Class 11 Geography (Fundamental of Physical Geography) Chapter 9 Atmospheric Circulation And Weather Systems

Key Concept: Basic Definitions

a) Cold front

[Solution Description] A cold front forms when a cold air mass moves into an area occupied by a warmer air mass, forcing the warm air to rise.

Key Concept: Pressure Decrease

c) 1 mb per 10 m

[Solution Description] The typical rate at which atmospheric pressure decreases with altitude is approximately 1 mb for every 10 m increase in height. As elevation increases, the column of air above becomes shorter, reducing its weight and thereby lowering the pressure.

Key Concept: Wind Type Identification

c) Katabatic wind

[Solution Description] A katabatic wind is formed when cold, dense air flows from higher elevations such as plateaus or ice fields down into valleys due to gravity.

Key Concept: Formation Conditions

c) Small variations in vertical wind speed

[Solution Description] The syllabus states that a necessary condition for the formation of tropical cyclones is a sea surface temperature higher than 27°C.

Key Concept: Basic Concept

b) Continental polar (cP)

[Solution Description] Air masses that originate from cold polar regions with dry characteristics are known as continental polar (cP) air masses. They acquire these traits because they form over land areas far from oceans or seas, making them cold and dry.

Key Concept: Pressure Measurement

c) 1,013.2 mb

[Solution Description] The average atmospheric pressure at sea level is typically given as 1,013.2 mb, based on standard atmospheric conditions.

Key Concept: Coriolis Effect

a) Deflects wind to the left

[Solution Description] In the southern hemisphere, the Coriolis force deflects moving air to the left. This is due to Earth’s rotation, which causes the path of moving air to curve as it travels across the surface.

Key Concept: Pressure Systems

d) It is enclosed by isobars with the highest pressure in the center.

[Solution Description] A high-pressure system is characterized by isobars that enclose the highest pressure in the center. This is because air moves out from high-pressure areas, making the central point have the highest concentration of atmospheric pressure.

Key Concept: Coriolis and Ocean Currents, Complex Interaction

c) Geostrophic balance results in currents running parallel to isobars

[Solution Description] In the mid-latitudes, the interaction between the Coriolis force and pressure gradient force plays a major role in determining the direction of ocean currents. The Coriolis force causes moving water to deflect relative to its initial trajectory depending on the hemisphere. When coupled with the pressure gradient force, which drives water movement from high-pressure to low-pressure areas, currents such as the Gulf Stream in the North Atlantic are formed. These currents stabilize geostrophically, flowing parallel to isobars without crossing them due to the perpendicular nature of Coriolis action.

Key Concept: Measurement Tools

b) Barometer

[Solution Description] The barometer is the instrument specifically designed to measure atmospheric pressure. There are two types: mercury barometer and aneroid barometer.

Key Concept: Wind Pressure

a) Temperature difference between slopes and valley floor

[Solution Description] During the day, the sun heats the mountain slopes more rapidly than the valley floor. The warm, less dense air rises along the slopes, creating a low-pressure area at higher altitudes compared to the valley floor. This temperature and pressure difference drives the valley breeze upwards.

Key Concept: Front Types

a) Cold front

[Solution Description] A cold front is formed when a cold air mass moves under a warm air mass, forcing the warmer air to rise. This often leads to cloud formation and precipitation.

Key Concept: Geostrophic Wind

c) When pressure gradient force is balanced by Coriolis force

[Solution Description] Geostrophic wind occurs when the pressure gradient force acting on air parcels is balanced by the Coriolis force. This balance results in wind blowing parallel to the isobars without any influence from friction.

Key Concept: ENSO Phenomenon

b) Warmer than average

[Solution Description] An El Nino event results in a significant warming of the sea surface temperatures in the central and eastern Pacific Ocean. This occurs because the usual trade winds weaken, allowing warm water to move eastward across the Pacific. This temperature anomaly disrupts normal weather patterns.

Key Concept: Weather Impact

b) Abrupt changes in temperature and precipitation

[Solution Description] Extra tropical cyclones cause abrupt changes in weather conditions, including temperature and precipitation, due to their movement and interaction between different air masses.

Key Concept: Basic Definition

d) High pressure

[Solution Description] The Subtropical High-Pressure Belt is primarily characterized by high atmospheric pressure, which is one of its defining features.

Key Concept: Basic Definition

c) The cyclone’s moisture supply is cut off

[Solution Description] Tropical cyclones dissipate when they make landfall primarily because the source of moisture from the ocean is cut off. Without this supply of moisture, the cyclone loses its energy and weakens.

Key Concept: Air Mass Interaction

c) Heavy precipitation

[Solution Description] When a dry, cold continental polar air mass meets a humid, warm maritime tropical air mass, the interaction often results in stormy conditions and heavy precipitation due to the differences in temperature and moisture content.

Key Concept: Seasonal Variation

c) It shifts north or south following the sun’s direct rays.

[Solution Description] The ITCZ is a region near the equator where the trade winds from the Northern and Southern Hemispheres converge. Its position shifts seasonally north or south following the sun’s zenith point due to the Earth’s axial tilt. During summer in the northern hemisphere, the ITCZ moves northward, and during winter, it moves southward. Thus, the shift in the ITCZ is primarily caused by shifts in the sun’s direct rays.

Key Concept: Pressure Variation

a) 1 mb per 10 m

[Solution Description] As elevation increases, atmospheric pressure decreases due to the thinning of the atmosphere. This change happens at approximately 1 mb for every 10 m increase in elevation.

Key Concept: Cyclonic Circulation

b) Clockwise

[Solution Description] Cyclonic circulation in the Southern Hemisphere is clockwise due to the Coriolis effect, which deflects moving particles to the left of their path.

Key Concept: Frontogenesis Process

b) Cold front

[Solution Description] Thunderstorms and rapid weather changes are usually associated with cold fronts. A cold front occurs when a cooler air mass moves into an area of warmer air, forcing the warm air to rise rapidly. This leads to the development of cumulonimbus clouds, which often result in thunderstorms and quick changes in weather conditions.

Key Concept: Local Winds

c) Land cools down faster than the sea

[Solution Description] A land breeze occurs at night when the land cools down faster than the sea. The cooler land air becomes denser and moves towards the warmer sea, creating a breeze from land to sea.

Key Concept: Basic Definition

c) Isobars

[Solution Description] Lines connecting places with equal atmospheric pressure are known as isobars. These lines help in identifying different pressure systems by showing areas of high and low pressure.

Key Concept: Basic Definition

b) 1,013.2 milibar

[Solution Description] The average atmospheric pressure at sea level is known to be 1,013.2 milibars.

Key Concept: Basic Concepts

b) Uneven heating of Earth’s surface

[Solution Description]

The general circulation of the atmosphere is mainly driven by the uneven heating of Earth’s surface, where the equator receives more solar energy than the poles. This differential heating creates convection currents that drive the large-scale movement of air.

Key Concept: Pressure Gradient

b) Strong

[Solution Description] Closely spaced isobars indicate a strong pressure gradient because the pressure changes rapidly over a small distance.

Key Concept: Force Interaction

c) Wind blows parallel to isobars

[Solution Description] When the pressure gradient force is balanced by the Coriolis force, the resulting wind is called geostrophic wind. The geostrophic wind blows parallel to the isobars because the forces are in equilibrium, and there is no net force causing the air to move across the isobars.

Key Concept: Basic Definition

b) The force due to differences in atmospheric pressure

[Solution Description] The pressure gradient force is defined as the force that results from differences in atmospheric pressure. It acts to move air from regions of high pressure to regions of low pressure.

Key Concept: Basic Definition

b) Thunder and Lightning

[Solution Description] Well-grown cumulonimbus clouds are large, towering clouds associated with thunderstorms. These clouds produce thunder and lightning due to intense convection processes within them.

Key Concept: Instrument Identification

b) Mercury barometer

[Solution Description] The instruments that measure atmospheric pressure are the mercury barometer and the aneroid barometer. Among the options provided, the mercury barometer is the correct choice.

Key Concept: Pressure Variation

b) 540.48 mb

[Solution Description]

From the given data, at 5 km above sea level, the pressure is approximately 540.48 mb.

Key Concept: Land and Sea Breezes

b) From land to sea

[Solution Description] Land breezes occur during the night when the land cools down faster than the sea. As a result, the cooler denser air from the land moves towards the sea where the warmer lighter air rises. So, the land breeze blows from land to sea.

Key Concept: Basic Definition

b) Low sea level pressure

[Solution Description] The equatorial low-pressure belt is characterized by a lower atmospheric pressure at sea level due to rising air and convergence of winds.

Key Concept: Friction vs. Coriolis

c) Frictional force affects speed, Coriolis affects direction

[Solution Description]

Frictional force primarily affects the wind speed by reducing it, especially near the earth’s surface where rough surfaces create more resistance to movement. On the other hand, the Coriolis force does not affect wind speed but instead alters the direction of the wind due to the rotation of the Earth. Hence, frictional force affects speed while Coriolis affects direction.

Key Concept: Cyclones and Anticyclones

c) Anticlockwise and inward

[Solution Description] In the northern hemisphere, due to the Coriolis force, cyclonic circulation around a low-pressure area is anticlockwise. The pressure gradient force directs winds towards the low pressure, while the Coriolis effect causes a deflection resulting in an anticlockwise spiral inward.

Key Concept: Cyclone Formation

c) Stationary

[Solution Description] An extra-tropical cyclone typically begins to form when there is a stationary front. This front separates different air masses that begin to interact and initiate cyclonic activity.

Key Concept: Pressure Gradient

d) It becomes stronger

[Solution Description] When isobars are close together, there is a rapid change in pressure over a small distance. The pressure gradient, defined as the rate of change of pressure with respect to distance, becomes stronger.

Key Concept: Cause and Effect

b) The land becomes cooler than the sea

[Solution Description] During the day, the land heats up faster than the sea, creating low pressure over the land and high pressure over the sea, resulting in a sea breeze. At night, the situation reverses as the land cools down more quickly than the sea, creating high pressure over the land and low pressure over the sea, thus causing the wind to blow from the land to the sea, known as a land breeze.

Key Concept: Front Characteristics

a) Cold front

[Solution Description] A cold front is characterized by a steep gradient in both temperature and pressure as the cold dense air pushes away the warmer air.

Key Concept: Basic Definition

b) Warm water off the coast of Peru

[Solution Description] El Niño is characterized by the warming of sea surface temperatures in the central and eastern tropical Pacific Ocean, particularly off the coast of Peru.

Key Concept: Land-Sea Breezes

b) The land heats faster than the sea

[Solution Description] A sea breeze is formed due to the differential heating between land and sea. During the day, the land heats up faster than the sea, creating low pressure over the land and high pressure over the sea. Air moves from high pressure to low pressure, causing a breeze from the sea towards the land.

Key Concept: Seasonal Shifts

a) It shifts northward

[Solution Description] During the Southern Hemisphere winter, solar heating is more intense in the Northern Hemisphere, causing the ITCZ to shift northward. This shift results from the fact that the ITCZ follows the area of maximum solar radiation, which changes with seasons.

Key Concept: Basic Definition

b) Sea breeze

[Solution Description] During the day, the land heats up faster than the sea, creating a low-pressure area over the land and a high-pressure area over the sea. This pressure difference causes air to move from the sea towards the land, resulting in a sea breeze.

Key Concept: Basic Definition

c) A tornado over water

[Solution Description] A waterspout is defined as a tornado that occurs over a body of water, typically forming when a funnel-shaped cloud extends from a thunderstorm to the surface of the water.

Key Concept: Pressure Systems

b) Lowest pressure with inward wind movement

[Solution Description] In a low-pressure system, the pressure is lowest at the center. The air moves inward towards the center of the low-pressure area due to the pressure gradient force. Once it reaches near the center, it rises. This inward flow generally results in convergent winds at the surface.

Key Concept: Cyclone Patterns

b) Anti-clockwise spiraling into the center

[Solution Description] In the northern hemisphere, cyclonic circulation around low-pressure areas is characterized by winds that move counterclockwise due to the Coriolis force deflecting winds to the right.

Key Concept: Cloud Formation

c) Altostratus

[Solution Description] As a warm front approaches, the warm air slowly rises over the cold air. This gradual rise leads to widespread cloudiness characterized by stratiform clouds such as nimbostratus and altostratus, rather than isolated cumulus clouds which form with steeper gradients like cold fronts.

Key Concept: Cyclone vs Anticyclone

a) Clockwise

[Solution Description] In the Southern Hemisphere, winds around an anticyclone move clockwise due to the Coriolis effect. This deflection arises because the Coriolis force causes moving air to turn left in the Southern Hemisphere, resulting in clockwise circulation around areas of high pressure (anticyclones).

Key Concept: Basic Definition

d) Eye

[Solution Description] The region of calm within a tropical cyclone is referred to as the eye. It is characterized by light winds and often clear skies, surrounded by the eye wall where the strongest winds occur.

Key Concept: Comparative Analysis

c) Katabatic winds are cold winds, while chinook winds are warm.

[Solution Description] Katabatic winds are cold due to descending from high elevations often over ice fields, while chinook winds warm adiabatically as they descend mountain ranges. Therefore, katabatic winds are typically cooler.

Key Concept: Basic Concepts

c) Enclosed by isobars with the lowest pressure at the center

[Solution Description] A low-pressure system is characterized by being enclosed by isobars that have the lowest pressure at the center.

Key Concept: Wind Patterns and ITCZ

d) Is an area of converging trade winds that leads to rising air currents and precipitation.

[Solution Description]

The Inter Tropical Convergence Zone (ITCZ) is a crucial part of global atmospheric circulation. It acts as a low-pressure zone where trade winds from both hemispheres meet. At this convergence zone, warm moist air rises due to high solar heating, leading to cloud formation and precipitation. This rising air forms the upward branch of the Hadley Cell circulation pattern, which subsequently influences tropical climates.

Key Concept: Equatorial Low

b) Low pressure

[Solution Description] The equatorial low-pressure area is characterized by relatively low atmospheric pressure due to the intense heating and rising air near the equator.

Key Concept: Cause Identification

b) Intense convection on moist hot days

[Solution Description] Thunderstorms often occur due to intense convection on moist hot days. This process involves warm air rising rapidly into the atmosphere, which can lead to the formation of storm clouds and precipitation.

Key Concept: Complex Wind Systems, Climate Impact

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] The assertion is based on an advanced understanding of how polar high-pressure belts interact with cyclone systems. Polar high-pressure belts exert a significant influence on atmospheric circulation patterns, which include steering winds that direct the movement of cyclones. Cyclones tend to move along the path dictated by these larger-scale wind patterns associated with high-pressure systems. Thus, both the assertion and reason are true, and the reason correctly explains how the polar high-pressure belts affect cyclone trajectories in the mid-latitudes.

Key Concept: Wind Patterns

b) Clockwise

[Solution Description] In the southern hemisphere, winds circulate clockwise around high-pressure systems due to Coriolis effect.

Key Concept: Weather Phenomena

b) Low pressure at the center

[Solution Description] A tornado is a severe thunderstorm feature known for its spiraling winds and very low pressure at the center.

Key Concept: Pressure Gradient

c) It becomes stronger when isobars are close together

[Solution Description] The pressure gradient force is defined as the rate of change of pressure with distance. It becomes stronger when isobars are close together because this indicates a rapid change in pressure over a short distance.

Key Concept: Terminology Focus

c) Hurricanes

[Solution Description] In the Atlantic Ocean, tropical cyclones are commonly referred to as hurricanes. This terminology is specific to certain regions and helps differentiate these systems based on their location.

Key Concept: Cyclone Structure

d) Eye

[Solution Description] The calm center of a tropical cyclone is known as the “Eye.” It is characterized by light winds and often clear skies, contrasting with the severe weather found in the surrounding Eye wall.

Key Concept: Basic Definition

d) A line connecting points of equal pressure

[Solution Description] An isobar is a line on a weather map that connects points with the same atmospheric pressure. These lines are used to represent constant pressure on maps and help in understanding pressure patterns and forecasting weather.

Key Concept: Cyclones and Anticyclones

b) Inward spiraling counterclockwise winds

[Solution Description] In the Northern Hemisphere, cyclones are characterized by inward spiraling winds that rotate counterclockwise. This counterclockwise movement is due to the Coriolis effect acting on the winds that are flowing towards the low-pressure center.

Key Concept: ENSO Impact

b) Decrease in fish catches due to reduced upwelling of nutrients.

[Solution Description] During El Nino events, warm surface waters are pushed toward the coast of Peru, reducing the nutrient-rich cold water that typically upwells along the coast. This reduction leads to less available nutrients for marine life, significantly affecting the productivity of fisheries in this region.

Key Concept: Air Mass Interaction

c) Thunderstorms

[Solution Description] When a Maritime Polar (mP) air mass, which is cold and moist, interacts with a Continental Tropical (cT) air mass, which is warm and dry, the meeting of these contrasting air masses can often lead to the development of storms or other forms of unsettled weather due to the clashing of differing temperature and moisture content.

Key Concept: Basic Definition

b) Low atmospheric pressure

[Solution Description] Subpolar low-pressure belts are regions of low atmospheric pressure located near 60° N and 60° S. These areas are characterized by cyclonic circulation, where air converges and rises.

Key Concept: Cyclone Winds

b) Anticlockwise

[Solution Description] In the southern hemisphere, cyclones are characterized by anticlockwise (counterclockwise) wind circulation due to the influence of the Coriolis force.

Key Concept: Air Mass Classification

c) Continental Polar

[Solution Description] Air masses originating from polar regions are typically colder due to their high latitude source. The dryness comes from forming over continental areas rather than oceans. Hence, the air mass that fits these characteristics is the Continental Polar air mass.