Upwardly buoyant air parcels can produce powerful thunderstorms, towering cumulonimbus clouds producing rain, hail, lightning, and thunder. When these thunderstorms rotate, they can also spawn tornadoes and cause some of the most severe weather on Earth. We will now describe the different stages of thunderstorm development, the formation of lightning and thunder, and will explain the circumstances and mechanism by which tornadoes can form at the base of a supercell thunderstorm.

How will weather change as our planet warms and our climate changes? Will there be more droughts? How will precipitation patterns be affected? Will there be more or fewer storms? Will tropical cyclones be more, or less, intense? Will all regions of the world be affected? In this last chapter, we will put in perspective all that we have learned about the atmosphere and apply it to the pressing issue of global warming.

Wind, clouds, rain... Most midlatitude weather is a result of the movement of warm tropical air poleward and cold polar air equatorward as the atmosphere acts to reduce the strong temperature gradients in the middle latitudes. The contrast of warm and cold air masses is most pronounced along warm and cold fronts, where most of the weather (clouds and precipitation) is found. We will now build a full picture of the midlatitude, or extratropical, cyclone, the weather system in which this air mass encounter is occurring.

Where should we start with our study of the atmosphere? How should we first approach the weather? Like many scientists, meteorologists first make observations. Then they raise questions, and try to answer them. In this first chapter, we will quickly describe four of the elements, also called variables, of weather that meteorologists regularly observe, measure, and chart on weather maps, before we return to each of them for a more thorough exploration in subsequent chapters. Three of these elements are fairly intuitive: when concerned with the weather, we like to know how warm or cold it will be (temperature), whether it will be windy or not (wind), and whether it will rain or not (precipitation). The fourth variable, atmospheric pressure, is less intuitive, but it may be the most important to a meteorologist, as we will soon discover.

Local weather is largely the result of large weather systems in motion. Thus, meteorologists gain insight into the manifestations of weather by studying maps and images of the atmosphere on a number of scales, from global, to regional, down to local scales. In this chapter, we will learn how weather information is represented on weather maps and images to reveal the two-dimensional dynamics of weather systems.

Wind is a manifestation of air in motion, from light breezes by the sea to powerful jet streams in the upper troposphere. But what causes wind? Motion is always initiated by forces, and wind is no exception. We will now explore how pressure differences result in forces that set air in motion, how additional factors come into play at different scales of motion, and how a given pressure distribution generates specific wind patterns.

Satellite images, land weather station measurements, and buoy measurements were obtained from the National Climate Data Center at the National Oceanic and Atmospheric Administration (NOAA). Radiosonde measurements were obtained from the Earth System Research Laboratory at NOAA. ERA-Interim numerical analyses were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) after Dee et al. (2011). Infrared transmission spectra were obtained from the Gemini Observatory website after Lord (1992). NASA images were obtained from the Earth Observatory.

Since they impact our daily life, we tend to think about wind and weather locally, but they are in fact interconnected with the dynamics of heat transfer in the atmosphere. We will now step back and apply our concepts of differential heating, pressure gradient forces, and geostrophic motion at a global scale, to explore the implications for what is called the general circulation of the atmosphere. This will set the broader context within which individual weather systems exist.