Water vapor is an odorless, colorless gas that mixes freely with the other gases of the atmosphere. Unlike oxygen and nitrogen—the two most abundant components of the atmosphere—water can change from one state of matter to another (solid, liquid, or gas) at the temperatures and pressures experienced on Earth ▼. Because of this unique property, water leaves the oceans as a gas and returns to the oceans as a liquid.

As you observe day-to-day weather changes, you might ask: Why is it generally more humid in the summer than in the winter? Why do clouds form on some occasion days but not on others? Why do some clouds look thin and harmless, whereas others form dark and ominous towers? Answers to these questions involve the role of water vapor in the atmosphere, the central theme of this chapter.

Water is the only substance that naturally exists on Earth as a solid (ice), liquid, and gas (water vapor). Because all forms of water are composed of water molecules (H2O), each consisting of one oxygen atom bonded to two hydrogen atoms, the primary difference among water’s three phases is the arrangement of these water molecules.

Ice, Liquid Water, and Water Vapor

Ice is composed of water molecules that form a tight, orderly network held together by mutual molecular attractions, as shown below ▼. As a consequence, the water molecules in ice are not free to move relative to each other but rather vibrate about fixed sites. When ice is heated, the molecules oscillate more rapidly. When the rate of molecular movement increases sufficiently, the bonds between the water molecules begin to break, resulting in melting.

In the liquid state, water molecules are still tightly packed but are moving fast enough that they are able to slide past one another. As a result, liquid water is fluid and takes the shape of its container.

As liquid water gains heat from its environment, some of the molecules acquire enough energy to break the remaining molecular attractions and escape from the surface, becoming water vapor. Water-vapor molecules are widely spaced compared to liquid water and exhibit very energetic random motion. Unlike a liquid, a gas will expand to occupy a container of any size, and it can also be compressed. For example, you can easily put more and more air into a tire and increase its volume only slightly. However, you can’t put 10 gallons of gasoline into a 5-gallon can.

Latent Heat

Whenever water changes state, heat is exchanged between water and its surroundings. Heat is absorbed, for example, when water evaporates (refer to ▲). Meteorologists often measure heat energy in calories. One calorie is the amount of heat required to raise the temperature of 1 gram of liquid water 1°C (1.8°F). Thus, when 10 calories of heat are added to 1 gram of water, the water molecules vibrate faster, and a 10°C (18°F) temperature rise occurs.

Under certain conditions, a substance can absorb heat without an accompanying rise in temperature. For example, when a glass of water with ice is warmed, the temperature of the ice–water mixture remains a constant 0°C (32°F) until all the ice has melted. If adding heat does not raise the temperature, where does this energy go? In this case, the added energy goes into breaking the molecular attractions between the water molecules in the ice cubes.

Because the heat used to melt ice does not produce a temperature change, it is referred to as latent heat. (Latent means “hidden,” like the latent fingerprints hidden at a crime scene.) This energy can be thought of as being stored in liquid water, and it is not released to its surroundings as heat until the liquid returns to the solid state.

Melting 1 gram of ice requires 80 calories, an amount referred to as latent heat of melting. Freezing, the reverse process, releases these 80 calories per gram to its surroundings as latent heat of freezing.

Evaporation and Condensation

We saw that heat is absorbed when ice is converted to liquid water. Heat is also absorbed during evaporation, the process of converting a liquid to a gas (water vapor). The energy absorbed by water molecules during evaporation is used to give them the motion needed to escape the surface of the liquid and become a gas. This energy is referred to as the latent heat of vaporization. During the process of evaporation, the higher-temperature (faster-moving) molecules escape the surface. As a result, the average molecular motion (temperature) of the remaining water is reduced—hence the common expression “evaporation is a cooling process.” You have undoubtedly experienced this cooling effect when stepping dripping wet from a swimming pool or bathtub. In such a situation, the energy used to evaporate water comes from your skin, and you feel cool.

The reverse process, condensation, occurs when water vapor changes to the liquid state. During condensation, water-vapor molecules release energy (latent heat of condensation) in an amount equivalent to what was absorbed during evaporation. When condensation occurs in the atmosphere, it results in the formation of such phenomena as fog and clouds (▼A).

Latent heat plays an important role in many atmospheric processes. In particular, when water vapor condenses to form cloud droplets, latent heat of condensation is released, warming the surrounding air and giving it buoyancy. When the moisture content of air is high, this process can spur the growth of towering storm clouds.

Sublimation and Deposition

You are probably least familiar with the last two processes illustrated in ▼—sublimation and deposition. Sublimation is the conversion of a solid directly to a gas, without passing through the liquid state. Examples you may have observed include the gradual shrinking of unused ice cubes in the freezer and the rapid conversion of dry ice (frozen carbon dioxide) to wispy clouds that quickly disappear.

Deposition refers to the reverse process: the conversion of a vapor directly to a solid. This change occurs, for example, when water vapor is deposited as ice on solid objects, such as grass or windows ▼. These deposits are called white frost or hoar frost, or simply frost. A household example of the process of deposition is the “frost” that accumulates in a freezer. As shown in ▲, deposition releases an amount of energy equal to the total amount released by condensation and freezing.

• Water exists in all three states of matter (solid, liquid, or gas) at the temperatures and pressures near Earth’s surface. The gaseous form of water is water vapor.

• The processes by which matter changes state are evaporation (liquid to gas), condensation (gas to liquid), melting (solid to liquid), freezing (liquid to solid), sublimation (solid to gas), and deposition (gas to solid). During each change, latent (hidden, or stored) heat is either absorbed or released.

• XXXXX: