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Lecture 7 Part 2: Water and Atmospheric Moisture - In-Depth Notes

Water's Unique Properties

  • Water is essential for life.

  • Less than 0.03% of Earth's water is stored in the atmosphere.

  • Water is the only substance that exists in all three states of matter: liquid, solid, and vapor.

  • Energy changes during phase transitions of water help drive atmospheric circulation and global weather patterns.

Surface Tension

  • Defined as the property of liquid surfaces that allows them to resist external force due to water molecule cohesion.

  • Water's relatively high surface tension results from strong hydrogen bonds between molecules, making it second only to mercury.

  • This high surface tension causes:

    • Water to form beads on surfaces.

    • Water droplets to coalesce into raindrops.

Rain Drop Formation

  • Cloud-condensation nuclei: tiny particles (approx. 2 μm in diameter) around which moisture droplets form.

  • Typical raindrops have a diameter of about 2000 μm.

Capillarity

  • Surface tension plays a vital role in capillarity, which is the ability of water to rise through narrow spaces against gravity.

  • Water forms a film around soil grains, making it accessible to plants despite gravitational forces.

Water's Surface Interaction with Atmosphere

  • Water molecules at the surface prefer to bond with each other, but fewer available molecules at the air-water interface lead to stronger bonds at that layer.

  • This phenomena creates a barrier through surface tension, impacting interactions between water and atmosphere significantly.

The Water Cycle

  • The hydrologic cycle is a continuous loop encompassing the movement of water between the atmosphere, hydrosphere, lithosphere, and biosphere.

Phase Changes and Heat Exchange

  • Phase Changes: (Molecular structure illustrated)

    • Sublimation: Energy absorbed.

    • Deposition: Energy released.

    • Vaporization: Energy absorbed.

    • Condensation: Energy released.

    • Freezing: Energy released.

    • Melting: Energy absorbed.

  • Water exhibits unique thermal properties:

    • Its density changes as it cools, expanding from 4°C to 0°C, which is unusual compared to most liquids.

Latent Heat

  • Latent Heat refers to the energy exchanged during phase changes of a substance.

  • Latent Heat of Evaporation:

    • Crucial for cooling processes (e.g., cooling skin by the evaporation of sweat).

    • Significant for Earth's energy budget.

Water Vapour

  • Water vapor is a significant greenhouse gas, trapping heat radiated from Earth.

  • It creates a positive feedback loop: as Earth warms, more water vapor is held in the air, thus enhancing warming.

Relative Humidity

  • Humidity quantifies water vapor in the air.

  • Relative humidity = (actual water vapor content / maximum possible at a given temperature) x 100%.

  • Changes in temperature directly impact relative humidity.

Saturation and Dew Point

  • Saturation: occurs at 100% relative humidity, leading to active condensation (clouds, fog, precipitation).

  • Dew-Point Temperature: the temperature at which the air becomes saturated.

Daily Relative Humidity Patterns

  • An inverse relationship exists: as temperature increases, relative humidity decreases.

Vapour Pressure

  • Vapour pressure is the portion of total air pressure exerted by water vapor.

  • At saturation vapor pressure, the air contains maximum water vapor at a given temperature.

  • For every 10°C increase in temperature, saturation vapor pressure nearly doubles.

Maximum Specific Humidity

  • Specific Humidity: the mass of water vapor in grams per kilogram of air at a given temperature.

  • Maximum Specific Humidity: the maximum mass of water vapor possible in 1 kg of air at a specific temperature.

Stability and Instability

  • Parcel: a body of air with specific temperature and humidity.

  • Stable Air Parcel: returns to its initial position when disturbed.

  • Unstable Air Parcel: ascends or descends after being disturbed.

Clouds

  • Clouds are visible aggregates of tiny moisture droplets and ice crystals in the air.

  • Initial formation comes from small moisture droplets around 20 μm in diameter.

  • Fog is defined as clouds in contact with the ground, reducing visibility to less than 1 km.

Cloud Formation

  • Clouds form when air parcels rise and cool, reaching 100% relative humidity, leading to condensation onto cloud nucleation sites.

Cloud Classes and Types

  • Low Clouds (up to 2000 m):

    • Stratus, Stratocumulus

  • Middle Clouds (2000-6000 m):

    • Nimbostratus, Altostratus, Altocumulus

  • High Clouds (6000-13000 m):

    • Cirrus, Cirrostratus, Cirrocumulus

  • Vertically Developed:

    • Cumulus (from near surface to 13000 m), Cumulonimbus

Cloud Formation Releases Heat

  • When water vapor condenses into droplets (e.g., in cumulus clouds), substantial latent heat is released, impacting atmospheric temperatures and weather processes.

Types of Fog

  • Different types include:

    • Radiation Fog

    • Advection Fog

    • Upslope Fog

    • Steam Fog

    • Precipitation Fog

    • Ice Fog

Note: Videos linked in the original lecture for visual reference.