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4: Water Vapor

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    46736

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    Introduction

    Water can exist as a solid, liquid, or gas under typical Earth conditions. Figure \(\PageIndex{1}\) illustrates the water cycle on Earth. As we learned earlier, the process by which liquid water becomes water vapor is called evaporation, and it requires energy. As the Sun heats the Earth, liquid water molecules from oceans, lakes, and rivers absorb energy and change into water vapor. Water vapor is also released by plants through a process called transpiration. The opposite process is called condensation, where water vapor cools at higher altitudes and becomes liquid water or ice, releasing energy. Tiny droplets condense on dust or sand particles in the air. Condensation is significant in atmospheric science because it is the process that allows clouds to form.

    Arrows show the movement of water from the ocean to the air and back to the land.
    Figure \(\PageIndex{1}\): Earth’s water cycle is the path that water takes as it moves around Earth. This includes times when it flows, such as during rainfall, and places where it's stored, such as in ice, groundwater, lakes, rivers, and the ocean. Liquid water from such reservoirs evaporates, condenses into clouds, and returns to the surface as precipitation. (Public Domain; The Water Cycle via NASA/JPL-Caltech).

    Clouds are composed of millions and billions of tiny liquid water droplets. How do they form? Why are they there? Before we can understand clouds in the atmosphere, we need to explore concepts like how humidity is defined and what saturation means.

    Learning Objectives

    By the end of this chapter, you should be able to:

    • Differentiate between relative humidity, specific humidity, absolute humidity, wet-bulb temperature, mixing ratio, and dew point.
    • Use the Heat Index to determine personal comfort.
    • Describe the conditions for saturation to occur.
    • Distinguish between Dew and Frost.
    • Describe how fog forms and the different types of fogs.
    • Distinguish between different types of clouds.

    • 4.1: Saturation
      This page explains saturation in evaporation and condensation within a closed jar, emphasizing that it occurs when both rates are equal, influenced by temperature. It introduces vapor pressure, particularly saturation vapor pressure, and its relationship with temperature as per the Clausius-Clapeyron equation. Higher temperatures enable more water vapor, and boiling happens when saturation vapor pressure equals atmospheric pressure, exemplified by water boiling at 100 °C at sea level.
    • 4.2: Humidity
      Some measures of atmospheric humidity, such as absolute humidity, are less useful because they vary with temperature and pressure. Specific humidity is highlighted as a more reliable measure, while relative humidity is mentioned as the most common, indicating the current water vapor ratio to maximum saturation.
    • 4.3: The Heat Index
      This page discusses the importance of moisture in the atmosphere, its effect on weather and stability, and the phases of water. It explains the lifting condensation level (LCL) for cloud formation and contrasts the moist adiabatic lapse rate with the dry rate, highlighting the role of latent heat. These concepts are vital for understanding atmospheric dynamics.
    • 4.4: Dew and Frost
      We explain how dew and frost form under specific conditions, with dew appearing on clear nights when temperatures reach the dew point, leading to condensation. In winter, frost can form through deposition or by freezing dew, with distinctions in their appearance and composition. It discusses the influence of temperature on their formation and removal, which is significant for morning transportation.
    • 4.5: Fog
      This page explores different types of fog, including radiation fog formed by overnight cooling, valley fog occurring in valleys, advection fog from warm air over cool surfaces, and evaporation fog, like steam fog. It also describes upslope fog from lifted air, freezing fog with frozen droplets, and ice fog seen in Arctic regions.
    • 4.6: Clouds
      We discuss 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, we highlight special clouds such as lenticular and mammatus, as well as fog, which is categorized based on cooling and moisture processes.
    • 4.7: Exercises
      Some exercises to review the topics covered in this chapter.

    Thumbnail: Water drops on a glass by Mark Zuid is licensed under CC BY 2.0.

    This page titled 4: Water Vapor was last modified on Wed, 02 Sep 2026 04:08:18 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Neel Desai and Alicia Mullens.