the meteorological tide is negligible. However, when weather conditions differ from “normal,” the meteorological tide will make a more significant contribution to the total tide. There are two situations which need to be considered.

Atmospheric pressure will affect the state of the tide. Atmospheric pressure is defined as the force exerted by the column of air above a given point. When the pressure is significantly higher than normal in a given location, the force will be greater there than in surrounding areas. Basically this means that higher pressure will “push down” harder on the water and will result in a lower water level than would be the case if the pressure was near normal. Conversely, if the pressure is significantly lower than normal, the air will not push down as hard on the water and the water level will tend to be higher than it would be with near-normal pressure.

Sustained winds will also affect the state of the tide, especially in coastal areas. If the wind is blowing from the same direction for a period of time (usually 12 hours or more) with at least moderate speed, it will tend to move the surface water in that direction. When this occurs near the coast, it will cause changes in the water level. For winds blowing in an onshore direction (i.e. toward the coast), greater amounts of water will be moved toward the coast, and this will result in a higher water level than would be the case if this sustained wind were not present. On the other hand, a sustained offshore wind (i.e. away from the coast) will lead to water levels lower than would be the case otherwise. It is often necessary to examine the wind field not only in a local area, but also in a somewhat wider region to determine the overall movement of surface water due to the wind. This is particularly true for bays and inlets where even though the wind may not be toward the local coastline, if surface water is being forced into (or out of) the bay by the regional wind pattern, water levels will be higher (or lower) in all areas.

So what does this all mean? Basically, tides can be expected to run higher than normal, meaning higher high tides and also higher low tides, whenever barometric pressure is lower than normal and/or sustained onshore winds are present. The effect will be greatest when pressures are very low and/or the sustained winds are quite strong and have persisted for a long time. This means that the water will be deeper than would be expected otherwise, the water’s edge will climb higher on beaches and other shorelines than would normally occur, and the clearance beneath bridges and other overhead obstructions will be less than would otherwise be expected. Conversely, when the pressure is higher than normal and/or sustained offshore winds are present, high tides will not be as high, and low tides will be lower, and the opposite of all the effects listed above will result. If the two factors are in opposition to each other, such as onshore winds which would lead to higher water levels combined with high pressure which would lead to lower water levels, the net result may be very little variation from normal depending on the relative magnitude of each effect.

Speaking of magnitude begs the question: How much of a difference in water level are we talking about? In most cases, the answer is not that much — generally less than a foot, and sometimes only a few inches. In extreme cases, though, such as a powerful ocean storm (meaning very low pressure) that produces gale- to storm-force onshore winds for more than a day, the effect can be several feet. Remember that there is a variation in the astronomical tides between the more extreme spring tides and the less extreme neap tides, and when extreme meteorological tides coincide with a spring tide, this can lead to tides running well above normal at the time of high tide, and in these situations tidal flooding in coastal areas will be at its worst. In exposed areas, when wave action is superimposed on the above normal tides, serious beach erosion can occur.

The effect of the meteorological tide can be more significant in areas of the world where the normal tidal range is not that great. In these areas, under extreme meteorological conditions, the normal tidal cycle can be masked entirely by the meteorological tide.

The bottom line is that mariners need to be aware of how the tide can vary from published tide tables. When conditions lead to the tide running lower than normal, make sure you look down, to ensure you have significant depth to keep from running aground. When conditions lead to the tide running higher than normal, make sure to look up to ensure that your mast and/or antennae will clear that bridge you are going under. Also, make sure you pull the dinghy far enough up the beach so it won’t float away on an unusually high tide.

About the Author:
Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at the Massachusetts Institute of Technology. After working as a meteorologist for nearly 10 years he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine in 1991.
    A large portion of his business at Locus Weather involves providing custom weather forecast services for oceangoing yachts, both racers and cruisers. Ken serves as an instructor for the Ocean Navigator School of Seamanship, and also as an adjunct instructor at the STAR Centers for Professional Maritime Officers in Dania, Fla., and Toledo, Ohio, and for MITAGS in Baltimore, Md. He has also taught meteorology at Maine Maritime Academy. He resides in Rockport, Maine, with his wife and two sons.

By Ocean Navigator