RNLI guide to weather and waves for swimmers

September 01, 2021

Along with the news, the weather is typically reported on every hour of the day. Nowadays it’s usually a permanent feature on the home screen of mobile phones and possibly one of the most frequent topics of conversation in Ireland… We love to talk about the weather!

Sources for weather forecasts

Before you head out for your swim always check a current weather update. There are many sources available with varying degrees of detail, but here are just a few examples:

  • Met Eireann – www.met.ie – Sea Area Forecast
  • RTE – TV and Radio
  • Local radio stations – often a more local forecast
  • Internet/Apps – e.g. Windy, Windguru, Magicseaweed
  • Local Noticeboards – harbour office, clubs, etc

Understanding weather forecasts

A weather forecast is a prediction on the conditions of the atmosphere at a given location and time, using a combination of technology and science.

A forecast will generally tell you the air temperature, wind direction and wind speed, and the type of precipitation, if any.

It’s important to note that wind direction refers to the source the wind is coming from not travelling to.

The weather forecast will also tell you of any changes due, and when they are expected.

The Met Eireann website also contains useful information to help you understand the terminology used in weather forecasts.

How weather can affect your swim

Temperature – The average winter sea temperature around Ireland is 6°C, rising to around 15°C or 16°C in summer.

Cold water shock, which can seriously affect your breathing and movement, is a concern in water colder than 15°C. Therefore, it is a present risk for most of the year.

To avoid the effects of this you should enter the water slowly and acclimatise. Swimming regularly can help your body acclimatise quicker and reduces the effects of cold water shock on the body.

The air temperature should help guide you on what clothing and accessories you might need to ensure you stay warm before and after your swim and to avoid hypothermia.

Visibility – This can be in reference to both sea and air conditions.

If there is poor visibility in the water; are their hidden hazards or rocks which might cause you harm? Can you easily see your entry and exit points e.g. steps or ladders?

Poor visibility above the water might be caused by precipitation, mist or fog and distort the direction you think you’re swimming in, or your distance from the shore/land.

It’s very important to make yourself visible in the water with a bright swim hat and tow float, but it’s even more important when conditions are less favourable.

Wind – waves are caused by wind. While waves can be fun they can also be very powerful and dangerous and pull you out to sea or tire you quickly.

Where you have an option of swimming locations it’s advisable to swim at the most sheltered location for your safety.

Understand waves

Powerful breaking waves have the potential to bring out the big kid in all of us. They are one of the most exciting and impressive features of our coastline and they are the primary force shaping coastal change.

It’s important for anyone who visits the coast to know the basics about waves so that they can keep themselves and others safe.

Waves are formed by friction when the wind blows across the surface of the sea, causing a swell as water particles rotate and move forwards. They can also be caused by seismic activity and boats moving in the water.

The movement of a wave up the beach is known as the swash, its movement down the beach is known as the backwash.

Depending on which is stronger, waves can be either constructive or destructive. Remember when entering or exiting the water, if the backwash is strong it can knock you off your feet.

Wave size and power

The size and power of a wave is influenced by three main factors:

  • How strong the wind is
  • How long it has been blowing
  • How far the wave has travelled (known as the fetch).

How steeply a beach slopes or shelves, and the topography of the sea bed near the beach, will also affect the size and type of wave.

Types of waves

Spilling waves

Spilling waves are softer and more consistent waves that break gradually as they approach the shore and are generally found on fairly flat beaches.

Dumping waves

Dumping waves break powerfully in shallow water and should be avoided. They most commonly occur at low tide and break quickly with a lot of force making them more dangerous. These can be found on steep shelving beaches.

Surging waves

When a wave breaks it loses some of its power and momentum. Watch out for surging waves - they don’t break, so they can knock you off your feet more easily and drag you into deeper water. Generally, these are found on very steep or shelving beaches.

Reading wave forecasts for swimming

Lots of apps and website - like Windguru, Met.ie, and Magicseaweed - share forecasting data relating to waves, as well as wind.

This advice is brought to you by the RNLI, whose volunteer lifeboat crews save lives at sea 365 days a year.