Why big, deep lakes stay cold below the surface

It's a hot afternoon, the lake is glassy, and the first few strokes feel lovely. Then you swim out a little further, your legs drop, and the water down there feels like a different season. You didn't imagine it. Deep lakes keep a store of cold water through the summer, and it can sit closer to the surface than you'd expect.

This guide explains why, and how to size up a lake before you swim.

Sizing up a lake

Ask two questions about every lake. The deeper and bigger the lake, the colder it will be, even in the summer months.

A warm day on the bank doesn't tell you what's going on under the surface. To see why, you need to look at how a lake warms.

Layers: what's going on beneath you

Lakes are heated from the top. In their review of lake stratification, Boehrer and Schultze explain that the surface takes the sun and the warmth of the air, while deeper water is shielded from those sources of heat. Left to itself, heat creeps downward through still water very slowly: they put it at around a month to move heat one metre by diffusion alone. Wind is what really mixes a lake, and wind can only stir the warm water down so far.

Once the surface warms in spring, the lake splits into layers. There's a warm, mixed top layer (the epilimnion), a colder layer below that the wind doesn't mix in (the hypolimnion), and a sharp change in temperature between them, the thermocline. In a lake that's deep enough, those layers hold all summer. The cold bottom layer is cut off from the air until cooler autumn and winter weather mixes the whole lake again.

The Outdoor Swimming Society (OSS) gives swimmers the numbers. It describes the thermocline as narrow, around a metre deep, and puts the water in the depths at around 4 to 7°C, while the surface in a warm UK summer can be 18 to 24°C. The warm layer can be anywhere from one to twenty metres deep.

That's the danger in a deep lake: warm enough on top to feel inviting, with very cold water not far below.

Where size comes in

Size works a little differently from depth, and the research is more nuanced than bigger simply meaning colder.

Boehrer and Schultze compared lakes in the same climate and found their daily average surface temperatures were very similar. The differences show up in two places. In spring, smaller lakes tend to gain heat faster. And in hot spells, smaller lakes peak a degree or two higher. Further down, at seven metres, the temperatures differ much more from lake to lake, depending on how each one mixes.

The OSS adds that on larger bodies of water the warm top layer tends to be deeper, because stronger wind mixes the heat further down.

That's a reason for caution in big lakes, particularly early in the season, when a big lake is slower to warm. In high summer the surface of a big lake may feel much like a small one. The cold is still there, though, below the thermocline.

Shallow lakes are the opposite case. Boehrer and Schultze describe lakes that aren't deep enough to form a cold bottom layer at all, so the wind mixes them from top to bottom. That has its own trade-offs, covered in small, shallow lakes.

What it means for your swim

Getting in. RoSPA describes cold water shock as a response to entering water at 15°C or lower that can leave you struggling to breathe and to swim. The OSS advises getting into cold water slowly and controlling your breathing first. If you haven't read it yet, cold water shock explains what happens in that first minute.

Swimming out. The RLSS advises swimming parallel to the shore, at a depth where you can stand, and keeping away from deeper water because it will be colder. The OSS also warns about sudden changes in depth, which is exactly what a big drop-off is.

Crossing. The OSS advises swimming along the bank rather than across larger lakes, so you can get out as soon as you feel cold. It notes that even strong swimmers get into trouble when they're a long way from safety.

Staying in. Cold works on your muscles as well as your breathing. The OSS warns that cold incapacitation can affect fit, expert swimmers too, so get out as soon as you start to feel cold, and know the signs of hypothermia.

Reservoirs follow the same physics. The OSS notes that in summer the warmer top layer of a reservoir may be shallow. There's more on reservoirs, and why so few allow swimming, in reservoirs and quarries.

What Wild's water temperature tells you

Wild estimates lake water temperature from published models and shows it as a band, such as cool or cold, rather than an exact reading. It's a guide to the water near the surface, and it can't tell you how deep the warm layer goes or how cold the water is underneath.

Use it alongside the depth of the lake, recent reports from swimmers who've been in, and your own feel of the water before you commit. The OSS advice: feel the water and judge it for yourself before getting in.

For everything else to check before a lake swim, see safe lake swimming: the complete guide.

Sources

1. Reviews of Geophysics (Boehrer and Schultze, 2008), Stratification of lakes
2. Outdoor Swimming Society, Understanding Thermoclines
3. Outdoor Swimming Society, Understanding Reservoirs
4. Outdoor Swimming Society, OSS Intermediate Tips for Safe Summer Swimming
5. Royal Life Saving Society UK, Top Recommendations for Safer Open Water Swimming
6. RoSPA, Open water swimming

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.