Why does water fall from clouds? How do cloud droplets turn into raindrops? Why are there different types of precipitation? These are some of the questions we will answer in this chapter, as we explore how cloud droplets, and in particular ice crystals, grow through various processes and turn into precipitating particles. From rain to hail and snow, we will describe specific conditions and processes in which various types of precipitation occur.

Among all the gases and aerosols constituting the atmosphere, some are particularly detrimental to human health and to the environment. Their concentration varies greatly from land to ocean, and from city to countryside. Weather phenomena and the stability of the atmosphere play a large role in mixing or concentrating these pollutants and also influence their formation. Thus, in describing and understanding the distribution of these polluting gases and aerosols, we will review and apply some of the fundamental principles encountered in this book.

Energy enters the Earth system in the form of solar radiation, preferentially heating the ground and the tropical latitudes. The resulting temperature contrasts set the atmosphere in motion, as heat is transferred upward and poleward. Weather is largely the result of this transfer of heat by atmospheric motions. In this chapter, we will explore the nature of heat and radiation, the origin of temperature contrasts on Earth, the mechanisms by which energy is transferred, and the implications for weather.

We now understand the basic mechanism behind condensation and the formation of dew: decrease the temperature until saturation is reached, and water vapor will start leaving the gas phase to form dew or frost. But what does it take for a cloud to form in the atmosphere? Why would the temperature decrease? What does water vapor condense upon in the absence of blades of grass and solid objects? In this chapter we will describe the cloud formation process and the conditions that are conducive to the formation of different types of clouds.

How is it possible to predict, often with great accuracy, upcoming weather? Is a 5-day forecast as good as a 2-day forecast? Can we predict the weather out to the distant future? How is a forecast made? Here we will explore the basic principles behind numerical weather forecasting, as well as the limits of numerical models in accurately predicting weather beyond a certain time. We will discuss the capabilities and limitations of computer models of the atmosphere, methods to address uncertainty, tools to make weather model forecasts more useful, and the role of human forecaster in analyzing and interpreting computer-generated weather predictions.

Water in its different forms creates the possibility of a wide array of atmospheric phenomena, from clouds to rainbows, and from lightning to hurricanes. Before we can describe these phenomena in detail, however, we need to understand how and when water can change phase, from solid (ice), to liquid (liquid water), and gas (water vapor). To that end, we will explore different aspects of the water cycle, in particular condensation and evaporation, leading to the formation of clouds and precipitation. We will define quantities of great importance to this exploration: saturation, relative humidity, and dew point temperature.