Swell direction and wave height

lamai

Just wanted to ask about something I am a bit unsure about. I surf on beaches in the north of Donegal. Take for example I am looking at a beach on magicseaweed that is not enclosed but just facing the Atlantic. The forecast is given as 6ft with a westerly swell. But when I get there the waves are not 6ft.I take it as these beaches are facing northerly they might actually get the full 6ft if the swell is northerly?

cornbb

The swell height is generally measured far out at sea by weather buoys: http://www.met.ie/latest/buoy.asp. Sites like magic seaweed (which I don't find to be particularly good) will try to predict the wave height on the shore based on this and other factors, but won't always get it right.

The actual wave height at your local break will depend on a number of other factors such as the tide, how sheltered it is, the topology of the sea bed, shifting sandbars, wind strength and direction, etc. Forecast sites will never tell you exactly what its gonna be like at the beach. Your best bet is to check various forecast sites (wind guru, met eireann etc) before going and comparing what they say to what the actual conditions are. That way, over time, you can learn to look at the forecast and figure out how things will actually be.

lamai

cornbb wrote: »

The swell height is generally measured far out at sea by weather buoys: http://www.met.ie/latest/buoy.asp. Sites like magic seaweed (which I don't find to be particularly good) will try to predict the wave height on the shore based on this and other factors, but won't always get it right.

The actual wave height at your local break will depend on a number of other factors such as the tide, how sheltered it is, the topology of the sea bed, shifting sandbars, wind strength and direction, etc. Forecast sites will never tell you exactly what its gonna be like at the beach. Your best bet is to check various forecast sites (wind guru, met eireann etc) before going and comparing what they say to what the actual conditions are. That way, over time, you can learn to look at the forecast and figure out how things will actually be.

Thanks for the reply, but in theory would a north facing beach with a 6ft westerly swell get a smaller wave than a 6ft northerly swell on the same beach?

rodento

Have a look at this site

https://www.fnmoc.navy.mil/wxmap_cgi/cgi-bin/wxmap_DOD_area.cgi?area=ngp_atlantic&set=SeaState



Magicseaweed/Windguru have a blind spot for southerly swells and class them as westerly often

These swells have to be pretty big to wrap around to get into some of the north facing beachs, hence you can get pretty despondent when you really on those forecasts alone

ben bennett

magic sea weed does not actually predict wave height, it gives you information on swell size , which is the height from the trough to the crest of the wave energy or swell traveling across deep water. as a very general rule you need to divide this in half even before you look at local factors as mentioned above. So normally if it says 6 feet you are looking at 3 ft if it gets directly into your bay with no interference etc.

however you also need to take into account the wave period which is the lenght of time between the crest of one swell wave and the next, ie the length of each wave in the swell. if you understand how a wave is formed then you see how the period is vital.

When the swell hits the land, the front of the swell wave slows up first with the back now travelling faster so it catches up with the front building up into a wave and breaking. if you have a swell period of say of 8 seconds and 1ft in height, compared to a the same height but say a 15 sec period, you see there is a lot more water behind the front of the second wave to build up so you get a bigger wave breaking on the shore.

cornbb

lamai wrote: »

Thanks for the reply, but in theory would a north facing beach with a 6ft westerly swell get a smaller wave than a 6ft northerly swell on the same beach?

As rodento alluded to, probably not. A swell coming from the west would lose some energy making its way into a beach/bay facing to the north.

rodento

Also you have to account for the ideal swell direction for the break, plenty of north facing spots fire on westerly swells but are absolute pants if the swell swings north

Wavefinder is the best guide for showing this, it has a little topo for each break showing the best swell directions to look for

http://www.wave-finder.com/surf/uk-ireland.php




Its easy to work out wind direction as well and it shows peaks...

spaceHopper

As a rule of tumb divide the swell height by 3 for most irish swell and you won't go far wrong. If you want to learn more then read LUD's FAQ google will find it easily it's hard going at the start but stick with it he breaks it down well