tide is the vertical motion of water
As the earth rotates on its axis, the changing gravitational pull from the moon powers two giant waves flowing around the coast of the United Kingdom. The distance between the peak and trough of these waves is just about 600 kilometers. When the peak reaches a beach it is high tide, and when the trough reaches a beach it is low tide. It takes around 6 hours 12 minutes and 30 seconds for the peak to reach the beach after the trough; and this is the time between low and high tides.
The two waves begin their course around the British isles at Lands End. One wave travels north to the west coast of Scotland, over and around the tip of Scotland, then on down the east coast. The second wave travels east, up the English Channel and the two waves meet just outside London at the Thames Estuary. When there is a trough (low tide) at the Thames Estuary, there is also a low tide in north Wales and north-east Scotland (i.e. Aberdeen). At the same time, there is a peak (high tide) along the west coast of Scotland and Yorkshire.
HOW TO SPOT TIDE
It is extremely difficult to spot tide in just a few minutes because the water moves insidiously to the naked eye. However, there are some clues that can help you in the absence of Google or tide tables.
Look out for...
line of detritus.
The high tide often leaves a visible line of seaweed, shells, driftwood and (unfortunately) plastic. Bear this in mind when planning beach cleans, a low or receding tide will be best!
The sand above the high tide will be rough and for the most part dry, whereas the sand in the intertidal zone is washed smooth by the receding tide.
If you are near piers, harbours, seawalls, or cliffs: look out for marine organisms and seaweeds - the high tide is the highest point these grow at and the surface above is almost always lighter.
WHAT IS A TIDAL DAY
There are some key timings for a semi diurnal tidal cycle (when there are two troughs and peaks a day); there is 06:12:30 between high and low tides, 12:25:00 between highs, and 24:50:00 for a full cycle. The hours represent quarter, half and full rotations of the earth and the minutes represent the simultaneous orbit of the moon. To make sense of this, you can take a step back and look at the United Kingdom from a global level.
So, first off, what makes the tide change? If you think of an imaginary line through the centres of the earth and the moon; the positions A and C experience the strongest gravitational pull – where the sea bulges out of the seabed and forms a high tide. On the other hand, positions B and C have the weakest gravitational pull – where the sea flattens against the seabed and forms a low tide. If these bulges stay in position relative to the moon, whilst the earth spins on its axis, the effect is a giant wave with two peaks and two troughs flowing around the globe.
If the United Kingdom is at position A at midnight (00:00:00) it would get to position B at 6 am and move from high to low tides. Fast forward another 6 hours, and at midday the United Kingdom will have another high tide. At 6pm the United Kingdom will have another low tide, and at midnight there will be another high tide. Or at least that would be the case if the moon stayed still whilst the earth completed one full rotation on its axis.
By the time the earth has spun around once, the moon has moved from its original position, to position X – and this means the earth has to spin for another 50 minutes before it can be realigned with the moon. This means that it takes 25 minutes to realign after a half rotation; which is why there are 12 hours and 25 minutes between high tides.
These timings are consistent in a semi diurnal cycle, but there is one main issue in this theory – land. If there were no obstacles (e.g. the United Kingdom and all other land masses) and the sea flowed over a completely smooth seabed, the tidal wave would flow as described above. However, because we have irregular coastlines and seabeds that interrupt the journey of the tidal wave, each continent and island have their own unique tides and waves.
Many parts of the Pacific experience a phenomenon called a mixed semi diurnal tide. The timings are the same as a normal semi diurnal tide, but the heights of these two tides differ immensely. Rarer still is a diurnal tide, which has one high and low tide in a lunar day - these can be found in the Gulf of Mexico, Antarctica and Western Australia.
WHAT IS A TIDAL MONTH
While the moon is arguably the most important factor in a tidal day; the sun is the most important factor in a tidal month – and this all depends on where the moon is on its 29.7 day orbit of the earth.
When the moon and sun are aligned with the earth together, the combined gravitational pull of the moon and sun is stronger, causing a greater tidal range. This means the high tides can be exceptionally high, and the low tides can be exceptionally low. These tides are called spring tides (confusingly they have nothing to do with the season of spring and refer to tides that “spring” forth with power), and happen twice a month – just after full and new moons. In springs the exceptionally low tides mean that a greater expanse of sand is exposed for longer – in some places beaches can double in size; allowing for huge beach parties to take place i.e. full moon parties.
A week after spring tides and a week before, we have neap tides. Neap (meaning without power). These neap tides happen when the moon is perpendicular to the alignment of the sun and earth’s alignment, in it's 1/4 and 3/4 phases, so the combined pull is weakest. Neap tides have a low tidal range, which means that difference between low and high tides is less pronounced.
If you look at a tide table, or better yet a tidal graph, you can decipher whether you are going from springs to neaps, or from neaps to springs. Look at the heights of high and low tides over a few days – if the numbers are going towards each other you are going from springs to neaps; and if the numbers are going away from each other you are going from neaps to springs. On a daily basis if the second high tide is higher than the first you are going from neaps to springs, and if the second high tide is lower than the first you are going from springs to neaps.
You can also work it out by looking at the moon. If the moon is full (you can see the whole moon) or new (you can't see any moon) then it will be a spring tide. If you can only see one side of the moon it will be neaps. In the northern hemisphere, if the right half is visible the moon is in 1/4 phase and the full moon will be the following week. If you can see the left half of the moon, it is in 3/4 phase and the new moon will be the following week. The opposite is true for the southern hemisphere. Whilst the exact times of high and low tides vary with location, the world experiences neap and high tides at the same time. You don't have to wait till darkness to see the moon, in between 1/4 and 3/4 moons, the moon is on the opposite side of the earth to the sun, so you can often see it during the day.
Because a daily tidal cycle is 24:50:00, high tide is fifty minutes later each day – this equates to six hours later each week. This also means that each spring and neap high tide is around the same time every tidal month. For Aberdeen this means on springs, high tides are almost always around 1.30am/pm (+- 50 minutes), and on neaps, high tides are almost always around 7.30am/pm (+- 50 minutes).
With this knowledge, you should be able to roughly work out the time of high tide by knowing the phase of the moon. Below we’ve added the spring and neap times (am/pm +- 50 minutes) for some of our most used surf spots:
Springs – 12
Neaps – 6
Springs – 1
Neaps – 7
Springs – 12 .30
Neaps – 6.30
Springs – 5
Neaps – 11
WHAT IS A TIDAL YEAR
As discussed, tides have daily and monthly cycles - but they also follow an annual cycle too. This is decided by the position of the earth in relation to the sun. During equinoxes, in March and September, the sun lies directly over the equator and so its gravitational pull is the greatest. During solstices, in December and June, the sun's gravitational pull on the ocean is weakest. If an equinox coincides with the moon also being directly over the equator, and closest to the earth in its orbit, especially high tides can occur. These especially high tides occur every 4.5 years.
Simple weather conditions can have a larger effect than equinoxes on our tides. Low air pressure and onshore winds during spring tides can create higher tides than any equinox.
Although the effect of equinox on tides is limited by many factors, it has a profound effect on our lives and the adventures we can pursue. In the northern hemisphere, the March equinox represents the start of spring and the introduction of warmer weather patterns. In the southern hemisphere, the March equinox represents the beginning of winter. As you move away from the equator the transition becomes more and more dramatic; this is seen at the Poles where it is either day or night for 24 hours a day at each solstice.
wHAT IS TIDAL rANGE
Tidal range is the difference in height between high and low tides; the bigger the number the larger the tide. The Mediterranean Sea has some of the world's smallest tides because of the Strait of Gibraltar. The strait is a mere 14 kilometres wide, and separates Africa and Europe, and also the Atlantic Ocean from the Mediterranean Sea. The strait is so narrow that the Atlantic tide wave cannot squeeze through the opening, creating extraordinarily weak tides in the Mediterranean Sea. In some parts of the Mediterranean the tide will rise by as little as 30 cm over six hours - in the Bay of Fundy in Canada the tide can rise by 30 cm in six minutes!
Although the tidal range in the Mediterranean Sea is so small it is essentially zero, there are places around the world where the tidal range is actually zero. These places are known as amphidromic points or tidal nodes. We have already talked about how there is a single tidal wave flowing around the world - but in practice there are several tidal waves within each ocean that rotate around amphidromic points. This sounds very complicated, and if the following analogy doesn't make it click, feel free to forget we even mentioned it.
Imagine you are coordinated enough to spin a CD on your finger tip. Now imagine you tilt the CD so it is spinning at a slant on your finger. The highest point is the high tide, the lowest point is the low tide and the central hole is the amphidromic point.
Amphidromic points are nearly always found in the ocean. One exception is New Zealand. Although tidal ranges can be high throughout the shores of New Zealand, one tidal wave is constantly flowing around North and South islands, taking 12 hours and 25 minutes to make an anticlockwise circumnavigation of the country. This is another exceptional fact about the tidal node of New Zealand - the wave flows anticlockwise. In amphidromic points out at sea, the tidal waves will flow anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. This is all down to the Coriolis effect, which is (simply put) when the rotation of the earth on its axis causes the ocean currents to rotate on their axis (tidal nodes). In the west coast of North America the tidal wave crashes to shore in California and continues up the continent to Alaska, before roaming anticlockwise around the pacific and returning to California. In the west coast of South America, the tidal wave travels down the coast of Chile in its clockwise rotation.
To figure out what direction the tidal wave is in your area, look at high tide times for beaches on your left and right - the one where high tide is first indicates the direction the wave is travelling from.
WHAT EFFECTS TIDAL HEIGHT
High and low tides are not always the height predicted in the tide table from the positions of the earth, moon and sun and their alignments. This is because the current weather conditions of the day can have a huge impact on the tide. The two main variables of this are air pressure and wind.
A low air pressure system and onshore winds will combine to create exceptionally high tides.
Low air pressure occurs when warm air rises, which decreases the pressure on the surface of the sea. A single milibar drop in air pressure can raise the local sea level by 1 centimetre. So what does a low pressure air system look like? Well, it’s pretty easy to spot – think atrocious weather and you’re pretty much there. As the air rises it also cools down, forming clouds that create wet and windy conditions.
A high air pressure system and offshore winds will combine to create exceptionally low tides.
High air pressure occurs when cold air sinks and exerts a greater force on the sea (squishing it to the seabed), which prevents the tide rising to its expected height. The weather conditions of high air pressure systems are also pretty easy to spot – think blue skies and low/absent winds i.e. a perfect summer day. If a high air pressure system happens during springs (a full or new moon), there will be extremely low tides – an offshore wind will lower the tide even more.