Precipitation (fact sheets 3&4) Flashcards
Precipitation
The water reaching a catchment as rainfall, snow, hail, dew or occult (i.e. mist, fog) forms.
Average annual rainfall for the world and Lancaster
1000m
Rainfall measurement is based on…
areal integration of a network of point rainfall measurements (individual rainguage catches)
Measuring rainfall
2 types
- storage gauge
2. rainfall recorders
two types of storage gauge
daily and monthly
Example of a daily storage gauge
met office mark 2 daily storage gauge
Errors associated with storage gauges
2
outsplash and insplash
adaptation to reduce in-splash
met office mark 2 daily storage gauge
the gauge orifice is located 1ft above the ground surface
adaptation to reduce out-splash
met office mark 2 daily storage gauge
drop between orifice and collection funnel of 4 inches
adaptation to reduce uncertainty in the collected area
met office mark 2 daily storage gauge
the orifice has a ‘knife edge’
adaptation to reduce evaporation of collected water
met office mark 2 daily storage gauge
the collection funnel has a narrow neck
adaptation to extreme rainfall
met office mark 2 daily storage gauge
the collection bottle sits within an outer can that will catch extreme rainfall (slightly less accurately though)
adaptation to further reduce evaporation and increase stability
(met office mark 2 daily storage gauge)
the gauge is dug into the ground
When are met office mark 2 daily storage gauge monitored in the UK
9am GMT / 10am BST
two types of rainfall recorders
siphoning-tank raingauge
tipping-bucket raingauge
siphoning-tank raingauge
collects water in a funnel, directs this into a cylindrical tank, as the tank fills a float is raised which lifts a pen on a chart (autographic chart)
what happens when a siphoning-tank raingauge reaches a certain water level?
the float-pen mechanism releases a catch and the tank tips over and empties from a pipe aided by a siphoning mechanisms
name of autographic chart produced by a siphoning-tank raingauge
hyetogram
disadvantage of a siphoning-tank raingauge
the chart requires digitising to obtain rainfall intensity data
Tipping-bucket raingauge
water collection
collects water in a funnel then delivers it to a pivoted double bucket device, one bucket is filled first, which causes it to tip, empty and bring the second bucket below the funnel
Tipping-bucket raingauge
data collection
as each bucket tips, a magnet passes a reed switch causing an electrical contact to close, the time of the voltage pulse can then be recorded with a datalogger
when are the tipping-bucket raingauge buckets set to tip?
after receiving 0.5mm of rainfall
Tipping-bucket raingauge are sensitive to
freezing
what is a reed switch
two pieces of metal which are very close together, close to a magnet, makes a circuit when closed
how to mitigate freezing of tipping-bucket raingauges
2
use resistors as heaters
or
accept freezing on cold days and have a check gauge which records monthly total next to it
how can raingauge sheltering be avoided?
sit the gauge at a distance 2x the height of the nearest obstacle, which is equivalent to a 30 degree angle between the gauge and the building top
how can turbulence at the gauge orifice be reduced?
2
sit the gauge within a Huddleston turf wall and slope the banks
or
gauge in a ground-level pit with an anti-splash grid around
how is precipitation in forest measured?
2
sitting rain gauges in clearings
connecting a canopy-level funnel to a standard raingauge (canopy gauge)
how is snowfall measured?
heated gauges at 1m above ground with shields around the gauge orifice to reduce turbulance effects
