Atmospheric pressure: The atmospheric air exerts a normal pressure upon all surfaces with which it is in contact and it is known as atmospheric pressure. Atmospheric pressure is also known as barometric pressure. The atmospheric pressure at sea level (above absolute zero) is called standard atmospheric pressure and its value is given as follows:

Standard atmospheric pressure = 101.3 kN/m² or kPa (1 kN/m² = 1 kPa)
= 10.3 m of water
= 760 mm of Hg

The important dimensionless numbers in fluid mechanics are
1. Reynold’s number
2. Froude’s number
3. Weber’s number
4. Euler’s number
5. Mach’s number or Cauchy’s number

Reynold’s number: Reynold’s number is the ratio of inertia force to the viscous force.

Froude’s number: Froude’s number is the ratio of inertia force to the gravity force.

Weber’s number: Weber’s number is the ratio of inertia force to the surface tension force.

Euler’s number: Euler’s number is the ratio of inertia force to the pressure force.

Mach’s number or Cauchy’s number: Mach’s number or Cauchy’s number is the ratio of inertia force to the elastic force.