Study Guide: Moisture, Clouds, and Precipitation

Changes of State of Water

• Calorie: Specific heat to raise the temperature of 1 gram of water by 1°C.

• Latent Heat: Hidden heat during phase changes.

  • Evaporation: Liquid to gas, 600 cal/g.

  • Condensation: Gas to liquid, releases latent heat.

  • Melting: Solid to liquid, 80 cal/g.

  • Freezing: Liquid to solid, releases latent heat.

  • Sublimation: Solid to gas, 680 cal/g.

  • Deposition: Gas to solid.

• Water Molecule Bonds: Strong bonds resist heat change, causing high thermal inertia and surface tension.

Humidity (Water Vapor in the Air)

• Saturated Air: Air full of water vapor, depends on temperature.

  • Warm air holds more moisture.

  • Water vapor adds pressure (vapor pressure).

• Measuring Humidity:

  • Mixing Ratio: Water vapor in air vs. dry air (g/kg).

  • Relative Humidity: Actual water vapor vs. capacity at a given temperature.

    • 100% RH = Saturation (dew point).

Relative Humidity

• Changes:

  • Adding Moisture: Increases RH.

  • Removing Moisture: Lowers RH.

  • Temperature Dependence:

    • High temp = low RH.

    • Low temp = high RH.

    • 100% RH = Dew Point (causes clouds, fog, and condensation).

Adiabatic Heating and Cooling

• Adiabatic Changes: Temperature changes without heat exchange with the environment.

  • Compression: Air warms.

  • Expansion: Air cools.

• Adiabatic Rates:

  • Dry Adiabatic Rate (DAR): 1°C per 100 meters.

  • Wet Adiabatic Rate (WAR): Slower cooling (0.5°C to 0.9°C per 100 meters) due to latent heat.

Processes That Lift Air

1. Orographic Lifting: Air rises over terrain, creates rainshadow deserts.

2. Frontal Wedging: Warm air rises over cool air, part of storm systems.

3. Convergence: Air flows together and rises due to low pressure.

4. Localized Convective Lifting: Unequal surface heating causes air to rise.

Stability of Air

• Absolute Stability: Cooler, dense air resists rising; fair weather.

• Absolute Instability: Warm, light air rises, causing bad weather.

• Conditional Instability: Stable for unsaturated air, unstable for saturated air.

Condensation

• Water vapor changes to liquid, forming dew, fog, or clouds.

• Needs surfaces to condense on (plants, windows, dust, etc.).

• 100% Relative Humidity needed for condensation.

Clouds

• Made of: Tiny water droplets or ice crystals.

• Cloud Types by Height:

  • Cirrus: High, thin clouds (>6000m).

  • Cumulus: Mid-level, fair weather clouds (2000-6000m).

  • Stratus: Low-level, sheet-like clouds (<2000m).

  • Cumulonimbus: Strong vertical development, storm clouds.

Fog (Ground-Level Clouds)

• Types of Fog:

  • Advection Fog: Warm air moves over cool surface.

  • Radiation Fog: Earth cools rapidly at night.

  • Upslope Fog: Humid air moves up a slope and cools.

  • Steam Fog: Cool air over warm water.

  • Frontal Fog: Forms when rain evaporates into cool air.

Precipitation

• Bergeron Process: Ice crystals collect water vapor, form snowflakes, may melt into rain.

• Collision-Coalescence Process: Large droplets form in warm clouds and collide.

• Types of Precipitation:

  • Rain: Free-falling water droplets.

  • Snow: Free-falling ice crystals.

  • Sleet: Frozen raindrops.

  • Glaze: Freezing rain.

  • Hail: Ice pellets with multiple ice layers, formed in strong updrafts of cumulonimbus clouds.