Water also absorbs heat when it evaporates. This heat supplies the energy needed to break the hydrogen bonds that hold molecules together. Because it is used to break the hydrogen bonds between individual molecules, this heat does not raise the temperature of the liquid water. It is known as latent heat, because it appears to be hidden (see the sidebar "Latent heat and dew point" below).
Heat energy that is absorbed when water evaporates is released when the hydrogen bonds form once again and the water vapor condenses into liquid. Water vapor condenses when it (or the air containing it) becomes satu-
Water can exist in three different states, or phases: as gas (water vapor), liquid (water), or solid (ice). In the gaseous phase, molecules are free to move in all directions. In the liquid phase, molecules join together in short "strings." In the solid phase, molecules form a closed structure with a space at the center. As water cools, its molecules move closer together and the liquid becomes denser. Pure water at sea-level pressure reaches its densest state at 39°F (4°C). If the temperature falls lower than this the molecules start forming ice crystals. Because these have a space at the center, ice is less dense than water and, weight for weight, has a greater volume. That is why water expands when it freezes and why ice floats on the surface of water.
Molecules bond to one another by the attraction of opposite charges and energy must be supplied to break those bonds. The molecules absorb this energy with no resulting change in their temperature, and the same amount of energy is released when the bonds form again. This energy is called latent heat. For pure water, 600 calories of energy are absorbed to change one gram (1 g = 0.035 oz.; 600 cal g-1 = 2,501 joules per gram; joules are the units scientists use) from liquid to gas (evaporation) at 32°F (0°C).
This is the latent heat of vaporization, and the same amount of latent heat is released when water vapor condenses. When water freezes or ice melts, the latent heat of fusion is 80 cal g-1 (334 J g-1). Sublimation, the direct change from ice to vapor without the water passing through the liquid phase, absorbs 680 cal g-1 (2,835 J g-1), equal to the sum of the latent heats of vaporization and fusion. Deposition, the direct change from vapor to ice, releases the same amount of latent heat. The amount of latent heat varies very slightly with temperature, so this should be specified when the value is given. The standard values given here are correct at 32°F (0°C). The diagram illustrates what happens.
Energy to supply the latent heat is taken from the surrounding air or water. When ice melts or water evaporates, the air and water in contact with them are cooled, because energy has been taken from them. That is why it often feels cold during a thaw and why our bodies can cool themselves by sweating and allowing the sweat to evaporate.
When latent heat is released by freezing and condensation, the surroundings are warmed. This process is very important in the formation of storm clouds. Warm air rises, its water vapor condenses,
Latent heat and adiabatic cooling and warming
Latent heat. As water changes between the gaseous, liquid, and solid phases, the breakage and formation of the hydrogen bonds linking molecules release or absorb energy as latent heat.
and the release of the latent heat of condensation warms the air still more, making it rise higher.
Warm air is able to hold more water vapor than cool air can, and the amount of water vapor air can hold depends on its temperature. If moist air is cooled, its water vapor will condense into liquid droplets. The temperature at which this occurs is called the dew point temperature. It is the tempera ture at which dew forms on surfaces and evaporates from them.
At the dew point temperature, the air is saturated with water vapor. The amount of moisture in the air is usually expressed as its relative humidity (RH). This is the amount of water present in the air, expressed as a percentage of the amount needed to saturate the air at that temperature.
rated, and saturation occurs when the air temperature falls below a threshold known as the dew point. When the temperature decreases, water molecules lose energy and move more slowly, and when they meet they remain close to one another long enough for hydrogen bonds to form between them. The relative humidity (RH) is the amount of water vapor present in a unit volume of air expressed as the percentage of the amount that is needed to reach saturation at that temperature. If the amount of water vapor remains constant, RH decreases as the air warms and increases as it cools.
Air cools if it is made to rise. It will rise by convection if it is heated by contact with a warm surface, or if it is forced to rise over high ground—a process called orographic lifting—or by frontal lifting if cold air pushes beneath warm air at a weather front and lifts the warm air. Regardless of the surrounding temperature, rising air cools and subsiding air warms. This is called adiabatic cooling and warming (see the sidebar "Adiabatic cooling and warming" below). Adiabatic cooling reduces the temperature of rising air, and if the temperature falls below the dew point temperature, water vapor
Air is compressed by the weight of air above it. Imagine a balloon partly inflated with air and made from some substance that totally insulates the air inside. No matter what the temperature outside the balloon, the temperature of the air inside remains the same.
Imagine the balloon is released into the atmosphere. The air inside is squeezed between the weight of air above it, all the way to the top of the atmosphere, and the denser air below it.
Suppose the air inside the balloon is less dense than the air above it. The balloon will rise. As it rises, the distance to the top of the atmosphere becomes smaller, so there is less air above to weigh down on the air in the balloon. At the same time, as it moves through air that is less dense, it experiences less pressure from below. This causes the air in the balloon to expand.
When air (or any gas) expands, its molecules move farther apart. The amount of air remains the same, but it occupies a bigger volume. As they move apart, the molecules must "push" other molecules out of their way. This uses energy, so as the air expands its molecules lose energy Because they have less energy they move more slowly.
When a moving molecule strikes something, some of its energy of motion (kinetic energy) is transferred to whatever it strikes, and part of that energy is converted into heat. This raises the temperature of the struck object by an amount related to the number of molecules striking it and their speed.
In expanding air the molecules are moving farther apart, so a smaller number of them strike an object each second. They are also traveling more slowly, so they strike with less force. This means the temperature of the air decreases. As it expands, air cools.
If the air in the balloon is denser than the air below, it will descend. The pressure on it will increase, its volume will decrease, and its molecules will acquire more energy. Its temperature will increase.
This warming and cooling has nothing to do with the temperature of the air surrounding the balloon. It is called adiabatic warming and cooling, from the Greek word adiabatos, meaning impassable.
Adiabatic cooling and warming. Effect of air pressure on rising and sinking air. Air is compressed by the weight of air above it. A "parcel" or"bubble" of air is squeezed between the weight of air above and the denser air below. As it rises into a region of less dense air, it expands. As it sinks into denser air, it contracts.
will start to condense to form clouds. The height at which this happens is known as the lifting condensation level. Condensation releases latent heat, which warms the adjacent air. This can be enough to make the air continue rising, with further condensation leading to towering clouds of the cumulus and cumulonimbus types.
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