Let A be a fixed point in space, at time t_{1}. P_{1}, P_{2} and P_{3} (all passing through A) are the three path lines traversed by three particles in an unsteady fluid flow. Let B, C and Dare the end points at time t_{2} of P_{1}, P_{2}, P_{3} respectively. If we join ABC D, we will get a line which is called the streak line. Thus a streak line at any instant of time is the locus of the temporary locations of all particles that have passed through a fixed point in the flow field.

In Lagrangian method, if a fluid particle (S) passes through a fixed point (S) in time t

The equation of streak line at a time t is given by

Equation (4.10) is the equation for a streak line referrent to a fixed point (5;).