1. What is Energy?
Energy is the property of a system that allows it to do work. Work is defined as a force acting on matter, causing motion. Energy comes in many forms, such as kinetic (motion), radiant (such as light), potential (such as the energy in a rock at the top of a hill), chemical (such as unburned gasoline), electric (the transfer of electrons), and magnetic (for example iron ions or compounds).
The source of energy, which powers the Earth's weather, is the sun. The sun is a million times the size of Earth (the radius of the sun is 696,000 km, the radius of the earth is 12,756 km) and that provides more energy to the surface of our planet than any other source. Although the sun is very large, it has a relatively low overall density of 1.41 g/cm3. Pure water, for reference, has a density of 1g/cm3. Most of the sun's mass (94%) is located in the inner half of the sun's radius.
At one time, it was thought that the sun was releasing energy because it was behaving like a gas under pressure, and was heating up. It was also thought that the sun was burning, as in a chemical reaction9.
The source of this energy is a nuclear reaction. Unlike a nuclear power plant or nuclear weapon that derive energy from fission (or splitting of large atoms) a fusion reaction occurs in the sun between hydrogen nuclei, resulting in a helium atom.
In the middle 25% to 30% of the radius of the sun, the core, 600 million metric tons of hydrogen nuclei are fused each second to form helium. Each time this occurs, a large amount of energy is released as a portion of the mass of the four original hydrogen nuclei (0.7%) does not end up in the helium nucleus. This transformation of mass to energy was originally described by Einstein in his famous E=mc2 formula. This small amount of lost matter releases an enormous amount of energy. For each fusion reaction (the term which describes the uniting of atomic nuclei) there are 4.3 x 10-12 joules of energy released10.
Gamma ray photons are released in the core as a result of these thermonuclear reactions. The temperature in this region of the sun is 1.55 x 107Kelvin. (10 million Kelvin). The Kelvin temperature scale has the same incremental value as the Celsius scale, but is 273.15 degrees higher, as Kelvin starts at "absolute zero". Absolute zero is a value equal to -273.15 Celsius. The sun is 5,800 Kelvin at the surface. The temperature at the core is 10 million Kelvin, due to the tremendous pressure11.
The sun releases 3.9 x 1026 joules of energy per second, and will continue doing so for another six billion years.
Conduction, convection, and radiative diffusion are three ways energy is transferred from the core of the sun to the surface, where it leaves as radiant energy. Conduction is not an effective way to transfer energy in materials with low densities.
When the photons leave the core, they move via radiative diffusion from the 10 million Kelvin core towards the 5,800 Kelvin surface. When the photons are over 2/3 of the way to the surface, convection takes over, and the circulating band of solar fluid transfers this energy to the surface. This process takes an astonishing 170,000 years to complete! This is a rate 20 times slower than a snail travels! Once at the surface, the photons travel easily through space at the speed of light, and some of this energy will strike Earth eight minutes later12.
The center of the sun is under extreme pressure - so much that the hydrogen and helium mixture (the two lightest elements) are 14 times the density of lead! The density of this region is 155 g/cm3, and under 3.4 x 1011 atmospheres of pressure!13 At sea level, there is one atmosphere of pressure, which is approximately 14.7 pounds per square inch (psi).
Temperature is the relative measure of heat energy and a measure of the average speed with which particles in a material move around. This is important as it determines energy transfer from one region to another. There are vast temperature differences among the materials that make up our planet, as well as across regions of Earth. This is why being able to quantify these molecular motions is so useful to meteorologists.
Students usually need refreshing on the units of Fahrenheit, Celsius, and Kelvin. Some web sites offer online conversions14 and these are useful for students to use to check their work once they have mastered using the following formulas.
To convert Celsius to Fahrenheit: C x 9/5 +32
To convert Fahrenheit to Celsius: (F-32) x 5/9
To convert Celsius to Kelvin add 273.15 to the Celsius value
3. The uneven heating of the Earth's surface
Radiation is the mechanism by which energy is transferred from the sun to the Earth. Photons are the smallest unit of electromagnetic energy (light is an example of visible electromagnetic radiation), and do not require a medium to travel through. They travel through space. When these photons strike the surface of the Earth, they transfer their energy to the ground. In turn, the ground warms up the air. This is done through conduction over a very narrow layer (about 1 mm) above the surface.
Conduction is the direct transfer of energy without movement of the medium itself (in this case, air). This only happens across a very small distance because air is a very poor conductor.
Once the air in this gap has been heated, further transport of the heated air is done by convection. Convection in the atmosphere occurs when currents of air transfer heat in a flowing manner from one area to another.
The surface of Earth is composed of different materials (which do not heat evenly) and is a major driving force of weather.
The equatorial latitudes receive much more solar radiation during the entire year as this part of the Earth more directly faces the sun in our annual revolution. The Northern and Southern Hemispheres experience seasons as a result of the tilt of the Earth's axis. This differential in energy distribution sets up the subsequent flow of energy via convection of air and water.
Water has a very high specific heat (it takes more energy to raise the temperature of water than most other substances), it therefore has a high heat capacity and serves as a reservoir of heat. This makes water very important in the transfer of thermal energy from warmer to cooler regions.
The Gulf Stream is an example of one such current on the East Coast of the United States. It is one of the strongest ocean currents, and ferries energy from the tropics along the Eastern Seaboard, and across the Atlantic to Northern Europe!
In contrast, on the West Coast of the United States cold water is drawn south from Alaska to Mexico by the California Current. Kuroshio, or the Japan Current, carries warm water along the Eastern coast of Asia, and up to the north Pacific15.
4. Air Pressure
Air pressure results from the motion and number of air molecules over a geographic location.
As air molecules move about at an average of 1,090 mph, they push against what they strike. While individually this results in a small amount of pressure, collectively (a one inch square box holds 4 x 1023 air molecules) this pressure has dramatic effects16.
The lower temperatures that are associated with higher elevations are a result of fewer collisions of air molecules. With less contact (among fewer molecules) the temperature is lower16.
The significance of this in our study of weather is that differences in air pressure cause wind, as air will move from areas of high toward low pressure.
Air pressure is measured using three different scales: millibar, kilopascal (which is 1/10 a millibar (thus millibar=hectopascal), and inches of mercury (Hg). This is the conversion among these units:
1013.25 mb = 1013.25 hPa = 29.92 in. Hg18
5. Atmospheric water vapor
Water vapor in the atmosphere runs the water (or hydrologic) cycle. The hydrologic cycle is the process by which water changes state among liquid, solid, and gas. In order for this to occur, energy is lost or gained (please see "latent heat" in glossary). This is the primary means by which solar energy is transferred around the globe. 90% of the water in the atmosphere evaporated from surface water (oceans, rivers, lakes, and other bodies of water).
Plants also play a role in the transportation of water into the atmosphere. Plants draw water from the ground (via their roots) through their vasculature by selectively allowing water to evaporate off their leaves. This process is called transpiration, and is responsible for moving the remaining 10% of the water into the atmosphere19.
Sinking air does not produce clouds20. Clouds form when water vapor is cooled, and condenses around small particles of dust or smoke in the upper atmosphere. So you know the air mass is moving up when you see clouds forming!
Water vapor plays a very important role in transferring energy around the globe. As we have discovered, there is an uneven distribution of thermal energy, and water vapor in the atmosphere, along with oceanic water currents, play an important role in distributing this energy.
6. Geographic location
Oceanic and atmospheric currents serve as distribution networks for energy around our planet. The geographic location therefore will play a very important role in the kind of weather experienced. Locations which are near large bodies of water will experience cooling or warming, depending on the relative temperature of the water to the land. If the location is served by a cold current (such as the California) the land near the sea will be cooler. The inland locations can be much warmer, as they receive less influence from the large thermal mass. Conversely, in times when the inland areas are very cold, the coastal areas can have their temperature moderated by the presence of a large body of water. This can be an ocean, or large lake, such as the Great Lakes21.
Our location at the coast of Long Island Sound makes teaching this portion of the standard fun! In coastal Connecticut we regularly have different weather from those who live further inland. Crops can be planted a full two weeks earlier closer to the coast due to the high heat capacity of the water in Long Island Sound.