Incoming Solar Radiation

The sun is the driving force of our weather and climate. Its diameter is about 865,000 miles or 109 Earths lined up side-by-side and is so large that it makes up 99.99 percent of the mass of our Solar System. According to astronomers, our sun is the most common type of star in the known universe and has existed for nearly 4.6 billion years.

The sun’s composition is about 92 percent hydrogen (the lightest element in the universe) and almost 8 percent helium (the second lightest element). Using these elements, the sun generates energy through a process called thermonuclear reaction. This energy is created when the sun fuses hydrogen atoms to make a heavier element called helium. This process not only creates new elements but also releases an enormous amount of energy.

The energy emitted by the sun is called electromagnetic energy, a range of wavelengths from the shortwave to the longwave spectrum of light, which travels at the speed of light. Recall from the module on tsunamis that a wavelength is the distance between two wave crests, and frequency is the number of wavelengths that pass a given point in a given amount of time.

The sun’s life expectancy is already half over. In another 4 billion years, it will consume all the hydrogen within it, leaving behind helium and a few other newly created elements. When the hydrogen runs out, in about 5 billion years, the sun will grow into a red giant, expanding its diameter well past Mercury and possibly Venus and Earth. The sun will continue to fuse helium into carbon until the star begins to release its outer layers to form a planetary nebula. Left behind will be the white-hot core of our star, called a white dwarf that will slowly fade out over billions of years.

Why would you want to wear a black shirt in the winter and a white shirt in the summer? Would you rather walk barefoot on black pavement or grass? The answers to these questions relate to the idea that different objects absorb and reflect energy differently on the planet. Darker objects tend to absorb more energy, while lighter objects reflect less energy. The percent of energy an object reflects is an object’s albedo. Because the planet has different albedos from cities, forests, deserts, oceans, rocks, glaciers, and more, the planet has uneven heating. The result is that of all the energy the Earth receives from the sun, only 51 percent is absorbed by the planet, while 49 percent is reflected into space.

Uneven heating of the planet also occurs because of the curvature of the Earth. The planet receives most of its energy near the equator, where the sun’s rays are directly hitting the planet. Towards the poles, the sun’s energy becomes more diffused and spread out over a greater area. The equator receives more energy from the sun than it can radiate back into space, producing a surplus of energy at 0 degrees latitude. At the poles, the planet radiates more energy out into space than it receives from the sun. There is a deficit of energy in these regions. The ultimate purpose of weather is to transfer the surplus of energy and heat from the equator to the poles, bring colder air toward the equator and find equilibrium.

Global Transfer of Heat

The source of energy for the weather on Earth is the sun. The earth-atmosphere energy balance is the balance of energy the earth absorbs, and the energy reflected into space. Roughly 51 percent of the energy the earth receives from the sun is absorbed, while approximately 49 percent of the sun’s energy is reflected into space. To balance the energy deficit at the earth’s poles and the energy surplus at the equator, the planet will transfer energy by conduction, convection, latent heat, and radiation.

Conduction

Conduction is the transfer of heat from a warmer object to a colder object through molecule interaction. As the sun heats the ground, energy is transferred to the atmosphere by conduction. However, the atmosphere is an inferior conductor of heat. In calm weather, the heated ground only warms the first few centimeters of air. The atmosphere’s temperature can be up to 50 percent colder five feet above the ground than at the surface. Since the atmosphere is such a poor conductor of heat, there must be other ways to transfer the energy.

Convection

Convection is the transfer of heat by the mass movement of a fluid (such as water and air). It occurs mostly in liquids and gases because they are free to move around. Heat is transferred upward and outward away from its heat source, and cooler air is brought in to replace the rising air. On a local scale, convection in the summer can produce afternoon thunderstorms. On a global scale, convection transfers energy between the equator and poles.

Radiation

The final process of transferring heat around the planet is called radiation. The energy coming from the sun comes in various wavelengths. A wavelength is the distance measured along a wave of energy from one crest to another. The shorter the wavelength, the higher the energy. The energy received from the sun passes through the atmosphere without warming it. All objects that absorb radiation from the sun radiate some of that energy back into space, but in a weaker form of energy. This weaker energy becomes longwave radiation and is typically observed as heat. All living objects (including humans) radiate longwave radiation. The Earth itself radiates the energy absorbed from the sun in the form of longwave radiation, which is sometimes called Earthlight.

Latent Heat

Latent heat is a powerful force in the weather. When water transforms from gas to liquid or solid, or vice versa, it is called a phase change. The heat required to change phases is called latent heat. For water to change from a liquid to a gas, energy/heat must be taken from external sources such as the surrounding atmosphere. Therefore, evaporation is a cooling process because the water is taking heat from the surrounding air to evaporate.

The process of evaporation and condensation transfers substantial amounts of heat around the planet. The heat released in a typical thunderstorm is equal to one of the original atomic bombs. The heat and energy within a hurricane are equal to thousands of nuclear weapons or could power the United States for over a year.