Precipitation by Sander Schimmelpenninck
sander@idirect.com

In meteorology, precipitation (precip for short) means the falling of liquid or frozen water that results when air becomes so moist and/or cold that it can no longer contain water in its vapour state. Meteorologists report two aspects of precipitation: type and accumulated quantity over a stated period.

The major breakdown of precip types is liquid, freezing, and frozen. Liquid, of course, is either rain or drizzle. The difference is size: raindrops are 0.5 mm or more across. Measuring drops in flight would be a daunting task, best left to mad scientists. For this purpose I keep a small mirror in my Stevenson screen. When in doubt, I briefly expose it shiny side up and estimate the drop size realizing that the drops spread out after hitting the glass. You'll also find that precip can be a mixture of rain and drizzle.

Rain is further subdivided in three ways: character, intensity, and state after falling. Character is either constant or showers. If you think that is clear-cut, you're wrong. Constant rain is most often associated with stratified, dreary clouds called nimbostratus that usually accompany a warm front. Showers typically fall from vertically-developed, billowing clouds called cumulus. Note my weasel words: `most often', `usually', and `typically'. What I gave you are textbook cases. Real life often fails to behave as advertised. For this an instructor in my Dutch Army Reserve-officer School had a reassuring solution: Schimmelpenninck, when the terrain and your topographical map don't match, the terrain is wrong. Don't lie awake nights over this distinction. When rain or snow vary noticeably in intensity over, say, 10 minutes, call it showers. Oh, I should add that drizzle is always considered constant. Say drizzle shower, and any meteorologist will start avoiding your company.

Rain intensity comes in three flavours. Light is 2.5 mm or less per hour, moderate 2.6-7.5, and heavy 7.6 or more. Did you notice this holdover from the inch, which equals 25.4 mm? If so, you get an A and you can skip the rest of this class.

Measuring rain calls for some of the oldest, simplest technology known to meteorologists, The basic instrument is the rain gauge: a device consisting of a bottom shell, a funnel, and a graduated measuring glass whose diameter is much smaller than that of the funnel mouth. That system does two things. First, the combination of large and small diameters magnifies rainfall. That makes it easy to read results to 0.2 mm which is the standard precision. Second, the small diameter of the funnel spout minimizes the evaporation of the rain you have caught.

Rainfall rates and changes therein greatly interest meteorologists. For this reason some stations have tipping-bucket rain gauges (TBRGs). As the name implies, rain exiting the intake funnel fills one of two buckets on a seesaw device. When the first bucket fills, the seesaw tilts, dumps the rain caught, and presents the other one. An electrical device times each tipping and thus shows how much fell when.

Placement is critical, as with other weather instruments. Put your gauge as far away s possible from obstructions like buildings and trees at least as distant as their height. We have all heard of rain  falling horizontally,  in hurricanes, but such violent winds seldom occur in Canada. Also, install your gauge such that the intake is 40 cm above ground, to meet the standard of Canada's Atmospheric Environment Service (AES, our sister of the US National Weather Service).

Remember another basic type: freezing precip. Linguists will notice the present participle: freezing. This phenomenon refers to rain, drizzle, and even fog that are liquid when airborne but freeze on contact with objects or the ground. I'll write about the upper atmosphere later in this series. For now, just understand that rain can fall from relatively warm air aloft while it=s freezing at or near the surface. When that happens, the water drops freeze on contact with pavements, buildings, tree branches and aircraft wings and control surfaces. In Canada, freezing precip is most common down East, where the confluence of the warm Gulfstream and the cold Labrador often makes a mess of things. In the Toronto area freezing precip normally occurs only a few times a year.

You can measure freezing rain by bringing your rain gauge inside and letting it thaw out. But also estimate the thickness of the glaze ice on objects like branches and metal fences or balustrades. That applies especially to those of you who participate in AES's Severe-weather Watcher program, a volunteer group.

Now for frozen precip, i.e. water that was already frozen before it reached the ground. Here things get trickier, because we're talking about a relatively large family.

The basic divide is structure: crystals or not crystals. (This recalls a Monty Python show about a race of two cars, a brown one and a not brown one.) When crystals are hexagonal, you have snow. When they're needles, columns, or plates and they remain suspended, you have ice crystals. They occur only in very cold weather and I mean by Canadian standards.

Non-crystalline frozen precip breaks down as follows. Snow pellets are spherical or conical, 2-5 mm across, they bounce, and they break up. Snow grains are flat and long, smaller than 1 mm, and they don't bounce. Ice pellets are spherical or irregular and 5mm or less across; they bounce. Think of them as small hail. Hail is more than 5 mm across, and bounces like crazy. Weather lore abounds with hail stories. However, a learned article I read said no qualified observer has reported anything bigger than orange-sized hail. If you ever see very large hail, venture out when it's safe and lay a ruler on the ground to estimate the size before the stuff melts. By `when it's safe, I refer to the conditions typically associated with large hail: severe thunderstorms and tornados.

That leaves us with snow. Surprisingly, AES's Manual of Surface Weather Observations (MANOBS) describes its rates if fall only in terms of visibility, which is also influenced by the fog that often accompanies snow. I prefer to judge rate of fall by the size and density of the snowflakes I see. If 3 cm or more falls in an hour, I call it heavy snow.

Meteorologists record both how much snow falls in a given period and how much has accumulated. For recent falls I recommend a snowboard or snow table, white to minimize heating by sunlight, wiped clean after each observation with a yardstick. (I'm metric, but I love English to much to write metrestick.) Take the average of three probes. Place the board in a typical snowfall area of your observation green. AES is testing a new design: a 40-cm square board with a 1-cm lip and a 40-cm dowel sticking up from the middle, so you can find the board after it's buried in snow. This prototype comes from my AES friend George, whose job it is to keep professional and volunteer observers on the ball. He recruited me in 1990, when I asked AES for a cast-off Stevenson screen.

Measuring fresh snowfall is easy when no rain has fallen. Otherwise, it's just a guess, based on what you saw out the window that day.

To determine the accumulated snow on the ground, the snow blanket, determine a typical `snow course': a range 10 metres long or so, in an area that usually attracts a representative buildup: no ridges or valleys. Determine the average of at least six probes with a snow ruler. Make sure it touches the ground. If a layer of ice has formed, get a disused walking-stick or a 1-metre long dowel with a tapered but not pointed end, and mark it at 10-cm intervals with a soldering iron.

Finally, understand the difference between snow, blowing snow, and drifting snow. The first kind falls from the sky. However, the wind can cause the other two. Blowing snow is picked up from the surface and carried above eye level. Drifting snow is the same recycled stuff at or below eye level. You cannot have blowing and drifting snow at the same time, because blowing snow always contains the drifting kind.

This article is Copyrighted 1999: Sander Schimmelpenninck and used with his expressed written permission.  No unauthorized use allowed without prior written consent by him.