data manipulation for plot

[kerry.wo@xxxxxxxxx]

Ferret Users,
I have adcp data in spd and dir at one location over water column and
time and I would like to plot a contour dir over time and depth only
when its corresponding spd is greater than a particular value.

How could I manipulate the data for the plot or how could I let those
dir to be a missing value when its corresponding spd is greater than a
value specified?

Kerry