Tips for understanding how weather and tides affect sea conditions

This is general advice based on our own experience and research. Any suggestions or corrections can be sent to swim@galwaytri.ie.

Always check the weather forecast and tide before you go swimming and understand what it means/meant for your swim. Think about how you feel after each swim and use this knowledge to predict how your next swim will go!

Weather

Useful websites/apps:

Keep in mind that Ireland is a tiny Island on one side of a massive ocean, so weather can change unexpectedly and forecasts are not always correct. Use your own best judgement before getting into the water no matter what the internet is telling you!

Wind speed has a huge effect on sea conditions. General rules of wind speeds:

  • Up to 3 metres per second (3 m/s) means calm conditions
  • 3-6 m/s will be choppy and sighting will be more difficult
  • Greater than 6 m/s means sea will be rough and potentially dangerous

However….wind direction also plays a part and affects different swimming spots differently, e.g. For Blackrock Salthill

  • Winds coming from northerly and easterly directions give the calmest conditions as the bay is generally sheltered from these. Even moderate to fresh breezes (3-6 m/s) have little effect on waves if the wind is coming directly from the North
  • Southerly and westerly winds have the biggest effect on conditions in Salthill. Water will be “bouncy” if even a very light breeze (1-3 m/s) is coming directly from the South

Water temperature fluctuates with, and lags behind, changes in air temperature, rainfall and wind. Today’s sunshine does not necessarily mean the water will be warm. Low air temperatures and/or high rainfall on previous days will affect your swim today. Winds from the east generally bring cooler (but clearer and less salty!) water to Salthill.

Tides

Understand your tides. There’s more to it than just knowing if it’s in or out!

Useful websites/apps:

Tides take 6 hours to come in and 6 hours to go out (approx.) but tide times change every day and are unique to every location.

Tide heights also change daily according to the lunar cycle (approx. 1 month). There are two extremes:

  • Spring tides
  • Neap tides

Spring tides happen roughly every two weeks throughout the year (at new and full moon) regardless of season.
Spring tides are when the highs are highest and the lows are lowest so the difference between high and low tide is at its greatest. This means a larger volume of water moves during Spring tides and give rise to stronger currents and potentially more dangerous conditions.

Neap tide occurs approx. 7 days after a Spring tide and during this the difference between high and low tide is at its smallest and currents are relatively weaker.

Regardless of day or type of tide, currents are always stronger mid-tide (i.e. 2-4 hours after high or low tide) as this is when the most amount of water is moving during that tide cycle.

Swell (wave height) is biggest when tide and wind are in opposite directions (e.g. in Salthill wind coming from south while tide is going out will generally give the biggest swells).

Combining all this knowledge, an example of the absolute best/worst times to swim in Blackrock Salthill are:

  • Best time: At either high or low tide during a neap tide with a 1-3 m/s northerly wind
  • Worst time: 2-4 hours after high tide during a spring tide with greater than 6 m/s southerly wind

…..but there are many variations in between these two extremes! Being more aware of weather and tides when you’re swimming will help you get the most out of your training and keep you safe.

Useful resources: