Why Do Ponds and Lakes Freeze, but the Sea Hardly Ever Does?
― Chilled seawater doesn't stay at the surface. It sinks.
"Because the sea is salty" is only half the answer. Salt lowers the freezing point by just about 2°C. What really matters is that chilled seawater becomes heavier and sinks. So the sea can't start freezing until it has cooled down together, not just at the surface but deep below it.
On a midwinter morning, a thin layer of ice covers the pond in the park. A puddle in a nearby rice field has frozen white too.
Later that day you go to the seaside. The waves are rolling in as usual. The air is just as far below freezing, yet there is no ice anywhere.
If a child asked you, "Why doesn't the sea freeze?", what would you say? If you stop at "because it's salty", you leave out the more important half.
There are only two reasons the sea resists freezing
When something is dissolved in water, the water freezes less easily. For ordinary seawater, freezing starts not at 0°C but at about −1.9°C.
In a pond, chilled water stays on top and freezes there. In the sea, chilled water becomes heavy and sinks, swapping places with the water below. For the sea to freeze, the whole mixed layer has to cool right through.
The first reason is well known. But a difference of 2°C alone would mean the seas in cold regions freeze easily every winter. The second reason is the one that does most of the work.
A pond freezes from the "lid"; the sea must wait until the "whole pot" is cold
Fresh water has an odd property. As you cool it, it is heaviest at 4°C. Cool it further and it actually gets lighter.
In a winter pond, the whole body of water first cools to around 4°C. After that, when the surface water cools further, it is lighter, so it doesn't sink. A thin cold layer forms on top, and only that layer reaches 0°C and freezes. Only the "lid" has to freeze.
Add salt, though, and the story changes. The more salt there is, the lower the temperature of greatest density falls. Once the salinity goes above about 2.5%, that temperature drops below the freezing point. Ordinary seawater is about 3.5% salt, well past this boundary.
So seawater keeps getting heavier as it cools, right up to the moment it freezes. Water chilled at the surface sinks at once, and slightly warmer water rises from below. Move the slider in Figure 1 and you can see where this swapping begins and ends.
What happens before the sea freezes?
When cold wind chills the sea surface, the chilled seawater sinks and water from below rises. This swapping continues down to a depth where salinity or temperature changes sharply. The layer above that depth is called the "mixed layer", and it is said to range from tens to hundreds of metres thick, depending on place and season.
For a pond, only a thin surface layer had to be cooled. In the sea, ice can't form at the surface until the water down to 100 metres has been cooled to −1.9°C. The amount of water to be cooled is vastly greater. Figure 2 sets the pond and the sea side by side.
Even so, the sea does freeze in the Arctic and Antarctic, and along the coast of the Sea of Okhotsk in Hokkaido. In the Sea of Okhotsk, fresh water from large rivers forms a low-salinity layer at the surface. Salinity rises sharply beneath this thin layer, so the mixing depth is thought to be kept shallow. Because there is less water to cool, the Hokkaido coast is said to be one of the southernmost places in the Northern Hemisphere where drift ice is seen.
When seawater freezes, only the water molecules line up first and become ice, and most of the salt is pushed out. So sea ice (called "kaihyo" in Japanese) is much less salty than the seawater it came from. The dense, salty water pushed out is heavy and sinks. This is thought to be one of the forces that drives the deep ocean currents of the whole planet.
Summary
The sea resists freezing for two reasons: salt lowers the freezing point to about −1.9°C, and chilled seawater sinks. A pond freezes once just a thin surface layer is cold. But the sea can't form surface ice until the whole layer mixed down to great depth has been cooled. "Because it's salty" is the doorway. The real answer is "because chilled water sinks".
A pond freezes once only its lid is cold.
The sea can't freeze until the whole pot is cold.
For the seasonal motions that follow from fresh water being heaviest at 4°C, see "Why does lake water turn over, top to bottom, in autumn?". For how salt lowers the freezing point, see "Why does salt on ice make it colder than 0°C?". And for why the sea is salty, try "Why is the sea salty?".
- Take two cups of the same shape. Put plain water in one, and 200 mL of water plus a little over 1 teaspoon of salt (about 7 g, about the same strength as seawater) in the other.
- Put both in the freezer side by side and check the surface every 30 minutes. The fresh water should start freezing from the surface first.
- Once the salt water has frozen, lick a little of the half-melted ice. The first water to melt out is salty, and the ice that's left may taste less salty than you'd expect.
A cup is shallow, so it can't reproduce the deep mixing of the sea. What you can check are two things: that the freezing point drops, and that salt is pushed out of the ice. When you taste anything from the freezer, use clean cups and edible salt.
For those who want to know more ― terms, formulas and links to textbooksWe mark which level each part belongs to, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school chemistry or earth science
- HS+Advanced high school material, or textbook sidebar content
- UniversityUniversity specialist subjects (physical oceanography, physical chemistry) not taught in high school
- ResearchTopics researchers are still investigating, not taught even at university as settled fact
Junior highTerms: this phenomenon has names
- Freezing-point depression: the effect where dissolving something in water lowers the temperature at which it starts to freeze (the freezing point).
- Temperature of maximum density: the temperature at which water is heaviest (densest). About 4°C for fresh water.
- Mixed layer: the layer from the sea surface down to the depth where sinking cold water and waves keep the water well stirred.
- Sea ice (kaihyo): ice formed by freezing seawater. Pieces that drift away from the shore are called drift ice.
Junior highHigh schoolChecking with formulas: the freezing point of seawater, and the days needed to cool it
First we work out how far the freezing point of seawater falls. Then we estimate how many days it takes to cool the mixed layer by 1°C.
| In symbols | ΔT ＝ Kf × m (and Q ＝ M × c × Δθ) |
| In words | Drop in freezing point ＝ a constant set by the kind of solvent × number of dissolved particles (moles) per 1 kg of water. Heat needed to cool ＝ mass of water × specific heat × temperature drop |
| Where it comes from | Dissolved particles dilute the water molecules' "readiness to freeze", and the effect depends only on the number of particles, not on what was dissolved (a property of dilute solutions). The heat formula comes from conservation of energy |
| Freezing-point depression constant of water Kf | 1.86 °C·kg/mol |
| Total dissolved particles (ions) in seawater m | about 1.0 mol per 1 kg of water |
| Mixed-layer thickness (an assumption here) | 100 m |
| Mass of 1 cubic metre of seawater | about 1000 kg |
| Specific heat of water c | about 4200 J/(kg·°C) |
| Heat escaping from the sea surface in a strong midwinter cold spell (an assumption here) | 200 J per second per square metre |
| Drop in freezing point | 1.86 × 1.0 ＝ 1.86 (°C) |
| Mass of mixed-layer water under 1 square metre of sea surface | 100 × 1000 ＝ 100000 (kg) |
| Heat removed to cool that water by 1°C | 100000 × 4200 ＝ 420000000 (J) |
| Time needed (seconds) | 420000000 ÷ 200 ＝ 2100000 (s) |
| Converted to days | 2100000 ÷ 86400 ≒ 24 (days) |
The freezing point comes out as 0 − 1.86, about −1.9°C, which matches the real value for seawater. And a 100 m mixed layer takes more than three weeks to cool by just 1°C, even in a strong cold spell. The top few centimetres of a pond can freeze overnight. This difference in quantity is what lies behind "the sea doesn't freeze easily".
High schoolHS+Properties set by particle count, and density changed by salt
High schoolFreezing-point depression is one of the "colligative properties", along with boiling-point elevation and osmotic pressure. Its size depends on the number of particles, not on what was dissolved. Table salt splits into two ions in water, so it works about twice as strongly as the same number of moles of sugar.
HS+The two lines in Figure 1 are approximations in which both fall roughly in proportion to salinity. The freezing point drops about 0.54°C per 1% of salinity, and the temperature of maximum density drops about 2.2°C per 1%. Because the latter slope is steeper, the two lines cross at a salinity of about 2.5% and about −1.3°C.
UniversitySea ice formation and thermohaline circulation
In physical oceanography, sinking caused by cooling of the sea surface is treated as "convection", and the depth of the mixed layer is set by the strength of the cooling, the wind and the structure of the salinity layers. In seas with low surface salinity, like the Sea of Okhotsk, a step in density (the halocline) holds convection shallow. The dense salty water (brine) pushed out as sea ice forms sinks, and around Antarctica and elsewhere it becomes the source of deep water. This is part of the planet-wide ocean circulation called "thermohaline circulation".
📖 Derivations of the formulas, and further reading: Freezing-point depression (Wikipedia) / Thermohaline circulation (Wikipedia)
ResearchWhat is still not well understood
- How will deep ocean currents change as sea ice declines? If less sea ice forms, less dense salty water sinks. How much the deep circulation will weaken is still being studied through both observation and modelling.
- The future of sea ice in the Sea of Okhotsk. The amount of drift ice is said to be declining over the long term, but it varies a lot from year to year, and research continues on how changes in river water and wind affect it.
- Brine channels inside the ice. Sea ice keeps a network of thin channels of salty water inside it. How their shape affects heat flow and the lives of organisms is not yet known in detail.
In other words, what this article says is also "an explanation of what is known so far". The link between sea ice and climate in particular is a field where the numbers may change in the future.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: changes of state and density | Water is heaviest at 4°C; chilled water sinks |
| High school | Chemistry: properties of dilute solutions | Getting −1.9°C from the freezing-point depression formula |
| HS+ | Earth science: structure of the ocean | Mixed layer; temperature of maximum density shifted by salinity |
| University | Physical oceanography | Convection, halocline, thermohaline circulation |
| Research | Climate and sea ice research | Sea ice decline and deep circulation |
| ― | Connection to daily life | Winter seaside scenery, drift ice tourism, freezer experiment |
- Wikipedia (Japanese), "Freezing-point depression" (凝固点降下)
- Wikipedia (Japanese), "Sea ice" (海氷)
- Wikipedia (Japanese), "Drift ice" (流氷)
- Wikipedia (Japanese), "Thermohaline circulation" (熱塩循環)
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, "Chronological Scientific Tables"), Maruzen Publishing (properties of seawater)
*This article is a general-audience science explanation. The figures given are rough guides to help you grasp the mechanism. Mixed-layer thickness and the heat escaping from the sea surface vary greatly with place, season and weather.