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. 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
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
