## Solar Radiation and the Earth's Energy Balance.

### What Is Energy?

Energy is a latent property of matter describing its ability to perform work. When energy is released it is converted to either work or to another form of energy. Energy is always conserved. The only way to form energy is to convert it from one form to another or to do work (which also requires energy).

#### 1. Energy is measured in units of work:

The MKS unit for energy is the Joule (J): 1 Joule is equal to the work spent by moving a body with a force of 1 Newton over a distance of 1 meter, or the work that could be gotten from bringing to halt a mass of 1 kg moving at a speed of 1 m/sec, or the work that has to be invested to increase the elevation of a body weighing 1 kg by 10.2 cm.

Other units of energy are the calorie and the Btu (British thermal unit). The calorie is defined as the energy spent raising the temperature of 1 gr of pure water by 1 degree Centigrade.

1 calorie = 4.185 Joules

The Btu is defined as the energy required to raise the temperature of 1 lb of pure water by 1 degree Fahrenheit.

1 Btu = 251.996 calories = 1054.6 joules

#### 2. Forms of energy relevant to climate:

• Thermal energy (or heat): the ability to do work through the release of heat. In climate science it is separated into two components: internal energy - the energy associated with molecular motion (or temperature) and latent energy - the energy associated with the evaporation/condensation of water vapor.
• Kinetic energy: the ability to do work due to motion.
• Gravitational (or potential) energy: the ability to do work through changes in a body's location within a gravitational field.
• Electromagnetic wave energy: energy embedded in electromagnetic wave motion (such as light, microwaves, or x-rays). When this kind of energy impinges on matter part or all of is converted to heat.
• Chemical energy: the ability to do work via a chemical interaction (i.e., change in molecular structure)

#### 3. Energy transformations in the climate system:

Energy is conserved but can be transformed from one form to another. All the forms of energy listed above exist within the climate system. The transformation of one form of energy to another is one of the main processes of climate. Generation of thermal energy is responsible for the change in temperature of matter. Kinetic energy is embedded in the motion of atmosphere (winds) and ocean (currents). Potential energy is the energy gained or released by changing the elevation of air or water parcels. Electromagnetic energy is the energy embedded in the rays of light emitted from the Sun. Chemical energy is the energy absorbed or released by chemical interactions within the climate system, such as when the gas Ozone is formed and destroyed in the atmosphere.

Thermal energy can be transformed into kinetic energy when heat is converted into motion. This kinetic energy can become potential energy when the motion results in the upward movement of air, say by encountering a topographic obstacle. Kinetic energy can also be converted back to heat when the motion of air or water encounters frictional resistance. Heat is also generated from electromagnetic energy when the latter is absorbed in matter. Heat is can in turn be converted into electromagnetic energy as is the case of a fire that emits light and infra-red radiation.

Text by Yochanan Kushnir, 2000. Revised 2004.