Overview

Seeing an isolated thunderstorm from a distance can be awe-inspiring, partly because of its size, partly because of the apparent sense of organization. At this macroscopic level, we may see distinct turrets and sharp, bumpy edges along one side of the storm, evidence of turbulent motions and the rapid penetration of moist, cloudy air into the dry surroundings. The other side of the storm, by contrast, may look diffuse and wispy, evidence of gentler air motions and less abrupt distinctions between cloudy and clear air. These visible macroscale features of mature storms evolved from smaller convective elements in response to the effects of energy conversions on the air motions. We realize that cloudy air is composed of many small, subvisible particles that eventually become precipitation, but it is the macroscale structure of the storm as a whole that compels us to understand the relevant energy conversions and the connections between the microphysics of condensate formation and the macrophysics of cloud development.

The conversion of energy from potential to kinetic is of fundamental importance to cloud formation and evolution. Sometimes this conversion occurs on the atmospheric mesoscale (as when air motions respond to pressure readjustments), sometimes at the microscale (as in the release of latent heat during condensation). We come to understand energy transformations, regardless of scale, through the discipline of thermodynamics. The fundamental principles of thermodynamics are here applied to clouds.

Overview

Clouds play important roles in the composition of the atmosphere and in the chemical quality of precipitation. Cloud particles form in the first place by condensation onto aerosol particles composed of diverse compounds. Then, they take up additional chemicals from the air, change their chemical properties, and eventually release modified compounds back into the air or transfer them to large, sedimenting particles. Clouds effectively cleanse the air through precipitation, which serves as the carrier of atmospheric chemicals to terrestrial and aquatic ecosystems. Clouds simultaneously depend on the chemicals in the air and influence the composition of the atmosphere through a variety of microchemical processes.

The term microchemistry in cloud physics parallels that of microphysics. Both disciplines deal with the particles making up clouds, but the emphasis in cloud microchemistry is on the chemicals contained in the particles, not on the particles themselves. Atmospheric trace chemicals influence cloud properties in important ways, and the cloud microphysics also determine the fates of atmospheric chemicals. Important goals of microchemical research include understanding source-receptor relationships, the chemical quality of precipitation, and the influence trace chemicals have on clouds and climate.

As soon as pollutants enter a cloud, they become intertwined with the cloud processes at both the macro- and microscales. The active dynamics of a large convective cloud, for instance, often serves to vent the planetary boundary layer, pulling pollutants into the free troposphere along with the moisture that serves as the fuel for cloud formation (see Fig. 13.1).

Clouds form in an atmosphere that is rich in water vapor and diverse chemical compounds. These components reside in the Earth's gaseous envelope and so become organized is ways that respond to gravitational attraction and to the steady input of energy from the Sun. If we are to understand the properties and behavior of atmospheric clouds, we must first understand the environment in which clouds form.

Composition

What actually makes up or constitutes the Earth's atmosphere? The answer to such a question depends on time, for the composition of the atmosphere has been and still is changing. For some compounds the changes are slow, for others they are rapid. Here, we explore how the atmosphere evolved to its present form, the fundamental nature of the substances making up the atmosphere, and how those substances are currently distributed throughout the atmosphere.

History of the atmosphere

Our atmosphere has evolved dramatically since the Earth formed some 4.6 billion (4.6 × 109) years ago. Initially, while the Earth was still hot from the gravitational accumulation of matter in the solar nebula and from the heavy bombardment of rocky planetesimals, the atmospheric composition was more akin to that of the Sun. It is during this early heavy-bombardment period that the Moon most likely formed and helped stabilize the Earth's rotational axis. The primary or proto-atmosphere contained much hydrogen and helium, reflecting the dominant composition of the stellar matter.

Clouds form when atmospheric conditions become appropriate. The necessary ingredients for cloud formation include an adequate supply of water vapor, aerosol particles, and a mechanism for cooling the air. Water, in condensed form, is, of course, the primary component of any cloud. The aerosol particles provide the sites for condensation by offering places where water vapor can adhere. Cooling lets condensation take place by causing the physical temperature to fall below the dew point and the concentration of water vapor to exceed the equilibrium value.

The properties of the resulting cloud depend on both macro- and micro scale processes. The macroscale air motions, driven by atmospheric pressure gradients, move the requisite water vapor and aerosol particles upward toward lower pressures, thereby cooling the air and generating excess water vapor. The microphysical processes, driven initially by aerosol abundance, determine how the excess vapor is utilized within the cloud. This chapter offers an overview of the macroscale processes that produce environments conducive to cloud formation. Subsequent chapters focus on the evolution of the microphysical properties.

Cooling mechanisms

A cloud, the visible aggregation of liquid or ice particles suspended in the atmosphere, requires cooling so that the partial pressure of vapor, pH2O ≡e, can exceed the equilibrium vapor pressure of the condensate, eeq(T), a function that increases monotonically with temperature T.

Thermodynamics is the study of energy and its transformations. Traditionally, thermodynamics has been used to understand the transfer of “heat” and the mechanical work that can be realized from it. Mechanical heat engines became important to society during the Industrial Revolution, but natural heat engines exist, too, although we do not often speak of thunderstorms and hurricanes in such terms. All such “engines” ultimately derive their organized, macroscopic motions from the random motions of the molecules making up the systems.

Thermodynamics is useful because it applies to many phenomena in the Universe. At the same time, the discipline can become abstract, especially when one is not sure exactly what part of the Universe is being considered. It is therefore important to define the components and variables of the system carefully. Traditionally, the “system” is the part of the universe we are interested in for a particular application. Any system is separated from the rest of the Universe by a “control surface” situated between it and its “surroundings” or “environment”. The “system” plus “surroundings” together make up the “universe”. The “state” of the system at any given time is specified by the magnitudes of all relevant macro-scopic variables, such as temperature, pressure, and volume of the system. In atmospheric physics, a commonly used system is a parcel of air, an amount of gas that is small enough to have uniform properties, but large enough to contain many molecules.

Importance of clouds

What would our world be like without clouds? Unimaginable – quite literally – for clouds are essential for our lives on earth. Humans, and for that matter most other land-dwelling species, would simply not exist, let alone thrive in the absence of the fresh water that clouds supply. The favorable climate we have enjoyed for thousands of years might also not exist in the absence of atmospheric water and clouds. A world without clouds would be different indeed.

Clouds contribute to the environment in many ways. Clouds, through a variety of physical processes acting over many spatial scales, provide both liquid and solid forms of precipitation and nature's only significant source of fresh water. Under extreme circumstances, however, clouds and precipitation may not form at all, leading to prolonged droughts in some regions. At other times and places, too much rain or snow falls, giving rise to devastating floods or blizzards. Liquid rain drops bring usable water directly to the surface, while simultaneously carrying many trace chemicals out of the atmosphere and into the ecosystems of the Earth. Chemical wet deposition thereby supplies nutrients (and sometimes toxic compounds) to both terrestrial and aquatic lifeforms, as well as the weak acids responsible for the weathering of the Earth's crust. The solid forms of precipitation contribute in additional ways to the world as we know it. Snow, for instance, forms the winter snowpacks that dramatically affect the radiation balance and climate of high latitudes on a seasonal basis.

The naming of cloud types began in the early 1800s. Jean Babtiste Lamarck, a French naturalist, proposed a naming scheme in about 1802, but his nomenclature did not catch on, perhaps because of the cumbersome French terms he used. Lamarck is, however, credited with proposing that clouds be identified with the level (high, middle, or low) at which they appear in the atmosphere. Then, within a year or so, Luke Howard, an English chemist/pharmacist, suggested that clouds could be grouped into four main categories: Cirrus (Latin for hair curl), Cumulus (heap), Stratus (layer), and Nimbus (rain). The simple Latin terminology likely helped Howard's categorization to be accepted more readily.

The cloud classification scheme used internationally today thus traces its roots back to the suggestions of Lamarck (for organization into three levels) and Howard (for use of Latin names). The names in Latin follow the convention used in biology: Genus species. Subspecies or varieties are often appended to distinguish one species of cloud from another. The names given to various cloud types, now as then, are all based on human observations from the surface. The modern methods of observing clouds (aircraft, radar, satellites, etc.) were not available to the early observers. The determination of cloud type is based on visual appearance only, mainly the shape/form and size of the cloud. The altitude of cloud base is difficult to judge precisely, but the form usually helps us distinguish high clouds from mid-level and low clouds.

Clouds contribute to our lives in both direct and indirect ways. Clouds are at once the most visible elements of the sky and the dominant contributors to the weather we experience every day. Less apparent, but perhaps even more important, are the roles clouds play in the global energy and water budgets that determine the climate of Earth. Through their ability to precipitate, clouds provide virtually all of the fresh water on Earth and a crucial link in the hydrologic cycle. Clouds are also the most effective agents cleansing the atmosphere, although some terrestrial and aquatic ecosystems pay the price for anthropogenic emissions of chemicals into the air. With ever-increasing importance being placed on quantifiable predictions, whether to forecast the local weather or to anticipate changes in global climate, we must learn how clouds operate in the real atmosphere, where two-way interactions with natural and anthropogenic pollutants are common.

Clouds have been the subject of observation for centuries, but serious systematic investigations began only a few decades ago. For all practical purposes, the study of clouds can be traced back to Luke Howard, the English pharmacist who began, around 1803, the system of naming cloud types that we still use today (see Appendix A). Speculation about the composition and nature of clouds persisted for many years. Direct observations from balloons and aircraft helped greatly to develop a base of empirical knowledge upon which the research community could later build testable hypotheses.

Overview

The electrification of large cumulonimbus clouds often leads to lightning, an exciting, yet sometimes frightening phenomenon of nature. The generation of electric fields of sufficient strength to cause electrical breakdown of the air involves a broad range of scales, all the way from the size of individual units of electrical charge (electrons and protons) to that of the cloud itself. Both the microphysical processes and the macroscale motions of air within thunderstorms must act in coordinated ways for charges to separate and large electric fields to develop. The subjects of charging mechanisms, electric-field evolution, and discharge events are each extensive in their own rights. The treatment here offers a basic overview to show the interconnectivity of the various scales and processes.

Electrical structure of thunderstorms and the atmosphere

Thunderstorms, by definition, produce thunder, the audible consequence of lightning. Ever since the pioneering work of Franklin and d'Alibard in the mid-1700s, it has been recognized that lightning is an electrical phenomenon resulting from excess charges in various parts of the parent cloud. Subsequent investigations by Wilson in the 1920s and many others have enabled us to develop a valid conceptual model of thunderstorm electrification.

The simplest charge structure of a thunderstorm is envisioned to be similar to that shown in Fig. 14.1. A convective storm during its mature stage of development exhibits an anvil that represents the upper outflow of cloudy air from the storm's interior.

Change occurs in nature when a system is not in equilibrium. Natural systems are frequently thrust out of balance, into a disequilibrium state that cannot persist in the long run. Sunlight shining on a puddle, for instance, heats the ground and forces the water to evaporate. Radiative transfer constitutes one way change and transformations are brought about. Atmospheric systems (e.g., clouds) routinely interact with their environments through exchanges of matter and energy, and they respond to external forces (e.g., pressure gradients). Environmental interactions shift the mechanical, thermal, and chemical balances that allowed a system to be at equilibrium in the first place, forcing the system to change, be transformed to another state. The atmosphere is never at equilibrium, so it is continually changing in one way or another. Indeed, clouds ultimately owe their existence to the disequilibrium forced by solar heating of the surface.

Deviations from equilibrium

Deviations from equilibrium drive changes in the observable properties of a system. A ladder that is kicked may suddenly topple, an example of mechanical forces that become imbalanced. A turkey placed in an oven gradually gets hot, an example of energy transfer because of a temperature difference. A mixture of air and natural gas (methane, CH4) exposed to a spark explodes, an example of one set of compounds (oxygen and methane) transforming to another (carbon dioxide and water) because of differences in the chemical potentials of the compounds involved.