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  1. #1

    Swell height vs. wave height

    What's the difference between the two on the buoy data? Since I've been looking at the buoys, which isn't very long, I've noticed that the wave data is always larger with a shorter period. So my guess is the wave data is local wind driven conditions and the swell data is the ground swell from wherever. Is that right?

    How does it translate to beach conditions? Obviously each beach is different but are there any rules of thumb?

  2. #2
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    Quote Originally Posted by clintopher View Post
    Obviously each beach is different but are there any rules of thumb?
    The amount of swell showing on the buoy (swell height and period) is more useful than the total wave data, (mean wave height and period) which includes all wave data in the mix, including (when its windy) the locally wind generated chop heading sideshore and offshore that generally show on the beach.

    If your buoy is 30 miles offshore and its reading
    mean wave height and period = 6 feet at 6 seconds
    swell height and period = 2 feet at 8 seconds

    I'd be expecting that 2' @ 8 seconds to be more like what you'll see for surf.
    Last edited by mitchell; Aug 12, 2012 at 11:36 AM.

  3. #3
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    The significant wave is roughly the wave height of the top 30 percent of all waves.
    The swell height on the buoy maps refers to the primary swell at the buoy location.

    With statistical approaches we can segregate wave magnitude, frequency, and direction at the buoy location into individual swell groups.

    Both significant wave height and primary swell height are telling in the breaking wave. As are secondary and tertiary swells. This intricate nature of the wave spectrum is one of the trickiest part of forecasting. You have to think of waves in terms of an entire 360 degree spectrum.

    Primary swell is usually the most telling, but additional swells can combine to become additive to the total breaking wave height.

    Some examples:
    Significant wave height: 5ft
    Primary swell: 3ft @ 12 seconds, ESE
    Secondary swell: 5ft @ 6 seconds, NW

    If this is an east facing break, the secondary swell is actually moving away from the coast, do the primary swell will be most telling.

    Significant wave height: 5ft
    Primary swell: 3ft @ 12 seconds, ESE
    Secondary swell: 4ft @ 9 seconds, SSE

    In this example the primary and secondary swells are additive to create a bigger overall significant wave height.

  4. #4
    Quote Originally Posted by mitchell View Post
    The amount of swell showing on the buoy (swell height and period) is more useful than the total wave data, (mean wave height and period) which includes all wave data in the mix, including (when its windy) the locally wind generated chop heading sideshore and offshore that generally show on the beach.

    If your buoy is 30 miles offshore and its reading
    mean wave height and period = 6 feet at 6 seconds
    swell height and period = 2 feet at 8 seconds

    I'd be expecting that 2' @ 8 seconds to be more like what you'll see for surf.
    That's not totally accurate in all cases. We are on the east coast. Unless you're talking about strong nor'easters, hurricanes most of the swell generated is very localized so significant wave height is more important than the swell measurement on the buoy.

    For example, right now here the swell height is "(.)7 feet" at 11 seconds and the significant wave height is 6 feet at 8 seconds out of the sw. It's waist to chest right now. Not less than 1 foot. Just pay attention to the direction of the swell. Significant wave height is more important than swell height here on the east coast ESPECIALLY in summer with fronts creating most of the wave action.

    I must add, If there's a signifcant medium period/ ground swell in the water than swell date on the buoy is your go to measurement!
    Last edited by shark-hunter; Aug 12, 2012 at 02:25 PM.

  5. #5
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    Quote Originally Posted by shark-hunter View Post
    We are on the east coast. Unless you're talking about strong nor'easters, hurricanes most of the swell generated is very localized so significant wave height is more important than the swell measurement on the buoy.
    I was just talking about sideshore/outgoing locally generated wind wave height not really mattering

    I agree that if the local wind waves coming onshore, then they add to the surf height.

  6. #6
    Quote Originally Posted by mitchell View Post
    I was just talking about sideshore/outgoing locally generated wind wave height not really mattering

    I agree that if the local wind waves coming onshore, then they add to the surf height.
    Oh ok. Yeah a great example would be A south swell 3 feet at 7 seconds and a nw swell 3 feet at 7 seconds. The buoy would show significant wave height 6 feet at 7 seconds. Of course we all know that's completely misleading for surfers, but very useful for mariners.