Chapter 2

Heating Earth’s Surface and Atmosphere

More than 99.9 % of the Energy That Heats Earth’s Surface Comes from the Sun

Earth – Sun Relationships

Earth intercepts only a minute percentage of the energy given off by the Sun—less than one two-billionth. This may seem like a small amount, but the entire Earth receives 174 billion megawatts of energy from the Sun. For comparison, one billion megawatts is enough energy to power New York City for more than ten years.

Solar energy is not equally distributed over Earth’s surface. The amount varies with latitude, time of day, and season of the year. The unequal heating of Earth creates winds and drive the ocean’s currents. These movements transport heat from the tropics toward the poles in an effort to balance the inequalities.

The consequences of these energy transporting processes are the phenomena we call weather.

Daily Path of the Sun

Earth has two principal motions—rotation and revolution.

Rotation – the spinning of Earth about its axis that produces the daily cycle of daylight and darkness.

Revolution—the movement of Earth in orbit around the Sun.

The Earth moves around the Sun at nearly 113,000 kilometers per hours on a slightly elliptical orbit. The Earth’s closest approach to the Sun is called the perihelion and is 91.5 million miles. The point in Earth’s orbit where it is farthest from the Sun is called the aphelion and is 94.5 million miles. Earth’s average distance from the Sun equals 93 million miles.

The most important reasons for variations in the amount of solar energy reaching a particular location are the seasonal changes in the angle at which the Sun’s rays strike the surface and changes in the length of daylight.

Rays Striking Earth at Low Angles Travel Through More Atmosphere and Are Subject to Greater Depletion

Fluctuations in Sun angle and length of daylight during the course of a year occur because Earth’s orientation to the Sun continually changes.

Earth’s axis of rotation is not perpendicular to the plane of its orbit around the Sun. The axis is titled 23½O from perpendicular this is called the inclination of the axis.

The tilt of the axis of rotation is responsible for the seasons. In the Northern Hemisphere the season of summer occurs when the Northern Hemisphere is tilted toward the Sun and it is winter when the Northern Hemisphere is tilted away from the Sun.

Earth-Sun Relationships

There are four important days of the years that help to describe the Earth-Sun relationship:

1) The summer solstice – Earth is in a position where the axis in the Northern Hemisphere is tilted 23 ½ o towards the Sun.

2) The winter solstice – Earth is a position where the axis in the Northern Hemisphere is tilted 23 ½ o away from the Sun.

3) The autumnal and vernal (spring) equinoxes – occur midway between the solstices, and the Earth is positioned in its orbit that the axis is tilted neither toward nor away from the Sun.

Mean Monthly Temperatures for Cities at Different Latitudes

Energy, Heat, and Temperature

The universe is made up of a combination of matter and energy. While matter is to grasp and understand, the concept of energy is abstract and is more difficult to describe and understand.

Energy can be defined as the capacity to do work.

Energy can be classified into two major categories: kinetic energy and potential energy. Kinetic energy is the energy associated with an object by virtue of its motion. Potential energy is the capability to do work.

Temperature is a quantity that describes how warm or clod an object is with respect to some standard measure.

Heat is the transfer of energy into or out of an object because of temperature differences between that object and its surroundings.

Mechanisms of Heat Transfer

It is important to remember that energy in the form of heat can only flow in one direction: from an object at a higher temperature to an object at a lower temperature

Heat can be transferred three ways: by conduction, convection, and radiation. These mechanisms of heat transfer can operate separately or simultaneously. These processes transfer heat between the Sun and Earth and between Earth’s surface, the atmosphere, and outer space.

Conduction – the transfer of heat through electron and molecular collisions from one molecule to another. Heat transfer by conduction will continue until both objects are at the same temperature. Materials that readily transfer energy by conduction are called conductors, while materials that do not readily transfer energy in this manner are called insulators.

Convection - heat transfer by the movement or circulation of a substances. Convection can only occur in fluids—liquids or gases. Heated fluids are less dense than cool fluids and will rise through the cooler fluid, transferring energy to the cooler surroundings, and as a result, become less heated. The now cool fluid is more dense and will begin to sink, and as it sinks becomes reheated and less dense. This cycle of heating, rising, cooling, sinking is called convective circulation. In the atmosphere the hot, rising current of air is a thermal. The term advection describes the horizontal movement of fluid in the convective circulation.

The Electromagnetic Spectrum

The third mechanism of heat transfer is radiation. Unlike conduction and convection, radiation does not require a medium in order to transfer energy. Radiation is the heat transfer mechanism by which solar energy reaches our planet.

The Sun is the ultimate source of energy that drives the weather machine here on Earth. The Sun emits a spectrum of radiant energy that includes visible light and a type of energy that causes the human skin to burn and tan. This array of energy is called radiation or electromagnetic radiation.

Electromagnetic energy waves all travel at the same speed but occur at a range of wavelengths—the distance from the crest of one wave to the crest of the next.

Radiation is often identified by the effect that it produces when it interacts with an object. Visible light is that portion of electromagnetic radiation to which the retina of our eyes are sensitive. Of slightly longer wavelengths than visible light is infrared radiation, which we cannot see but our skin can detect as heat. Electromagnetic radiation slightly shorter wavelengths is ultraviolet radiation. It is this radiation from the Sun that is responsible for human skin burning and tanning.

Intensity and Wavelength of Radiation from the Sun and Earth

Laws of Radiation

1. All objects continually emit radiant energy over a range of wavelengths.

2. Hotter objects radiate more total energy per unit area than do colder objects.

3. Hotter objects radiate more energy in the form of short wavelengths radiation than do cooler objects.

4. Objects that are good absorbers of radiation are also good emitters.

Distribution of Incoming Solar Radiation

When radiation strikes an object, there are usually three different things that can happen at the same time:1. Some of the energy may be absorbed.

When energy is absorbed by an object, the temperature of the object increases. The amount of energy absorbed by an object depends on the intensity of the radiation and the object’s absorptivity. Darker colors have greater absorptivity than lighter colors.

2. Substances that transparent to certain wavelengths or radiation, such as water and air, may transmit energy.Radiation that is transmitted does not increase energy going into the object.

3. Some radiation may “bounce off” the object without being absorbed or transmitted.Reflection and scattering are responsible for redirecting incoming solar radiation.

Reflection is the process whereby light bounces back from an object at the same angle at which it encounters a surface and with the same intensity.

Scattering occurs when a beam of energy is broken into a large number of weaker rays.

The fraction of radiation that is reflected by an object is called its albedo. The albedo for Earth as a whole is 30%. Clouds are largely responsible for most of Earth’s albedo.

Small dust particles and gas molecules in the atmosphere scatter some of the incoming solar radiation in different directions. The result, called diffuse light, accounts for the brightness and even the blue color of the daytime sky.

The Greenhouse Effect

Gases are selective absorbers of energy, meaning that they absorb strongly in some wavelengths, moderately in other, and only slightly in still others.

When a gas molecule absorbs radiation, this energy is transformed into internal molecular motion, detectable as a rise in the temperature of the gas. So, the gases that are the most effective absorbers of radiation play the primary role in heating the atmosphere.

The only significant absorbers of incoming solar radiation are water vapor. Oxygen, and ozone. The absorption of UV energy in the stratosphere accounts for the high temperatures there.

The atmosphere as a whole is a relatively efficient absorber of infrared radiation emitted by Earth. Water vapor and carbon dioxide are the principal absorbing gases, with water vapor absorbing about 60% of the radiation emitted by Earth.. Water vapor accounts for the warm temperatures of the lower troposphere, where it is most highly concentrated.

If Earth had no atmosphere, it would experience an average surface temperature of below freezing. The atmosphere warms the planer and makes Earth livable. The greenhouse effect is the result of the atmosphere absorbing energy and heating Earth’s surface to a comfortable level.

Earth’s Heat Budget

On the whole, the amount of incoming solar radiation is nearly equal to the amount of outgoing radiation from Earth, so the average worldwide temperature remains constant.

The balance of incoming and outgoing radiation is not maintained at each latitude. Averaged over the entire year, a zone around Earth between 38ON latitude and 38OS latitude receives more solar radiation that is lost to space. The opposite is true for higher latitudes, where more heat is lost through radiation emitted by Earth than is received from the Sun.

The tropics are not getting warmer and the polar regions getting colder, however. The atmosphere and the oceans act as giant thermal engines transferring surplus heat from the tropics to the poles.

This energy imbalance and transfer drive the winds and ocean currents and so are responsible for much of the weather the Earth experiences.