Fjaltring wave buoy daily means for Jan 2002

Wave period is measured in seconds. It describes how often waves pass the buoy, but an irregular sea contains many wave periods at once. Each metric summarises that mix in a different way.

Peak period (T_p)

The period at the strongest peak in the wave-energy spectrum. It identifies the dominant wave system and is usually the most useful period for judging the main swell or wind-sea system. It can jump when two systems contain similar energy.

Average period

A mean across the whole sea state. Providers may derive it from individual waves or from the wave spectrum, so its exact method can vary. It gives a broad sense of the sea's overall rhythm.

Mean period Tm02

A spectral mean calculated from the spread of energy across all frequencies. Shorter waves have more influence on it, so it is useful for describing the overall chop and wave spacing. It is the spectral measure closest to the zero-crossing period.

Zero-crossing period (T_z)

The average time between successive upward crossings, or successive downward crossings, of the mean water level in the measured surface record. It represents the spacing of typical individual waves and is useful for comparing how busy or orderly the sea is.

Energy period (T_e)

A spectral mean weighted towards the longer-period waves that carry energy farther. It is the period used with significant wave height to calculate wave power.

Maximum wave period (T_max)

The period of the highest individual wave in the buoy's sampling record. It describes that extreme wave, not the longest period present in the sea state.

Reported period

A period retained from a legacy source that did not identify the measurement method. It preserves the historical record, but should not be treated as directly equivalent to a named period type.

Longer periods usually point to more developed swell, while shorter periods often indicate local wind waves. Height and direction still matter, and one period value cannot separate every swell in a mixed sea. On daily and monthly tables, "mean" is the average of that named metric over the displayed interval.

kW/m means kilowatts of wave power per metre of wave crest. It describes the rate at which the observed sea state carries energy past the buoy.

Unlike the shore-directed forecast value, buoy wave power is the total power at the observation point. It is not projected towards a particular beach and does not predict breaking wave height.

In deep water, wave power can be calculated with:

P ≈ 0.49 × H_s² × T_eP is approximately 0.49 times significant wave height squared times wave energy period.

P is power in kW/m, H_s is significant wave height in metres, and T_e is wave energy period in seconds.

• Provider-reported power is used when available.
• Otherwise, a value is calculated only when both significant height and a genuine energy period are available. The ≈ symbol marks calculated values.
• A dash is shown instead of substituting peak, Tm02, zero-crossing or other period measurements.
• Daily and monthly figures use the mean of hourly-normalized observation power values; they are not calculated from mean height and period.

Wave direction, spectrum, water depth and local bathymetry still matter. Wave power is best read alongside height, period and direction rather than as a standalone surf-quality score.