Natural quantities often have both periodic and secular variations.

In fluid dynamics, a flow with periodic variations is known as pulsatile flow.

The periodic variation in the activity of Comet Halley during the 1985/1986 apparition.

UT2 is a smoothed version of UT1, filtering out periodic seasonal variations.

Terraces record natural, periodic variations driven by cycles such as the Milankovitch cycle.

A probe measuring the temperature or velocity at a fixed point within this flow would indicate a periodic variation (cf.

The trend is a decline, probably due to water stress and when this is removed (Figure ) the roughly periodic variation is clear.

Over the short term the instrument detects a roughly periodic variation.

This almost exactly matches up with the periodic variation present in the Mercury-transit data.

In addition there are periodic variations, as Earth moves within the Solar system.