# Tau-value

From AMS Glossary

## tau-value

The time rate of changes of D-values at a fixed point defined by relation, where Δ

*t*is the change in time and Δ_{t}*D*is the change in D-value during this time interval.Tau-values are expressed in terms of feet per hour. Tau-value lines are drawn on 4D charts and compose the time dimension of these charts.