Science behind virga, rain or snow that never reaches the ground

Skies have remained largely dry over the past week across Northwest Missouri and Northeast Kansas, or have they? Meaningful rain has yet to hit the ground in many areas for at least 10 days, yet clouds did produce rainfall in the form of light showers earlier this week. Most of that rain evaporated before ever reaching the surface, a common phenomenon called virga. 

Virga can look deceiving on doppler radar, appearing as light or even moderate precipitation in the form of rain or snow showers. As doppler radar emits microwaves into the atmosphere to detect precipitation, it does so at an upward angle from the radar site. Because virga is present above surface level, it is detected by radar before evaporating on its way down. 

Virga is more pronounced on radar in areas farther away from the radar site. For example, extreme Northwest and North Central Missouri are a fair distance away from the radars in Kansas City, Topeka, Omaha, and Des Moines. This makes virga radar returns more apparent within the region, as the radar beam is emitted higher in the atmosphere.

Viewed from ground level, virga has a somewhat distinct appearance. Described and defined by the National Weather Surface as 'Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.'

Earlier this week, light rain showers fell into a relatively dry air mass over the Mid-Missouri River Valley resulting in virga. Dry air is in better supply going into the fall, winter and early spring season, making virga more common during these times of year. When combined with a sunrise in just the right spot, virga can also add another layer of unique texture and color to a pastel sky.