# Froude number

The

**Froude number** is the reciprocal of the square root of the

Richardson number.

It is usually used in the context of the Boussinesq approximation and
is defined as

where is a representative speed,

*g' * the reduced gravity
(see

Boussinesq approximation), and a representative
vertical lengthscale. Strictly, this is known as the densimetric Froude
number.

It is usually used by modellers who wish to nondimensionalize a speed.

For example, the leading edge of a gravity current moves with a front
Froude number of about unity.