type is a tipping bucket gage. Rain falls from the gage mouth into a small, conical
bucket. When a certain mass of water (usually a millimeter of precipitation) fills
the bucket, it tips to empty while its place is taken by an empty bucket. The tips regis-
ter as pulses to an electronic device so that the amount and timing of the rain are reg-
istered. The rate of rainfall is measured in depth per hour or day. In the United States,
inches per hour define rainfall rates with upper rates called cloudbursts and exceed-
ing 4.00” (10” cm) per hour.
Measurement of snow is more problematic. The traditional method was to mea-
sure with a stick in places where there didn’t appear to be drifting. The most
modern method is to have the snow automatically melted into a heated gage
containing a tipping bucket. Snowfall intensities are measured by noting runway
visibilities in fractions of kilometers or miles.
The liquid equivalent (water equivalent) is the depth of accumulated water
adding melted frozen forms to the measured rainfall. In the past this was not
readily done, so the U.S. National Weather Service adopted a 10:1 ratio for sites
where the snow was not melted for measurement. That is, for each 10” of snow
1” of liquid was added to precipitation totals. In reality there are “wet” (3:1) and
“powder” (30:1) snowfalls that complicate matters. The water equivalent is of cru-
cial interest in places like the mountainous American West where most precipita-
tion comes as snow and melts over the warm season to supply water needs. No
matter what the actual ratio, it bears noting that it takes large snowfall totals to
equal the precipitation coming from one summertime thunderstorm.
Precipitation catches over Earth are highly varied. Mt. Wai’ale’ale on the island
of Kaua’i, Hawaii, USA is officially the wettest at over 11,600 mm (460”) a year,
while worldwide there are several other mountainous locations that are similar but
not well measured. There are some generalities that can be applied to world pre-
cipitation. The highest precipitation catches are in the tropical regions of the world
followed by places that experiencemiddle latitude cyclones. The wettest places
have good water vapor supplies and consistent lift due to topography and passing
low pressure disturbances. Dry places are defined by the relative lack of these
above factors and the dryness is usually enhanced by one of several other factors.
The Atacama coastal desert of western South America is said to be the driest
continental area on the planet with rain occurring every few years at individual
locations (seeDeserts). Dry places tend to be dominated by subtropical highs or
inland location or blockage by mountains.
Some authors refer to permanently frozen places as cold or polar deserts. These
places have very little water vapor in the air so there is not frequently meaningful
precipitation. For instance, in the snow and ice world at the South Pole, the pre-
cipitation averages about 25 mm per year. Melted down to liquid equivalent, polar
precipitation totals rival the lowest ones in other deserts of the planet.
Precipitation 265