Physical Geography E-Book

Michael Craghan

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley Self-Teaching Guides
• Sprache: Englisch

Seitenzahl: 300

Veröffentlichungsjahr: 2011

Beschreibung

Learn physical geography at your own pace What is atmospheric pressure? How does latitude indicate the type of climate a specific place will have? Where are volcanic eruptions or strong earthquakes most likely to occur? With Physical Geography: A Self-Teaching Guide, you'll discover the answers to these questions and many more about the basics of how our planet operates. Veteran geography teacher Michael Craghan takes you on a guided tour of Earth's surface, explaining our planet's systems and cycles and their complex interactions step by step. From seasonal changes to coastal processes, from effluvial basins to deep sea fissures, Craghan puts the emphasis on comprehension of the topics. He also includes more than 100 specially commissioned illustrations and 50 photographs to help clarify difficult concepts. The clearly structured format of Physical Geography makes it fully accessible, providing an easily understood, comprehensive overview for everyone from the student to the amateur geographer to the hobbyist. Like all Self-Teaching Guides, Physical Geography allows you to build gradually on what you have learned-at your own pace. Questions and self-tests reinforce the information in each chapter and allow you to skip ahead or focus on specific areas of concern. Packed with useful, up-to-date information, this clear, concise volume is a valuable learning tool and reference source for anyone who wants to improve his or her understanding of physical geography.

Leseprobe

Table of Contents

Title Page

Copyright Page

Acknowledgments

Introduction

Chapter 1 - Earth and Sun

Objectives

Size and Shape of Earth

Rotation, Poles, Equator

Latitude

Revolution around the Sun

Tilt and Reference Latitudes

Revolution, Alignment, and Day Length

Seasonal Changes

Chapter 2 - Insolation and Temperature

Objectives

Insolation Intensity

Seasonal Changes to Insolation

Heating, Reradiation, and Yearly Temperature Cycles

Heating and Daily Temperature Cycles

Continentality and Altitude

Chapter 3 - The Atmosphere and Atmospheric Water

Objectives

Atmospheric Layers

Atmospheric Constituents

Intercepting Insolation

Temperature and the Atmosphere

Air Pressure

Evaporation

Relative Humidity

Saturation and Condensation

Adiabatic Heating and Cooling

Chapter 4 - Pressure and Wind

Objectives

Air Pressure

Isobars and Pressure Centers

Pressure Force

Coriolis Force

High-Pressure Circulation

Low-Pressure Circulation

Friction and Coriolis

High-Pressure Circulation with Ground Friction

Low-Pressure Circulation with Ground Friction

Pressure Systems and Vertical Air Movement

Chapter 5 - General Circulation of the Atmosphere

Objectives

Up, Down, Up, Down; East, West, East

General Circulation Model

Heating and the Equator

Descending Air at 30°: Subtropical High Pressure

Trade Winds

Intertropical Convergence Zone

Westerlies

High Latitudes

Seasonal Changes

Chapter 6 - Air Masses and Storms

Objectives

Air Masses

Fronts

Air Mass Interaction at Fronts

Extratropical Cyclones I

Extratropical Cyclones II

Tropical Storms and Hurricanes

Chapter 7 - Climate

Objectives

Climate Elements

Climate Question 1: Latitude—Seasonality

Climate Question 2: Latitude—General Circulation

Seasonal Movement of Subtropical High Pressure

Climate Question 3: Continentality

An Exercise in Climate Determination

Climographs

Real Climates

Climate Question 4: Terrain

Chapter 8 - Plate Tectonics

Objectives

Geologic Time

Random and Systematic Distributions

Distributions of Geographic Phenomena

Layers of Earth

Asthenosphere and Lithosphere

Chapter 9 - Plate Interactions

Objectives

Lithospheric Plates

Continental-Oceanic Crust Plate Collisions

Oceanic-Oceanic Crust Plate Collisions

Continental-Continental Crust Plate Collisions

Oceanic Crust Covered Plate Splits

Continental Crust Covered Plate Separates

Oceanic-Continental Crust Plate Separates

Diagnostic Geomorphology and Crustal Changes

Chapter 10 - Volcanoes and Earthquakes

Objectives

Plates, Volcanoes, and Earthquakes

Explosive Volcanoes

Shield Volcanoes

Earthquakes

Measuring Earthquakes

Faults

Evidence for the Theory of Plate Tectonics

Chapter 11 - Weathering

Objectives

Denudation

Physical Weathering

Chemical Weathering

Water

Gravity

Rock Weathering to Soil

Soil Horizons

Soil-Forming Factors

Chapter 12 - Groundwater

Objectives

Water at Earth’s Surface

Infiltration

Groundwater Movement

Saturation and the Water Table

Groundwater at the Surface

Chapter 13 - Streams

Objectives

Overland Flow

Watersheds

Water and Sediment Transport

Stream Meandering

Cutoff Meander

Creation of a Floodplain

Chapter 14 - Wind and Ice

Objectives

Wind and Ice

The Wind

Sediment Transport

Dune Creation

Accumulation of Snow and Ice

Moving Ice

Glacial Melting

Glacial Landforms

Chapter 15 - Waves and Tides

Objectives

Waves

Wave Creation

Refraction in Shallow Water

Breaking Waves and the Longshore Current

Groins

Tides

Moon Cycles and Spring and Neap Tides

Tidal Impacts

Appendix 1: The Ancient Explanation of Earth-Sun Relationships

Appendix 2: Coriolis Force

Index

This book is printed on acid-free paper.

Copyright © 2003 by Michael Craghan. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Craghan, Michael

Physical geography : a self-teaching guide / Michael Craghan.

p. cm.—(Wiley self-teaching guides)

Includes index (p. ).

ISBN 0-471-44566-5 (pbk.)

1. Physical geography. I. Title. II. Series.

GB59.C74 2003

910’.02—dc22

2003057677

Acknowledgments

Many thanks to Patricia Craghan, Elizabeth Maddalena, Pat Dunne, and my neighbors at 180 First for their good ideas and their faith. My appreciation goes to Allan Frei, Karen Nichols, Karl Nordstrom, Norbert Psuty, Dave Robinson, Michael Siegel, and other colleagues in Geography. Thank you to the people at John Wiley & Sons, especially Harper Coles, Jeff Golick, and Kimberly Monroe-Hill, who recognized the need for this book and encouraged me to work on it and brought forth just what I was imagining. Thank you also to Patricia Craghan, Andrew Craghan, Karen Caprara, N.W.U., and the Middle Atlantic Center for Geography and Environmental Studies for their assistance with photography and the production of this book. And to all of my family and friends: see, all those nights I really did go home to work on a book.

Introduction

Physical geography is the study of the forces that influence the surface of Earth. This book is intended to explain how geographic processes function and why they generate characteristic responses. Climate and geomorphology are the principal divisions in physical geography, and that is reflected in this book. The first part focuses on climatology, the study of atmospheric functions and their consequences. Some processes, such as the general circulation of the atmosphere, or the revolution of Earth around the Sun, are planetary in scale. Others, such as condensation or terrain effects on temperature, are more localized. The second part of the book is concerned with the solid earth. Geomorphology is the study of the processes that affect the surface of Earth and the landforms that are produced. Some of the processes are internal, such as plate tectonics, while others, such as stream flow, are external. Many geomorphic processes are driven by atmospheric or climatic forces. Always keep in mind that the surface of Earth and the atmosphere above it are constantly interacting with and influencing each other. Although each topical chapter may be studied in isolation, it is necessary to understand system linkages to fully appreciate environmental operations. At the end of each chapter I connect its themes with other sections in the book.

This book focuses on the aspects of physical geography that people are likely to encounter in their lives: the topics that pass the “Why should I care?” test—not the arcane elements or trivia. I have tried to select subjects that are prevalent or that are responsible for large proportions of system operations. Because of this book’s purposes, topics are discussed at a very basic level, and I acknowledge that some things are greatly simplified. Readers should be aware that any subsection of these chapters would offer a lifetime of research opportunities to an Earth scientist. Because concepts, not details, are the foci of this work, its lessons should be applicable everywhere, although there is a bit of a North American bias to the presentation.

One of the appealing things about studying physical geography is its obvious relevance to society. When you consider the weather or climate, or when you read about a flood or an earthquake, you are thinking about how people are affected by environmental processes. Physical geography has real-world applications in fields such as disaster planning, agriculture, engineering, and environmental management. You will be able to open a good newspaper nearly every day and see how the topics in this book cross over into the social and political domains.

1

Earth and Sun

Objectives

In this chapter you will learn that:

• Earth is approximately 25,000 miles around.

• Earth rotates on its axis, which generates night and day.

• Latitude is an angle measurement used to identify a location on the surface of Earth.

• It takes Earth one year to revolve around the Sun.

• Seasons are caused by how the tilt of Earth’s axis affects the orientation of the planet as it revolves around the Sun.

• Hours of daylight are determined by Earth’s orientation with the Sun.

Size and Shape of Earth

Earth is a planet—it is a large body that moves around the Sun. It is not a perfect sphere, but Earth is a spherically shaped object. Earth has these approximate dimensions:

Figure 1.1. Earth is about 4,000 miles from its center to the surface (8,000-mile diameter) and approximately 25,000 miles around.

• Radius: 4,000 miles (6,400 km)

• Diameter: 8,000 miles (12,800 km)

• Circumference: 25,000 miles (40,000 km)

These values can vary slightly due to differences in surface topography and because Earth is not an exact sphere. If you could drive nonstop around the equator at 60 mph it would take seventeen days to make the trip.

What is the approximate distance around Earth (its circumference)? ________________

Answer: 25,000 miles (40,000 km)

Rotation, Poles, Equator

One feature of this planet is its rotation—it spins. It takes one day for Earth to rotate on its axis (one day exactly, because that is the definition of a day: one spin on its axis). Spinning leads to a reference system based on the axis of rotation. The North and South Poles are at the ends of the axis of rotation and thus can be used as unique reference points. If Earth did not spin (and thus had no rotation axis), then any place would be as good as any other for describing location.

Figure 1.2. Because Earth rotates, we can identify the North Pole and the South Pole as special spots. A place on Earth will rotate once around and find itself back in the same position a day later.

Rotation also produces another feature of interest: the equator. The equator is in a plane perpendicular to the axis of rotation, and it divides the spherical Earth into halves. All of the points on one side of the equator are closer to the North Pole than to the South Pole. All of the points on the other side are closer to the South Pole. The half of Earth closest to the North Pole is called the Northern Hemisphere (half a sphere). The half of Earth closest to the South Pole is the Southern Hemisphere.

Figure 1.3. The equator is in a plane perpendicular to the axis of rotation, and it separates Earth into two halves: the Northern Hemisphere and the Southern Hemisphere.

What is the line that divides Earth into a half that is closer to the North Pole and another half that is closer to the South Pole? ________________

Answer: the equator

Latitude

Once the two poles and the equator have been identified, then a system of measurement called latitude can be established. Latitude is an angle measurement from the equator to a point on Earth’s surface. The angle is measured from the center of Earth at the point where the rotation axis intersects the plane of the equator.

The latitude system has some simple qualities:

• All points on the equator are 0° away from the equator.

• The North Pole is 90° away from the equator.

• The South Pole is 90° away from the equator.

• If the angle is measured toward the North Pole it is called north latitude.

• If the angle is measured toward the South Pole it is called south latitude.

• North and south are important! You must state whether a place has north or south latitude to properly identify it.

Figure 1.4. All points that are the same angle away from the equator and the center of Earth have the same latitude.

Figure 1.5.

What are the latitudes of points A, B, C, D, E, F, and G in Figure 1.5? ________________

Revolution around the Sun

At the same time that it is rotating on its axis, Earth also is following a path around the Sun. Earth is a planet that rotates on its axis and also revolves around the Sun.

It takes one year for Earth to revolve around the Sun (one year exactly, because that is the definition of a year: one trip around the Sun). This journey also takes 365¼ days (i.e., one year). So Earth will rotate on its axis 365¼ times in the time it takes for the planet to go around the Sun and return to its departing point.

The path that Earth travels along is an ellipse—but it is very close to being a circle. The nearly circular path is used to define a geometric feature called the plane of revolution. Although the planet orbits within the plane of revolution—this is going to affect almost everything on Earth—Earth’s axis of rotation (the line running from the South Pole through the North Pole) always points toward the North Star.

Figure 1.6. It takes Earth one year to complete its nearly circular revolution around the Sun. Earth’s axis is always tilted toward the North Star.

For the North Pole to be continuously directed toward the North Star, Earth’s axis has to be tilted 23½° away from perpendicular to its plane of revolution around the Sun. The direction and angle of the tilt will always be the same: the axis is always aligned toward the North Star. As a result of its constant aim to the North Star, the alignment of the axis with the Sun is always changing. For part of its revolution around the Sun, Earth’s North Pole generally leans toward the Sun, and for the other part of a year it leans away from the Sun.

• In December, the North Pole leans away from the Sun.

• In June, the North Pole leans toward the Sun.

Figure 1.7. Earth’s axis is tilted 23½° away from perpendicular to its orbit in the plane of revolution.

Figure 1.8. In this view from above Earth’s plane of revolution you can see that the North Pole is always pointed toward the North Star. This causes the orientation of Earth with respect to the Sun to always be changing.

• In March and September, the line from Earth to the Sun is perpendicular to the South Pole-North Pole axis.

Because the North Pole is always pointing to the North Star, Earth’s Northern Hemisphere is directed ________________ the Sun in June and ________________ the Sun in December.

Answer: toward; away from

Tilt and Reference Latitudes

This tilt of Earth’s axis creates five special latitude lines. These five lines are the equator, two “tropics,” and two “circles.” Because tropics and circles are lines of latitude, they are in planes that are perpendicular to Earth’s rotation axis and parallel to the plane of the equator.

• 66½°N is the Arctic Circle. As Earth rotates on its axis, all of the places on the North Pole side of this line will always be on the same side of perpendicular as the North Pole.

Figure 1.9. There are five special lines of latitude that are produced by Earth’s 23½° angle of tilt to its plane of revolution around the Sun.

• 23½°N is the tropic of Cancer. As Earth rotates on its axis, all of the places on the North Pole side of this line will always be above the plane of revolution around the Sun. No point that is north of this line will ever rotate to be directly on the plane of revolution.

• The equator is at 0° latitude. As Earth rotates on its axis, all points at the equator will spend half of each day above the plane of revolution and half below it, and half of each day on the North Pole side of perpendicular and half on the South Pole side.

• 23½°S is the tropic of Capricorn. As Earth rotates on its axis, all of the places on the South Pole side of this line will always be below the plane of revolution around the Sun. No point that is south of this line will ever rotate to be directly on the plane of revolution.

• 66½°S is the Antarctic Circle. As Earth rotates on its axis, all of the places on the South Pole side of this line will always be on the same side of perpendicular as the South Pole.

Which two lines mark the farthest places north and south that can be directly on Earth’s plane of revolution around the Sun? ________________

Answer: the tropic of Cancer (23½°N) and the tropic of Capricorn (23½°S)

Revolution, Alignment, and Day Length

As Earth travels around the Sun, Earth’s tilt toward the North Star will create four days when Earth-Sun alignment is in a special condition. In June and December, there are solstices. A solstice is the moment when the Sun is directly overhead at one of the tropics. This is the farthest point north or south of the equator that the Sun can be directly overhead. A solstice also is the day when a hemisphere is aimed either most directly toward the Sun (summer solstice) or away from the Sun (winter solstice). In September and March, there are equinoxes. An equinox is the moment when the Sun is directly over the equator. Solstices and equinoxes mark the extremes of orientation and a changeover with respect to Sun conditions.

June Solstice

On the day of the June solstice, the North Pole is tilted as close as it gets toward the Sun and the South Pole is tilted as far away as it gets. It is summer in the Northern Hemisphere and winter in the Southern Hemisphere. On this day:

• The Sun will be directly overhead at 23½°N (tropic of Cancer), and it is strongest at that latitude.

• All points in the Northern Hemisphere will get more than 12 hours of sunlight; they spend more than half of the day rotating on the sunlit side of the planet. All points in the Southern Hemisphere will get fewer than 12 hours of sunlight.

• All points on the equator will spend 12 hours rotating on the sunlit side of Earth and 12 hours rotating on the dark side of Earth.

• All points north of the Arctic Circle (66½°N) will spend the entire 24-hour day rotating on the sunlit side of Earth.

• All points south of the Antarctic Circle (66½°S) will spend the entire 24-hour day rotating on the dark side of Earth.

September Equinox

On the day of the September equinox, the Sun is directly overhead at the equator. It is the first day of autumn in the Northern Hemisphere and the first day of spring in the Southern Hemisphere. On this day:

Figure 1.10. The June solstice.

Figure 1.11. The September equinox.

• The Sun is most directly overhead at the equator.

• All points on Earth will rotate on the sunlit side of the planet for 12 hours and rotate on the side away from the Sun for 12 hours.

December Solstice

On the day of the December solstice, the South Pole is tilted as close as it gets toward the Sun and the North Pole is tilted as far away as it gets. It is winter in the Northern Hemisphere and summer in the Southern Hemisphere. On this day:

Figure 1.12. The December solstice.

This classic photograph from December 7, 1972, was taken by a crew member on Apollo 17 near the time of the December solstice. Note how the part of Earth that is sunlit and visible to the astronauts ranges from nearly all of Antarctica (at bottom) to the Mediterranean Sea (top) at about 40°N latitude. (Image AS17-148-22721 courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center.)

• The Sun will be directly overhead at 23½°S (tropic of Capricorn), and it is strongest at that latitude.

• All points in the Southern Hemisphere will get more than 12 hours of sunlight; they spend more than half of the day rotating on the sunlit side of the planet. All points in the Northern Hemisphere will get fewer than 12 hours of sunlight.

• All points on the equator will rotate 12 hours on the sunlit side of Earth and rotate 12 hours on the dark side of Earth.

• All points south of the Antarctic Circle (66½°S) will spend the entire 24-hour day on the sunlit side of Earth.

• All points north of the Arctic Circle (66½°N) will spend the entire 24-hour day rotating on the dark side of Earth.

March Equinox

On the day of the March equinox, the Sun is directly overhead at the equator. It is the first day of spring in the Northern Hemisphere and the first day of autumn in the Southern Hemisphere. On this day:

• The Sun is most directly overhead at the equator.

• All points on Earth will be on the sunlit side of the planet for 12 hours and on the dark side for 12 hours.

Figure 1.13. The March equinox.

“In-Between” Days

Figure 1.14. On August 1, the Earth-Sun relationship will be “in between” the conditions from the June solstice and the September equinox. Top: The Sun will be most directly overhead in the Northern Hemisphere, somewhere between the tropic of Cancer and the equator (it will actually be at about 18°N). On this day, all places in the Northern Hemisphere spend more than half a day on the sunlit side of Earth. Bottom: Don’t be misled by two-dimensional depictions of the situation. The Sun is directly overhead at 18°N latitude. Earth is still tilted 23½° away from perpendicular with respect to its plane of revolution around the Sun. The North Pole is still aimed at the North Star.

Because of the way Earth is tilted with respect to its plane of revolution, the Sun can never be directly overhead north of 23½°N (tropic of Cancer) or south of 23½°S (tropic of Capricorn). The Sun can only be overhead in the tropics (between Cancer and Capricorn). The Sun will be directly overhead at the equator on the two equinox days of the year (in March and September). Days that are not an equinox or a solstice (i.e., the other 361) are simply “in the middle.” If you interpolate between the solstice and equinox extremes, you should be able to figure out Earth-Sun relations for any day. Here are the basic principles:

• The Sun must be overhead somewhere between the tropics of Cancer and Capricorn.

If it is March-September, the Sun will be directly overhead a place in the Northern Hemisphere.

If it is September-March, the Sun will be directly overhead a place in the Southern Hemisphere.

The closer a date is to a solstice, the closer to a tropic the Sun will be overhead.

The closer a date is to an equinox, the closer to the equator the Sun will be overhead.

• Hours of daylight will be controlled by which hemisphere the Sun is “in” (i.e., in which hemisphere it is overhead) and then by latitude.

In a hemisphere in which the Sun is overhead, the closer to a pole a place is in that hemisphere, the more hours of daylight there will be at that place.

There are 12 hours of daylight every day of the year at the equator.

In a hemisphere in which the Sun is not overhead, the closer to a pole a place is in that hemisphere, the fewer hours of daylight there will be at that place.

Why would a place like New York City (lat. 41°N) have about 15 hours of daylight in June but only about 9 hours in December? ________________

Answer: The Northern Hemisphere is tilted most directly toward the Sun in June, and a place at 41°N spends most of a day on the illuminated part of Earth. In December, the Northern Hemisphere is tilted away from the Sun, and a place at 41°N spends most of a day on the dark side of Earth.

Seasonal Changes

Seasons are produced because Earth revolves around the Sun with a tilted axis, which directs different parts of the planet toward or away from the Sun at different stages of the journey.

Seasons have nothing to do with how close Earth and the Sun are to each other. If proximity was the cause of seasons, then it would be the same season in the Northern and Southern Hemispheres on the same day.

Yearly Sunlight Variations because of Earth’s Revolution and Tilt

What causes seasons? ________________

Answer: The tilt of Earth’s axis changes the alignments of the two hemispheres and the Sun over the course of a year.

SELF-TEST

1. How much is Earth’s rotation axis tilted away from perpendicular to the planet’s plane of revolution around the Sun?

a. 0°

b. 23½°

c. 78½°

d. 90°

2. The moment when the Sun is directly over either of the two tropics is called a(n) ________________.

a. solstice

b. equinox

c. season

d. ellipse

3. The Sun is directly above the tropic of Cancer (23½°N) in which of these months?

a. February

b. April

c. June

d. August

4. Earth’s North Pole is always pointed toward the ________________.

5. The Sun is directly above the equator in the months of ________________ and ________________.

6. The reason that it is hot in the summer is because that is when Earth is closest to the Sun. (True or False)

7. There are 12 hours of daylight at the equator every day of the year. (True or False)

8. How does Earth’s rotation on its axis cause night and day?

9. With respect to Earth-Sun relationships, how are tropical latitudes different from middle and high latitudes?

10. For February 1, describe in general terms the latitude zone where the Sun will be directly overhead.

ANSWERS

1. b

2. a

3. c

4. North Star

5. March; September

6. False

7. True

8. Rotation spins a place into the sunlit half of Earth, then around out of the sunlight to the dark side of the planet.

9. The Sun can be directly overhead in tropical areas (between 23½°N and 23½°S), but it can never be 90° overhead in the middle or high latitudes.

10. The Sun will be directly overhead in the Southern Hemisphere, between the equator and the tropic of Capricorn.

Links to Other Chapters

• Latitude affects the length of day and the duration of daily insolation (chapter 2), which affects heating and temperature (chapter 2).

• Earth’s rotation on its axis will produce night and a day with changing sunlight intensity, which will affect daily temperature patterns (chapter 2).

• Variations in heating help establish the general circulation of the atmosphere (chapter 5).

• Earth’s spin on its axis contributes a Coriolis force, which affects how the wind moves across the surface of Earth (chapters 4, 5).

• Seasonal variations in heating are a major factor in climate (chapter 7).

• Temperature and seasons affect geomorphic forces such as weathering (chapter 11), soil formation (chapter 11), and glacial processes (chapter 14).

2

Insolation and Temperature

Objectives

In this chapter you will learn that:

• Solar energy does not hit all places on Earth with the same intensity.

• The more intense the Sun’s rays are, the more energy the ground will absorb and the warmer it will be.

• Earth radiates as much energy away to space as it gets from the Sun.

• January and July usually are the coldest and hottest months, respectively, even though insolation is least intense in December and most intense in June in the Northern Hemisphere.

• Solar energy does not hit a place with the same intensity all day long.

• The coldest time of a typical 24-hour day is a bit after the Sun rises. The hottest time of a typical day is midafternoon.

• Continental places heat up faster and cool off quicker than coastal places.

Insolation Intensity

Energy that comes from the Sun to Earth is called incoming solar radiation, insolation. Insolation intensity changes at a given place from day to day during a year because the Earth-Sun orientation changes as Earth revolves around the Sun. Within a day, insolation intensity changes from minute to minute because a place’s solar alignment is changing as Earth rotates on its axis. The Sun is stronger at noon than at early morning, and it has no power at all after sunset. At any given moment, different places on Earth will have different insolation intensities depending on (1) latitude or (2) time of day. Each of these two factors influences how directly the Sun will shine onto a place.

What two actions of planet Earth affect the amount of insolation being received at a given place? ________________

Answer: The rotation of Earth on its axis and the revolution of Earth around the Sun each change the alignment of a place with respect to incoming solar radiation.

Seasonal Changes to Insolation

The place where the Sun is most directly overhead will receive more insolation energy than other places. Over the course of a year, the place where the Sun is directly overhead is changing every day. The Sun is directly overhead at the tropic of Cancer (23½°N) at the June solstice. The Sun is directly overhead at the tropic of Capricorn (23½°S) at the December solstice. At the March and September equinoxes, the Sun is directly overhead at the equator.

At times between solstices and equinoxes, the Sun will be overhead in tropical areas, the zone between 23½°N (tropic of Cancer) and 23½°S (tropic of Capricorn). The Sun will pass directly over a tropical place twice a year: once as the Sun moves from being overhead at the equator to being overhead at the tropic and then again on the return from the tropic to the equator. The Sun will never be overhead outside of the tropics; the Sun will be most overhead in middle and high latitudes on the day of that hemisphere’s summer solstice.

Figure 2.1.Top: A flashlight shining directly down onto a floor will have all of its light energy concentrated in a small, bright circle. If the flashlight is directed at an angle to the floor, the energy will be distributed over a larger dimmer area. Insolation is similarly concentrated or dispersed depending on the angle that it strikes the surface. Bottom: Insolation that reaches Earth at a high angle is concentrated and intense. As the angle of insolation decreases, the sunlight is dispersed and its intensity drops.

When insolation reaches the surface of Earth, some of that solar energy is absorbed and turned into heat. Heating and warming from the Sun will vary over the course of a year as insolation intensity changes. The yearly changes in insolation intensity cause the temperature patterns we associate with seasons. It is hotter in summer because insolation is more direct and because there are more hours of daylight (a lot of high-intensity sunlight). It is cold in winter because insolation comes at a lower angle and there are fewer hours of daylight (little, weak sunshine).

Figure 2.2.A: On the day of the June solstice, insolation is straight overhead at the tropic of Cancer, 23½°N. B: On the day of the December solstice, insolation is straight overhead at the tropic of Capricorn, 23½°S. At the (C) March equinox and the (D) September equinox, the Sun is straight overhead at the equator. Insolation intensity and therefore heating intensity will be greatest when the Sun is most directly overhead.

In what month will New York City (41°N) receive the most intense insolation? ________________

Answer: June

Heating, Reradiation, and Yearly Temperature Cycles

At the same time that the Sun’s energy is coming into the half of the planet that is in daylight, heat energy is being radiated away from the planet to space. There is a balance between incoming solar radiation and outgoing heat radiation. If Earth received more insolation than it radiated away, it would get hotter and hotter. If Earth received less insolation than it radiated away, it would get colder and colder.

There is a planetwide energy balance, but at any given moment a particular place can be out of equilibrium. The planetary radiation balance is not evenly distributed. When the Sun is high overhead (in summer), a place receives more insolation energy than it loses from radiating heat away, so it gets progressively hotter. When the Sun is weak and less direct (in winter), a place radiates away more energy than it receives as insolation, so it gets progressively colder.

Over the course of a year, a place will have different insolation inputs that will affect how hot or cold it will be. Let’s consider the yearly temperature changes for a midlatitude, Northern Hemisphere city somewhere in the lower forty-eight United States.

Figure 2.3. Insolation intensity is strongest when the Sun is directly overhead. In the tropics, the Sun will be directly overhead twice each year. In the middle and high latitudes, the Sun can never be directly overhead, but it is highest in the sky on the day of the summer solstice.

Figure 2.4. Earth continuously receives energy from the Sun—it is always daylight on half of the planet. Earth also gives energy off into space (in the form of invisible heat energy). All areas of Earth radiate energy, both the illuminated and the dark parts.

1. At the solstice in December, insolation will be at its lowest level. The Sun will be directly over the tropic of Capricorn in the Southern Hemisphere, and the Northern Hemisphere will be tipped as far away from the Sun as it gets. The angle of the incoming sunlight will be low, and the duration of daylight will be at its shortest. Our city will be receiving the least amount of solar energy and it will be losing radiated heat energy to outer space, so the temperature will be cold.

2. In January, insolation is slightly higher although still low. The Sun is not heating the city enough to make up for heat lost to radiation. Even though the insolation is getting stronger, it is still relatively weak, and the city is still getting colder. The average yearly temperature will be at its minimum in January.