How to play Surfer's Wait
- Choose a test and read the spacing, the speed, or the period on the card.
- Use T = λ ÷ v, λ = v·T, and f = 1 ÷ T; a minute holds 60 s ÷ T waves.
- Type your prediction and press Start. The swell rolls in, sped up.
- Compare the orange lines with the crests' dots on the right, or the orange marks with the crests in the sea.
The period of a wave
The period T of a wave is the time it takes for one whole wave to pass a point: for a surfer waiting beyond the breakers, the time between one crest lifting the board and the next. During one period the wave travels exactly one wavelength, so T = λ ÷ v, or turned round, λ = v·T and v = λ ÷ T. The frequency is the period upside down, f = 1 ÷ T: an 8 s swell has a frequency of 0.125 Hz, and 7.5 waves arrive each minute. Surfers and sailors care a lot about the period. Swell made by a distant storm has long periods, 10 to 20 s, long wavelengths, and moves fast; it arrives in smooth, powerful lines. A local wind chop has short periods of a few seconds and feels messy. The period is also the easiest number to measure: stand still, count the seconds between crests, and you have it, whatever the waves are made of.
A focused learning model
The swell is an ideal sine wave of one period, rolling in without breaking or losing height; the surfer only rises and falls. Heights are drawn larger than lengths. Time runs faster than real time by the factor shown, so each run lasts about 4 s; the clock shows the real time. The first crest reaches the surfer half a period after the start. Answers within 2% are right.