Temperature and depth at Higher Level: This statement is Higher Level only. It explains why water is warm above and cold below, why the layers stay apart, and why lakes freeze from the top down.
Practise this as you read
- Describe a temperature-depth graph with numbers.
- Explain why ice forms on top and life survives below.
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Warm on top, cold below: Sunlight heats only the top of a lake or sea. Warm water is lighter than cold water, so it stays on top, and the water gets colder the deeper you go.
The points to remember
- The sun warms only the surface, so warm water sits above and cold water below.
- Warm water is less dense, so it floats on the colder, denser water.
- Near the top, wind keeps the water mixed, so its temperature stays steady.
- Below that, temperature falls quickly with depth, then more slowly.
- Deep ocean water is cold everywhere: about 2 to 5 °C, even under the tropics.
Remember it as: Sun on top, cold at the bottom.
Real example: in June 2023 an Argo float in the Atlantic near the equator measured 29 °C from the surface to 50 m. By 100 m the water was about 17 °C, and at 1000 m only 5 °C.
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Because warm water floats on cold water, the layers resist mixing. Scientists call a body of water with layers like this stratified.
Layers that do not mix
- Water of different density does not mix easily: the light layer floats on the heavy one.
- So the warm and cold layers stay apart for long periods: persistent stratification.
- Wind can only stir the top layer; it cannot push light water down into heavy water.
- Little mixing means nutrients stay trapped in the deep water and oxygen from the air does not reach it.
- So different organisms live at different depths.
Remember it as: Light on heavy: the layers stay put.
Top layer
- Warm and light.
- Stirred by wind.
- Gets oxygen from the air.
Deep layer
- Cold and dense.
- Hardly moves.
- Cut off from the air.
Real example: Lake Tanganyika in Africa is 1,470 m deep and warm all year. Its layers almost never mix, so below about 150 m the water has had no oxygen for a very long time and almost nothing lives there.
Water has one strange property: it is heaviest not when it is coldest, but at 4 °C. This decides how lakes freeze in winter.
Densest at 4 °C
- Fresh water is densest at 4 °C.
- In autumn, surface water cooled to 4 °C sinks, until the whole lake is at 4 °C.
- Water cooled below 4 °C is lighter, so it stays on top.
- So a lake freezes from the surface downwards, at 0 °C.
- Ice floats (it is less dense than water) and insulates the water below.
- Fish and other life survive the winter in the 4 °C water at the bottom.
Remember it as: 4 °C sinks, colder floats, ice on top.
So in winter the layers turn upside down: the coldest water lies at the top, just under the ice, and the 4 °C water lies at the bottom.
Real example: Lake Baikal in Siberia freezes over with ice about 1 m thick from January to May. Underneath, omul and Baikal seals live through the winter; the seals keep breathing holes open in the ice.
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How this comes up: Paper 2, Section A: a depth graph with 'Describe the variation in temperature' [2], or a short question on why lakes freeze from the top.
The figure above shows water temperature measured at different depths by an Argo float in the tropical Atlantic Ocean in June 2023.
Describe the variation in temperature from 0 to 1000 m depth shown on the graph.
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