tau-value

tau-value

[′tau̇ ‚val·yü]
(meteorology)
The time rate of change of D value at a fixed point defined by the relation τ =(Δt D)/(Δ t), where Δ t is the change in time and Δt 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 4-D charts and constitute the time dimension of these charts.
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