Our rating exists to give you an easy way to spot good quality swell at a glance with a simple rating of one to five stars. Not all surf sessions are created equal and size alone doesn't dictate quality. Generally swells from more distant, more powerful storms create the best surf. With more powerful, more organised waves. Our rating is biased towards this sort of 'ground swell'. This sort of swell also creates steeper faces, more likely to barrel or provide a better quality of ride on most surf craft.
The total number of stars is the rating of the swell without the wind taken into effect. We then grey out stars if the wind is blowing onshore in a way that'll decrease the quality of the surf. So the overall rating for the hour is the total number of just the dark blue stars. So in the example above the 6am wind is cross/onshore and a 'would be' four star rating is degraded to two stars. By the evening the swell has dropped in size but the wind spun offshore so the overall rating is now three solid stars.
For big wave locations we use a black star, this uses a scale more familiar to big wave surfing and takes more account of overall breaking wave size because of the special characteristics of this sort of wave.
While an 'at a glance' rating is really useful for both experienced surfers wanting to scan the forecast quickly and beginners needing some additional help, it can't take into account every small local variable. An experienced surf forecaster will prefer to create his or her own final assesment from the swell data in the 'advanced' section and the wind forecast rather than relying solely on this rating. The rating system uses this same information but without reference to the kind of subtle local knowledge you can accumulate from carefully correlating the forecast to your experiences in the surf.