Why September swims better than June
Water has an enormous heat capacity, so the sea runs about six weeks behind the air. Almost every swimming decision follows from that one fact.
At a glance
- The lag
- About six weeks behind air temperature
- Mediterranean peak
- Late August into September, not July
- Comfortable swimming
- From about 22 degrees for most people
- What it means
- September beats June by four or five degrees
Water takes roughly four times as much energy to warm as air does, and the sea is very deep. So a coastline heats slowly through spring and cools slowly through autumn, and the peak sea temperature arrives weeks after the peak air temperature. It is a single physical fact and it reorganises the travel calendar around it.
The size of the effect
On a typical northern Mediterranean coast, air temperature peaks in late July and the sea peaks in late August. In June the air can be 27 degrees and the sea 20, which is bracing rather than pleasant. In September the air is 26 and the sea is 24 or 25, which is the best swimming of the year.
The comparison people make in the shop is against the air temperature in the forecast, which is why so many holidays are booked into the wrong half of the summer. Both months look identical on a weather app. They are four or five degrees apart in the only measurement that matters if you intend to be in the water.
The comfort thresholds
- Below 18 degrees: cold water swimming, exhilarating, brief, not what most people booked.
- 18 to 21: swimmable with commitment, and it stops being fun when you get out.
- 22 to 24: comfortable for most people, the point at which you stay in.
- 25 and above: warm, effortless, and the water stops cooling you at all, which in real heat becomes a disadvantage.
Where the rule bends
Shallow, enclosed water heats and cools far faster than open coast, so a lagoon, a bay or a shallow shelf can be pleasant weeks before the sea nearby. Upwelling reverses the rule entirely along certain coasts, notably western Portugal and parts of the Canary Islands, where cold water rises from depth and keeps the sea at 18 or 19 degrees in August while the air is at 30.
Large lakes follow the same physics with a shorter lag, usually three to four weeks, because they are shallower. Alpine lakes peak in August and are genuinely cold in June whatever the valley temperature says.
Check the sea temperature, not the air temperature, for any trip whose point is the water.
The other end of the lag
The same physics makes early summer excellent for everything that is not swimming. Late May and June give the most comfortable air temperatures for walking, the longest days, the landscape before it burns off, and a sea that keeps the coast cooler than it will be in August. Anyone going to walk, cycle, look at things or eat outdoors is better off in June, and anyone going to swim is better off in September.
Which resolves the argument about which is the better month. They are better months for different holidays, and the sea temperature is how you tell which one you are taking.
Spotted an error? Tell the desk