Class 11 Geography chapter 10 water in the atmosphere notes

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Water in the atmosphere 

• water vapour varies from 0-4 percent by volume in atmosphere 
• Water vapour present in atmosphere is called humidity 
• Actual amount of water vapour present in air is called absolute humidity 
• The percentage of moisture present in the atmosphere compared to its full capacity at a given temperature is called as relative humidity 
• Measured in gram per cubic metre 
• Air containing moisture to its full capacity is called saturated at the temperature is called as dew point 

Evaporation and condensation 

• Evaporation is the process by which water is transformed from liquid to gases state the temperature at which the water start vaporising is referred to as the latent heat of vaporization 
•  if the water content in the air is low, the potential of the air of retaining moisture increases in moving air there is unsaturated air  Movement of air replaces the saturated layer with the unsaturated layer
• Thus ,The greater the movement of air the greater is the vaporisation 
• The transformation of water vapour into water is called condensation it is caused by loss of heat if it directly converts into solid form it is known as sublimation 
• in free air condensation results from cooling around very small particles termed as hygroscopic condensation nuclei 
• Condensation is influenced by the volume of air its temperature pressure and humidity 
• It takes place 

1. when the temperature of the air is reduced to give point with its volume remaining constant and
2.  when both the volume and temperature are reduced and 
3. when moisture is added to the air through evaporation 
4. the most favourable condition of condensation is the decrease in air temperature 

• Condensation takes place when the dew point is lower than the freezing point as well as higher than the freezing point

Frost

• Definition: Frost forms when the air temperature falls below freezing point (0°C) and water vapor directly turns into ice crystals on cold surfaces (a process called deposition).
• Formation Process:
• Frost occurs on clear, calm nights when the ground loses heat rapidly through radiation.
• Surfaces cool below the dew point, which must also be below 0°C.

 Dew

• Definition: Dew forms when water vapor condenses into liquid droplets on cool surfaces after the air temperature falls to the dew point but remains above freezing (0°C).
• Formation Process:
• Dew forms during calm, clear nights when the ground cools rapidly through radiation, causing air near the surface to become saturated.
• Appearance: Small, clear water droplets on grass, leaves, or other objects, often seen in the early morning.
• Significance: Indicates high humidity and aids plant hydration in dry areas.

Fog

• Definition: Fog is a dense layer of tiny water droplets suspended in the air that reduces visibility to less than 1 kilometer.
• Formation Process:
• Air near the surface cools to its dew point temperature, causing water vapor to condense into tiny droplets.
• Fog forms in areas with high humidity, such as valleys, coastal areas, or during winter.
• Types of fog include:
• Smog – when smoke is mixed with fog 
• Characteristics:
• Dense and cloud-like; significantly reduces visibility.
• Common in winter, especially in valleys or near water bodies

Mist

• Definition: Mist is a thin veil of water droplets suspended in the air, with visibility ranging between 1 and 2 kilometers.
• Formation Process:
• Mist forms under similar conditions as fog but with less cooling and when the air contains more moisture.
• Water droplets in mist are smaller and more dispersed than in fog.
• Characteristics:
• Appears as a light, hazy layer, often seen near water bodies or early in the morning.
• Mist contains more moisture than fog, making it less dense but widespread.

Key Differences Between Frost, Dew, Fog, and Mist

• Dew and frost are surface-level condensation phenomena; dew forms as liquid water, while frost forms as ice.
• Fog and mist are airborne condensation phenomena. While both are clouds near the surface, mist contains more moisture and is less dense than fog, resulting in higher visibility.
• All these processes depend on cooling, saturation, and the presence of condensation nuclei in the atmosphere.

Cloud

• Clouds are classified into four types based on their form: Cirrus, Cumulus, Stratus, and nimbus 

1. Cirrus Clouds

Definition:

• High-altitude clouds that appear thin, white, and wispy, often resembling feathers.

Characteristics:

• They are formed at high altitude of 8,000 - 12,000 m
•  Light, delicate appearance with no precipitation.
• Indicate fair weather but often signal an approaching weather change, like a warm front.

2. Cumulus Clouds

Definition:

• White, puffy clouds with a flat base and a dome-shaped top, often likened to cotton balls.

Characteristics:

• They are formed at height of  4000 -7000 m 
• Associated with fair weather, but they can grow vertically into storm clouds (Cumulonimbus).
• Form due to convection as warm air rises and cools.

3. Stratus Clouds

Definition:

• Clouds that form a uniform, layered, grayish layer covering large parts of the sky. They are featureless and often bring overcast weather.

Characteristics:

• Found at up to 2,000 meters (low altitude).
• This are either formed by loss of heat or mixing of air masses with different temperature .

4. Nimbus Clouds

Definition:

• Nimbus clouds are dense, dark clouds associated with precipitation. The term “nimbus” refers to clouds that bring rain.

Characteristics:

• Sometimes they could be very low as to touch the ground 
• They are opaque to rays of sun 

• A combination of these four basic types can give rise to the following types of clouds:
• high clouds – cirrus, cirrostratus, cirrocumulus;
• middle clouds – altostratus and altocumulus;
• low clouds – stratocumulus and nimbostratus 
• clouds with extensive vertical development – cumulus and cumulonimbus.

• 

Precipitation 

• The release of moisture is known as precipitation this may take form in rainfall or in snowfall The other forms of precipitation include sleet and hail

1. Sleet

Definition:

• Sleet is a type of precipitation that occurs when raindrops freeze into ice pellets before reaching the ground.

Formation Process:

• Sleet forms in cold weather when there is a layer of warm air above a freezing layer near the surface.
• Raindrops fall through the warm layer, then pass into the freezing layer below, where they turn into ice pellets.

Characteristics:

• Sleet consists of small, transparent ice pellets.
• It often bounces upon hitting the ground and can create slippery surfaces.
• Typically occurs in winter or during cold weather transitions.

2. Hail

Definition:

• Hail consists of large, solid ice balls or lumps that form in strong convective clouds.

Formation Process:

• Hail forms in intense updrafts inside thunderstorms.
• Supercooled water droplets are lifted repeatedly within the cloud by strong updrafts, freezing into layers of ice around a nucleus (like a dust particle or frozen droplet).
• As the ice grows larger, it eventually becomes too heavy for the updraft to hold, and it falls as hailstones.

Characteristics:

• Hailstones can vary in size, ranging from small pellets to large stones over 5 cm in diameter.
• Associated with severe weather like thunderstorms and strong winds.
• Typically occurs in summer or warm seasons due to convective activity.

Types of rainfall 

1. Convectional Rainfall

Definition:

• Convectional rainfall occurs due to the heating of the Earth’s surface, which causes warm, moist air to rise rapidly, cool, and condense into rain.

Formation Process:

• Sun heats the Earth’s surface, warming the air above it.
• The warm, moist air rises (convection), expands, and cools as it ascends.
• Cooling reaches the dew point, causing condensation and cumulous cloud formation.
• Further cooling leads to precipitation in the form of rain.

Characteristics:

• Usually occurs in equatorial regions.
• Accompanied by thunderstorms and heavy downpours.
• Happens mostly during the afternoon when surface heating is strongest.

Examples:

• Found in tropical rainforests, such as the Amazon Basin, Congo Basin, and Southeast Asia.

2. Orographic (Relief) Rainfall

Definition:

• Orographic rainfall occurs when moist air is forced to rise over a mountain barrier. As the air rises, it cools, condenses, and causes rain on the windward side.

Formation Process:

• Moist air moves toward a mountain range.
• The air is forced to ascend along the slopes, cooling adiabatically.
• Water vapor condenses, forming clouds and precipitation on the windward side.
• On the leeward side, the descending air becomes warmer and drier, creating a rain shadow area.It is also known as relief rain 

Characteristics:

• Uneven distribution of rainfall across regions.
• Heavy rainfall occurs on the windward side, while the leeward side experiences arid or semi-arid conditions.

Examples:

• Western Ghats: Windward side receives heavy rainfall during the monsoon, while the leeward side (Deccan Plateau) lies in the rain shadow.

 3. Cyclonic (Frontal) Rainfall

Definition:

• Cyclonic rainfall occurs when two air masses of different temperatures and densities meet along a front. Warm air is forced over the cold air, leading to condensation and precipitation.

Formation Process:

• A cyclone or depression forms when warm, moist air meets cold, dense air.
• The warm air rises over the cold air due to its lower density.
• As the warm air ascends, it cools, condenses, and forms clouds, resulting in rainfall.

Characteristics:

• Associated with cyclones and temperate regions.
• Rainfall is often widespread and prolonged.
• Accompanied by cloudy skies and moderate to heavy rain.

Examples:

• Occurs in the Indian Ocean during cyclonic storms.

World distribution of rainfall 

World Distribution of Rainfall

Different places on Earth get different amounts of rainfall in a year and in different seasons.

General Patterns of Rainfall:

• As we move from the equator to the poles, rainfall generally decreases.
• Coastal areas receive more rainfall than inland areas.
• Oceans get more rainfall than land because they have more water to evaporate.

Regional Rainfall Patterns:

• Between 35° and 40° latitude (North & South): The eastern coasts get more rain, and it decreases as we move westward.
• Between 45° and 65° latitude (North & South): The western coasts get more rain due to strong winds (westerlies), and it decreases towards the east.
• Mountains near the coast: The side facing the wind (windward side) gets more rain, while the opposite side (leeward side) gets less.

Annual Rainfall Zones:

• Heavy Rainfall (Above 200 cm per year): Found in the equatorial region, mountain slopes on the western coasts in cooler areas, and monsoon coastal regions.
• Moderate Rainfall (100–200 cm per year): Found in inland areas and some coastal regions.
• Low Rainfall (50–100 cm per year): Found in central tropical regions and the interiors of temperate lands.
• Very Low Rainfall (Below 50 cm per year): Found in deserts, rain-shadow areas, and high-latitude regions near the poles.

Seasonal Rainfall:

• Some areas, like the equatorial belt and western coasts in cool temperate zones, get rain evenly throughout the year.
• In other places, rainfall is concentrated in specific seasons, making water availability uneven.

This distribution of rainfall affects agriculture, water supply, and climate in different parts of the world.

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