HOW TO DETECT AND FORECAST VIRGA
METEOROLOGIST JEFF HABY
Virga is hydrometeors that evaporate or sublimate before reaching the earth's surface. The effect of
evaporation and sublimation are enhanced when the air the hydrometeors are falling through is initially
relative humidity (RH)). Also, small hydrometeors have a better chance of evaporating or
sublimating as compared to larger hydrometeors.
Virga is more likely when precipitation is falling from a relatively high cloud base and the lower tropospheric
RH ambient environment is fairly dry. Virga appears as streaks of hydrometeors fallings from a cloud base.
With good visibility, the hydrometeor curtain along with the lower edge where evaporation or sublimation
has eliminated the liquid or solid state of the hydrometeors is evident.
VIRGA ON RADAR:
When the lower troposphere is dry, light precipitation echoes on radar (commonly coded as shades of
light green) will not reach the surface. The precipitation has to either be moderate to heavy (commonly
coded as shades of dark green, yellow and red) or occur for several hours in order to reach the surface
because of the dry initial conditions. Virga is common on radar in winter when light snow or rain is
detected aloft but the lower troposphere is initially dry. The leading edge of a rain or snow band
will often be virga on radar.
When the lower tropospheric RH (surface to 800 mb relative humidity) is low (i.e. less than 50% in part of that
layer), light precipitation can be expected to evaporate or sublimate before reaching the ground. It may
take light hydrometeors several hours to saturate the air. The air saturates aloft and saturates
gradually toward the surface. If hydrometeors do not occur for several hours when the lower
tropospheric RH is initially low, it is likely that none of the hydrometeors will
reach the surface.
When the air is warm and dry, the air has a much higher capacity to evaporate liquid water. In a dry and
warm lower troposphere, even moderate or moderately heavy rain falling from high-based clouds may not
reach the surface. This commonly occurs with the initial hydrometeors that fall from high-based
thunderstorms that develop in the interior western U.S. in the summer. The
produced from the virga can result in very
strong convective wind gusts at the surface.