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 dry (low relative humidity (RH)). Also, small hydrometeors have a better chance of evaporating or sublimating as compared to larger hydrometeors. DETECTING VIRGA: 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. FORECASTING VIRGA: 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 evaporative cooling produced from the virga can result in very strong convective wind gusts at the surface. |