Backwater
A rise of water level in a stream caused by a natural or artificial obstruction.

Background levels (of risk)
In risk analysis, inherent natural or normal levels of risk in addition to risk from any specific factor.

Barometer
Instrument for measuring atmospheric pressure. (H)

Barometric pressure
The pressure exerted by the atmosphere as a consequence of the force of gravity.

Barrage
Barrier across a stream provided with a series of gates or other control mechanisms to control the water-surface level upstream, to regulate the flow or to divert water supplies into a canal. (OFDA)

Bayes theorem
A theorem that provides the logical basis for updating a probability on the basis of new information.

Beaufort scale
Scale of wind force, measured from O (calm) to 12 (hurricane force wind).

Biological disaster
Disaster caused by the exposure of living organisms to germs and toxic substances.

Bomb
See “ejecta”.

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