Determine the mixing ratio (w, in g/kg), which is the value of the saturation mixing ratio line passing through the dewpoint (Td), at a given pressure.
The virtual temperature is then computed as follows:
Tv ~ T + w/6
In this example:
T = 5°C and Td = -6°C.
Thus, w = 3.0 g/kg.
Tv = T + w/6
= 5 + 3/6
= 5.5°C