Unit 22
Water and oceans.
The Earth's water on the move: the water cycle, groundwater, ocean currents, El Niño, and tsunamis.
Before this unit
The unit, lesson by lesson
Lessons
5 lessons · 36 practice questions · about 2 hours
The water cycle
The Sun evaporates water from the sea, the wind carries it inland, and a mountain lifts the air until it cools to its dew point. Work out the cloud base from the cooling rate, find the freezing level, and see why there is desert behind mountains.
Groundwater and aquifers
Under your feet, between the grains of sand, there is water. Use Darcy’s law to work out how slowly it creeps, find how much a well can give before it runs dry, and see what drought and over-pumping do to an aquifer.
Ocean currents
The wind turns the sea’s surface, and density drives a deep loop round the world. Find out which water sinks, find how much salt it takes, and see what meltwater does to the current that warms Europe.
El Niño
The trade winds have piled warm water in the western Pacific. Weaken them and watch the water and rain move east, strengthen them for a La Niña, and see how sea and air strengthen each other.
Tsunamis
A quake lifts the sea floor, and the wave it makes travels at $\sqrt{g d}$, set by the depth alone. Work out how fast it crosses an ocean, find why the shallow water takes most of the time, and use Green’s law for how much it grows near the coast.