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Absorptance
The ratio of radiation absorbed at a surface to the radiation incident on that surface. The absorptance varies with radiation wavelength and angle of incidence.
For an opaque surface: absorptance + reflectance = 1.
For glazing: absorptance + reflectance + transmittance = 1.

Air change rate
See Air changes per hour.

Air changes per hour (ac/h)
Number of times in an hour a volume of air equal to the volume of a room or building is renewed with outside air.

Air leakage
The uncontrolled flow of air through the building envelope.

Airtightness
Characterises the outcome of measures taken to contain air leakage through the building envelope.

Altitude angle
The angular height of a point above the horizon.

Auxiliary heating (or backup or conventional heating)
The heating appliances installed in a building to cover part of the heating load; the useful heat contributed by these appliances (kWh/year or kWh/month).

Azimuth angle
The angular distance between true south and a point on the horizontal plane: negative to the east, positive to the west.