Wave height vs wave period for coastal boaters

What wave height and wave period actually measure

Wave height is the vertical distance from a wave trough to its crest. Wave period is the time between successive crests passing a fixed point. Height is usually shown in metres or feet. Period is shown in seconds. These describe different parts of the same picture: how tall the waves are and how closely they follow one another. The National Weather Service explains these wave measurements.

For a coastal trip, read both before comparing one forecast hour with another. A smaller height with a shorter period can describe a very different sea from a slightly larger height with a longer period. Start by checking that both forecasts use the same units and the same definition of period.

Significant wave height is an average of the larger waves

Most marine forecasts report significant wave height. A useful way to understand it is to arrange the recorded waves by height, take the tallest third, and average that group. Buoy and model calculations often estimate this from the distribution of wave energy. The resulting number describes the sea as a whole. It does not give the height of every wave. NDBC explains how significant wave height is calculated.

A forecast showing 1 metre therefore includes waves below and above 1 metre. The Met Office explains that individual waves can reach about twice the significant height. Treating the displayed height as a maximum would miss that variation. Read the Met Office explanation of offshore wave height.

What changes between a 4 second and an 8 second period?

In deep water, a longer period corresponds to a longer wavelength. At the same height, spreading the wave over more horizontal distance makes it less steep. Short waves pack the rise and fall into a smaller distance. NDBC uses the relationship between height, wavelength and period in its wave steepness calculations.

The following is a hypothetical comparison of two simple wave trains in deep water. Each row describes one wave system, with the same height and direction. These are teaching examples, not observations or forecasts for a particular coast.

The second example has more distance between crests. That explains why height alone cannot describe wave shape. It does not establish a preferred period for every coastal passage. The National Weather Service specifically distinguishes short wave chop from the effects of longer waves as they reach shallow water. Its wave detail guide explains why both matter.

Average wave period and dominant wave period are different

Dominant period, also called peak period, identifies the strongest peak in the wave energy spectrum. Average period summarizes a broader range of waves. They can differ substantially when several wave systems are present. NDBC also notes that providers calculate average period in different ways. See NDBC's definitions of dominant and average period.

Suppose one forecast displays an average period of 5 seconds and another displays a peak period of 9 seconds. Those numbers alone do not show that the second forecast predicts more widely spaced seas. You are comparing different measurements. Look for the labels APD, DPD, mean, average or peak before interpreting the difference. Keep that definition consistent when comparing a buoy reading with a forecast.

Look at wind waves and swell separately

Wind waves develop under the local wind. Swell has travelled away from the weather that generated it. A place can have swell even when the local wind is light. Several swells can also arrive together, with local chop on top. Australia's Bureau of Meteorology describes these different wave systems and how they overlap.

This is where the forecast breakdown becomes useful. Read the height, period and direction for the wind waves, then do the same for each swell component. NDBC provides these measurements separately because each part of the sea can have its own pattern. See how NDBC separates wind waves and swell.

Keep combined wave height separate from component height. A combined height and a dominant period describe the overall sea through two different statistics. They do not mean every wave has that height and period. The dominant period can belong to one component while the combined height includes energy from the others. Component heights are combined using their squared values, rather than simply added together. NDBC's wave glossary explains combined height and period.

Bring wave direction into the comparison

On NDBC reports, wave direction tells you where the waves come from. A west swell comes from the west. Wind waves and swell may arrive from different directions, so read each direction alongside its own period and height. NDBC lists its direction conventions here.

For your next coastal passage, write down the wave forecast at the start, an exposed section and the destination, using the hours you expect to reach them. Include the component breakdown wherever it is available. This gives you a useful comparison to carry into a weather window assessment. If you are choosing between morning and afternoon, compare those same locations at the matching arrival times. Our guide to comparing departure times works through that process.

Sources and further reading

• National Weather Service: Coastal Waters Forecasts With Wave Detail
• NDBC: How wave height and periods are calculated
• Met Office: Beach forecast and tide times explained
• NDBC: Wave steepness algorithm
• Bureau of Meteorology: Waves and swell
• NDBC: Separating wind waves and swell
• NDBC: Glossary of terms for wave information
• NDBC: Measurement descriptions and units