Atmosphere and Weather Concepts

  • Hydrologic Cycle:

    • Approximately 3.75 imes 10^{16} gallons of water in vapor phase in the atmosphere at any time.

    • This amount gets recycled 40 times per year.

    • Average residence time of a water vapor molecule in the atmosphere is about 9 days.

  • Saturation:

    • Definition of saturation: Limiting value of humidity. Approaches by:

    1. Adding more water vapor to the air.

    2. Cooling the air (reducing the average kinetic energy of molecules).

  • Measures of Humidity:

    1. Absolute Humidity (A.H.): A.H. = rac{ ext{Mass of water vapor} }{ ext{Volume of the air containing vapor} }

    2. Specific Humidity (S.H.): S.H. = rac{ ext{Mass of water vapor} }{ ext{Total mass of air containing vapor} }

    3. Mixing Ratio:
      M.R. = rac{ ext{Mass of water vapor} }{ ext{Mass of dry air that contains vapor} }

    4. Saturation Mixing Ratio: rac{ ext{Mass of water vapor} }{ ext{Mass of dry air from a saturated parcel} }

  • Dew Point Temperature:

    • The temperature to which air must be cooled at constant pressure to reach saturation. Example: If air is saturated at 10 degrees Celsius, it must be cooled to that temperature under constant pressure.

  • Dew Point Depression:

    • The difference between the temperature and the dew point.

  • Wet Bulb Temperature:

    • Temperature of unsaturated air that becomes saturated through evaporation.

    • The temperature that air is cooled at constant pressure by evaporating water into it until it becomes saturated.

Condensation and Cloud Formation

  • Condensation:

    • Occurs when water vapor condenses onto particles such as dust or salt (called condensation nuclei). The smaller the particle, the easier it is for vapor to condense.

  • Haze Formation:

    • Very small condensation nuclei form tiny droplets.

    • Can survive in unsaturated air but cannot grow significantly.

  • Fog Formation:

    1. Cool the Air: Radiative cooling at night can lead to fog formation.

    2. Advection Fog: Air cools when it moves over a colder surface.

Precipitation Formation

  • Clouds and Precipitation:

    • Precipitation develops from cloud droplets, requiring approximately 1 million droplets to form an average raindrop.

    • Droplet growth depends on the relationship between actual vapor pressure and equilibrium vapor pressure.

    • If actual vapor pressure > equilibrium vapor pressure → droplet grows.

    • If actual vapor pressure < equilibrium vapor pressure → droplet shrinks.

  • Factors Influencing Equilibrium Vapor Pressure:

    1. Curvature: More vapor molecules are required for equilibrium over a curved surface compared to a flat surface.

  • Types of Precipitation:

    1. Rain: Liquid Water (T > 0°C).

    2. Snow: Frozen Water (T < 0°C).

    3. Sleet: Frozen Raindrops.

    4. Freezing Rain: Liquid water that freezes on surfaces with a temperature below 0°C (occurs with inversion).