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Cold water shock: be prepared before boating

Falling into cold water (under 15 °C) can have extreme effects on your body and its ability to function—and these can kick in much faster than you think. If you participate in water activities you need to be prepared for these risks all year round.

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How dangerous is cold water?

Falling into cold water unexpectedly (such as when a boat capsizes) can be life-threatening. Impacts on the body begin immediately. Studies show up to 60 per cent of fatalities occur in the first 15 minutes.¹

The 1:10:1 principle describes three phases of effects on the body.

• One minute—cold shock response: Falling into cold water can trigger an immediate 'cold shock response' which includes increased heart rate and a sharp, involuntary inhalation or gasp.
• Ten minutes—cold incapacitation: The water can numb extremities and weaken strength and coordination, making it difficult to swim, wave, or grab a throw ring.
• One hour—hypothermia: When body temperature drops below 35 °C, hypothermia (dangerously low body temperature) sets in, which can be fatal without quick rescue and treatment.

The water temperature considered to be 'cold' depends on the specific circumstances but can generally be defined as lower than 15 °C. A person's survival time will depend on their physical condition, clothing, access to flotation aids, and other weather and wave conditions.

How can you be prepared?

Good preparation is essential for activities where falling into the water may be a risk. The Australian and New Zealand Safe Boating Education Group recommends that boaters wear a life jacket and appropriate clothing, learn rescue techniques and first aid, and carry safety equipment. Find out about precautions and first aid for hypothermia.

The five vital weather safety checks will help you prepare for or avoid conditions that could increase the risk of falling into the water.

What influences coastal water temperatures?

Cold water can be a risk in many waterbodies including lakes, dams, and rivers. This section focuses on the factors influencing sea surface temperatures in Australia's coastal waters.

Latitude

Sea surface temperatures depend strongly on the angle at which the sun's rays strike the Earth's surface and are generally warmest at the equator and gradually cool toward the poles.

Image: Sea surface temperatures shown in MetEye

Time of year and day

Sea surface temperatures change throughout the year due to changes in the amount of solar radiation reaching the ocean surface. The ocean warms more slowly than the land so there is a time-lag between changes in the air temperature and water temperature (usually around three months, depending on location). So the ocean can still feel quite cold during early summer. The lowest sea surface temperatures are usually observed in early spring and the highest in early autumn.

Temperatures also vary slightly during the day and tend to be highest in the late afternoon. However, cloudiness may moderate water temperatures by reducing the amount of direct heating.

Water depth

In shallow, calm waters, the sun warms the water quite efficiently. However, beyond the surf zone, due to the deeper waters and wave action, the sun’s radiation doesn’t provide any noticeable warming effects.

Wind and water mixing

Periods of sustained winds can move warmer or colder water towards the coastline. Under the surface, upwelling and downwelling circulate water of different temperatures. An example of this is the Ekman transport effect—a wind-driven process that brings colder water to the coastline.

Tides can influence sea temperatures in the short term by moving warmer or colder water into and out of estuaries and bays. During high tides, cooler marine water mixes into warmer coastal areas. At low tide, rivers and streams flowing into the bays and estuaries can have a greater influence. Further, at low tide, water temperatures may rise as the water becomes shallower.

Currents

Australia's sea temperatures are influenced by ocean and coastal currents including:

• the East Australian Current which transports warm tropical waters from the Coral Sea southwards to interact with the cooler waters of the Tasman sea; and
• the Leeuwin Current which sweeps down Australia's west coast, bringing warm tropical water to Western Australia.

Find out more about ocean currents.

Image: Forecast sea surface temperature and currents

River run-off, rainfall or snow melt

Waters close to the coast can be affected by changes in the amount of fresh water flowing into the ocean, and the extent to which this is mixed by winds or tides. Coastal waters near river entrances can experience cooling due to the inflow of colder water after heavy rain or snow melt.

More information

You can find observed sea surface temperatures in our online mapping tool MetEye. These observations are derived automatically from temperature sensors in the ocean, which are blended with measurements from meteorological satellites. Because temperatures are averaged over a large area, the sea surface temperatures shown in MetEye and other maps may differ slightly from what you experience at the beach.

• Forecasts for sea surface temperatures (and currents)
• Marine safety issue: cold water (NSW Centre for Maritime Safety)
• 1. MAST 2014; Maritime Safety Victoria 2015