9: Clouds and Precipitation
- Page ID
- 55050
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- 9.1: Cloud Condensation Nuclei
- This page discusses how air saturation leads to water vapor condensation into liquid water, facilitated by aerosols, or cloud condensation nuclei (CCN). It notes the composition of aerosols, including dust, sea salt, and organic compounds, and emphasizes their role in cloud formation. The presence of aerosols reduces the humidity needed for clouds to form, with specific aerosols like Ice Nuclei promoting the creation of ice crystal clouds.
- 9.2: Cloud Naming Conventions
- This page discusses the classification of clouds by appearance, shape, altitude, and water phase. It presents two main categories: cumuliform (vertical motion) and stratiform (stable). Examples include cumulus, cumulonimbus, nimbostratus, and cirrus. Clouds can be warm, cold, or mixed phase, with altitude prefixes like "cirro" and "alto." Additionally, it highlights special clouds such as lenticular and mammatus, as well as fog, which is categorized based on cooling and moisture processes.
- 9.3: Cloud Droplets
- This page covers the processes of cloud droplet and raindrop formation, highlighting the importance of cloud condensation nuclei (CCNs) and two main precipitation theories: collision-coalescence in warm clouds and the ice phase process in cold clouds. It also discusses ice crystal formation mechanisms, including homogeneous and contact freezing. The Wegener-Bergeron-Findeisen process illustrates the growth of ice crystals by utilizing surrounding liquid droplets.
- 9.4: Precipitation Types
- This page discusses the ice phase process in global precipitation, detailing types such as rain, snow, sleet, and hail, all influenced by environmental conditions and the journey of ice crystals. It explains how variations in temperature and air mass interactions determine the type of precipitation. Subsequent chapters will delve deeper into these atmospheric dynamics.
Chapter 9: Clouds and Precipitation — Introduction
Clouds are among the most visible signs of activity in Earth’s atmosphere. Their formation brings together many of the ideas introduced in the previous chapter. Water evaporates from Earth’s surface and enters the atmosphere as water vapor. As air rises and cools, it may approach saturation, creating conditions in which water vapor can condense. But reaching saturation is only the beginning of the story. To understand how clouds and precipitation form, we need to examine what happens to water once condensation begins.
Cloud formation depends on tiny particles in the atmosphere known as cloud condensation nuclei. Water can condense onto these particles to form the microscopic droplets that make up a cloud. Depending on atmospheric temperature and other conditions, clouds may contain liquid water droplets, ice crystals, or a mixture of both. The altitude, atmospheric stability, and movement of air also help determine the shapes and types of clouds that develop. Learning to recognize these cloud types can therefore provide clues about what is happening in the atmosphere and what weather may follow.
Cloud droplets and ice crystals are initially far too small to fall to the ground as precipitation. They must grow through interactions with other droplets and crystals before becoming large enough to overcome the upward motions of the surrounding air. Different processes dominate this growth in warm and cold clouds, eventually producing rain, snow, sleet, or hail depending on the conditions within the cloud and the atmosphere below it.
In this chapter, we will follow water from its first condensation onto microscopic particles through the development of clouds and, ultimately, precipitation. In doing so, we will connect the concepts of humidity, saturation, and atmospheric stability from Chapter 8 to the clouds and precipitation we observe in everyday weather.