The Three States of Water

Water can exist in three states: solid (ice), liquid (water), and gas (water vapour).

Water exists in the air in the form of water vapour, clouds, fog, and precipitation.

A schematic diagram of the three states of water. Arrows show the ways that any one state can change into either of the other two states. Heat energy is absorbed or released, depending on the direction of change.

The Three States of Water

Global Water Balance

Since our planet contains only a fixed amount of water, a global balance must be maintained among flows of water to and from the lands, oceans, and atmosphere.

This balance also controls the proportion of water found in the different reservoirs of the hydrosphere.

The oceans and glaciers contain most of the water. The remaining freshwater, upon which we are so dependent, comprises the least.

Figures give average water flows in and out of world land areas and world oceans. Values are given in thousands of cubic kilometres. Global precipitation equals global evaporation.

Global Water Balance

The amount of water vapour present in the air is called humidity. Specific humidity is a measure of the actual quantity of water vapour held by the air. Relative humidity compares the amount of water vapour present to the maximum amount that the air can hold at that temperature, expressed as a percentage.

Humidity

The maximum specific humidity of a mass of air increases sharply with rising temperature.

Relative humidity changes with temperature because the capacity of warm air to hold water vapour is greater than that of cold air. In this example, the amount of water vapour stays the same, and only the capacity changes.

Humidity

The Adiabatic Process

Physicists use the term adiabatic process to refer to a heating or cooling process that occurs solely as a result of pressure change. That is, the change in temperature is not caused by heat flowing into or away from a volume of air, but only by a change in pressure on a volume of air.

*See the Animation on the Adiabatic Process found on WileyPLUS or your text's website.

A schematic diagram of adiabatic cooling and heating that accompanies the rising and sinking of a mass of air. When air is forced to rise, it expands and its temperature decreases. When air is forced to descend, its temperature increases.

Adiabatic decrease of temperature in a rising parcel of air leads to condensation of water vapour into water droplets and the formation of a cloud.

The Adiabatic Process

Clouds

A cloud is made up of water droplets or ice particles suspended in air.

Clouds are grouped into families on the basis of height. Individual cloud types are named according to their form.

Clouds

Types of Clouds

Cirrus in Parallel Bands

Altostratus Layer

Stratocumulus Clouds

Cirrocumulus Clouds

Altocumulus Clouds

Cumulus Clouds

Precipitation

Precipitation can form in two ways:

1) Water droplets condense, collide, and coalesce into larger and larger water droplets that fall as rain. This is referred to as coalescence.

2) Ice crystals form and grow in a cloud that contains a mixture of both ice crystals and water droplets. This occurs where the cloud temperature is below zero degrees Celsius, but despite that some droplets are still liquid in form.

Growth of raindrops is called coalescence.

Precipitation

Types of Precipitation

The processes responsible for precipitation in the atmosphere create different forms of precipitation.

Rain falls from warm clouds where saturated air has risen quickly and cooled, leading to condensation. Snow, however, forms in cool clouds where ice crystals collide with super cooled water droplets. If they fall through air that is below freezing, they remain as snow flakes. If the air is above freezing, they begin to melt and fall as sleet. Hail falls from tall cumulonimbus clouds associated with thunderstorms. Large hailstones grow as the updrafts in the clouds successively add layer upon layer of condensing water.

Orographic Precipitation

In orographic precipitation, through-flowing winds move moist air up and over a mountain barrier. The term "orographic" means related to the mountains.

*See the Animation on Orographic Precipitation found on WileyPLUS or your text's website.

Convectional Precipitation

In convectional precipitation, unequal heating of the surface causes an air parcel to become warmer and less dense than the surrounding air. Because it is less dense, it rises. As it moves upward, it cools, and condensation with precipitation may occur.

Successive bubbles of moist condensing air push upward in the thunderstorm cell. Their upward movement creates a corresponding downdraft, expelling rain, hail, and cool air from the storm as it moves forward.

*See the Animation and Video Clip on Convectional Precipitation (Thunderstorms) found on WileyPLUS or your text's website.