Understanding marine forecasts
New Zealand is an island nation, and many kiwis get a lot of pleasure, wellbeing and wealth from the oceans which surround us. As part of SeaWeek, we would like to help you better understand the marine forecasts which enable us to enjoy and explore our marine environment safely. Marine weather forecasts provide information about how the weather and ocean will look in the near future.
You can use the marine weather forecast to plan a weekend fishing, a sailing race, a surf trip, a swimming event or even a walk on the beach. A marine weather forecast can provide information such wind, waves, tides and the temperature of the ocean.
Wind
The wind forecast refers to speed and direction of the wind. Direction, such as north-west, is the direction the wind is expected to come from. Speed of the wind is normally measured in knots. One knot is 1.85 kph. Wind speed is not constant – it is changing all the time. The wind forecast also includes gusts, which is a brief increase in the strength of the wind, often only lasting a few seconds. Sailors use wind forecasts to plan their tactics in a sailing race to ensure they are the first ones over the finish line.
Waves
Snapshot of Raglan Entrance forecast from SwellMap.
A marine weather forecast will provide the sea state, which is the overall description of the ocean waves. It can include Sea (or wind sea) and one or more Swells.
Sea describes the waves or chop generated by local wind conditions. Sea waves are short crested and come from a range of angles. The sea wave height may change significantly in conjunction with changing wind conditions. In stronger winds, sea waves will whitecap and may become steep. If you are planning a day fishing, wind sea is a good variable to check, as choppy conditions make for a bumpy ride.
“I love to swim in the Raglan estuary,” says Aitana Forcén-Vázquez, MetOcean Solutions’ Business Development Manager. “But swimming in a place that is so influenced by the tides requires a bit of preparation. Our little MetOcean swim squad is always checking the tides to choose the best time to go swimming. When high tide is approaching, it’s time to head to the water. We do not want to be too late or we will be swimming against the current!"
Swell refers to waves that develop over a longer time (persistence), and over a larger area (fetch). Swell usually originates several kilometres away and can travel long distances. Swell typically has long organised crests, and if you are planning a surf trip along the coast this is the variable you want to check.
Period is the time between waves (between successive wave crests) in seconds. Swell waves have longer periods (time between waves passing by) than wind waves, which makes them more energetic.
When big swells arrive to the coast, they generate those beautiful surf waves that we see in Taranaki and Raglan. When there is swell and chop on the same day, you are more likely to experience sea sickness. Also check sea and swell directions, if they are opposing, boating conditions are more uncomfortable.
Tide
Tides are very long-period waves that causes the rise and fall in sea level resulted from gravitational attraction of the moon and the sun. When the highest part, or crest, of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. All around the New Zealand coastline the tidal regime is semi-diurnal, meaning that on most days two high and two low tides will occur at any given location.
A spring tide—also known as a "King Tide", occurs during full or new moons, when the Earth, sun, and moon are nearly in alignment. This means that high tides are a higher and low tides are a lower than average.
This week is New Zealand Association for Environmental Education (NZAEE) SeaWeek – Kaupapa Moana - New Zealand’s annual national week celebrating the sea. Seaweek 2022 is about “Connecting with our seas, Toi Moana ~ Toi Tangata”. Check out https://www.seaweek.org.nz/ for more information.