Greenwich wave buoy observations on Sat, Apr 19, 2025

Metrics run down the rows and observation times run across the columns. Swipe the table or use the arrow buttons to move through time. Hourly times use your device time zone.

Colours

Height, period and wind colours follow the scales in the Colour key. A colour does not mean conditions are safe or suitable for surfing.

Dash

No usable value was available for that metric and time. The provider may not have reported it, or there may be a gap in the observations.

QC

SurfHog rejected the value during quality control. Reasons include an invalid range, an isolated spike, an implausible rate of change or an unchanged sensor reading.

Approximate symbol, ≈

SurfHog calculated the wave-power value from significant wave height and energy period instead of using power reported by the provider.

Night shading

On hourly tables, shaded cells mark darkness at the buoy's location. Shading does not indicate missing or poor-quality data.

Latest marker

The coloured edge and Latest label identify the newest received observation on the current hourly view.

Daily and monthly means

These tables average the named observation metric over the displayed day or month. Direction uses a circular mean so north stays next to northwest and northeast. Wave power uses hourly-normalised values, as described in its own guide.

Question-mark buttons

Open the guide for that family of measurements. The period, height, direction and wave-power guides explain what each value measures and when it is useful.

Wave heights are vertical distances. An irregular sea contains many individual waves, so each height metric describes a different part of the buoy's sampling record.

Significant wave height (H_s)

The characteristic height of the larger waves. It is roughly the average height of the highest third, and modern buoys often estimate it from the wave spectrum. Use it as the main measure of overall sea size. Individual waves can be much higher.

Average wave height

The mean crest-to-trough height of all individual waves in the sample. It includes the smaller waves, so it is normally lower than significant wave height.

Maximum wave height

The greatest crest-to-trough height of one individual wave in the sample. It is useful for spotting extremes, but it is less stable than significant wave height.

Maximum crest height

The highest crest above the mean water level during the sample. It describes the upper half of an extreme wave.

Maximum trough height

The depth of the lowest trough below the mean water level, shown as a positive distance. It describes the lower half of an extreme wave.

The maximum crest and maximum trough may come from different waves, so they should not be added together. On daily and monthly tables, a mean maximum wave height is the average of observation-level maxima, not the single highest wave across the whole interval.

Wave directions show where the waves come from, measured clockwise from true north. A west direction means waves are travelling from west to east.

Peak wave direction

The direction of the waves at the peak period, where the wave spectrum contains its strongest energy peak. It usually describes the dominant swell or wind-sea system.

Mean wave direction

An energy-weighted direction across the broader wave spectrum. In a mixed sea it can fall between separate wave systems, so read it with peak direction and directional spread.

Directional spread

The angular width of wave energy around the mean direction within one provider sample. A small spread means the waves are tightly aligned. A large spread means they arrive from a wider range of directions.

Reported wave direction

A direction retained from a legacy source that did not identify the measurement method. It preserves the historical record but is not directly equivalent to a named direction type.

Concentration

SurfHog's measure of how consistent the observation directions were across a day or month. A value near 100% means they stayed close together. A low value means they varied widely. This is not the provider's directional spread measurement.

"Variable" replaces a daily or monthly mean when concentration is too low for one direction to represent the interval. Direction at an offshore buoy does not by itself predict the angle or size of waves at a beach because refraction, shelter and local bathymetry can change them before they break.

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.