Water in the Atmosphere

The air around us contains water vapour, varying from zero to four percent by volume. This moisture plays a critical role in weather phenomena, existing in three forms: gaseous, liquid, and solid. It is constantly exchanged between the atmosphere, oceans, and continents through processes such as evaporation, transpiration, condensation, and precipitation.

Learning Outcomes:

1. Understand the role of water vapour in the atmosphere.
2. Learn the processes of evaporation, transpiration, and condensation.
3. Explore the formation of clouds, fog, frost, and dew.
4. Understand precipitation types and their distribution.

Humidity and Its Measurements

Humidity refers to the amount of water vapour in the air. There are multiple ways to measure it:

1. Absolute humidity: The actual water vapour content in the air, measured in grams per cubic meter. It depends entirely on temperature.
2. Relative humidity: The percentage of moisture in the air compared to its full capacity at a given temperature. As air temperature changes, its capacity to hold moisture also changes.
3. Saturation: Air that is fully saturated with moisture at a given temperature cannot hold any more moisture. The temperature at which this occurs is known as the dew point.

Evaporation and Condensation

Evaporation is the process by which water transforms from a liquid to a gaseous state due to heat. The rate of evaporation depends on the following:

1. Temperature: As temperature increases, water absorption and retention capacity also increase.
2. Air movement: Faster moving air replaces the saturated air layer with an unsaturated layer, increasing evaporation.
3. Humidity: Less moisture content in the air creates a higher potential for absorption.

On the other hand, condensation transforms water vapour back into a liquid when air loses heat. Factors influencing condensation include:

1. Air cooling: When moist air cools, it reaches a point where it cannot hold any more moisture, causing condensation.
2. Contact with cold surfaces: When warm air meets a cold surface, condensation occurs.
3. Condensation nuclei: Small particles like dust, smoke, and salt from the oceans serve as nuclei around which water condenses.

Forms of Condensation

Condensation results in various forms, depending on temperature and location:

1. Dew: Water droplets that form on cooler surfaces like grass or plant leaves when the dew point is above freezing.
2. Frost: Minute ice crystals that form on cold surfaces when condensation occurs at or below freezing temperatures.
3. Fog and Mist: When moist air cools suddenly, it condenses on fine dust particles, creating fog or mist. Fog occurs when condensation happens close to the ground, while mist contains more moisture and occurs often in mountain regions. Smog forms in urban areas where fog mixes with smoke.
4. Clouds: Clouds form when condensation occurs at higher altitudes. They are classified based on their height and appearance into cirrus, cumulus, stratus, and nimbus clouds.

Cloud Types and Characteristics

Clouds can be classified into four main types based on their appearance and altitude:

1. Cirrus clouds: Thin, feather-like clouds found at high altitudes (8,000 – 12,000 meters), always white in color.
2. Cumulus clouds: These resemble cotton wool, forming at mid-altitudes (4,000 – 7,000 meters), usually in patches.
3. Stratus clouds: Layered clouds covering large portions of the sky, formed by the loss of heat or the mixing of different air masses.
4. Nimbus clouds: Dense and dark clouds, often close to the ground, causing heavy rainfall. They are opaque and sometimes appear to touch the ground.

Additionally, combinations of these cloud types give rise to various cloud formations, such as:

• High clouds: Cirrus, cirrostratus, cirrocumulus.
• Middle clouds: Altostratus, altocumulus.
• Low clouds: Stratocumulus, nimbostratus.
• Vertical clouds: Cumulus, cumulonimbus.

Precipitation

Precipitation occurs when condensed water droplets become heavy enough to overcome the resistance of air and fall to the earth. It takes various forms:

1. Rainfall: The most common form, where liquid water falls to the ground.
2. Snowfall: Occurs when the air temperature is below 0°C, leading to the formation of snowflakes.
3. Sleet: Frozen raindrops that form when raindrops fall through a subfreezing layer near the ground.
4. Hailstones: Rounded pieces of ice formed when raindrops pass through colder layers and freeze multiple times, creating concentric layers of ice.

Types of Rainfall

Rainfall can be classified based on its origin into three types:

1. Convectional Rain: This occurs when air rises due to heating, cools, and condenses to form cumulus clouds. It is often accompanied by thunderstorms and is common in equatorial regions and continental interiors.
2. Orographic (Relief) Rain: When moist air is forced to ascend over mountains, it cools and condenses on the windward side, leading to precipitation. The leeward side remains dry, forming a rain shadow area.
3. Cyclonic (Frontal) Rain: Associated with cyclonic storms, this type of rainfall occurs when warm and cold air masses meet, and the warmer, lighter air rises over the denser, cold air, causing precipitation.

World Distribution of Rainfall

Rainfall distribution varies across the globe due to factors like latitude, proximity to oceans, and mountain ranges:

1. Rainfall generally decreases from the equator to the poles.
2. Coastal areas receive more rainfall compared to interior regions because oceans are significant sources of water vapour.
3. The eastern coasts of continents receive more rainfall in tropical regions, while in temperate regions, western coasts receive more rainfall due to westerlies.
4. Mountain ranges influence rainfall, with the windward slopes receiving more rain than the leeward sides.

Global Precipitation Regimes

Global precipitation regimes are categorized based on the total annual rainfall:

1. High rainfall areas: The equatorial belt, windward slopes of mountains, and coastal areas of monsoon regions receive over 200 cm of rainfall annually.
2. Moderate rainfall areas: Interior continental areas and coastal regions receive rainfall ranging from 100 to 200 cm annually.
3. Low rainfall areas: Central parts of tropical lands and temperate regions receive rainfall between 50 and 100 cm per year.
4. Dry regions: The rain shadow zones and high-latitude areas receive less than 50 cm of annual rainfall.

Important Note: Rain shadow areas remain dry due to the blocking of moist air by mountain ranges, preventing the leeward side from receiving rainfall.

Comparative Table: Precipitation Types

Important Note: Precipitation not only influences climate but also affects agriculture, water resources, and ecosystems.

MCQ

Which of the following is a form of solid precipitation?

1. Hail
2. Rain
3. Mist
4. Fog
   Answer: Hail