TIDE

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...

a 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!

sand texture.

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. 

vertical structures.

 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.


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, 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.

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. 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:

Thurso:
Springs – 12
Neaps – 6
Peterhead:
Springs – 1
Neaps – 7
Fraserburgh:
Springs – 12 .30
Neaps – 6.30
Newquay:
Springs – 5
Neaps – 11

HOW DO WIND & AIR PRESSURE AFFECT TIDE

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.