Tides and Currents Questions and Answers

Answer: C

Correct answer: semidiurnal

The form factor is \( F = (K_1 + O_1)/(M_2 + S_2) \), comparing the diurnal constituents with the semidiurnal ones. Below 0.25 the regime is semidiurnal, from 0.25 to 1.5 mixed mainly semidiurnal, from 1.5 to 3 mixed mainly diurnal, and above 3 diurnal. A value of 0.1 is well inside the first band, so two nearly equal high waters a day.

Answer: D

Correct answer: the moon advances in its orbit while the Earth is rotating

A lunar day is the time between successive transits of the moon over the same meridian. In the twenty-four hours the Earth takes to turn once, the moon has moved about thirteen degrees along its orbit in the same direction, so the Earth must turn a little further to catch up. That extra rotation is what makes the lunar day the longer of the two.

Answer: A

Correct answer: 50 minutes

The moon advances roughly 13 degrees a day and the Earth turns 15 degrees an hour, so catching up takes about \( 13/15 \) of an hour, or some 50 minutes. This is why high water occurs about 50 minutes later each day, and why the two high waters of a semidiurnal regime are separated by 12 hours 25 minutes rather than 12.

Answer: B

Both are true and the reason gives the cause, and it is why tide tables list a different time each day. The pattern repeats approximately with the lunar rather than the solar cycle. Tidal times drift steadily through the calendar day.

Answer: C

Harmonic analysis decomposes an observed record into constituents of known astronomical period. Prediction then recombines them with the amplitude and phase determined for that port. Sixty or more constituents are used at a major port.

Answer: D

Correct answer: 12.42 hours

M2 is the principal lunar semidiurnal constituent, and its period is half a lunar day: half of 24 hours 50 minutes, or 12 hours 25 minutes, which is 12.42 hours. It is the largest constituent almost everywhere, which is why the interval between successive high waters is close to that figure at most ports.

Answer: A

Correct answer: the amplitude and phase of each constituent at that particular port

The periods of the constituents are fixed by astronomy and are the same everywhere; what differs from port to port is how strongly each responds and how much it is delayed, which depends on the shape and depth of the basin. Harmonic analysis extracts those two numbers per constituent from a record, and prediction then runs the sum forward indefinitely.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the difference. The tide is forced by the positions of the sun and moon, which are known centuries ahead, so once a port’s constituents have been determined its tides can be tabulated for any future date. Waves are forced by the weather, whose predictability runs out in days, so wave conditions can only be forecast.

Answer: C

The first two are correct and describe harmonic prediction. The third is not so, since surge is a meteorological effect that must be forecast separately and added to the astronomical tide. Weather cannot be predicted years in advance.

Answer: D

A year separates constituents whose periods are close together, including the annual ones. Shorter records support fewer constituents and give correspondingly poorer predictions. Nineteen years is needed to resolve the nodal cycle fully.