What Is The Difference Between High Tide & Low Tide?
When comparing the difference between high tide and low tide, we get two very different answers depending on whether you want the literal numerical interpretation, or whether you are looking for similarities and differences.
From a navigational perspective, however, the literal answer is the most important.
The tidal range is the difference between the tidal height at high tide and the tidal height at low tide in any given location. Areas with a large tidal range have a greater difference between their high water height and low water height than areas with a small tidal range.
The tidal range is the literal interpretation of the difference between high tide and low tide, but when we dive into it a little deeper we find that there are a number of other, non-literal, differences as well.
Some are particularly important from a navigational perspective.
| High Tide | Low Tide |
|---|---|
| Water level is higher | Water level is lower |
| Before high tide, water is flowing in (flooding) | Before low tide, water is flowing out (ebbing) |
| After high tide, water is flowing out (ebbing) | After low tide, water is flowing in (flooding) |
| Under-keel clearances are larger | Under-keel clearances are smaller |
| More of the land is covered by the sea | Less of the land is covered by the sea |
From a boater’s perspective, the biggest difference between high tide and low tide is the amount of water under your boat.
When the tide is high, you will have more water under your boat than you will at low tide.
This means that there are more places that you can safely navigate in your boat at any given draft.
The other major difference between high tide and low tide for boating is the different effects that it will have on currents.
You need to know whether the tide is flooding or ebbing to be able to understand the effect it will have on your boat handling.
The main exception to this is at slack water when you expect there to be no current running at all.
Read More: When Is Slack Tide?
Tidal Range
Tidal Range is the range over which the water level will change on any given tide.
| Time | Height |
|---|---|
| 0330 | 3.1m |
| 0933 | 1.2m |
| 1551 | 3.1m |
| 2159 | 1.3m |
The table above is a sample tide table for a random day in a fictional location.
To calculate the tidal range, you calculate the difference between the height of the tide at high water compared to the height of the tide at lot water.
| Tide | Type | Tidal Range |
|---|---|---|
| 0330 – 0933 | Ebb | 1.9m |
| 0933 – 1551 | Flood | 1.9m |
| 1551 – 2159 | Ebb | 1.8m |
In the above table, you can see that the tidal ranges have been calculated by working out the difference between the height of the tide at high water and low water.
Notice how the tidal range is not constant for every tide. It changes according to whether the tides are spring tides or neap tides.
Spring tides have larger tidal ranges, and neap tides have lower.
You can read all about spring tides and neap tides in my other article if you are interested: What Is The Difference Between Spring Tides & Neap Tides?
What is high tide?
High tide is when the height of tide above chart datum reaches a maximum before it starts falling again.
High tides are caused by the gravitational effects of the moon and sun pulling on the global body of water that makes up the world’s oceans.
As the body of water is pulled in one direction, it causes a high tide.
At high tide, the sea reaches high up the coastline, covering the inter-tidal areas on the foreshore.
What is low tide?
Low tide is when the height of tide above chart datum drops to a minimum before it starts rising again.
Low tides are caused when a high tide occurs elsewhere, pulling the water body away, by gravity, from a particular area.
At low tide, inter-tidal areas are left exposed, occasionally causing a “low tide smell”.
Read more: Why Does Low Tide Smell?
Similarities and differences between high tide and low tide
There are a number of similarities and differences that you will notice between high tide and low tide, some being more obvious than others.
Water level
The most obvious between high tide and low tide is the water level.
At high tide, the water level is at its maximum, and at low tide, it is at its minimum.
You’ll most likely notice it along a coastline, where low water will expose a lot of seaweed in inter-tidal areas.
Further out, the water level still changes but it may not be quite so noticeable unless you are on a boat or a ship and monitoring your under-keel clearance.
Currents
High water and low water usually coincide with the slack tide. This means that there is no current running as the water level is neither rising nor falling.
You do, however, need to be aware of whether you are at high water or low water because it will tell you what you expect to happen next.
At high water, you know that the tide will soon start to ebb, and you can expect the currents to start to increase away from the land.
Similarly, at low water, you know that the tide will soon start to floss, so you would expect the currents to start to flow towards the land.
Navigational channels
As a consequence of the change in water levels between high tide and low tide, navigational channels may be very different.
Depending on the tidal range in your location, at low tide, you might expect navigational channels to become smaller, as the water shrinks away from their edges.
Similarly, at high tide, you would expect the same channels to become larger, as the shallowing water at the edges is increased by the height of the tide.