Chapter+12-Moisture%2C+Clouds%2C+and+Precipitation

Earth & Space Science Survey

Chapter 12: Moisture, Clouds, and Precipitation

• Image: Fog "tsunami" taken of Panama City Beach, February 2012

Understanding Water Vapor

• Definition: Water vapor is an odorless, colorless gas capable of changing states of matter under Earth's temperatures and pressures.
• Importance: Essential for understanding atmospheric processes.
• Unique Properties of Water:
  • Exists as solid, liquid, and gas in the atmosphere.
  • Composed of 2 hydrogen atoms bonded to 1 oxygen atom.
  • Molecules are in constant motion; motion increases with temperature.
  • Differences among states of matter arise from molecular arrangements.

States of Matter

Solid (Ice)

• Molecular Arrangement: Tight, orderly, with strong molecular attractions.
• Movement: Molecules vibrate in fixed positions.
• Melting Process: As temperature increases, bonds are broken, transitioning ice to liquid water.

Liquid (Water)

• Molecular Arrangement: Tightly packed, moving fast enough to slide past one another.
• Evaporation Process: Fast-moving molecules escape, increasing water vapor and lowering temperature.

Gas (Water Vapor)

• Molecular Arrangement: Widely spaced, exhibiting energetic, random motion.
• Compressibility: Water vapor is compressible.
• State Change Mechanism: Changes in state involve alterations in molecular distances and interactions without altering the substance itself.

Energy and Heat in Phase Changes

• Heat Requirements:
  • Energy (measured in calories) is absorbed or released during changes of state.
  • Definition of Calorie: Amount of heat to raise the temperature of 1 gram of water by 1°C.
  • Ex: Adding 10 calories raises water temperature by 10°C.
• Latent Heat:
  • Heat added or removed during state changes without temperature change.
  • Example: Ice-water mixture remains at 0°C until all ice melts.
• Latent Heat Values:
  • Melting: 80 calories absorbed/g of ice at 0°C to become liquid water.
  • Fusion (Freezing): 80 calories released/g of water at 0°C to become ice.
  • Vaporization: ~600 calories absorbed/g of water at 100°C to become gas.
  • Condensation: ~600 calories released/g of water vapor at 100°C to become liquid.
  • Sublimation: 680 calories released when solid converts directly into gas.
  • Deposition: 680 calories absorbed when gas converts directly into solid.

Water Vapor in the Atmosphere

• Proportion in Atmosphere: Water vapor varies from 0.1% to 4% of atmospheric gases.
• Key Role: Most significant gas for understanding atmospheric processes.
• Humidity: General term for the amount of water vapor present in the air.

Vapor Pressure

• Definition: Part of total atmospheric pressure attributed to water vapor content.
• Evaporation Dynamics: As water evaporates, vapor pressure increases, leading to saturation when evaporation equals condensation.
• Temperature Dependency: Air's water vapor capacity increases with temperature.

Measuring Water Vapor Content

Methods:

1. Mixing Ratio: Mass of water vapor in a unit of air compared to dry air.
   • Expressed in mass units (kg) and unaffected by pressure/temperature.
   • Time-consuming due to required atmospheric sampling.

Relative Humidity

• Definition: Ratio of actual water vapor present to saturation value at a given temperature.
• Indicators of Saturation: Changes when humidity levels increase or decrease, influenced by temperature shifts.

Dew Point Temperature

• Definition: Temperature to which air must be cooled for saturation.
  • Significance: Measure of actual moisture content; high dew points indicate moist air; low dew points indicate dry air.
  • **Climate Depend