Water In The Atmosphere

Evaporation And Condensation

Water exists in the atmosphere in three states: solid (ice), liquid (water droplets), and gaseous (water vapour). The processes of evaporation and condensation are fundamental to the atmospheric water cycle.

Dew

Definition: Dew is the deposition of water vapour directly onto a surface as liquid water droplets. It forms when a surface cools to or below the dew point of the surrounding air.

Formation:

• On clear, calm nights, surfaces (like grass, leaves, or cars) lose heat rapidly through radiation.
• As the surface cools, it cools the layer of air in contact with it.
• When this air layer reaches its dew point temperature (the temperature at which it becomes saturated with water vapour), the water vapour in the air condenses into liquid water droplets on the cool surface.

Conditions Favourable: Clear skies, calm winds, high humidity, and radiative cooling of surfaces.

Frost

Definition: Frost is the deposition of ice crystals directly onto a surface when the surface cools to or below the frost point of the surrounding air.

Formation:

• Similar to dew formation, frost occurs when a surface cools significantly during the night.
• However, frost forms when the surface temperature drops to or below the freezing point of water (0°C or 32°F) before condensation occurs.
• Water vapour in the air undergoes deposition directly from a gas to a solid (ice) state without passing through the liquid phase.

Conditions Favourable: Clear skies, calm winds, temperatures at or below freezing, and sufficient moisture in the air.

Fog And Mist

Definition: Fog and mist are essentially clouds that form at or near the Earth's surface. They consist of tiny liquid water droplets or ice crystals suspended in the air. The distinction between fog and mist is often based on visibility:

• Fog: Reduces visibility to less than 1 kilometer (0.62 miles).
• Mist: Reduces visibility, but to a lesser extent than fog (typically between 1 and 2 kilometers).

Formation Processes: Fog forms when air cools to its dew point, causing water vapour to condense into tiny droplets. Common formation mechanisms include:

• Radiation Fog: Forms on clear, calm nights when the ground cools rapidly by radiation, cooling the air above it to the dew point. Most common in valleys.
• Advection Fog: Forms when warm, moist air moves horizontally (advects) over a cooler surface (like cold ocean currents or snow-covered land). The air cools by contact, reaching its dew point.
• Upslope Fog: Forms when moist air is forced to rise up a slope or mountain. As it rises, it expands and cools adiabatically, reaching its dew point and condensing.
• Evaporation Fog (Steam Fog): Forms when cold air moves over warmer water. The water evaporates into the cold air, becomes saturated quickly, and condenses, forming a fog that looks like steam rising from the water surface.
• Precipitation Fog: Occurs when rain falls through a layer of cold air, evaporating and saturating the air below, leading to fog formation.

Clouds

Definition: Clouds are visible masses of tiny liquid water droplets or ice crystals suspended in the Earth's atmosphere, formed by the condensation of water vapour.

Formation: Clouds form when moist air rises, expands, and cools. As the air cools, its relative humidity increases. When the air reaches its dew point (saturation), water vapour condenses onto tiny particles called condensation nuclei (e.g., dust, salt, pollen).

Classification of Clouds: Clouds are primarily classified based on their altitude and appearance:

• High Clouds (above 6,000 meters): Composed mainly of ice crystals.
• Middle Clouds (2,000 - 6,000 meters): Composed of water droplets and/or ice crystals.
• Low Clouds (below 2,000 meters): Composed mainly of water droplets.
• Clouds with Vertical Development: Clouds that extend through multiple altitude levels.

Cirrus

Appearance: High-altitude clouds (typically above 6,000 meters) that appear thin, wispy, feathery, or hair-like. They are composed entirely of ice crystals.

Characteristics: Often appear white and delicate. They do not produce precipitation that reaches the ground but can indicate approaching weather changes, such as a warm front.

Cumulus

Appearance: Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes, or towers, of which the bulging upper part often resembles a cauliflower. The sunlit parts are mostly brilliant white; their base is relatively dark and nearly horizontal.

Formation: Formed by convection (rising thermals of warm, moist air). Fair-weather cumulus clouds are small and do not produce precipitation. However, they can grow vertically into towering cumulus and eventually cumulonimbus clouds, which produce thunderstorms.

Stratus

Appearance: Grayish cloud layer with a fairly uniform base, often resembling fog that hasn't reached the ground. They appear as a featureless sheet or layer covering the entire sky.

Characteristics: Low-altitude clouds (below 2,000 meters). They can produce drizzle or light snow, but significant precipitation is rare. They often bring overcast and gloomy weather.

Nimbus

Meaning: "Nimbus" is a Latin word meaning "rain cloud." It is used as a prefix or suffix to denote clouds that produce precipitation.

Examples:

• Nimbostratus (Ns): A dark gray, thick, amorphous cloud layer associated with continuous, widespread, light to moderate precipitation (rain or snow). They are mid-level clouds that can extend into lower altitudes.
• Cumulonimbus (Cb): Large, dense, towering vertical clouds often associated with thunderstorms. They produce heavy precipitation, hail, lightning, and strong winds. The prefix "cumulo-" indicates their puffy, convective form, and "nimbus" indicates precipitation.

Precipitation

Definition: Precipitation is any product of the condensation of atmospheric water vapour that falls under gravity from clouds. The key requirement is that the water droplets or ice crystals grow large enough to overcome updrafts and fall to the Earth's surface.

Growth of Cloud Droplets/Ice Crystals: For precipitation to occur, cloud droplets or ice crystals must grow significantly larger than their initial microscopic size. This happens through two main processes:

1. Collision-Coalescence Process: Occurs in warmer clouds (above 0°C). Cloud droplets collide with each other as they are moved by air currents. If they collide with enough force and stick together (coalesce), they can grow large enough to fall as rain.
2. Bergeron Process (Ice-Crystal Process): Occurs in colder clouds (below 0°C) where both supercooled water droplets (liquid water below freezing) and ice crystals coexist. Ice crystals grow rapidly at the expense of the supercooled droplets because the saturation vapour pressure over ice is lower than over water. These larger ice crystals then fall, and can melt into raindrops as they descend through warmer air.

Types Of Rainfall

Rainfall is categorized based on the process that causes the air to rise, cool, and condense:

Convectional Rain

Mechanism: Occurs when the sun heats the Earth's surface, causing the air near the ground to become warm, less dense, and rise rapidly (convection). As the air parcel rises, it expands and cools adiabatically. If it reaches its dew point and continues to rise in an unstable atmosphere, cumulus clouds develop and can grow into cumulonimbus clouds, producing heavy, short-lived showers with lightning and thunder.

Characteristics: Often occurs in the afternoon, especially in tropical and sub-tropical regions during summer. Characterized by isolated heavy downpours, often accompanied by thunderstorms.

Orographic Rain

Mechanism: Also known as relief rainfall. Occurs when moist air is forced to rise as it encounters a physical barrier, such as a mountain range. As the air is lifted up the windward side of the mountain, it cools adiabatically, its relative humidity increases, and condensation leads to cloud formation and precipitation. After passing over the mountain crest, the air descends on the leeward side, warms adiabatically, and its relative humidity decreases. This creates a "rain shadow" area on the leeward side, which is much drier.

Characteristics: Concentrated on the windward slopes of mountains. The leeward side is drier and experiences less precipitation.

Cyclonic Rain

Mechanism: Also known as frontal rainfall. Occurs along the boundaries (fronts) between different air masses, typically in mid-latitude regions associated with extra-tropical cyclones. When a warmer, less dense air mass advances and overrides a cooler, denser air mass (warm front), or when a colder, denser air mass advances and forces a warmer air mass upwards (cold front), the warm, moist air is lifted. This lifting causes cooling, condensation, and precipitation.

Characteristics: Often widespread and prolonged, with varying intensity depending on the type of front. Warm fronts typically bring steady, light to moderate precipitation, while cold fronts can produce more intense, showery precipitation.

Other Forms of Precipitation:

• Snow: Occurs when temperatures throughout the atmosphere are at or below freezing, and ice crystals grow and aggregate.
• Sleet: Raindrops that freeze into ice pellets as they fall through a sub-freezing layer of air near the surface.
• Hail: Solid precipitation in the form of irregular lumps of ice, formed within strong thunderstorm updrafts where ice particles are repeatedly lifted and coated with supercooled water.
• Drizzle: Very fine water droplets, smaller than raindrops, that appear to float rather than fall. Associated with stratus clouds.

World Distribution Of Rainfall

The distribution of rainfall across the globe is highly uneven and is primarily controlled by the general atmospheric circulation, pressure systems, prevailing winds, and proximity to oceans.

Factors Influencing Rainfall Distribution:

• Latitude and Atmospheric Circulation:
  • Equatorial Regions (ITCZ): Receive abundant rainfall throughout the year due to the convergence of trade winds and strong convection. This leads to lush tropical rainforests.
  • Subtropical High-Pressure Belts (around 30° N/S): Characterized by sinking air, which inhibits cloud formation and leads to dry conditions and deserts.
  • Mid-Latitudes (around 60° N/S): Receive significant rainfall associated with the convergence of polar and tropical air masses along the polar front and the passage of extra-tropical cyclones.
  • Polar Regions: Receive very little precipitation as the cold air holds very little moisture. What precipitation does fall is usually in the form of snow.
• Continentality: Inland areas tend to be drier than coastal regions because they are farther from moisture sources (oceans) and are often influenced by continental air masses.
• Ocean Currents: Warm ocean currents can increase the moisture content of the air that moves over them, leading to higher rainfall on adjacent coastlines. Cold currents have the opposite effect.
• Prevailing Winds: Winds blowing from oceans inland bring moisture and contribute to rainfall, especially on windward coasts. Winds blowing from continental interiors are usually drier.
• Topography (Orographic Effects): Mountain ranges create significant rainfall gradients. Windward slopes receive high amounts of precipitation (orographic rainfall), while leeward slopes (rain shadow areas) receive very little.
• Monsoon Systems: Regions influenced by monsoons experience distinct wet and dry seasons, with a dramatic increase in rainfall during the summer monsoon.

Major Rainfall Zones:

• Equatorial Region: High rainfall throughout the year (e.g., Amazon Basin, Congo Basin, Southeast Asia).
• Tropical Regions (West Coasts): Often dry due to subtropical high-pressure belts (e.g., Atacama Desert, Namib Desert).
• Tropical Regions (East Coasts): Receive rainfall from trade winds and tropical cyclones (e.g., eastern Australia, parts of Brazil).
• Monsoon Regions: Experience distinct wet summer seasons (e.g., India, Southeast Asia).
• Temperate Regions: Receive rainfall throughout the year, often with distinct wet seasons (e.g., Western Europe, Eastern North America).
• Mediterranean Climate Regions: Have dry summers and wet winters, with rainfall occurring mainly due to the influence of extra-tropical cyclones during the cooler months.
• Continental Interiors: Generally have lower rainfall, with higher seasonal variations.
• Polar Regions: Very low precipitation, mainly as snow.
• Deserts: Extremely low rainfall due to persistent high-pressure systems and rain shadows (e.g., Sahara, Arabian Desert, Gobi Desert).

Example of Rainfall Distribution:

India, for instance, experiences a pronounced monsoon climate. The southwest monsoon (June-September) brings heavy rainfall to most of the country, while the northeast monsoon (October-December) brings some rain to the southeastern coast.