Active Solar System: A system that uses mechanical devices and an external energy source in addition to solar energy, to collect, store, and distribute thermal (heat) energy.

Advection: The horizontal transport of air along the earth's surface felt as the wind. Also serves to transfer temperature and humidity characteristics from one region to another (e.g., the sea breeze is one form of advection).

ASHRAE: American Society of Heating, Refrigeration and Air Conditioning Engineers

Angle of Incidence: The angle created by incoming radiation and a line perpendicular to an intercepting surface.

Bead Wall: A form of movable insulation using pea-sized balls of polystyrene alternately blown into and vacuumed out between a double-glazed panel during the diurnal cycle.

BTU (British Thermal Unit): A measurement of energy representing the amount of heat needed to raise 1 pound of water by 1 degree Fahrenheit. About the same amount of energy released by a single lit match.

Clerestorey: A vertical roof window placed that is used for light, ventilation, and to collect heat.

Conduction: The distribution of heat or coolness through a material. The rate of conduction is dependent on the ability of molecules to receive and pass on energy.

Convection, Natural: Heat transfer through a medium, such as air or water, by currents that result from the rising of lighter, warmed medium and the sinking of heavier, cool medium.

Convection, Forced: Heat transfer through a medium, such as air or water, by current, caused by a device powered by an external energy source.

Cooling Degree-Day: Refer to "degree-day."

Degree-Day: Term referring to the number of units (degrees) that the average outdoor temperature falls below or exceeds a base value (usually 65° F) in a given period of time. Each degree that the mean daily temperature is above the base value is called a cooling degree day (CDD) unit. Each degree that the mean daily temperature is below the base value is called a heating degree-day (HDD) unit. The monthly CDD & HDD values are simply the sum of the individual CDD and HDD units for the days in the month. Degree-days are reasonably good indices of the heating and cooling requirements of buildings.

Diurnal: Daily

Glazing: A glass, plastic or transparent/translucent covering.

Greenhouse Effect: The build-up of heat in an interior space caused by energy input through a transparent membrane such as glass.

Heat Capacity: A measurement of the specific heat of a material multiplied by its density. See "specific heat."

Heating Degree-Day: Refer to "degree-day"

Heat Sink: A body which is capable of accepting and storing heat or cool and, therefore, may also act as a delayed heat source.

Hybrid Solar System: A system that combines passive solar collection with active elements for transport of heat to an isolated storage system.

Infiltration: The uncontrolled flow of air into, and out of, a building through cracks, openings, doors, or other areas which allow air to move.

Insolation (Incident Solar Radiation): A term denoting the amount of solar radiation (all wavelengths) which strikes the earth. The basic unit of measurement is the Langley, a unit of heat energy equivalent to one calorie falling on one square centimeter of surface. One BTU per square foot is the equivalent of 0.27125 Langley.

Inversion: A reversal of the normal atmospheric situation where the temperature decreases with height. Under inversion conditions, the atmosphere actually gets warmer with height. Atmospheric conditions associated with inversion conditions are the absence of wind and hazy, smoggy views associated with trapped air pollutants.

Microclimate: The climate of a defined local area, such as a house or building site, formed by a unique combination of wind, topography, solar exposure, soil and vegetation of the site.

Precipitation: Water, in either liquid or solid form, which falls from clouds. Includes rain, hail, sleet, snow, etc.

Radiant Energy: Low temperature infrared energy emissions from the sun.

Radiation: Along with convection and advection, one of the three major processes in transferring heat energy from place to place. In the radiation process, heat is transferred in the form of waves of energy. The sun transfers its heat and light energy to the earth by this process. The earth loses heat to space by this process.

Radiation Fog: A low-lying fog formed when the ground loses heat to space by radiation to the point where it becomes cooler than the air, which lies above. The overlying air is then cooled to the dew point by contact with the cold surface causing a dense fog to form. Tule fogs in the Central Valley are formed by this process.

Relative Humidity: A measure of the ability of air to hold gaseous water vapor. The warmer the air, the more water it can hold. Relative humidity is usually expressed as a percentage of saturation: 0% = totally dry air, and 100% = fully saturated air. Relative humidity becomes important for human comfort as at high values perspiration is not readily evaporated and thus discomfort is felt. Relative humidity is also important from the standpoint of the atmosphere's ability to lose heat to space at night (water vapor absorbs and retains heat exceedingly well). Drier climates thus have a greater potential for night ventilation cooling than do more humid ones.

Roof Ponds: Passive heating/cooling system. Enclosed water above and in immediate contact with ceiling elements, with movable insulation, which expose ponds to the winter sun for heating, and to the night sky for summer cooling.

Solar Constant: The amount of heat energy delivered by solar radiation to a square foot of material set perpendicular to the sun's rays for one hour at the outer edge of the earth's atmosphere.

Solar Radiation: See "insolation."

Specific Heat: The amount of heat, measured in BTU's for a given mass. a material can hold when its temperature is raised one degree Fahrenheit.

Thermal Chimney: An air heating device that provides a constant vacuum which may be used to vent the house, bring warm air from collectors, or pull air from cool towers, earth tubes or rock bed coolers. This creates what is called a "stack effect."

Thermal Energy: Heat energy

Thermal Storage Mass: A thermal element that has heat or cooling storage capacity in a given assembly or system. Water storage tanks. concrete floors, rocks, and masonry are examples of thermal mass.

Trombe Wall: A concrete, stone or masonry wall that has vents at regular intervals both along the floor and just below the ceiling. The exterior. south-facing side is dark in color and fronted with glass. Air is warmed between the glass and wall and circulates by convection through the vents. It is named after Dr. Felix Trombe, one of it, developers.

Vestibule: An enclosed, double entryway into a structure.

Water Walls: A passive heating and cooling technique for collecting and controlling solar energy, usually fronted by glass. Water walls are usually water-filled containers exposed to or protected from the sun depending on mode (heating or cooling). Wintertime use is to collect and store heat, which is used to warm a living space. Summer use is to absorb unwanted internal heat, thereby cooling the interior, and dissipating the heat at a later time.