It's Raining — What does that mean? Part 1

There is an easy answer, and I think everyone would get it right!

But without more information, we don't really know much about this rain or what it might imply. For beneficial marine weather work, or even for planning picnics, we need to know more about how rain is defined. In fact, the first thing we learn is that the water falling from the sky is not always "rain."  

This is Part 1 of 3 parts.

Part 1 Covers definitions and background 

Part 2 Gives an example of rain at the border between Light Rain and Moderate Rain

Part 3 Covers global rain, rain gauges, and violent rain.

Part 1. Terminology

Checking with the National Weather Service we find there are effectively three categories of liquid precipitation: rain, drizzle, and showers, and each of these is further categorized by duration and intensity. The definitions have evolved because they are crucial to the understanding and application of this precipitation to weather analysis—or to picnic planning or to agriculture, or perhaps even to the existence of life on this planet. In short, it can be important.

Some sources argue there are only two categories, drizzle and rain, but our exposure to the science is mostly from forecasts and these refer to either rain or showers stressing or implying an important distinction between the two. I don't recall seeing any forecasts for drizzle, but we do get forecasts for thunderstorms, which bring “heavy rain.” Periodically we might see drizzle referred to as “mist” when it reduces the visibility to below

The definitions

Rain. Precipitation in the form of liquid water droplets greater than 0.5 mm (0.02 inches) in diameter. If the drops are widely scattered, the drop size may be smaller.

Light rain. Rate of fall greater than a trace and up to 0.10 inch an hour, but not more than 0.01 inches in 6 minutes.

The Observer's Handbook adds: Scattered drops that do not completely wet an exposed surface, regardless of duration, [up to] to a condition where individual drops are easily seen; slight spray is observed over the decks; puddles form slowly; sound on roofs ranges from slow pattering to gentle swishing; steady small streams may flow in scuppers and deck drains.
Visibility 1 km (0.5 nmi) or more.

Moderate rain. Rate of fall is between 0.11 to 0.30 inch per hour, but not more tan 0.03 in 6 minutes.

The Observer's Handbook adds: Individual drops are not clearly identifiable; spray is observable just above deck and other hard surfaces; puddles form rapidly; sound on roofs ranges from swishing to gentle roar.
Visibility less than 1 km (0.5 nmi) but not less than 0.5 km (0.25 nmi, 550 yds).

Heavy rain. Rate of fall greater than 0.30 inches per hour.

The Observer's Handbook adds: Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of several inches is observed over hard surfaces; visibility is greatly reduced; sound on roofs resembles roll of drums or distant roar.
Visibility less than 0.5 km (0.25 nmi, 550 yds).

Sometimes rain is further characterized by its duration.

Continuous rain. Intensity changes gradually, if at all.

Intermittent rain. Intensity changes gradually, if at all, but precipitation stops and starts at least once within the hour preceding the observation.

Drizzle. Uniform precipitation composed exclusively of fine drops (diameter less than 0.5 mm or 0.02 inch) very close together. Drizzle appears to float while following air currents, although unlike fog droplets, drizzle falls to the ground. Drizzle drops are too small to appreciably disturb still water puddles.

Showers. Precipitation from a convective cloud (cumuliform) that is characterized by its sudden beginning and ending, changes in intensity, and rapid changes in the appearance of the sky. The implication here is that “rain” in contrast comes from stratiform clouds (nimbostratus).

Thunderstorms. Though thunderstorms are often foretasted without reference to rain, it is implied that they will most likely bring heavy rain. Thunderstorms are a result of cumulonimbus clouds, which by their vary name means rain. There are of course smaller squalls with only moderate rain, or you may meet a squall stage with just light rain left over, and there is a category of squalls, low precipitation (LP) supercells, that do not have much liquid rain at all, but generally we can safely assume that a forecast with thunderstorms means that somewhere in the region it will have heavy rain, but it will be just underneath these cumulus clouds.

The affected area of convective weather (showers and thunderstorms) is given as:

Isolated. 10 to 20% of the forecast zone. Same as “few.”

Scattered. 30 to 50% of the forecast zone.

Numerous. 60 to 70% of the forecast zone.

Likely. Greater than 80% of the forecast zone.

Measurable precipitation means a total of at least 0.01 inch (ie the first tip of a tipping bucket rain gauge, which are calibrated in 0.01 inches per tip)

Probability of precipitation (PoP). See PoP Explained. To actually see these numbers forecasted you need to view the hourly weather graph on a standard NWS forecast page. See Work Horses and Secret Sources.

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NWS rain symbols. I have added the red to emphasize the intensities of the rain, which follow the definitions above. We will follow up with more discussion of these later. Note they use the word "slight" for "light." It means the same, and everywhere else they use "light." A simple confusion that is maintained by the government as a way to support navigation schools. The word "violent" in code 83 is unique. If you have been in a tropical squall you will  know the term. It is essentially heavy rain in strong wind.

Stand by for more discussion of rain intensity and now to identify it.

Looking ahead, you might find interesting the great data from UW rain guages at

http://www-k12.atmos.washington.edu/k12/grayskies/nw_weather.html. Select Cumulative Rain at the ATG building roof at UW and other options of choice, and compare with forecasts and reports over the same period. We will be doing some of that in following articles.

We have also ordered a precision rain gauge for Starpath and we will link it to a video camera so we can get some practical feeling for what different levels of rain looks like.

Dec 25, 2012 UPDATE: Part 2 is now online.

References

National Weather Service Observer's Handbook No. 1—Ships Synoptic Code and Observing Methods, 2-50 May 2010 (online in full)

Weather of the Pacific Northwest by Cliff Mass, UW Press, 2008 

Modern Marine Weather by David Burch, Starpath Publications, 2008

Refraction in a sink

Refraction plays a key role in celestial navigation because starlight coming in from the vacuum of outer space bends when it enters the atmosphere. The speed of light is slower in air, so the light bends (refracts), and as a result all the stars we see are actually not quite as high as they appear. We then apply a refraction correction to get them back to the right angle.

It is easy to see this effect in a bathroom sink by adjusting your sight angle to just not see the drain, then turn on the water and fill the sink, as shown below.

You are seeing the light bend toward you as it leaves the water surface.  Then let the water run out and the drain goes away.  Seems a nice way to demonstrate this effect.

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Below is a schematic illustrating how this takes place, but as always with some hand waving needed to skip over  more fundamental issues. We think of light as a wave, with the lines AB being a wave front.  A fundamental point is light travels faster in air than in water.

 When the A1 side of the wave hits the surface and enters the air, the B1 side is still under  water. The A1 side starts to move faster now than the B1 side. So by the time B1 reaches B2, A1 has traveled quite a bit farther up to A2, and thus the wave front bends at the surface.

This simple picture shows why the angle you look at the drain (or star) makes a big difference. Look straight down at the drain (straight up to a star) and of course you do not see any change.  Light will always bend away from the surface entering into the faster medium

How to learn the present winds offshore — West Coast of Vancouver Island, BC

These notes apply to any ocean, but we will use specific examples for west coast of Vancouver Island, BC for those following the OAR NW expedition, which has  reached Port Hardy now at the top of the island, waiting for a good weather window to head south on the ocean side.

There are several ways to learn about actual ocean winds (as opposed to forecasted winds, which I will address later).

Buoys and Lighthouses

In some cases there are buoys offshore that report winds back on a regular basis. In our example there are 3 of them. The red dots at the EC marine weather page.  There are also 4 land based stations along the coast on the same link.

 

Satellites

We can also learn the winds from the ASCAT instrument on the satellite MetOP-A that provides data in this region several times a day as it passes overhead.  Read this data from the Ocean Surface Winds Team website. This is wonderful data, actual measurements, not predictions, but there are some nuances to using it.

For more details on the use of the data see an earlier article on the subject at Starpath article on ASCAT

Ship reports

Ships at sea participating the VOS program report weather to the NWS every 6h, and some times as often as every hour. Starpath has set up a free service for mariners to get access to this near live data by email. Most vessels have email underway these days, so this is a very handy service.  On land we can use it to get more insight into local weather at sea for any location. See Starpath Free Ship Reports program

Here is a sample location you can check for the top of Vancouver Island:  50.8N, 128.5W

Note if this does not work and you have followed the instructions, then it might be a mail format issue. This works most dependably with plain text format, ie not html.  For Macs: Mail/Preferences/Composing/Mail Format=Plain text

Again, these are ways to see the actual wind. Later we will add some notes on forecasted winds.

GPS Accuracy in Tight Quarters

With a clear view of the sky, GPS can usually give us a fix within 20 or 30 yards, even without a WAAS satellite locked in. But on a long trip along coasts, in and out of narrow passages, the accuracy can vary depending on how much of the sky is showing. Often we do not pay attention to this detail, but it is printed out on the GPS display in some form giving reliability, usually in terms of feet.

The OAR NW team decided to pull into Snug Harbor for some repairs they discovered needing after getting under way.  You can see info on the start at www.oarnorthwest.com.

The track shown there is from their Yellowbrick transmitter, a slick device that relays back their position, COG and SOG, and brief text messages every 15 minutes by txt msg to a cell phone at home base.  They apparently decided to check out this system by keeping it on at a fixed location overnight to see what the spread in positions might look like.

The results are shown below.

 I added the scale to the picture. Each tick mark is 10 yards. Red rings are at 20, 60 and 170 yds. These are fixes every 15 minutes. We see that most of the signals are within 20 yards, as expected. There are outliers, but even these are mostly within 60 yds. We see just one rather far out at 170 yds. 

This is strong testimony for this device because the unit had only some 10% of the usable sky to see through two large windows. The main message is, if we subsequently see the boat on land as it traverses a narrow channel we should not panic. Wait another 15 min to see if this was just one of these fixes from the far wings of the statistical distribution of fixes.

In narrow passes and along steep cliffs, or back packing with your Yellowbrick, you have GDOP to deal with (geometric dilution of position). Position accuracy depends on the geometry of the satellites in view. It is the same issue of getting a bearing fix from two land marks next to each other.  Fixes are best when the targets (or satellites in this case) are more spread around the horizon.  On a hotel balcony, or walking or rowing along the side of a steep cliff, notably less than half the sky is visible, which might limit your accuracy.

A good way to check this when in doubt is refer to the satellite display on the GPS unit. There is usually one in the set up menu. It will look some thing the following. It is a radar like display. The center is overhead in the sky, the circumference is the horizon. North is usually at the top of the page.A satellite located about half way out at 3 o'clock would be 45 deg high bearing due east.

Here is a sample of a good distribution (but normally there would be more satellites... i just found this pic online). 

And here is one from my Garmin 301 looking out a window. It does not give a strong fix in these conditions, but even a weak fix these days is pretty good. Generally the unit will just say tracking or can't find a fix if this gets this bad.

We see satellite 09 (about 15 deg high) and 27 (about 30 deg high) are the strongest, but they are both in the same NW direction. It has locked on to 11, but the signal is weak.  Note that is knows where the others are, but it cannot connect to them. When there is only two satellites in about the same direction the fix will not be as good. Knowing this does not give you a solution, but just an understanding of the limits involved. You might learn, however, that if you just move another few feet (on land to get away from a building) or another quarter mile on the water to get past a bluff, then you would have access to the satellites that you can tell from this picture are being blocked.

Most GPS units have such a display, but if not there are numerous computer or smartphone apps that do  this type of display and analysis.  One we like a lot for the PC is called GpsInfo.exe, a free utility from US GlobalSat.  It also does a good job of locating the com port with GPS signals as well as showing a hyper terminal type display of the incoming signals (again, most ECS programs have such a utility, but we have still found this tool useful. The instructions imply it has to be installed, but they must refer to a version that they ship with their GPS units. This link brings a 100-kb, stand-alone file that runs as is, no install, no interactions with your registry. Just realized we have not tested to see if this works on 64-bit.  Will try tomorrow.