Middle school year 3 · third unit
The Hydrosphere and Ocean Circulation
The Earth is called the water planet, yet the water we can drink is astonishingly little. And the sea, which seems to lie still, differs in saltiness and in temperature at every depth, and never stops flowing and rising and falling. Go down into it.
With so much water on Earth, why must we save it?
A board to arrangeCall all the water on Earth 100 L. Drag the gold drop to pour into the next container, taking some away each time.
Is the sea equally salty everywhere?
A board to runOne kilogram was taken from each of three seas. Drag the burner under a beaker to boil it dry.
How cold is the deep sea?
A board to adjustIt does not get colder evenly on the way down. Take the submersible and drag it downward.
In winter the east coast of Korea is milder than the west
A board to arrangeThat is because the sea has paths that flow like rivers. Take the buoy and set it on the sea.
The mudflats are covered by the sea twice a day
A board to adjustIt is the Moon that pulls the sea. Take the Moon and move it round the Earth.
What to take away from this unit
- Of the Earth's water, 97.5% is sea water and 2.5% is fresh. Most fresh water is ice and groundwater; the water of lakes and rivers that is easy to use is very little. That is why water resources must be saved and managed.
- Salinity is the grams of salts dissolved in 1 kg of sea water, about 35 psu on average. It is higher where evaporation is great and lower where rain, rivers or melted ice mix in. The proportions between the salts are the same in every sea (the law of constant proportions).
- By temperature the sea is divided into the mixed layer, the thermocline and the deep layer. The mixed layer is stirred by wind and even in temperature; the thermocline keeps the water above and below from mixing.
- An ocean current is sea water flowing steadily in one direction. Round Korea flow the Yellow Sea Warm Current and the East Korea Warm Current, both branching from the Kuroshio, and the North Korea Cold Current coming down from the north. Where warm and cold currents meet, the fishing is good.
- The tide is the sea coming in and going out about twice a day. Twice a month come the spring tide, with the largest range, and the neap tide, with the smallest.