Background Information
The Sun
Our Sun, the centerpiece and literal star of our solar system, is a massive sphere of hot plasma heated to incandescence by nuclear fusion reactions in its core. It emits electromagnetic radiation mainly as visible, infrared, and ultraviolet light. At birth, it was composed of about 71% hydrogen, 27% helium, and less than 2% oxygen, carbon, neon, iron, and all other elements combined. The Sun contains 99.8% of all the mass in our solar system, leading some astronomers to describe our solar system as "the Sun plus some debris." In comparison, the mass of the Sun is about 330,000 times the mass of the Earth. Thus, the volume of the Sun could fit approximately 1,000 planets the size of Jupiter, the giant planet in our solar system, or about 1 million planets the size of Earth.2
The mass of the Sun produces a gravitational force strong enough to lock into orbit the eight planets, along with their moons and all other objects in between and some beyond, such as comets and asteroids. The farthest known object in our solar system, discovered in 2018, is a planetoid aptly named Farfarout (2018 AG37) and is 132 astronomical units (AU) from the Sun. One astronomical unit is about 93 million miles, the distance from the Earth to the Sun. It takes about 1,000 Earth years for Farfarout to complete its orbit around the Sun!3
Our Sun classifies as a medium-sized yellow dwarf star. Despite its dominance in our solar system, there are stars up to 300 times more massive in the Milky Way galaxy where our solar system resides. The most petite star in the Milky Way, EBLM J0555-57 discovered in 2017, is roughly the size of Saturn but also about 85 times the mass of Jupiter.4 It is believed that it is the smallest a star can get and still contain enough mass for energy-generating hydrogen fusion.
Formation of the Sun
Our Sun formed in a nebula of dust and gas (mostly hydrogen and helium with small amounts of all other elements.) about 4.5 billion years ago. As the gas and dust began to collapse under its own gravity, it entered the protostar stage. In this stage, the center got denser and hotter as it developed an opaque, pressure-supported core. During the protostar phase, it continued to contract while gathering material from the formative molecular cloud around it. Protostars continue to do so until enough material accumulates to create a temperature exceeding 10 million K, the temperature needed for efficient hydrogen fusion to occur. This can take between 100,000 to 100 million years, depending on the size of the forming star. For a star like our Sun, it took about 50 million years. Once hydrogen fusion begins, any leftover material in the surrounding nebula can eventually form other solar bodies such as planets, asteroids, or comets. If a protostar does not accumulate enough mass for hydrogen fusion, it will become a brown dwarf star.5
Fortunately for us, our Sun did achieve hydrogen fusion and became a dwarf star. At that point, the Sun entered its main-sequence phase. It will spend most of its life in equilibrium between the inward pull of its gravity and outward expansion of pressure from gas heated by nuclear reactions when four hydrogen nuclei fuse to form helium. Our Sun will remain in this stage for about five billion more years until it has exhausted the hydrogen in its core. Eventually, the Sun will swell in size, possibly far enough to consume Earth, as it enters the red giant stage. By then, all evidence of life on Earth will have been obliterated (even though life as we know it will have ceased long before). The increased pressure in the core will cause it to become hot enough for helium fusion. Once helium is exhausted, the Sun will have shed its outer layers, and the Sun's core will become a white dwarf about the size of Earth. The white dwarf will cool over billions of years and eventually will become too dark to see.
Solar Energy
All matter with a temperature above absolute zero (where all atomic or molecular motion stops) radiates energy across a range of wavelengths in the electromagnetic spectrum. The peak wavelength of radiated energy becomes shorter as an object gets hotter. The hottest things in the universe radiate mostly gamma rays and X-rays, while cooler objects emit mostly longer-wavelength radiation, including visible light, thermal infrared, radio, and microwaves. For example, the surface of the Sun has a temperature of about 5,800 Kelvin (5,526ºC, or about 10,000ºF). At that temperature, most of the radiated energy is visible, infrared, and ultraviolet light. At Earth's average distance from the Sun (about 93 million miles), the average intensity of solar energy reaching the top of the atmosphere facing the Sun directly is about 1,360 watts per square meter. This amount of power is known as the total solar irradiance (also known as the solar constant).
The Sun does not always shine at a constant level of brightness. Instead, the Sun brightens and dims slightly, taking 11 years to complete one solar cycle. In addition, the Sun undergoes various changes in its activity and appearance, with levels of solar radiation going up or down, as well as the amount of material the Sun ejects into space and the size and number of sunspots and solar flares. These changes have various effects in space, Earth's atmosphere, and Earth's surface but are usually not severe enough to affect living things on Earth or Earth's climate. Human technology, however, is at risk. Satellites and electronic devices on the surface that we depend on today for our infrastructure and quality of life are susceptible to damage with increased solar activity.
Solar Energy & the Earth
Almost all (99.9%) of the energy on Earth originates from the Sun. It is our primary source of heat and prevents our atmosphere and water from completely freezing. It is necessary for plant growth (photosynthesis) and therefore the food, shelter, and oxygen needed by other living things. It powers Earth's hydrologic cycle and other Earth systems that play a role in weather and climate. We also depend on it to meet our energy needs, as even unearthed fossil fuels were formed millions of years ago from buried plants that absorbed and stored energy derived from the Sun.
What if the Sun suddenly disappeared? If this were to happen, the daylight side of the Earth would darken after 8½ minutes. The Moon and planets would no longer be visible. Solar system objects, including Earth, would continue in a straight path along their trajectory until they collide with something or encounter the gravitational force of a larger entity. Within days, global temperatures would fall below freezing. Most plant life would die due to no photosynthesis, and soon after, any animal life that depends on them. Eventually, the water on Earth would freeze, yet deeper bodies of water such as oceans might experience an insulating effect and not freeze completely. At least not for a few thousand to millions of years. Some microscopic life would persist, as well as some life sustained by geothermal activity such as ocean vents. The atmosphere would eventually freeze, therefore exposing the surface to any radiation from space. Over a few million years, the Earth would ultimately cool to a stable –400°F, the temperature at which the heat radiating from the planet's core would equal the heat that the Earth radiates into space. Of course, the sudden disappearance of our Sun is an impossibility, but without it, we simply would not be here, and our home planet would be inhospitable and nearly unrecognizable.6
I previously mentioned that our Sun radiates shortwave energy mainly in the form of visible, infrared, and ultraviolet light. Only a fraction of the Sun's energy output reaches Earth. And even then, not all the Sun's energy that reaches the top of the atmosphere reaches the surface of the Earth. About 30% of the solar energy that arrives at the top of the atmosphere is reflected to space by clouds, particles in the atmosphere, or reflective ground surfaces like sea ice and snow. This means approximately 70% of incoming solar energy is absorbed. Atmospheric dust, ozone, and greenhouse gases such as water vapor, carbon dioxide, and methane absorb about 23% of the incoming solar energy. Only 48% passes through the atmosphere and is absorbed by the surface.7 The solar energy that reflects off Earth's surfaces emits back as longwave radiation. This flow of incoming and outgoing energy is Earth's energy budget. For Earth's temperature to be stable over long periods, incoming and outgoing energy must be in equilibrium.8
Although the Earth's atmosphere is thin in comparison to the size of the planet, much like an eggshell compared to the egg, it is enough to keep our world warm and protected as if wrapped in a blanket. Our Moon has an exosphere of trace elements instead of an atmosphere like Earth due to its low mass and weak gravitational and magnetic fields. Lunar surface temperatures can range from a high of up to 250ºF during a lunar day to a low as far as –387ºF during a lunar night because its atmosphere does not retain heat.9 It is also quiet because sound cannot travel through the air as it does on Earth. The sky is black because the light does not scatter through the atmosphere. The surface shows the aftermath of many impact craters because objects collided with the surface rather than burning up in the atmosphere. There is also no protection from harmful radiation, which is part of the reason why lunar astronauts must wear suits that protect their entire bodies. We could expect similar conditions here on Earth if we did not have an atmosphere.10
Solar Energy Distribution on Earth
Figure 1 The Earth rotates on its axis, resulting in a day and a night. Credits: MIT OpenCourseWare |
The reasons for the Earth experiencing seasons are rotation, revolution, sphericity, axial tilt, and axial parallelism. Earth is a sphere that rotates on its axis once every 24 hours.11 Therefore, we have a day and night on Earth. If the Earth did not rotate, then a full day would span six months and a whole night another six months. |
| The Earth takes 365.24 days to orbit the Sun. Earth's orbit around the Sun is not a perfect circle. It is an elliptical, or oval-shaped, orbit. At aphelion, the Earth is farthest from the Sun at about 94.5 million miles. At perihelion, the Earth is closest to the Sun at about 91.4 million miles. One might assume that we have changing seasons and diverse climate zones on Earth because of our distance from the Sun. After all, the closer one gets to a source of heat, the warmer one becomes. And the opposite is true as well; the farther away, the colder it gets. While there is a difference of over 3 million miles, that is not much relative to the entire distance. |
Figure 2 Aphelion is when Earth is farthest from the Sun, while perihelion is when Earth is closest. Credits: NASA |
Aphelion (when Earth is farthest from the Sun) occurs in July, and perihelion (when we are closest) occurs in January.12 For those of us who live in the Northern Hemisphere, where it is summer in July and winter in January, that may seem backward, but it does provide a clue that Earth's distance from the Sun is not the leading cause of the seasons.
Figure 3 Incoming solar radiation hits Earth at different angles. Credits: NWClimate.org |
The spherical shape of the Earth is another reason why we have uneven heating. A smooth, flat object would experience an equal amount of radiation from top to bottom. However, since the Earth is a sphere, the curvature of the Earth influences the angle and intensity of incoming solar radiation. As a result, the rays of solar energy are most direct and intense in the equatorial regions and less severe (and cooler) toward the poles.13 |
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The Earth is also tilted on its axis by about 23.5 degrees as it rotates. This tilt means that at one point in Earth's orbit around the Sun, the northern part of the planet is pointing toward the Sun, while the southern end points away from the Sun. This tilt also explains why different parts of the Earth have differing amounts of daylight at varying times of the year. |
 Figure 4 The tilt of Earth causes seasons. Credits: NASA |
To tie this into the seasons, we also need to look at axial parallelism. While Earth is rotating on its axis at an angle of 23.5 degrees, it is always (at least in our lifetime) tilted in the same direction toward the North Star (Polaris). The tilt of the planet remains in a constant direction as Earth orbits the Sun. In December, the Southern Hemisphere points toward the Sun, and it is summer. Meanwhile, the Northern Hemisphere has pointed away from the Sun, and it is winter. Six months later, Earth has revolved halfway around the Sun, so now the Northern Hemisphere is pointed toward the Sun (summer), and the Southern Hemisphere has pointed away (winter).
In addition, the land heats and cools faster than water. This uneven heating influences both ocean and atmospheric convection currents. Uneven heating of land and water produces wind systems. Differences in air pressure due to varying temperatures create wind as high-pressure areas move to low-pressure areas. The same happens in the ocean, where uneven heating and cooling produce the convection of ocean currents.
The Greenhouse Effect
| The process that occurs when gases in the Earth's atmosphere trap heat, much like the Earth under a blanket, is known as the greenhouse effect. Students may relate more to a blanket analogy rather than a greenhouse, but either way – it is a process that keeps the Earth warm. Without greenhouse gases in our atmosphere, the Earth's average temperature would drop to as low as 0º Fahrenheit (14º Celsius). Earth would be an icy wasteland, hospitable only to organisms that could tolerate such conditions.14 |
Figure 5 The glass walls of a greenhouse trap the Sun's heat. Credits: NASA/JPL-Caltech |
Key to this understanding is the natural balance that makes our planet unique in our solar system to support an abundance of life. However, too much greenhouse gas can have a detrimental effect. The planet Venus provides an example of a planet where greenhouse gases have significantly increased the atmospheric and surface temperature. Too much heat would be trapped, and to continue the blanket analogy, it would be like wearing a winter coat under a large pile of blankets on an already hot day.
Greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons (CFCs). They absorb heat and radiate it out in all directions, very much like a stone heated by the Sun. Carbon dioxide has increased under the current balance as human activity releases more and more into the atmosphere each year. This increase in carbon dioxide correlates to a rise in temperature. Since warmer air can hold more water, this adds more water vapor to the atmosphere (and an increase in rainstorms in some regions).
A phenomenon known as albedo is a familiar concept that dark-colored objects left in the Sun get warmer than light-colored objects. Anyone who has walked barefoot on the white painted lines of a dark asphalt parking lot to reduce burning their feet is familiar with this concept. This difference in reflective surfaces has an impact on weather and climate on a planetary scale. In the Arctic regions, it can result in the melting (or build-up) of sea ice and glaciers. Sunlight is the primary driver of Earth's climate, with about 340 watts per square meter of energy from the Sun reaching the Earth. About one-third of that energy reflects to space, and the remaining energy is absorbed by land, ocean, and atmosphere. Exactly how much sunlight is absorbed depends on the reflectivity of the atmosphere and the surface. Albedo can range between 0 (nothing reflected) and 1 (completely reflected like a mirror). Ocean water albedo is about 0.06, or 6 percent. Sea ice can range from 0.25 to 0.8, or 25 to 80 percent. Sea ice melts as global temperatures rise. This change to the Earth's albedo as the result of melting ice further exacerbates climate change. The same can be said for man-made changes to the Earth's surface, such as removing vegetation and adding buildings while paving roads. Under current climate conditions, urban areas already experience a "heat island effect" where daytime temperatures are 1ºF to 7ºF warmer than outlying areas and nighttime temperatures about 2ºF to 5ºF higher.15
Weather Versus Climate
Weather is simply a description of the short-term atmospheric conditions, such as the temperature, humidity, level of cloudiness, and precipitation. Weather is what you get, but the climate is what you expect. Climate is the average, prevailing pattern of conditions of a region over a long period of time. The current standard considers an area's weather conditions during different parts of the year over thirty years. Can you expect snow in the winter? How hot does it get in the summer? Is it typically raining in the spring? These are some questions one would ask that are related to a region's climate. Knowing a region's climate can be helpful when choosing building materials or planning infrastructure or when considering what crops are likely (or unlikely) to thrive in a region. For visitors, knowing a location's climate can help them select appropriate clothing to pack.
Earth's Climate Zones
The Sun, Earth's orbit around the Sun, Earth's rotation and tilt on its axis, and the absorption or reflection of incoming solar energy are all factors of Earth's global climate system. Still, there are additional surface-level factors as well. Latitude, elevation, proximity to mountains or large bodies of water, ocean circulation, and long-term atmospheric circulation all influence a region's climate.
In 1884, German-Russian geographer, meteorologist, climatologist, and botanist Wladimir Koppen began to develop a climate classification system that is still widely used today and is known as the Koppen Climate Classification System.16 It has been revised several times since then by Koppen and others. Even other climate classification systems, such as the Trewartha system, further divide the middle latitudes. The Koppen system divides climates into five main groups. They are Tropical, Dry, Temperate, Continental, and Polar.17 Each of these main groups divides into sub-groups based on temperature and seasonal precipitation. Understanding the main features of these climate zones is sufficient for third graders to understand.
Tropical
Tropical or megathermal regions are warm throughout the year, and the lowest monthly mean temperature exceeds 64ºF/18ºC. These areas can be found between 15ºN to 15ºS latitudes. These areas typically have significant precipitation because warm air can hold more water. Some of the subgroups are rainforest, monsoon, and tropical savannas.18
Dry
Dry or arid climates receive little precipitation, and what little precipitation there is usually evaporates quickly. This is the only Koppen climate classification based on precipitation rather than temperature. First, a precipitation threshold is established based on the total rainfall during the spring and summer months (April-September in the Northern Hemisphere, October-March in the Southern Hemisphere). If a region's annual precipitation exceeds 50% of the precipitation threshold, it classifies as a semi-arid steppe. If less than 50%, it ranks as an arid desert. Dry climates are typically between 50° N and 50° S, but they are mainly in the 15º–30° latitude belt in both hemispheres.19 Deserts can be hot or cold deserts.
Temperate
A temperate or moist sub-tropical mid-latitude climate is typical along the edge of continents. Temperate climates have only moderate temperature differences between summer and winter. Seasonal changes are not as extreme as in dry climates. If the average temperature of the warmest month is higher than 50ºF/10°C and the coldest month is between 65ºF/18°C and 32ºF/0°C, then it is considered a temperate climate.20
Continental
Moist continental mid-latitude climates are, as the name implies, usually found in the interior of continents. They have one month with an average temperature below 32ºF/0ºC and one month averaging above 50ºF/10ºC. The typical range is from 40º to 75º latitudes, but they are rare in the Southern Hemisphere.21
Polar
Polar climates are cold, and every month has an average temperature below 50ºF/10ºC. Polar climates include tundra regions generally north of 70ºN with average temperatures between 32ºF/0ºC and 50ºF/10ºC. Also included are ice cap climates, which dominate at the poles in Antarctica and inner Greenland, where average monthly temperatures remain below 32ºF/0ºC.22
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