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4: Water Vapor

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    9553

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    Water vapor is one of the gases in air. Unlike nitrogen and oxygen which are constant in the bottom 100 km of the atmosphere, water-vapor concentration can vary widely in time and space. Most people are familiar with relative humidity as a measure of water-vapor concentration because it affects our body’s moisture and heat regulation. But other humidity variables are much more useful in other contexts.

    Storms get much of their energy from water vapor — when water vapor condenses or freezes it releases latent heat. For this reason we carefully track water vapor as it rises in buoyant thermals or is carried by horizontal winds. The amount of moisture available to a storm also regulates the amount of rain or snow precipitating out.

    What allows air to hold water as vapor in one case, but forces the vapor to condense in another? This depends on a concept called “saturation”.

    • 4.0: Vapor Pressure at Saturation
      This page explores atmospheric pressure, focusing on water vapor's behavior and saturation, detailing concepts like vapor pressure and saturation humidity. It explains the Clausius-Clapeyron equation's role in relating saturation vapor pressure to temperature. The impact of altitude on boiling point, saturation vapor pressure, and the need for longer cooking times in high-altitude conditions is also discussed, including a formula demonstrating boiling temperature decrease with elevation.
    • 4.1: Moisture Variables
      This page elaborates on calculating key moisture variables like specific humidity, mixing ratio, and density of saturated water vapor using temperature and pressure. It covers related concepts such as relative humidity and dew point, along with their definitions and equations. The significance of these metrics in meteorology is highlighted, with practical applications demonstrated through sample calculations and psychrometric graphs.
    • 4.2: Total Water
      This page examines key metrics of water in the atmosphere, such as liquid water content (LWC) and mixing ratios, essential for understanding atmospheric moisture. It introduces the concept of precipitable water, vital during storms. Additionally, it details the calculation of water column depth in the troposphere using specific equations, resulting in a depth of 0.057 m, highlighting the impact of large mixing ratios on water accumulation.
    • 4.3: Lagrangian Budgets
      This page explores the properties of moist air, emphasizing its unique characteristics compared to dry air, including latent heat and cloud formation. It covers the significance of total-water conservation during adiabatic processes and details the moist and dry adiabatic lapse rates, their equations, and modeling techniques using computer tools.
    • 4.4: Water Budget at a Fixed Location
      This page examines the water budget in a hypothetical air cube, emphasizing important terms like horizontal advection and precipitation while introducing reasoning methods in atmospheric science. It covers the derivation of water vapor flux equations, conversion to kinematic forms, and the influence of environmental factors on evaporation rates.
    • 4.5: Humidity Instruments
      This page covers hygrometers, instruments for measuring humidity, and differentiates them from hydrometers. It details various types like dew-point, hair, and psychrometers, as well as advanced devices such as radiosondes and differential absorption lidars (DIAL). The summary includes their operation principles, accuracy, and applications, emphasizing technological advancements in humidity measurement, including modern electronics and remote sensing technologies utilizing satellites and GPS.
    • 4.6: Review
      This page covers measures of water vapor content in the air, including vapor pressure, mixing ratio, and relative humidity. It explains the relationship between air temperature and saturation humidity, and differentiates between Lagrangian and Eulerian frameworks for humidity dynamics. The significance of these concepts in atmospheric processes such as cloud formation and precipitation is also highlighted.
    • 4.7: Homework Exercises
      This page includes exercises designed to enhance understanding of meteorological concepts, focusing on humidity, temperature, and atmospheric processes. Activities involve data collection on weather variables, calculations of saturation vapor pressures, and applying thermodynamic equations like Tetens’ formula and the Clausius-Clapeyron equation.

    This page titled 4: Water Vapor is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Roland Stull via source content that was edited to the style and standards of the LibreTexts platform.