Global Water Cycle: Distribution and Phase Changes

Global Water Cycle: Distribution and Processes

• Oceans contain 97.2% of Earth's water, most of which is saline. Ocean currents influence global heat distribution and weather patterns. Salinity varies due to evaporation and freshwater input.
• Ice sheets and glaciers store 2.15% of Earth's water, primarily in Antarctica and Greenland. Melting contributes to sea-level rise, altering ocean currents.
• Groundwater constitutes 0.63% of Earth's water and serves as a crucial freshwater source, with recharge rates that vary based on geological formations and precipitation patterns.
• Rivers, lakes, and atmospheric water vapor make up less than 0.02% of the total water. River flow depends on precipitation, snowmelt, and evapotranspiration.

Phase Changes of Water

• Evaporation changes water from liquid to gas (water vapor), driven by solar energy. Evaporation rates depend on temperature, humidity, wind speed, and surface area. Evapotranspiration combines evaporation and plant transpiration.
• Condensation converts water vapor from gas to liquid when air cools, forming clouds, influenced by condensation nuclei (e.g., dust particles).
• Precipitation returns water to the Earth's surface as rain, snow, sleet, or hail, influenced by atmospheric circulation, temperature gradients, and topography.
• Sublimation is the direct change from solid (ice/snow) to gas (water vapor), significant in cold, dry areas and important for glacier mass balance.
• Deposition is the direct change from water vapor to solid (ice/frost), common in cold, humid conditions, contributing to frost and ice crystal formation.

Humidity and Dew Point

• Humidity measures the amount of water vapor in the air (relative, absolute, specific humidity).
• Relative humidity is the percentage of water vapor relative to the maximum the air can hold at a given temperature.
• Dew point is the temperature at which air saturates and condensation begins, directly related to absolute humidity.
• Vapor pressure measures the partial pressure exerted by water vapor.
• Mixing ratio is the mass of water vapor per unit mass of dry air, not as affected by temperature changes as relative humidity.

Atmospheric Stability and Lifting Mechanisms

• Stable air resists vertical movement; displaced air returns to its original position, inhibiting cloud formation and precipitation.
• Unstable air readily rises; displaced air continues to rise, promoting cloud formation and precipitation, common in convection.
• Lifting Condensation Level (LCL) is the altitude where rising air becomes saturated, crucial for cloud formation.
• Convectional lifting: air rises due to unequal heating.
• Orographic lifting: air rises over mountains, causing windward precipitation and leeward rain shadows.
• Convergent lifting: air rises due to converging air masses, common in tropical regions.
• Frontal lifting: air rises due to colliding air masses with different temperatures.

Air Pressure: Measurement and Atmospheric Dynamics

• Air pressure is the force exerted by air molecules per unit area, decreasing exponentially with altitude.
• A barometer measures air pressure; types include mercury and aneroid barometers.
• Standard Atmosphere serves as a reference for average vertical temperature and pressure.
• High-pressure systems (anticyclones) have high air pressure, fair weather, and sinking air. They rotate clockwise in the Northern Hemisphere.
• Low-pressure systems (cyclones) have low air pressure, stormy weather, and rising air. They rotate counterclockwise in the Northern Hemisphere.
• Isobars connect points of equal air pressure, visualizing pressure gradients.

Influence of Temperature and Humidity on Air Pressure

• Temperature: Warmer air is less dense, resulting in lower pressure.
• Humidity: Humid air is less dense than dry air, resulting in slightly lower pressure.

Wind and Air Pressure: Convergence and Divergence

• Convergence: air flows together, increasing surface pressure and causing air to rise, often leading to cloud formation and precipitation.
• Divergence: air flows apart, decreasing surface pressure and causing air to sink, associated with clear skies.

Ideal Gas Law and Adiabatic Processes

• Ideal Gas Law (PV = nRT): relates pressure (P), volume (V), number of moles (n), temperature (T), and the ideal gas constant (R).
• Adiabatic processes: occur without heat exchange, influencing temperature changes in rising/sinking air.
• Dry Adiabatic Lapse Rate: 10°C per 1000m, is the rate at which dry air cools as it rises.
• Moist Adiabatic Lapse Rate: 6°C per 1000m, is the rate at which saturated air cools, affected by latent heat release.