where is the velocity vector. This equation implies that is normal to the equipotential lines and is directed from high to low potential.
a If there is also liquid water in the air this is modified as
where is the liquid water mixing ratio (in grams of liquid water per gram of dry air). The virtual temperature is thus defined because it allows the ideal gas law to be used for situations in which the air is not dry, i.e. moist air of temperature T behaves identically to dry air of temperature .