Atmospheric Processes and Global Climate Change

Weather Processes and Phenomena: Moisture Processes, Causes and Types of Precipitation

Moisture Processes

Moisture is a fundamental component of weather and precipitation. The atmosphere can hold varying amounts of water vapor, which is invisible. The amount of water vapor the air can hold depends on temperature; warmer air can hold more moisture. Key moisture processes include evaporation, transpiration, condensation, and sublimation.

• Evaporation: The process by which liquid water changes into water vapor. This occurs from bodies of water (oceans, lakes, rivers) and soil.
• Transpiration: The process by which water is released from plants into the atmosphere as vapor.
• Condensation: The process by which water vapor changes back into liquid water, forming clouds. This typically occurs when air cools.
• Sublimation: The process by which ice or snow changes directly into water vapor without first melting into liquid water.

Causes of Precipitation

Precipitation occurs when water droplets or ice crystals in clouds become heavy enough to fall to the Earth's surface. This requires sufficient moisture and a mechanism for lifting air.

1. Orographic Lifting: Air is forced to rise over mountains. As it rises, it cools and condenses, leading to precipitation on the windward side of the mountain. The leeward side often experiences a rain shadow.
2. Frontal Lifting: When warm and cold air masses meet, the warmer, less dense air rises over the colder, denser air. This can occur along warm fronts, cold fronts, and stationary fronts.
3. Convectional Lifting: Uneven heating of the Earth's surface causes air to become buoyant and rise. This is common in tropical regions and leads to the formation of cumulonimbus clouds and heavy showers.
4. Convergence: When air flows together from different directions, it is forced to rise. This can occur in low-pressure systems.

Types of Precipitation

Precipitation can take various forms depending on the temperature profile of the atmosphere.

Type of Precipitation	Description	Temperature
Rain	Liquid water droplets falling to the Earth's surface.	Above 0°C
Drizzle	Very small, fine water droplets falling to the Earth's surface.	Above 0°C
Snow	Ice crystals falling to the Earth's surface.	Below 0°C
Sleet	Raindrops that freeze as they fall through a layer of cold air.	Near 0°C, with a layer of freezing air aloft
Freezing Rain	Raindrops that freeze upon contact with a sub-zero surface.	Near 0°C, with a sub-zero surface temperature
Hail	Balls or irregular lumps of ice that are repeatedly lifted and dropped within a thunderstorm.	Below 0°C, with strong updrafts

The type of precipitation experienced in a particular location is influenced by factors such as latitude, altitude, proximity to large bodies of water, and prevailing wind patterns.