It is the difference between the projected water level and mean low water. For example - High water is +2.5 and low water is -.5 means there will be a difference of 3 feet between high tide and low tide.
On the South Shore of LI the difference is usually about 2 feet and on the North Shore of LI it can be as great as 8 feet in the Western Sound. It also changes based on the lunar cycle and winds.
Use the numbers to determine what the currents will probably be like. The greater the difference in the numbers between high and low tides the stronger the currents will be.
You will also notice there is a higher number during full and new moon periods. This also indicates a stronger current.
At Rich Johnson's Surf Seminar last fall John Skinner gave a very good explanation of how he uses the tide tables to determine current strengths. Hopefully he'll see this thread and comment on it (you out there John? )
Doughboy explained it very well. The way to take advantage of the numbers is to learn what they mean for an area you're interested in by always noting or logging what they are for every trip. Do this enough and the numbers will take on meaning. For example, for a certain inlet jetty a high of 4.0 tells me I'll have almost no eeling window and I better have a good supply of my heaviest bucktails. If it's at a time of year when the fish are only responding well to eels, then the 4.0 would keep me out of that spot and cause me to fish a different place that has a history of producing well with the extra water and current. I'm always watching those numbers. They're extremely useful.
Those expanations given just don't cut it (sorry guys). The numbers do not represent speed of the current. What they do stand for is the HEIGHT of water above or below the depth shown on a chart. Now in order to determine the depth of water, the height given on your tide chart is added to the depth shown on your nautical chart-OR- subtracted when a minus(-) sign appears. Boaters use these numbers when navigating shallow areas to make sure there will be enough water under the boat! Confused? Here's an example:
Question: What will the water depth be at 0600 if your nautical chart shows a depth of 5'(keeping in mind all those numbers on a chart are given at LOW tide) Simply add 4.5 to 5 to give 9.5'
Now at low tide you add .4 to 5 to give 5.4' (piece of cake right?)
Hope you're not getting confused. Like I said before, Doughboy had a nice explanation. The important thing is that the relative heights of those numbers do correlate to current speeds. Not perfectly, because the other non weather-related variable besides the tidal range is the amount of time it takes to go from high to low tide and vice versa. If you need convincing, correlate a tide table and current chart for a given area and I think the relationship will be clear.
An excellent book on these sorts of things is "The Fisherman's Ocean" by Dr. David Ross.
I think you missed part of my post. I said that the numbers are the projected hieght of the tide (I say projected because wind can have a great effect on the tide height as well) above or below mean high water. Which, as you indicated, is the water depth used on all of our charts.
I was just going on to explain that you can use those numbers to figure what the current will be like in an area. As the difference between the two numbers increases, during a period of full or new moons, so does the current.
I meant no disrespect for your explanation it's just that upon reading it I wasn't sure if great1 got the full gist of how to actually APPLY the numbers. Many people think that those numbers equal water depth which is incorrect.For your example you used the number(-.5) which when applied to a tidal chart would indicate that the height of water at low tide would be one half of a foot lower than the depth shown on a nautical chart (which you know usually occurs at new and full moons).