Class 11 Geography (Fundamental of Physical Geography) Chapter 10 Water In The Atmosphere

Key Concept: Definition Recall

a) Dew point

[Solution Description] The dew point is the temperature at which air becomes saturated with moisture, leading to condensation.

Key Concept: Humidity Types

c) Relative Humidity

[Solution Description] Relative humidity is defined as the ratio of the current amount of water vapor present in the air to the maximum amount of water vapor the air can hold at the same temperature, expressed as a percentage.

Key Concept: Dew Point and Condensation, Real-World Application

b) Formation of dew or fog

[Solution Description] When the air temperature drops to match the dew point, the air becomes saturated, and condensation occurs because the relative humidity reaches 100%. In this scenario, fog or dew formation is expected as water vapor condenses into liquid droplets on surfaces. The sudden drop in temperature aligns with the dew point condition facilitating condensation.

Key Concept: Layer Interaction

b) They solidify into sleet

[Solution Description] When raindrops descend from a warmer air layer and enter a colder, subfreezing layer near the ground, they solidify into small ice pellets known as sleet. This happens because the temperature in the lower layer is below freezing point, causing the liquid raindrops to freeze.

Key Concept: Temperature Effect

b) Increases

[Solution Description] As the temperature of air increases, its capacity to hold water vapour also increases because warmer air can contain more moisture than cooler air, due to increased energy levels that allow more water molecules to remain in vapor state.

Key Concept: Cloud Altitude

b) 8,000 – 12,000 m

[Solution Description] Cirrostratus clouds are high-altitude clouds and they are generally found at altitudes ranging from 8,000 to 12,000 meters.

Key Concept: Temperature Effect

b) Increases evaporation rate

[Solution Description] An increase in temperature raises the kinetic energy of water molecules, allowing more molecules to escape into the air as vapor. Thus, higher temperatures enhance the rate of evaporation due to increased molecular movement.

Key Concept: Cloud Types

d) Nimbus

[Solution Description] Nimbus clouds are known for their dense and heavy appearance, often giving them a dark color. They frequently appear to be low enough to seem as if they are touching the ground.

Key Concept: Rainfall Types

d) Convectional

[Solution Description] Convectional rainfall occurs when the sun heats the Earth’s surface, causing the air above it to rise due to convection. As this warm air ascends, it cools and condenses, forming clouds and resulting in precipitation.

Key Concept: Basic Concepts

d) Evaporation

[Solution Description] The process where water changes from its liquid form to a gaseous state is known as evaporation. Evaporation is influenced by factors such as temperature and air movement.

Key Concept: Cloud Types

d) Cirrus

[Solution Description] The characteristics mentioned in the question match with Cirrus clouds. They are known for being located at high altitudes and having a thin, feathery appearance.

Key Concept: Types of Rainfall

c) Convectional

[Solution Description] Convectional rainfall occurs when the sun heats the Earth’s surface, causing the air near the ground to rise. This rising air cools and condenses into clouds, leading to thunderstorms.

Key Concept: Basic Definition

a) Transformation of water vapour into liquid

[Solution Description] Condensation is the transformation of water vapour into liquid, caused by the loss of heat.

Key Concept: Basic Definition

c) Rainfall caused by rising warm air

[Solution Description] Convectional rain occurs when air is heated by the Earth’s surface, causing it to rise in convection currents. As it rises, it cools and condenses to form clouds, resulting in rainfall.

Key Concept: Rainfall Mechanism

b) Intense heating of the Earth’s surface

[Solution Description] Convectional rainfall occurs primarily due to the intense heating of the earth’s surface, which causes rapid air rising. The sun heats the ground, and the warm air rises because it is less dense. As it ascends, the air cools, leading to condensation and cloud formation. Eventually, this results in precipitation.

Key Concept: Condition Recognition

b) During nighttime with clear skies

[Solution Description] Fog forms when the temperature of the air drops to its dew point, usually under calm and clear conditions, often at night or early morning.

Key Concept: Convectional Rain Characteristics

b) Intense surface heating causing air to rise.

[Solution Description]

Convectional rain is caused by the intense heating of the Earth’s surface. The warm air rises due to its lower density compared to the surrounding cooler air, causing cooling and eventually leading to condensation and rainfall. This type of rain is common in equatorial regions with strong solar heating.

Key Concept: Cloud Formation Process

c) Stable atmosphere with slow rising air masses ahead of a warm front.

[Solution Description] Altostratus clouds typically form in stable atmospheric conditions when large air masses rise slowly due to an approaching warm front; this results in their formation over wide areas with no significant precipitation.

Key Concept: Cloud Types

c) Cirrus

[Solution Description] Cirrus clouds are found at high altitudes and are characterized by their thin and wispy appearance.

Key Concept: Fog and Air Currents

c) Air is calm and stable, keeping fog near the ground

[Solution Description] Fog persists when air is calm and stable, allowing it to remain close to the ground. In such conditions, there’s less vertical mixing of the air, preventing the fog from dispersing quickly. High humidity and clear skies during nighttime can also contribute to the persistence of fog by facilitating cooling at the surface.

Key Concept: Condensation Conditions

b) With a decrease in air temperature

[Solution Description] Condensation occurs when the air is cooled to its dew point, leading to saturation. A decrease in temperature allows the air to hold less water vapor, causing excess moisture to condense.

Key Concept: Cloud Formation Conditions

b) High altitude and low temperature

[Solution Description] Cirrus clouds form at high altitudes, typically between 8,000 – 12,000 meters, and are composed predominantly of ice crystals due to the cold temperatures at these altitudes.

Key Concept: Humidity Concept

b) Absolute humidity

[Solution Description] Absolute humidity is defined as the weight of water vapour per unit volume of air, expressed in grams per cubic metre. It represents the actual quantity of water vapour present in the air.

Key Concept: Comparative Analysis

c) Increased humidity resulting in lush vegetation

[Solution Description] The leeward side, known as the rain-shadow area, is characterized by dry conditions. Air descends, warms, and increases its moisture capacity, resulting in less rainfall. Therefore, increased humidity is not typical for the leeward side.

Key Concept: Frost Conditions

c) Dew point at or below freezing point

[Solution Description] Frost forms when the temperature falls below freezing and the dew point is at or below the freezing point, leading to deposition of ice crystals on surfaces.

Key Concept: Simple Calculation

a) 30 g

[Solution Description] To calculate the total water vapor in 2 kg of air with a specific humidity of 15 $g/kg$, multiply the specific humidity by the mass of the air:

$\text{Total Water Vapor} = 15 \, \text{g/kg} \times 2 \, \text{kg} = 30 \, \text{g}$

Thus, there are 30 grams of water vapor in 2 kilograms of air.

Key Concept: Rainfall Types

b) Orographic

[Solution Description] Orographic rainfall occurs when moist air is lifted as it moves over a mountain range. As the air rises, it cools and condenses, leading to precipitation on the windward side of the mountain.

Key Concept: Humidity Variation

b) Decreases

[Solution Description]

When the temperature of a saturated air mass increases, its capacity to hold moisture increases, but since its actual moisture content remains constant, the relative humidity decreases. Relative humidity is defined as the percentage of actual water vapor present compared to the maximum possible at that temperature.

Key Concept: Basic Concept

c) To transform liquid into gas

[Solution Description] The latent heat of vaporization is primarily responsible for converting water from a liquid state to a gaseous state during the process of evaporation.

Key Concept: Cloud Dynamics, Cloud Interaction

b) Increasing humidity with a warm front

[Solution Description] Altostratus clouds often cover the entire sky and can lead to continuous precipitation if they thicken enough. The transformation into nimbostratus typically occurs as a result of increasing moisture levels in the atmosphere, leading to thicker cloud coverage and potentially more significant precipitation. This process is also facilitated by an incoming weather front that provides additional uplift and moisture.

Key Concept: Factors Affecting Concepts

d) Color of container

[Solution Description] Factors affecting the rate of evaporation include temperature, humidity, wind speed, and surface area. Among the given options, color of the container does not influence the rate of evaporation directly.

Key Concept: Relative Humidity Calculation

c) 60%

[Solution Description] Relative humidity is given by the formula:

$\text{Relative Humidity} = \left( \frac{\text{Actual Water Vapour}}{\text{Max Water Vapour Capacity}} \right) \times 100%$

Here, Actual Water Vapour = 12 g/m$^3$ and Max Water Vapour Capacity = 20 g/m$^3$. Substituting these values,

$\text{Relative Humidity} = \left( \frac{12}{20} \right) \times 100\% = 60%$

Therefore, the relative humidity is 60%.

Key Concept: Basic Definition

c) Rain produced when air rises over mountains, cools, and condenses

[Solution Description] Orographic rain occurs when moist air is forced to ascend over a mountain, leading to cooling, condensation, and rainfall on the windward side.

Key Concept: Comparative Analysis

c) Low temperature and high humidity.

[Solution Description] Fog forms when the air temperature drops to the dew point temperature and water vapor condenses into tiny droplets suspended in the air. High humidity with cooling leads to this condition.

Key Concept: Dew Point

c) The temperature at which air reaches saturation

[Solution Description] Dew point is the temperature at which air becomes saturated with moisture, and any further cooling leads to condensation. It’s crucial for understanding humidity levels.

Key Concept: Complex Scenario, Real-World Application

c) Relative humidity reaches 100\%; dew point is reached

[Solution Description] As the air is cooled from 25°C to 15°C at constant pressure without adding or losing water vapor, its capacity to hold moisture decreases. Therefore, relative humidity will increase as the temperature approaches the dew point. At some point, the air may become saturated, reaching a relative humidity of 100\%, and the dew point will be reached.

Key Concept: Condition Identification

c) Air temperature at or below freezing point

[Solution Description] One of the necessary conditions for the formation of frost is that air temperature must be at or below freezing point.

Key Concept: Conditions for Dew

d) Subfreezing air layer near the ground

[Solution Description] Sleet formation requires raindrops to pass through a subfreezing layer of air near the ground, causing them to freeze before reaching the surface.

Key Concept: Basic Concepts

a) Water vapor

[Solution Description] The water in its gaseous form in the atmosphere is known as water vapor. It is an essential component of the air and plays a crucial role in weather and climate.

Key Concept: Basic Concept

a) Small pellets of ice

[Solution Description] Hailstones are large pellets of ice that form in the upper regions of thunderstorms. They have a layered structure and can vary in size from small to very large.

Key Concept: Real-World Application

b) Decreases

[Solution Description] When the air temperature increases while keeping the absolute humidity constant, the capacity of the air to hold moisture increases as well. This means that the percentage of moisture content compared to its full capacity decreases. Therefore, the relative humidity decreases.

Key Concept: Rainfall Causes

c) Interaction of different air masses

[Solution Description] Cyclonic rain occurs due to the interaction of different air masses, which leads to the development of an extra tropical cyclone.

Key Concept: Formation Conditions

a) Warm air above freezing point overlying a subfreezing layer

[Solution Description] Sleet forms when warm air above freezing point overlies a subfreezing layer near the ground. This allows raindrops to freeze into ice pellets, thus forming sleet.

Key Concept: Smog Definition

b) Fog mixed with smoke

[Solution Description] Smog is a combination of fog and smoke. It occurs when fog mixes with smoke, particularly in urban and industrial areas.

Key Concept: Basic Concept

d) Heat

[Solution Description] Heat is the primary factor that causes water to evaporate, transforming it from a liquid state into a gaseous state.

Key Concept: Basic Process

c) Evaporation

[Solution Description] The transformation of water from a liquid to a gaseous state due to heat is known as evaporation.

Key Concept: Cloud Composition

b) Ice crystals

[Solution Description] Cirrus clouds are composed mainly of ice crystals because they form at high altitudes where temperatures are quite low.

Key Concept: Basic Cloud Function

c) Stratus

[Solution Description] Stratus clouds are layered clouds that cover large portions of the sky. They form due to heat loss or mixing of air masses.

Key Concept: Rainfall Types

b) Orographic rainfall

[Solution Description] Precipitation occurs when moist air ascends along mountain slopes, cooling and condensing to form rain, typically seen on windward slopes.

Key Concept: Dew Formation

b) Above freezing point

[Solution Description] Dew forms when the air is cooled to its dew point, and this dew point must be above freezing since dew consists of liquid water droplets. If the dew point were below freezing, frost would form instead.

Key Concept: Comparative Analysis

c) It measures the weight of water vapour per unit volume of air.

[Solution Description]

Absolute humidity measures the actual mass of water vapor present in a cubic meter of air, whereas relative humidity is expressed as a percentage indicating how close the current air is to being fully saturated at a particular temperature.

Key Concept: Humidity Variation

c) Early morning in a rainforest

[Solution Description] Relative humidity tends to increase when temperatures are cooler because the air can hold less water vapor. Among the options, early morning hours usually present lower temperatures and limited evaporation, thus having higher relative humidity.

Key Concept: Basic Concept

d) Relative humidity

[Solution Description] Relative humidity is defined as the percentage of moisture present in the air compared to the maximum amount it can hold at a given temperature.

Key Concept: Ideal Conditions

c) Calm air

[Solution Description] Ideal conditions for dew formation include calm air, clear skies, high relative humidity, and cold, long nights as these conditions facilitate maximum cooling of surfaces and saturation of air with moisture.

Key Concept: Basic Concept

a) Evaporation

[Solution Description] The process in which liquid water changes to water vapour is known as evaporation. This occurs due to an increase in temperature and energy, allowing water molecules to escape into the air.

Key Concept: Cloud Types

c) Cumulus

[Solution Description] Cumulus clouds are known for their puffy, white, cotton-like appearance. They usually form at low levels in the atmosphere.

Key Concept: Snowflake Structure

c) Molecular structure of water

[Solution Description] Snowflakes form as hexagonal crystals because of the molecular structure of water, which has a bent shape with two hydrogen atoms bonded to one oxygen atom at an angle. When water freezes, each oxygen atom bonds with four other atoms in a tetrahedral arrangement, creating a six-sided or hexagonal pattern due to the hydrogen bonds.

Key Concept: Dew and Frost Formation

d) The temperature relative to freezing point

[Solution Description] Dew forms when the temperature is above freezing and condensation occurs, while frost forms when the temperature drops below the freezing point, causing deposition of water vapor as ice crystals.

Key Concept: Definition Recall

b) Windward slope

[Solution Description] The windward slope of a mountain receives greater rainfall because it is where the moist air ascends, cools, condenses, and forms precipitation as a result of orographic processes.