For cables in tunnels, this is the velocity of the air inside the tunnel.

For cables in troughs, this is the velocity of the air above ground (optional parameter).

Symbol
$V_{\mathrm{air}}$
Unit
m/s
Used in
$C_{\mathrm{av}}$
$H_{\mathrm{ts}}$
$\mathrm{Re}_{\mathrm{air}}$