Chapter 11 Water In The Atmosphere

The atmosphere contains water in various forms, collectively known as atmospheric moisture. While the amount of water vapour in the air is relatively small, varying from 0 to 4% by volume, it is a critical component of the atmosphere and plays a vital role in weather and climate phenomena. Water in the atmosphere exists in three states: gaseous (water vapour), liquid (water droplets in clouds, rain), and solid (ice crystals in clouds, snow, hail).

Atmospheric moisture is continuously exchanged between the atmosphere and the Earth's surface (oceans, continents, plants) through the processes of:

• Evaporation: Transformation of water from liquid to gaseous state, primarily from water bodies.
• Transpiration: Release of water vapour from plants into the atmosphere.
• Condensation: Transformation of water vapour from gaseous to liquid or solid state, forming clouds, fog, dew, frost, etc.
• Precipitation: Release of condensed water from the atmosphere to the Earth's surface as rain, snow, hail, etc.

This continuous movement and transformation of water is known as the hydrological cycle.

The amount of water vapour present in the air is referred to as humidity. Humidity can be expressed in different ways quantitatively:

• Absolute Humidity: The actual mass of water vapour present in a unit volume of air. It is typically measured in grams per cubic meter ($g/m^3$). The air's capacity to hold water vapour is highly dependent on its temperature; warmer air can hold more water vapour. Absolute humidity varies significantly from place to place depending on temperature and moisture sources.
• Relative Humidity: The ratio of the actual amount of water vapour in the air to the maximum amount of water vapour the air can hold at a given temperature and pressure, expressed as a percentage. Relative humidity indicates how close the air is to saturation. As air temperature changes, the air's capacity to hold moisture changes, which affects the relative humidity even if the absolute humidity remains constant (e.g., if temperature drops, relative humidity increases). Relative humidity is generally higher over oceans (due to abundant moisture source) and lower over continents (especially inland and arid regions).

When the air contains the maximum amount of water vapour it can hold at a specific temperature and pressure, it is said to be saturated. At this point, the air cannot absorb any more moisture. The temperature at which a given sample of air becomes saturated is called its dew point.

Evaporation And Condensation

The amount of water vapour in the atmosphere is controlled by the opposing processes of evaporation and condensation.

Evaporation is the physical process by which liquid water changes into water vapour (a gas). Heat energy is the primary driver of evaporation; the energy required to change water from liquid to gas without changing its temperature is called the latent heat of vaporization. Warmer temperatures increase the rate of evaporation and the air's capacity to hold water vapour. If the air is dry (low moisture content), it has a greater potential to absorb moisture through evaporation. Air movement (wind) also enhances evaporation by replacing moist air layers close to the surface with drier air, allowing more water to evaporate.

Condensation is the process by which water vapour changes back into liquid water or solid ice. Condensation occurs when moist air is cooled to its dew point or below, causing the excess water vapour to transform into a liquid or solid state. Condensation is essentially the reverse of evaporation and involves the release of latent heat. If water vapour directly transforms into a solid (ice) without passing through the liquid state, it is called sublimation (in the context of deposition from gas to solid, sometimes called deposition or desublimation).)

For condensation to occur in free air, cooling is typically needed. Water vapour condenses onto tiny solid particles suspended in the air, called hygroscopic condensation nuclei. These particles, which can include dust, smoke, pollen, and especially salt particles from the ocean, attract and absorb water, facilitating the formation of water droplets. Condensation can also happen when moist air comes into contact with a surface colder than its dew point.

Condensation depends on factors like the degree of cooling, the relative humidity, and the volume, temperature, and pressure of the air. The most common way for condensation to occur is through a decrease in air temperature, bringing the air to or below its dew point.

After condensation, atmospheric moisture takes various forms depending on the temperature and location of condensation:

Dew

Dew forms when water vapour condenses directly onto cold solid surfaces near the ground, such as grass blades, leaves, or car roofs (Figure 11.1 shows dew droplets). It occurs when the surface cools by radiation at night to a temperature at or below the dew point of the surrounding air. The dew point must be above freezing ($0^\circ C$) for dew to form as liquid water droplets. Ideal conditions for dew formation include clear skies (allowing rapid cooling by radiation), calm air (preventing mixing with warmer air above), high relative humidity, and long nights (allowing sufficient time for cooling).

Image depicting dew, which is moisture condensed as liquid droplets on surfaces cooled below the dew point.

Frost

Frost is similar to dew but forms when condensation occurs below the freezing point ($0^\circ C$) of water. This means the dew point of the air is at or below freezing. Instead of forming liquid water droplets, the water vapour transitions directly into tiny ice crystals (through deposition or desublimation) on the cold surfaces (Figure 11.2 shows frost crystals).

Image showing frost, which consists of ice crystals formed by the deposition of water vapour on surfaces cooled below freezing.

The conditions favorable for frost formation are the same as for dew: clear skies, calm air, high relative humidity, and cold, long nights. The crucial difference is that the surface temperature, and consequently the dew point, must be $0^\circ C$ or lower.

Fog And Mist

Fog and Mist are clouds that form very close to the ground surface, significantly reducing visibility. They occur when an air mass containing a considerable amount of water vapour is cooled to its dew point, causing condensation to happen on microscopic dust, smoke, or salt particles (condensation nuclei) suspended in the air.

• Fog: A dense concentration of tiny water droplets or ice crystals suspended in the air near the surface, reducing visibility to less than 1 kilometer. Fog is often thicker and drier than mist. It commonly forms when warm, moist air moves over a cold surface or when cold and warm air masses mix. In urban and industrial areas, smoke and pollution particles increase the number of condensation nuclei, contributing to fog formation (sometimes forming 'smog', a mixture of smoke and fog).
• Mist: Similar to fog but less dense, with visibility generally greater than 1 kilometer but less than 2 kilometers. Mist contains more moisture than fog, with each condensation nucleus having a thicker layer of water around it. Mists are frequent in mountainous areas where moist air rises and cools as it encounters cold slopes.

Clouds

Clouds are visible masses of minute water droplets or tiny ice crystals suspended in the atmosphere at considerable elevations above the Earth's surface (Figure 11.1 & 11.2, labeled as images of dew and frost in the original text, are not clouds. Figure 11.1 and 11.2 below are typical cloud images). They form when moist air rises, cools adiabatically (due to expansion at lower pressure), reaches saturation, and water vapour condenses or deposits onto condensation nuclei.

Image showing thin, wispy cirrus clouds high in the sky.

Image showing puffy, cotton-like cumulus clouds.

Clouds are classified based on their height, appearance (form), density, and transparency. The four main types of clouds based on form and appearance are:

Cirrus

Cirrus clouds (Ci) are high-altitude clouds, typically forming between 8,000 and 12,000 meters ($8,000 - 12,000 \, m$). They are thin, wispy, and feathery in appearance, composed entirely of ice crystals. Cirrus clouds are always white and often indicate fair weather, though they can signal the approach of a warm front.

Cumulus

Cumulus clouds (Cu) are detached, puffy clouds that look like cotton wool or cauliflower (Figure 11.2 above shows an example). They typically form at lower to middle altitudes (4,000-7,000 m base) due to convection. They have flat bases and rounded tops. Cumulus clouds often indicate fair weather but can develop into larger clouds associated with precipitation.

Stratus

Stratus clouds (St) are layered or sheet-like clouds that cover large areas of the sky, often resembling fog lifted above the ground. They are typically low-level clouds, formed when layers of air cool uniformly or when different air masses mix. Stratus clouds can produce light drizzle or mist. They can make the sky appear grey and overcast.

Nimbus

Nimbus is a term used to describe clouds that produce precipitation. Nimbus clouds are dense and opaque, often appearing dark grey or black due to their thickness. They can form at various altitudes. Nimbostratus (Ns) are layered precipitation clouds covering the entire sky, bringing continuous rain or snow. Cumulonimbus (Cb) are towering cumulus clouds with extensive vertical development, reaching high altitudes, associated with heavy showers, thunderstorms, and sometimes hail.

Combinations of these basic types define more specific cloud types based on height and form (e.g., Cirrostratus, Altocumulus, Stratocumulus). High clouds (above 6,000 m) are primarily Cirrus, Cirrocumulus, and Cirrostratus. Middle clouds (2,000-6,000 m) include Altocumulus and Altostratus. Low clouds (below 2,000 m) include Stratus, Cumulus (with limited vertical extent), Stratocumulus, and Nimbostratus. Clouds with significant vertical development that span multiple height zones are Cumulus (congestus) and Cumulonimbus.

Precipitation

Precipitation is the release of moisture from the atmosphere to the Earth's surface in liquid or solid form. It occurs when the condensed water droplets or ice crystals within clouds grow large enough that the air resistance can no longer support them against gravity, causing them to fall.

Precipitation takes various forms depending on the temperature profile of the atmosphere as the moisture falls:

• Rainfall: Precipitation in the form of liquid water droplets. This is the most common type of precipitation in warmer climates and seasons.
• Snowfall: Precipitation in the form of ice crystals (snowflakes). Snow forms when the temperature throughout the entire atmospheric column from the cloud to the ground is at or below freezing ($0^\circ C$). Moisture is released as hexagonal ice crystals that aggregate into snowflakes.
• Sleet: Precipitation consisting of pellets of ice, often formed when raindrops or partially melted snowflakes fall through a layer of sub-freezing air near the ground surface and refreeze before hitting the surface.
• Hailstones: Larger, rounded or irregular lumps of ice that fall from cumulonimbus clouds during thunderstorms. Hail forms when water droplets or ice pellets are repeatedly carried upwards into very cold regions of the cloud by strong updrafts, accumulating layers of ice before finally falling to the ground. Hailstones have a layered internal structure.

Types Of Rainfall

Rainfall is primarily triggered by processes that cause moist air to rise and cool to saturation. Based on the mechanism of uplift, rainfall is classified into three main types:

Convectional Rain

Convectional rain occurs when the ground surface is intensely heated, causing the overlying air to warm, become less dense, and rise rapidly in vertical convection currents. As the air rises, it cools, reaches its dew point, and condenses to form cumulus clouds, which can grow into towering cumulonimbus clouds. Convectional rainfall is typically heavy but of short duration, often accompanied by thunder and lightning (thunderstorms). It is common in hot, humid regions like the equatorial belt, particularly during the warmer parts of the day or during summer in continental interiors of the Northern Hemisphere where surface heating is strong.

Orographic Rain

Orographic rain, also known as relief rain, occurs when a moist air mass is forced to rise as it encounters a mountain range or elevated terrain. As the air ascends the windward slope of the mountain, it cools due to expansion at lower pressures, reaches saturation, and moisture condenses, leading to cloud formation and precipitation. The side of the mountain facing the incoming wind (windward slope) receives the majority of the rainfall. As the air descends the leeward (downwind) slope, it warms due to compression, and its capacity to hold moisture increases. This results in significantly less rainfall, creating a dry region known as a rain-shadow area on the leeward side of the mountain.

Cyclonic Rain

Cyclonic rain (also known as frontal rain in mid-latitudes) is associated with the lifting of air within weather systems characterized by low pressure and converging winds, particularly cyclones. In extra-tropical cyclones, rain occurs along frontal boundaries where warm, moist air is lifted over colder, denser air. Warm fronts produce broad areas of steady, moderate precipitation as warm air gently overrides cold air. Cold fronts cause a narrow band of intense showers or thunderstorms as cold air aggressively undercuts and lifts warm air. Tropical cyclones also produce cyclonic rainfall, characterized by torrential rain concentrated in the eyewall and spiral rainbands due to vigorous uplift and convection around the low-pressure center.

World Distribution Of Rainfall

Rainfall distribution across the globe is highly uneven, varying significantly in amount, intensity, and seasonality.

General patterns of rainfall distribution:

• Latitude: There is a general decrease in rainfall from the equator towards the poles. The equatorial belt, with high temperatures and intense convection, receives high rainfall. Subtropical regions dominated by descending air associated with subtropical highs tend to be dry (deserts). Mid-latitudes receive moderate rainfall due to frontal activity and cyclonic storms. Polar regions are characterized by low precipitation due to cold temperatures and limited moisture content in the air.
• Coastal vs. Interior: Coastal areas generally receive more rainfall than the interior of continents due to the proximity to moisture sources (oceans) and the influence of sea breezes and maritime air masses.
• Oceans vs. Landmasses: Rainfall is typically higher over oceans compared to landmasses because oceans are the primary source of atmospheric moisture through evaporation.
• Influence of Winds and Relief:
  • In the latitudes between 35° and 40° North and South (transition zones), rainfall is often heavier on the eastern coasts of continents and decreases towards the west, influenced by prevailing winds and pressure systems.
  • In the mid-latitudes (between 45° and 65° North and South), the prevailing Westerlies bring moisture from the west, causing rainfall to be heaviest on the western margins of continents and decrease eastward (orographically influenced by western mountain ranges).
  • Where mountain ranges are oriented parallel to the coast, the windward slopes receive abundant orographic rainfall, while the leeward side falls in a rain-shadow area and receives much less precipitation.

Based on the total annual precipitation, major precipitation regimes include:

• Heavy Rainfall (Over 200 cm): Found in the equatorial belt, the windward slopes of mountains along the west coasts in the cool temperate zone, and coastal areas in monsoon regions.
• Moderate Rainfall (100 - 200 cm): Occurs in interior continental areas in certain latitudes and some coastal regions.
• Low Rainfall (50 - 100 cm): Characterizes the central parts of tropical lands and the eastern and interior parts of temperate lands.
• Very Low Rainfall (Less than 50 cm): Typical of rain-shadow zones, desert regions in the interior of continents (especially subtropical deserts), and high latitudes (polar regions).

Seasonal distribution is also important; some regions (like the equatorial belt or western parts of cool temperate regions) receive rainfall relatively evenly throughout the year, while others experience distinct wet and dry seasons (like monsoon regions or Mediterranean climates).

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