Chapter5

Chapter 5: Atmospheric Moisture

Evaporation and Condensation

  • Evaporation: Process where molecules break free from liquid water.

  • Condensation: Opposite of evaporation, where water vapor becomes liquid.

  • Sublimation: Change directly from ice to water vapor without liquid phase.

  • Deposition: Reverse process from water vapor to ice.

Saturation and Equilibrium

  • Hypothetical jar with pure water shows:

    • Evaporation begins, water vapor starts accumulating.

    • Increasing water vapor increases condensation rate.

    • Equilibrium state: Occurs when rates of condensation and evaporation are equal, known as saturation.

Humidity and Vapor Pressure

  • Humidity: Measure of water vapor in the air.

  • Vapor Pressure: Portion of total atmospheric pressure due to water vapor; influenced by temperature and density of water vapor molecules.

  • Saturation Vapor Pressure: Maximum water vapor capability of air; only depends on temperature.

Humidity Measurements

  • Absolute Humidity: Density of water vapor in grams per cubic meter of air.

  • Specific Humidity: Mass of water vapor in a given mass of air.

  • Saturation Specific Humidity: Maximum specific humidity analogous to saturation vapor pressure.

  • Mixing Ratio: Mass of water vapor relative to other gases; the maximum is the saturation mixing ratio.

Relative Humidity

  • Relative Humidity (RH) Measure: RH = (specific humidity / saturation specific humidity) x 100%

    • More water vapor in warm air than cold; depends on actual moisture content and temperature.

    • Increasing temperature allows more vapor, reducing RH ratio.

Dew Point

  • Dew Point: Temperature at which air must cool to become saturated.

    • Air is unsaturated when above dew point; at this point, air temperature equals dew point, leading to saturation.

    • Frost Point: Dew point below 0 °C.

Nucleation

  • Homogeneous Nucleation: Formation of droplets by chance collisions of water vapor molecules under supersaturated conditions.

  • Heterogeneous Nucleation: Droplet formation on hygroscopic particles, called condensation nuclei.

    • The nuclei dissolve into the water, forming a solution.

Supercooled Water and Ice Crystals

  • At temperatures between 0 °C and -4 °C, surplus water vapor condenses to form supercooled water.

  • Formation of ice crystals near 0 °C requires rare ice nuclei with a six-sided structure.

Measuring Humidity

  • Sling Psychrometer: Measures humidity with wet bulb and dry bulb thermometers.

    • Wet bulb depression helps determine dew point and relative humidity.

Heat Index

  • Combines humidity and high temperatures to indicate apparent temperatures.

    • Danger Levels:

      • 41 °C to 54 °C: Muscle cramps or heat exhaustion likely.

      • Above 54 °C (129 °F): Extremely dangerous; possible heat stroke.

Diabatic and Adiabatic Processes

  • Diabatic Process: Energy added/removed from a system (e.g., air warmed by conduction).

    • Energy moves from higher to lower temperatures (2nd law of thermodynamics).

  • Adiabatic Process: Temperature changes without heat added or removed.

    • Dry Adiabatic Lapse Rate (DALR): Cools at approximately 1.0 °C/100 m (5.5 °F/1000 ft).

Lifting Condensation Level

  • When air rises and cools to dew or frost point, condensation begins at the Lifting Condensation Level (LCL).

  • Saturated Adiabatic Lapse Rate (SALR): Cools at about 0.5 °C/100 m (3.3 °F/1000 ft), not constant depending on specific conditions.

Environmental Lapse Rate

  • Environmental Lapse Rate (ELR): Temp change through still air; affects rising balloons and air temperature at different altitudes.

Dew and Frost Formation

  • Dew: Liquid condensation occurs during cool, windless nights; Frost: Similar process but occurs below 0 °C, forming ice crystals.

    • Frozen dew: Formed when saturation creates liquid dew and then freezes.

Types of Fog

  • Fog: Cloud at ground level.

    • Radiation Fog: Occurs due to cooling after nighttime radiation loss.

    • Advection Fog: Warm, moist air over cooler surfaces.

    • Upslope Fog: Forms via adiabatic cooling as air rises.

    • Precipitation Fog: Evaporation from falling raindrops.

    • Steam Fog: Cold, dry air mixes with warm, moist air.

Dew Point and Ascent

  • Dew Point Lapse Rate: Decreases by about 0.2 °C/100 m (1.1 °F/1000 ft) as air rises.

  • As unsaturated air is lifted, its temperature approaches dew point at 0.8 °C for every 100 m.

Conclusion

  • The next chapter focuses on cloud development and forms.