The vertical speeed indicator senses the difference between the instataneous static pressure and the static pressure few seconds ago.
The vertical speed indicator (VSI), called also variometer or a vertical velocity indicator, indicates whether the airplane is climbing, descending, or in level flight. The rate of climb or descent is indicated in feet per minute. If properly calibrated, the VSI indicates zero in level flight.
The VSI does not display immediately an accurate indication of a climbing or descending rate. There exists a lag or a time between the initial changes in the rate of climb/descent and the display of this new rate.
Note that turbulence can extend the lag period and cause erratic and unstable rate indications. In some aircraft a vertical speed indicator (IVSI) exists, which incorporates accelerometers to compensate for the lag in the typical VSI.