Forget about it - this is swell so small and weak in the very early stages of being generated from strong wind that it can almost never be surfed. Sea in this state will look lumpy and bumpy but you’ll struggle to see individual waves. Because this sort of swell is unable to travel far from the storms that create it you can guarantee that there will be strong local winds when you see these kind of waves forecast.
Much of what’s written about 1 - 4 seconds still holds for swells in this range. But you will start to see the odd weak ridable wave face if you’re very desperate. Again it’ll be almost impossible to find this sort of swell without the strong onshore winds that create it so expect really sub par conditions for surfing.
This would be typical wind swell considered surfable by many surfers, especially those in areas that don’t get great waves. Typically this sort of swell will still be in the path of the strong winds that created it, but it will hang around for a short time if the wind direction changes so offshore conditions are possible. The waves will generally be weaker and jumbled up without clear sets. However a good sand bank or bit of reef can generate decent waves and large storm swells at the upper end of this range can produce decent waves at the right spot with the right local winds. The surf produced by these swells will normally be slightly smaller in height than the swell that creates it, losing power as it enters shallow water, but a good reef can create a wave face larger than the swell size at the upper end of this period range.
Swells in this range will often be starting to head away from the storms that create them. It’s possible for them to travel in open ocean for some distance. They can often create good quality surf. These swells won’t bend or ‘refract’ into hard to reach surf spots and are less likely to barrel when smaller than longer period swells. On average sandy beaches these mid period swells can create some of the best conditions, a reef or point break needing the swell to refract can prefer a longer period of swell. The waves produced by these swells will often be about the height of the swell itself given the right direction on the average beach. If other conditions are favourable swell in this period range will definitely be worth checking out.
These swells are definitely ‘groundswell’ - normally created some considerable distance from the beach by powerful storms. They most often arrive without the storm that created them, making for a good possibility of calm local conditions. They’re powerful, they will bend or refract around headlands or into more sheltered coves and create, especially when smaller, hollow barrelling waves on even average sand bottomed beaches. These swells will have more defined sets and look a lot more ‘lined up’ than lower period swells. Typically this sort of swell is great for creating surf and the waves created will often be bigger than the swell height as the swell refracts to focus it’s energy in shallow water and the wave shape changes. This sort of swell period would be typical of the peak of an Indonesian swell for example and can create absolutely perfect conditions if other factors are aligned.
Extremely powerful swells generated by distant storms and often travelling the breadth of the largest oceans to reach the beach. These swells can refract considerably as they approach shallower water, bending into sheltered coves, around significant headlands and focusing their energy on beach to create, with the right local conditions, ridable waves considerably larger than the swell height at the right spots. A good example of scenarios in the upper end of this range would be powerful southern Hemisphere swells travelling all the way to California with maximum periods in the 21 seconds range. At just a couple of feet this sort of swell can create 6 - 9ft waves on the beach.
There's always a catch. In this case it's that the 'swell period' is a way of describing the most powerful waves in a swell, but a swell will have a mix of waves of different periods and you can't tell much about that mix from just the one number. You could be looking at a thumping pure 16 second period swell that'll have perfect sets of lined up waves, or a mixed up powerful sea with 16 second swell at the top end but wind junk in lower periods creating bumpy confused conditions. Swell period is still one of the most useful bits of information in your tool kit, but be aware of this shortcoming.