June 17 – In A Fog

Today’s Factismal: Fog is not considered to be precipitation by meteorologists.

If you’ve taken a fifth grade science course, then you’ve probably learned about the water cycle (or, if it was in a fancy school district, the hydrologic cycle). In this cycle, water evaporates from ocean, rivers, and lakes, goes high into the air to form clouds, and comes back down as rain and snow. It is a beautiful, simple model. And like most such things, it is too simple and not nearly beautiful enough.

When you ask a meteorologist about the hydrologic cycle, then you’ll get the full, juicy story. Water doesn’t just evaporate from lakes, rivers, and oceans; oh, no! It also comes out of plants that have sucked water up from the ground (sometimes from several hundred feet underground), used it during photosynthesis and then sweat it out as part of their temperature regulation in a process known as transpiration. Over the course of a year, a single large oak tree can “sweat” out enough water to fill two swimming pools! Transpiration from plants and evaporation from the soil itself may account for as much as 67% of all precipitation.

This fog is not precipitation (My camera)

This fog is not precipitation
(My camera)

OK, you say; so the water sources are a bit more varied than we thought. But at least we know what precipitation is. However, this turns out to be another of those places where non-scientists and scientists use terms differently. To a meteorologist, it is only precipitation if the air becomes so saturated in water vapor that the water comes out and condenses around a small particle (that’s the “precipitate” part) and then (here’s the tricky part) falls under gravity. If the water drops are too small to fall, as they are in mists and fogs, then it technically isn’t precipitation even if it is on the ground (e.g., dew). But if it falls and evaporates on the way down, it is precipitation even though it stays in the air; meteorologists call this type of precipitation “virga”.

Virga falling from a cloud in Florida (My camera)

Virga falling from a cloud in Florida
(My camera)

And the hydrologic cycle gets more interesting still once we consider all of the types of precipitation that we can get. There’s virga and rain and hail and snow and sleet and graupel and drizzle, to name but the seven best known. And here’s the truly interesting part: meteorologists still have to rely on people on the ground to help them discover what kind of precipitation is falling where. Though some progress has been made in using radar to discriminate between the various types of precipitation, radars don’t see very well near the ground (all those pesky buildings get int he way). So they need observers to tell them what is falling where, be it thundersnow or nonaqueous rain.

The drizzle on Uluru is a form of precipitation (My camera)

The drizzle on Uluru is a form of precipitation
(My camera)

If you’d like to help, then why not download the National Severe Storms Laboratory’s free mPING (Meteorological Phenomena Identification Near the Ground) app? It is available on both Android and Apple devices. All you have to do is use the app to send a report whenever you see precipitation; the app will even help you decide what type of precipitation it is. To find out more, go to the National Severe Storms Laboratory mPING webpage:

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