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Water is the most important molecule on our planet. Viewed from space, one of the most striking features of our  planet is the water, in both liquid and frozen forms, that covers approximately 75% of the Earth’s surface. It is the only known substance that can naturally exist as a gas, a liquid, and solid within the relatively small range of air temperatures and pressures found at the Earth’s surface.


In all, the Earth’s water content is about 1.39 billion cubic kilometers , with the bulk of it, about 96.5%, being in the global oceans. As for the rest, approximately 1.7% is stored in the polar icecaps, glaciers, and permanent snow, and another 1.7% is stored in groundwater, lakes, rivers, streams, and soil. Only a thousandth of 1% of the water on Earth exists as water vapor in the atmosphere.

The volume of all water would be about 332.5 million cubic miles (mi3), or 1,386 million cubic kilometers (km3). A cubic mile of water equals more than 1.1 trillion gallons. A cubic kilometer of water equals about 264 billion gallons.

• About 3,100 mi3 (12,900 km3) of water, mostly in the form of water vapor, is in the atmosphere at any one time. If it all fell as precipitation at once, the Earth would be covered with only about 1 inch of water.
The 48 contiguous United States receives a total volume of about 4 mi3 (17.7 km3) of precipitation each day.

• Each day, 280 mi3 (1,170 km3)of water evaporate or transpire into the atmosphere.

• If all of the world's water was poured on the United States, it would cover the land to a depth of 90 miles (145 kilometers).

• Of the freshwater on Earth, much more is stored in the ground than is available in lakes and rivers. More than 2,000,000 mi3 (8,400,000 km3)of freshwater is stored in the Earth, most within one-half mile of the surface. But, if you really want to find freshwater, the most is stored in the 7,000,000 mi3 (29,200,000 km3) of water found in glaciers and icecaps, mainly in the polar regions and in Greenland.

And this is how much fresh water there is, compared to Earth and the total amount of water:


Despite its small amount, this water vapor has a huge influence on the planet. Water vapor is a powerful greenhouse gas, and it is a major driver of the Earth’s weather and climate as it travels around the globe, transporting latent heat with it. Latent heat is heat obtained by water molecules as they transition from liquid or solid to vapor; the heat is released when the molecules condense from vapor back to liquid or solid form, creating cloud droplets and various forms of precipitation.

South Bank University Graphic


Water's chemical description is H2O, that is one atom of oxygen bound to two atoms of hydrogen. The hydrogen atoms are "attached" to one side of the oxygen atom, resulting in a water molecule having a positive charge on the side where the hydrogen atoms are and a negative charge on the other side, where the oxygen atom is. 

Since opposite electrical charges attract, water molecules tend to attract each other, making water kind of "sticky." The side with the hydrogen atoms (positive charge) attracts the oxygen side (negative charge) of a different water molecule.

  • Water is unique in that it is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth. Earth's water is constantly interacting, changing, and in movement.
  • Water freezes at 32o Fahrenheit (F) and boils at 212o F (at sea level, but 186.4° at 14,000 feet). In fact, water's freezing and boiling points are the baseline with which temperature is measured: 0o on the Celsius scale is water's freezing point, and 100o is water's boiling point. Water is unusual in that the solid form, ice, is less dense than the liquid form, which is why ice floats. Water contracts until it reaches 4 C then it expands until it is solid. Solid water is less dense that liquid water because of this.
  • Water has a high specific heat index. This means that water can absorb a lot of heat before it begins to get hot. This is why water is valuable to industries and in your car's radiator as a coolant. The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans.
  • Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin film. Surface tension is responsible for capillary action, which allows water (and its dissolved substances) to move through the roots of plants and through the tiny blood vessels in our bodies.
  •  water's properties:
    • Weight: 62.416 pounds per cubic foot at 32°F
    • Weight: 61.998 pounds per cubic foot at 100°F
    • Weight: 8.33 pounds/gallon, 0.036 pounds/cubic inch
    • Density: 1 gram per cubic centimeter (cc) at 39.2°F, 0.95865 gram per cc at 212°F


The States of Water

Water has three states. Below freezing water is a solid (ice or snowflakes), between freezing and boiling water is a liquid, and above its boiling point water is a gas. 

 Water changing from solid to liquid is said to be melting. When it changes from liquid to gas it is evaporating. Water changing from gas to liquid is called condensation . Frost formation is when water changes from gas directly to solid form. When water changes directly from solid to gas the process is called sublimation.


Water Measurements

  1 gallon = 4 quarts = 8 pints = 128 ounces = 231 cubic inches
  1 liter = 0.2642 gallons = 1.0568 quart = 61.02 cubic inches
  1 million gallons = 3.069 acre-feet = 133,685.64 cubic feet

Three temperature scales are in common use in science and industry 

Two of those scales are SI metric:

The degree Celsius (°C) scale was devised by dividing the range of temperature between the freezing and boiling temperatures of pure water at standard atmospheric conditions (sea level pressure) into 100 equal parts. Temperatures on this scale were at one time known as degrees centigrade, however it is no longer correct to use that terminology. [In 1948 the official name was changed from "centigrade degree" to "Celsius degree" by the 9th General Conference on Weights and Measures (CGPM).]

The kelvin (K) temperature scale is an extension of the degree Celsius scale down to absolute zero, a hypothetical temperature characterized by a complete absence of heat energy. Temperatures on this scale are called kelvins, NOT degrees kelvin, kelvin is not capitalized, and the symbol (capital K) stands alone with no degree symbol. [In 1967 the new official name "kelvin" and symbol "K" were set by the 13th General Conference on Weights and Measures (CGPM).]

The degree Fahrenheit (°F) non-metric temperature scale was devised and evolved over time so that the freezing and boiling temperatures of water are whole numbers, but not round numbers as in the Celsius temperature scale.


Some baseline temperatures in the three temperature scales:

temperature kelvin degree Celsius degree Fahrenheit
symbol K °C °F
boiling point of water 373.15 100. 212.
melting point of ice 273.15 0. 32.
absolute zero 0. -273.15 -459.67


Boiling Points Of Water

Altitude (feet)

Boiling Point  

Altitude (feet)

Boiling Point
°F °C   °F °C











































































Water Distribution

For human needs, the amount of freshwater on Earth—for drinking and agriculture—is particularly important. Freshwater exists in lakes, rivers, groundwater, and frozen as snow and ice. Estimates of groundwater are particularly difficult to make, and they vary widely.


One estimate of global water distribution:
(1000 km3)
Percent of Total Water Percent of Fresh Water
Oceans, Seas, & Bays 1,338,000 96.5 -
Ice caps, Glaciers, & Permanent Snow 24,064 1.74 68.7
Groundwater 23,400 1.7 -
        Fresh (10,530) (0.76) 30.1
        Saline (12,870) (0.94) -
Soil Moisture 16.5 0.001 0.05
Ground Ice & Permafrost 300 0.022 0.86
Lakes 176.4 0.013 -
        Fresh (91.0) (0.007) .26
        Saline (85.4) (0.006) -
12.9 0.001 0.04
Swamp Water 11.47 0.0008 0.03
Rivers 2.12 0.0002 0.006
Biological Water 1.12 0.0001 0.003
Total 1,385,984 100.0 100.0



  • The total water supply of the world is 326 million cubic miles. A cubic mile of water equals more than one trillion gallons. 

  • About 3,100 cubic miles of water, mostly in the form of water vapor, is in the atmosphere at any one time. If it all fell as precipitation at once, the Earth would be covered with only about 1 inch of water. 

  •  Each day, 280 cubic miles of water evaporate or transpire into the atmosphere. 

  • Of the freshwater on Earth, much more is stored in the ground than is available in lakes and rivers. More than 2,000,000 cubic miles of fresh water is stored in the Earth, most within one-half mile of the surface. Contrast that with the 60,000 cubic miles of water stored as fresh water in lakes, inland seas, and rivers. The most is stored in the 7,000,000 cubic miles of water found in glaciers and icecaps, mainly in the polar regions and in Greenland.




Almost a billion people on the planet don’t have access to clean, safe drinking water. 3.5 million people die each year from water-related disease


Unsafe water and lack of basic sanitation cause 80% of diseases and kill more people every year than all forms of violence, including war. Children are especially vulnerable, as their bodies aren't strong enough to fight diarrhea, dysentery and other illnesses.

90% of the 42,000 deaths that occur every week from unsafe water and unhygienic living conditions are to children under five years old. Many of these diseases are preventable. The UN predicts that one tenth of the global disease burden can be prevented simply by improving water supply and sanitation.