For Seeking Heat Capacity
We can all tell when the cold season is upon us. The outside air temperatures drop, yet at the same time the ocean is much milder, even feeling like a warm spa depending on the discrepancy between the ambient air temperature and the water.
The difference can be so stark that, in some locations, ocean temperatures can be double that of the air outside.
While most surfers are familiar with this phenomena, you may not know why there is a difference in temperature. So today we ask the question: Why doesn't the water temperature drop at a similar rate to the air when transitioning from summer to winter.
The answer comes down to the heat capacity of various substances and materials. Heat Capacity is the amount of energy needed to raise one gram of material 1° Celsius under constant pressure.
Water has the highest Heat Capacity of all common substances on Earth, meaning it requires more energy to change its temperature by 1° compared to anything else. Whether that be an increase in temperature or drop in temperature, the energy needed for this is greater than any other substance.
This makes water a great thermal buffer and herein lies the answer to the question. Air on the other hand has a much lower Heat Capacity requiring less energy to increase or decrease its temperature.
Looking at specifics, the Heat Capacity of air is 1,000 joules/kilogram, while water is 4,179 joules/kilogram. Adding salt (to make sea water) lowers the Heat Capacity, though its influence is minimal and only drops the specific Heat Capacity to ~ 4,000 joules/kilogram
So when comparing the Heat Capacity of air to sea water we see that it takes four times the amount of energy to heat the ocean 1° compared to air.
Coming out of winter the water has usually reached its low point in temperature and as the air and atmosphere start to warm due to increased solar energy, we see a lag in the heating of the ocean owing to this difference in Specific Heat Capacity. The solar energy is heating the air four times quicker.
Conversely, come late summer/early autumn a peak in ocean temperature is usually seen across most regions.
Once the solar energy reduces into autumn and winter we see the atmosphere dropping in temperature quicker than that of the ocean, owing the fact that the atmosphere will lose its heat four times faster than that of the ocean.
Now there are other nuances that come into play, such as global circulation of currents as well as upwelling 'hot spots' which can experience some of Australia's coldest waters in summer. The Mid North Coast under sustained north-easters and the South East region of South Australia under strong south-east winds are a couple of great examples.
Then there's the East Australian and Leeuwin Currents, which are both major transporters of heat from the northern climes. This southward flow of tropical water creates an even stronger lag between outside temperatures and the ocean temperatures with relatively warm water seen well into winter.
One final factor to consider is also the depth of the water - i.e the shallower the water basin, the quicker it will react to solar heating and cooling owing to there being less of it to heat/cool. This is why Port Phillip Bay gets colder and warmer than Bass Strait. Similarly the South Australian gulfs are cooler/warmer at their peaks compared to the nearby Southern Ocean.
// CRAIG BROKENSHA