Atmospheric

Part 7 - Water

Water is a chemical compound with two atoms of hydrogen combined with one atom of oxygen to make one molecule of water (H2O for short).  Life evolved in the seas and, arguably, water is the most important compound for all life on Earth.

It makes up about 60% of the human adult body. About 73% of the brain and heart consist of water, and even the bones are 31% water. So, we really never left the sea we just took it with us.

Water has some unique properties. At sea level, it freezes at 0°C and boils at 100°C. As it evaporates from the liquid state (water) to the gaseous state (water vapour) it removes heat from the water (known as the latent heat of evaporation). Which is why the temperature of boiling water remains constant (at 100°C at sea level) until all of the water in a container has evaporated. The latent heat is returned when the vapour condenses back to water.

Because of the lower pressure, at an altitude of 10,000 feet (3048 m) water boils at 89.8°C. It will also evaporate at lower temperature dependent on the amount of water already in the air (the relative humidity).  So, water evaporates easily into the atmosphere when it is hot and condenses out, as rain or snow, when it cools. The atmosphere thus acts as a giant heat transporter carrying warm water vapour from the tropical oceans to the cooler northern and southern extremities of Earth and returning cooler, drier air to tropical regions.

When air, saturated with water vapour, cools, it forms tiny water droplets that remain suspended in the air as clouds or (at ground level) fog. If the droplets coalesce they form rain or, if the temperature is below freezing they may link together in the complicated hexagonal crystals of snow flakes or pellets of ice known as hail. 

As water can store more heat energy per kilogram (kg) than any other material, the oceans serve as giant heat reservoirs that moderate the climate near oceans. The oceans also transport heat with slow moving streams of warm water from tropical regions with under currents of cool water (carrying oxygen and other nutrients) moving in the opposite direction. The most well known example is the Gulf Stream (from the Gulf of Mexico) that keeps Iceland, and most of northern Europe, much warmer than they would be otherwise.

In some places inland and in desert areas, when there is no cloud cover, the air temperature can range below freezing at night to unbearably hot during the day.

Water is also a very good solvent for a wide range of gases and other elements and compounds of elements. It absorbs oxygen and carbon dioxide from the air which is why plants and oxygen breathing organisms evolved in sea water.

When rain water reaches land (or ice and snow melts) it begins to dissolve chemical compounds such as sodium chloride (common salt), calcium and iron oxide and carries these to the sea or to an area where the water again evaporates, depositing the dissolved compounds (minerals) in salt flats. When water evaporates it leaves behind all chemical compounds dissolved in the water. Which is why sea water becomes progressively more salty over time and rain or snow is almost pure water.

Unlike most other compounds, frozen water (ice) is less dense than liquid water and it floats on the surface of liquid water. This means that water seeping into rock fissures and then freezing, expands, exerting enough pressure to split the rock. This is one of the processes that gradually reduces rock to sand and clay particles. Other processes that form soil include waves pounding on shorelines, water running over rocks and glaciers (rivers of ice) slowly grinding rocks together. 

Snow on the ground contains a lot of air which makes it an excellent insulator. Snow on top of ice insulates like a blanket and prevents seas, lakes and rivers from freezing ever more deeply. Many animals survive frigid winters beneath the snow and every Canadian and Alaskan Inuk knows how to make an iglu (snow house, igloo).