Chpt 6 AtmosWater

Learning Objectives

  • Describe the heat properties of water, including energy transfer during phase changes between solid, liquid, and gas.

  • Describe expressions of humidity: vapor pressure, specific humidity, relative humidity, and dew-point temperature.

  • Explain adiabatic lapse rates and discuss stable, unstable, and conditionally unstable atmospheric conditions.

  • Identify the requirements for cloud formation and explain the major cloud types, including fog.

  • Describe precipitation processes in the atmosphere and enumerate types of precipitation at the ground surface.

Chapter 6 Outline

Water’s Unique Properties

  • Phase Changes and Heat Exchange

  • Rates of Evaporation and Condensation

Humidity

  • Vapor Pressure

  • Specific Humidity

  • Relative Humidity

Atmospheric Stability

  • Adiabatic Processes

  • Atmospheric Conditions

Clouds and Fog

  • Cloud Formation

  • Cloud Types and Identification

  • Processes that Form Fog

Precipitation

  • Precipitation in Warm Clouds

  • Precipitation in Cold Clouds

  • Precipitation Reaching the Surface

Water's Unique Properties

  • Pure water is colorless, odorless, and tasteless; it rarely occurs in nature due to being a good solvent.

  • Liquid water density is 1 g/cm³ and makes up 70% of the human body.

  • Water exists as solid (ice), liquid (water), and gas (water vapor).

Phase Changes and Heat Exchange

Types of Phase Changes

  • Freezing and melting

  • Condensing and evaporating

  • Depositing and sublimating

  • Physical state changes do not affect temperature.

Heat Exchange Values

  • Latent Heat of Melting: +80 cal (absorbed)

  • Latent Heat of Vaporization: +540 cal (absorbed)

  • Latent Heat of Freezing: -80 cal (released)

  • Latent Heat of Condensation: -540 cal (released)

Humidity

  • Humidity indicates the amount of water vapor in the air, influenced by air and water vapor temperatures.

  • Relative Humidity: Ratio of current water vapor to max capacity at a given temperature.

  • Saturation Vapor Pressure: Maximum vapor pressure exerted by water vapor at a given temperature; increases with temperature.

Atmospheric Stability

  • Stability indicates an air parcel's response to vertical displacement:

    • Stable: Resists displacement

    • Unstable: Continues to rise until density and temperature match surrounding air

    • Conditionally Unstable: Characteristics depend on environmental conditions.

Adiabatic Processes

  • Adiabatic Cooling/Warming: Changes in air temperature due to expansion or compression without heat exchange.

    • Dry Adiabatic Rate (DAR): Average cooling rate of dry air is 10 °C/1000 m.

    • Moist Adiabatic Rate (MAR): Average cooling rate of saturated air is about 6 °C/1000 m.

Clouds and Fog

  • Clouds are collections of water droplets and/or ice crystals; fog is a low-altitude cloud.

  • Warm clouds (above 0°C) and cold clouds (below –40°C) differ in formation and temperature.

Cloud Formation

  • Occurs through adiabatic cooling by vertical lift, reaching the dew-point temperature, leading to saturation and condensation.

Cloud Types

  • Classified by altitude:

    • Low (up to 2000 m)

    • Middle (2000–6000 m)

    • High (6000–13000 m)

  • Classified by form:

    • Cirroform (hairlike)

    • Stratiform (layered)

    • Cumuliform (dense)

Precipitation

  • Precipitation occurs when droplets/crystals grow large enough to fall under gravity.

    • In warm clouds: Collision-coalescence mechanism.

    • In cold clouds: Bergeron process.

Types of Precipitation at Surface

  • Rain: Liquid water droplets.

  • Snow: Ice crystals.

  • Freezing rain: Supercooled liquid droplets freeze upon contact.

  • Sleet: Ice pellets.

Summary of Concepts

  • Water's properties, phase changes, humidity definitions, atmospheric stability conditions, cloud types, and precipitation mechanisms are critical in understanding weather dynamics.