Chapter 10 Water In The Atmosphere

Introduction

The atmosphere contains varying amounts of **water vapour**, ranging from virtually none in very dry conditions to about four percent of the air's volume in humid regions. This water vapour is an essential component of the atmosphere and plays a crucial role in generating various weather phenomena.

Water exists in the atmosphere in all three physical states:

• **Gaseous:** Water vapour (invisible).
• **Liquid:** Water droplets (forming clouds, rain, mist, dew).
• **Solid:** Ice crystals (forming clouds, snow, frost, hail).

Moisture enters the atmosphere primarily through **evaporation** from bodies of water (oceans, lakes, rivers, soil moisture) and through **transpiration** from plants (the release of water vapour from leaves). This creates a continuous cycle of water exchange – the **hydrological cycle** – involving the atmosphere, oceans, and continents through processes like evaporation, transpiration, condensation, and precipitation.

Humidity

**Humidity** is the general term used to describe the amount of water vapour present in the air. It can be quantified in different ways:

• **Absolute Humidity:** This measures the actual mass of water vapour contained within a specific volume of air. It is typically expressed in **grams of water vapour per cubic meter of air ($\text{g/m}^3$)**. Absolute humidity varies geographically, being generally higher in warm, moist areas and lower in cold or dry areas.
• **Relative Humidity:** This is a ratio, expressed as a percentage, comparing the amount of water vapour currently in the air to the maximum amount of water vapour the air *can* hold at that specific temperature and pressure. The air's capacity to hold water vapour is directly dependent on its temperature; warmer air can hold significantly more water vapour than colder air. Therefore, if the temperature of a parcel of air changes, its capacity changes, and its relative humidity will also change even if the absolute humidity remains the same. Relative humidity is typically highest over oceans (source of moisture) and lowest over continents (especially dry landmasses).

Relative humidity is calculated as: $$ \text{Relative Humidity} (\%) = \frac{\text{Actual water vapour content (Absolute Humidity)}}{\text{Maximum water vapour capacity at current temperature}} \times 100 $$

Saturated Air And Dew Point

Air is said to be **saturated** when it holds the maximum possible amount of water vapour it can at its current temperature and pressure. At saturation, the rate of evaporation equals the rate of condensation, and the air cannot absorb any additional moisture.

The **dew point** is the temperature to which a given parcel of air must be cooled (at constant pressure and water vapour content) for it to become saturated. If the air is cooled further below the dew point, the excess water vapour will condense into liquid water or sublimate into ice (if the dew point is below freezing).

Evaporation And Condensation

The amount of water vapour in the atmosphere is regulated by two opposing processes: evaporation (adding moisture) and condensation (removing moisture in gaseous form).

Evaporation

**Evaporation** is the process by which liquid water is transformed into water vapour (a gas). The primary driver of evaporation is **heat energy**. The heat required to change water from a liquid to a gaseous state is called the **latent heat of vaporisation**.

Factors that increase the rate of evaporation:

• **Higher Temperature:** Increases the energy of water molecules, allowing more to escape as vapour.
• **Lower Humidity:** When the air is less saturated (lower relative humidity), it has a greater capacity to absorb more water vapour.
• **Wind (Air Movement):** Wind blows away the layer of saturated air that forms just above the water surface, replacing it with drier, unsaturated air, thus facilitating further evaporation.

Condensation

**Condensation** is the process by which water vapour (a gas) transforms back into liquid water or ice (a solid). Condensation occurs when moist air is cooled to its dew point or below, causing it to become saturated and the excess water vapour to change state. Condensation involves the **release of heat energy** (the latent heat of condensation, which is the opposite of latent heat of vaporisation).

Condensation requires cooling of the air parcel. In the free atmosphere, this cooling often causes water vapour to condense onto tiny particles suspended in the air. These particles are called **hygroscopic condensation nuclei** because they have an affinity for water and readily absorb moisture. Common examples include dust, smoke, and salt crystals from ocean spray.

Condensation can also occur when moist air comes into contact with a surface colder than its dew point (leading to dew or frost formation). The process of condensation is influenced by the air's volume, temperature, pressure, and humidity. The most favorable condition for condensation in a large air mass is a decrease in its temperature.

Condensation can happen in several ways:

• Cooling air to its dew point while keeping its volume constant.
• Cooling air while reducing its volume (increasing pressure slightly).
• Adding more moisture to the air until it reaches saturation, even if the temperature doesn't change significantly (e.g., evaporation from a lake adding vapour to overlying air).

The key requirement is that the air reaches saturation. Once saturation is reached and cooling continues, condensation occurs.

Forms Of Condensation

When condensation occurs, the water vapour or moisture in the atmosphere can take various forms, depending on the temperature at which condensation happens and the location where it takes place (on surfaces or in the free air).

Dew

**Dew** forms when water vapour condenses directly onto solid surfaces near the ground, rather than on condensation nuclei within the air itself. This typically occurs on cool objects like grass blades, leaves, car roofs, or stones. For dew to form, the surface temperature must cool down to or below the air's dew point, and the dew point must be **above the freezing point of water ($0^\circ\text{C}$)**. Ideal conditions for dew formation include clear skies (allowing rapid surface cooling by radiation), calm air (to prevent mixing with warmer air aloft), high relative humidity, and long, cold nights.

Frost

**Frost** is similar to dew but forms when condensation occurs on cold surfaces at a temperature **at or below the freezing point ($0^\circ\text{C}$)**. In this case, the dew point is also at or below freezing. Instead of forming liquid water droplets, the water vapour changes directly into minute ice crystals through a process called deposition (or desublimation). White frost forms under similar conditions as dew (clear sky, calm air, high humidity, long nights), but with the critical difference of sub-freezing temperatures at the surface.

Fog And Mist

**Fog** and **mist** are essentially clouds that form at or very near the ground surface. They occur when a mass of air containing a significant amount of water vapour is cooled to its dew point, causing condensation to take place on microscopic condensation nuclei (dust, smoke, salt) suspended in the air.

The main difference between fog and mist lies in visibility and moisture content:

• **Fog:** Reduces visibility significantly, typically to less than 1 kilometer. It contains a lower density of water droplets than mist for a given number of nuclei. Fog often occurs in valleys (due to cold air drainage), over cold surfaces, or where warm, moist air moves over a colder surface (advection fog) or mixes with cold air. In urban or industrial areas, smoke particles act as abundant nuclei, contributing to fog formation and sometimes creating **smog** (smoke + fog).
• **Mist:** Contains more moisture than fog, with each condensation nucleus having a thicker layer of water around it. Visibility is generally better in mist than in fog, typically ranging from 1 to 2 kilometers. Mist is common in mountainous regions as warm, moist air rises and cools, or over coastlines.

Both fog and mist are composed of tiny suspended water droplets (or ice crystals at very low temperatures) that reduce visibility near the ground.

Clouds

**Clouds** are visible masses of minute water droplets or tiny ice crystals suspended in the free air at significant elevations above the Earth's surface. They form when moist air rises and cools below its dew point, leading to condensation or deposition onto condensation nuclei. Clouds take on various shapes and forms depending on the altitude at which they form, their composition (water droplets, ice crystals, or mixed), density, and vertical extent.

Clouds are broadly classified into four main types based on their appearance and general altitude: **Cirrus, Cumulus, Stratus, and Nimbus**. Combinations of these terms are used to describe more specific cloud types, often also indicating height (e.g., cirro- for high, alto- for middle, strat- for low).

• **Cirrus (Ci):** High-altitude clouds (above 8,000 m or 26,000 ft) composed entirely of ice crystals. They are thin, wispy, feathery, and white, often appearing as streaks across the sky.
• **Cumulus (Cu):** Puffy, detached clouds with flat bases and rounded, domed tops resembling cotton wool. They form through convection currents, typically at middle altitudes (4,000-7,000 m or 13,000-23,000 ft), but can have significant vertical development. They indicate rising air.
• **Stratus (St):** Layered, sheet-like clouds that cover large areas of the sky, often appearing gray and featureless. They form when air rises gently over a wide area or when different air masses mix. They are typically low-altitude clouds but can occur at various heights.
• **Nimbus (Ni):** These are rain-bearing clouds. The term 'nimbus' is added to other cloud types to indicate precipitation (e.g., cumulonimbus, nimbostratus). They are dense, dark gray or black, and often appear shapeless due to their thickness and association with active precipitation. **Nimbostratus** are layered rain clouds, while **Cumulonimbus** are towering rain clouds with strong vertical development.

Common combinations indicating height ranges:

• **High Clouds (above 6 km):** Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs).
• **Middle Clouds (2 km - 6 km):** Altocumulus (Ac), Altostratus (As).
• **Low Clouds (below 2 km):** Stratocumulus (Sc), Stratus (St), Nimbostratus (Ns).
• **Clouds with Vertical Development (extending through multiple layers):** Cumulus (Cu), Cumulonimbus (Cb).

Figure 10.1 and 10.2 would typically show photographs or diagrams of these cloud types to aid visual identification.

Precipitation

When condensation continues in clouds, the water droplets or ice crystals grow in size. When they become too large and heavy for the air's resistance (updrafts) to support them against gravity, they fall to the Earth's surface. This release of moisture from the atmosphere is called **precipitation**. Precipitation can occur in various forms, either liquid or solid, depending primarily on the temperature profile of the atmosphere between the cloud and the ground.

• **Rainfall:** Precipitation in the form of liquid water droplets. This occurs when the temperature throughout the atmosphere, from the cloud to the ground, is above freezing ($0^\circ\text{C}$).
• **Snowfall:** Precipitation in the form of fine flakes of ice crystals. This happens when the temperature throughout the atmosphere, from the cloud to the ground, is below freezing ($0^\circ\text{C}$), and water vapour is converted directly into hexagonal ice crystals (deposition) that aggregate into snowflakes.

Sleet And Hail

Other forms of precipitation include sleet and hail, which are less common and more sporadic than rain or snow:

• **Sleet:** Consists of frozen raindrops or refrozen melted snowflakes. Sleet forms when precipitation (initially rain or melted snow) falls from a warmer layer of air (above freezing) through a sub-freezing layer of air near the ground. The droplets or melted flakes freeze into small pellets of ice before reaching the surface.
• **Hail:** Consists of hard, rounded pellets or irregular lumps of ice called hailstones. Hail forms within intense thunderstorms (cumulonimbus clouds) with strong updrafts. Water droplets or ice crystals are carried upwards into very cold parts of the cloud, where they accumulate layers of ice as they collide with supercooled water droplets. They are then brought down by downdrafts, potentially partially melting, and carried up again by updrafts, adding more ice layers. This process repeats until the hailstones become too heavy to be supported by the updrafts and fall to the ground. Hailstones can have a layered structure due to this process.

Types Of Rainfall

Rainfall can be classified into three main types based on the mechanism that causes the air containing moisture to rise and cool, leading to condensation and precipitation:

1. **Convectional Rainfall:** Occurs when the ground is heated intensely, warming the air above it. This warm air becomes less dense and rises rapidly as convection currents. As it rises, it cools, leading to condensation and the formation of towering cumulonimbus clouds. This results in short-lived, intense rainfall, often accompanied by thunder and lightning. Convectional rain is common in tropical regions (especially near the equator) and continental interiors during the warmer parts of the day or year.
2. **Orographic (Relief) Rainfall:** Occurs when moist air is forced to rise as it encounters a topographic barrier, such as a mountain range. As the air ascends the windward slope, it cools adiabatically, reaches saturation, and the water vapour condenses, forming clouds and causing precipitation. The **windward slopes** (facing the incoming moist wind) receive heavy rainfall. As the air descends the leeward slope (the side sheltered from the wind), it warms adiabatically, its capacity to hold moisture increases, and it becomes drier. This descending air creates a **rain-shadow area** on the leeward side, which receives significantly less rainfall and is often much drier.
3. **Cyclonic (Frontal) Rainfall:** Occurs in association with low-pressure systems (cyclones), particularly along the fronts (boundaries) between different air masses in the middle latitudes (extra tropical cyclones). When warmer, moist air meets colder, denser air, the warmer air is forced to rise over the cold air. This uplift cools the warm air, causing condensation and precipitation. The nature and intensity of the rain depend on the type of front (warm, cold, occluded). (This type of rainfall is discussed in detail in Chapter 9 on Atmospheric Circulation and Weather Systems).

World Distribution Of Rainfall

Precipitation is not uniformly distributed across the globe. Different regions receive vastly different amounts of rainfall annually, and the seasonal patterns also vary. These variations are influenced by factors like latitude, proximity to oceans, mountain ranges, pressure belts, and wind systems.

General patterns of rainfall distribution include:

• Rainfall generally **decreases steadily from the equator towards the poles**. The equatorial belt receives high rainfall due to intense heating and convection.
• **Coastal areas** typically receive more rainfall than the **interior parts of continents**, especially on the side facing moist winds from the ocean, due to the availability of moisture and triggering mechanisms like orographic lift or convection near coastlines.
• Rainfall is generally **higher over the oceans** than over landmasses, as oceans are the primary source of atmospheric moisture.
• Between approximately $35^\circ$ and $40^\circ$ North and South latitudes, the **eastern coasts** of continents tend to receive heavier rainfall, decreasing westward.
• Between approximately $45^\circ$ and $65^\circ$ North and South latitudes, influenced by the prevailing **westerlies**, the **western margins** of continents receive higher rainfall, which decreases eastward towards the continental interior.
• Where mountain ranges run parallel to the coast, the **windward slopes** receive significantly more rainfall than the **leeward slopes** (rain-shadow areas), due to orographic uplift.

Based on annual precipitation amounts, major global precipitation regimes include:

• **Heavy Rainfall (Over 200 cm/year):** Characterizes the equatorial belt, windward slopes of mountains in the cool temperate zones (western coasts), and coastal areas affected by monsoons.
• **Moderate Rainfall (100 - 200 cm/year):** Found in continental interiors and some coastal areas.
• **Low Rainfall (50 - 100 cm/year):** Occurs in central parts of tropical lands and the eastern and interior parts of temperate lands.
• **Very Low Rainfall (Less than 50 cm/year):** Typical of rain-shadow zones, continental interiors in high latitudes, and desert regions.

The **seasonal distribution** of rainfall is also critical for judging its effectiveness for purposes like agriculture. Some regions, like the equatorial belt and the western parts of cool temperate regions, experience relatively even rainfall throughout the year. Other regions, like monsoon lands or Mediterranean climates, have highly seasonal precipitation patterns.

Exercises

Multiple Choice Questions

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Answer The Following Questions In About 30 Words

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Answer The Following Questions In About 150 Words

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Project Work

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