Here a simplified discussion of the transformations between ice, water, and water vapor is presented, as a tutorial. First, these changes are considered in closed containers which are surrounded by air at various temperatures. The interchange of heat energy needed to bring about the changes in state of the water is emphasized. Next, in order more closely to resemble water vapor in the atmosphere, the water-to-vapor transformation is discussed without a container separating the water from the air, but rather mixed together. Again, the energy interchange is stressed. Finally, we discuss very simply how these transformations might be involved in creating rainfall, and winds or larger patterns of weather.
Introduction. In the physical world, a substance can exist either as a solid (at low temperature), a liquid (at a higher temperature) or a gas or vapor (at a still higher temperature). An example familiar to us all is water.
Solid-Liquid-Vapor Phase Changes. Here, to start with (please see Note 1) let’s talk only about pure water, so we’re not including mixtures or solutions of water or its vapor with other substances. Why is solid water, ice, found at low temperatures, liquid water at intermediate temperatures and the gaseous form, water vapor, at the highest temperatures? There are two aspects to an answer.
Heat Energy. First, think about the ice itself, or the liquid water in a closed container or the water vapor in a closed container, surrounded by air for example. There is less heat contained in the air when its temperature is low, and more heat contained in it when its temperature is high. Heat is one form of energy. The air can transfer some of its heat energy to the ice or to the water or vapor through the containers, or in reverse the ice or water or vapor can transfer some of its heat energy to the air surrounding it. (Heat can only flow from a warmer to a colder temperature.)
Intrinsic Energy. Second, the physical states of solid, liquid and gas themselves contain differing amounts of intrinsic, or latent, energy. The ice contains the least, the water more, and the water vapor the most (see the graphic below).
Liquid Water-Vapor Interconversion. The same processes occur when liquid water is vaporized to a gas. The molecules in the liquid water still have some bonds between the molecules, but not as many as there were in the ice. The molecules in the water vapor have essentially no bonds between them, i.e., they are isolated. In order to convert from liquid water to the pure water vapor inside the container, more heat energy has to be provided from the surrounding air to break the remaining bonds between the molecules in the liquid, and permit them to escape from the liquid as independent molecules of water vapor (right, upper arrow). (For simplicity, the graphic ignores the fact that the vapor and its container are much larger than the liquid.) Thus the water vapor inside the container has still more intrinsic, or latent, heat than the liquid water. This process is also reversible. If the air were to cool down enough, the water molecules in the vapor would release their intrinsic heat to the surrounding air, condensing back into the liquid (right, lower arrow).
Transfer of heat energy. It’s crucial to understand from this explanation that the more important process is the transfer of heat energy between the surroundings and the substance. As people, we can see the change from ice to liquid water, and from liquid water to vapor, and back. But we don’t see the energy driving these visible changes. It’s actually the transfer of the heat energy that’s important, even if we can’t see it.
When we put an ice cube into a drink, the liquid of the drink transfers its heat to the ice, supplying the intrinsic heat needed to melt the ice. But as a result, the liquid drink has less heat energy remaining, i.e., it has cooled down. We observe the melting ice cube, and the cooling of the drink. But we don’t directly sense the heat transfer.
A Model: Compressed Gas and Gravity. It might be helpful to think of the following idealized setup as a model. Suppose a marble is at the bottom of a sloping ramp between two horizontal platforms (see the graphic below, left). A cylinder and piston contain a gas under high pressure, and the piston rod extends out of the cylinder, with a cup that can push the marble up the ramp from the low platform to the higher platform. Here, the low platform is a model for our solid, ice, and the higher platform is a model for our liquid, water. If the gas in the cylinder is allowed to expand, the piston pushes the marble from the lower platform (ice) up the ramp to the higher platform (liquid water; see the graphic, right). The expanding gas transfers its energy of compression to the marble in the form of intrinsic gravitational energy. The marble on the upper platform has more gravitational (latent) energy than it had on the lower platform (upper arrow).
This process is reversible (lower arrow). Let’s suppose that gravity, or some additional force, could make the marble roll down the ramp, giving up its intrinsic gravitational energy. The lost gravitational energy goes to increasing the compression energy of the gas by pushing the piston back into the cylinder again. In this example, the energy of compression of the gas is exchanged back and forth with the gravitational energy of the marble.
Liquid Water and Water Vapor in the Atmosphere. Now let’s return to focus on water, and only on the exchange between liquid water and gaseous vapor. Also, we’ll now open up the container so that air can be mixed with any water vapor that escapes from the liquid. This is a good way to think of liquid water and water vapor in the earth’s atmosphere. On a molecule-by-molecule basis, to a first approximation the intrinsic energy to vaporize a molecule of water from the liquid is more or less the same at all temperatures.
Since the liquid water and the water vapor are no longer enclosed in a container, some liquid water can transfer to water vapor and mix with the air, at all temperatures. But between the melting point of ice and the boiling point of water, there is a 100 °C (180 °F) difference in temperature. So as the temperature of the water and the air above it gets warmer, more heat can be supplied to the liquid water. More molecules of water acquire the intrinsic heat needed to vaporize, and more water vapor enters the air over the surface of the liquid water. In other words, as the environment’s temperature increases, the overall capacity of the air above liquid water to hold water vapor increases.
Roughly, at temperatures at which the earth’s weather is determined, a change of 1 °C (1.8 °F) in temperature changes the capacity of the air to hold water vapor by about 6% (Note 2). This means that if the temperature increases by 1 °C, an amount of heat energy must be supplied to provide the intrinsic energy for 6% more water to vaporize into the air if that air is saturated with water vapor; if the temperature falls by 1 °C, the water content of the air at saturation falls by about 6%, releasing that intrinsic heat energy back into the air. This intrinsic, or latent, heat of vaporizing liquid water, or condensing water vapor, is actually quite high. It is much higher than for other chemical compounds related to water, such as ammonia (Note 3) or hydrogen fluoride.
Global Warming and its Effect on Water Vapor Content of the Atmosphere. Global warming scenarios predict some areas on the surface of the earth having more rainfall, and more violent storms, than prior to the onset of warming. At present, the long-term increase in global temperature is 0.7 °C (1.3 °F) higher than the temperature prior to the start of the industrial revolution. Water can evaporate from the surface of the earth from oceans, fresh water lakes and rivers, and from moist lands and green fields and forests. Oceans occupy about 71% of the surface of the earth. Evaporation from the surface of the oceans and land masses contributes a significant amount of water vapor in the atmosphere. At the current level of the increase in global temperature, a simplified global average of the increase in the amount of moisture in the atmosphere could be about 4% more than before warming began (Note 4).
Our discussion above should make plausible the way that global warming can lead to weather patterns with more rainfall in some regions. A higher moisture content in the air provides the potential, upon cooling, for more moisture to condense into precipitation, by releasing its intrinsic heat content to the cooler air. In this way global warming is understood to lead potentially to heavier rainfall in some areas. (These comments do not represent all the factors that are included in modeling global warming. They simply strive to convey some of the simpler processes that are involved in translating global warming into altered patterns of climate and weather.)
Cloud Formation and Loss, and Heat Transfer. Now consider what you see when you look at a cloud, observing its ever-changing shape and appearance. The cloud that is visible is actually made of condensed droplets of liquid water, i.e., water that has already released its intrinsic heat to the surrounding air, warming the air up in the process. In contrast, water vapor in the air surrounding the cloud is itself invisible. A cloud sometimes loses a part of its wispy condensate, vaporizing back into water vapor, thereby absorbing its intrinsic heat for vaporization from the surrounding air, and cooling the air. This simplified description should help us understand that cloud formation and loss involve important localized changes in air temperature and heat content.
Global Warming and Winds. In the preceding paragraph, we discussed how water vapor and cloud formation involve reciprocal exchanges of heat energy, with cycles of warming and cooling of the air. On a larger scale, when air masses of differing temperatures encounter each other, one result is generation of wind (wind also has other origins). In certain conditions we could imagine that more turbulent winds could arise from global warming because of the increased moisture content and the resulting increase in exchanges of intrinsic heat with the heat of the air. This could produce a tendency for stronger winds, some of which could lead to more violent storms. (Other factors not considered here can also contribute to this tendency.)
Conclusion. Here we’ve shown that transformation of water between ice, liquid and vapor is accompanied by significant exchanges of heat with the air in the immediate environment of the changes. In addition, the capacity of the atmosphere to hold water vapor increases with the higher global average temperature that prevails as a result of global warming. This enhances the intensity of the heat exchange processes, which potentially can lead to more, and stronger winds, and more, and more intense precipitation as snow or rain. It is possible that global warming exacerbates the damaging effects of severe precipitation. The following post discusses this in greater detail.
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Note 1. This presentation is not meant to be comprehensive or technically accurate. It is intended to make plausible the ways in which global warming can affect certain changes in weather phenomena.
Note 2. Handbook of Chemistry of Physics, 53rd Ed., 1972-1973, CRC Press, Cleveland , OH
Note 3. Handbook of Chemistry of Physics, 53rd Ed., 1972-1973, CRC Press, Cleveland , OH
Note 4. This simplified statement ignores differences that might arise between the tropics and the poles, and the possible effects from the fact that some polar oceans are covered with ice.
© 2011 Henry Auer
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