M4 wave buoy daily means for May 2021Daily means from the M4 wave buoy for May 2021, including available wave height, period, direction, wind and sea temperature data.
May 2021
741 observations · 31 of 31 calendar days · 100% day coverage Period Summary Programmatically generated summary. Hog Says 741 wave-buoy observations covered all 31 days. Typical wave height was 7.2 ft (10th–90th percentile 4.4–12 ft), while mean peak wave period was 10.2 seconds (10th–90th percentile 8.1–12.2 seconds). Typical wind was 15.3 mph (10th–90th percentile 5.9–28.8 mph) and mean water temperature was 11°C (10th–90th percentile 10.4–11.6°C). 5 of 4,446 checked values (0.1%) were excluded by quality control. Across 9 comparable years, mean wave height was above the same-month mean of 7 ft and mean wave period was above the same-month mean of 8.4 seconds.
Generating summary…
Generated only from the observations and comparisons shown on this page.
Typical Significant Wave Height 7.2 ft
Mean: 7.8 ft
P10–P90: 4.4 — 12 ft
Recorded extremes 2.3 — 17.7 ft
Mean Observed Wave Period 10.2secs
P10–P90: 8.1 — 12.2secs
Recorded extremes 4.2 — 15.1secs
Available for 736 of 741 hourly samples
Typical Observed Wind Variable, 15.3 mph
Mean: 15.9 mph
P10–P90: 5.9 — 28.8 mph
Recorded extremes 1.4 — 40.1 mph
Circular concentration: 5%
Mean Water Temperature 11°C
P10–P90: 10.4 — 11.6°C
Recorded extremes 10.3 — 12.2°C
Compared With Previous Mays Historical baseline from 6,605 observations across 9 years.
Wave Height 7.8ft 0.8ft above mean Historical mean 6.955ft
Wave Period 10.2s 1.8s above mean Historical mean 8.4s
Water Temperature 11°C 0.2°C below mean Historical mean 11.2°C Chart showing grouped wave observations for M4. Use the table on this page for a text version of the same measurements. To open hourly observations, use a linked day name in the table.
Sorry, there is no wave data for Sat, May 1, 2021
About M4 wave buoy M4 is an offshore data buoy in the Atlantic approaches (55.0000°N, 10.0000°W). It supports marine forecasting by providing real‑time sea‑state and weather observations in open water. It typically reports directional waves and meteorology.
Its current data source reports wave height, wave period, wave direction, directional spread, wind conditions and sea temperature.
Provider-reported wave power. Approximate wave power calculated from significant wave height and energy period. Approximate mean including provider-reported and calculated wave power.
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Colours move through the scale as each number increases. They do not by themselves mean that conditions are good, safe or suitable for you.
Wave & swell height 0–40+ ft
0 5 10 15 20 25 30 40+ Wave & swell period 0–22+ s
0 4 8 12 16 20 22+ Wind speed 0–75 mph
0 10 20 30 40 50 60 75 The displayed numbers remain the source of truth; colour is an additional visual cue.
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 (Hs ) 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 (Tp ) 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 (Tz ) 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 (Te ) 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 (Tmax ) 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 × Hs 2 × Te P is approximately 0.49 times significant wave height squared times wave energy period.
P is power in kW/m, Hs is significant wave height in metres, and Te 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.
Learn how SurfHog combines, checks and summarizes observations in the data and methodology notes .
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NOTE: Automated quality checks remove individual measurements outside valid ranges or with suspect spikes, rates of change or frozen sensor values before they are shown. Automated checks cannot identify every instrument or reporting fault, so use this real-time data at your own risk.