Tuesday, May 22, 2012

It's Raining… What does that mean?

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."

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.


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.


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

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