Rain measurement: Difference between revisions

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→‎Webtags report: make clear the main screen also shows 'this season'
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*For virtually all gauges, drops will stick to the sides of the collecting device, resulting in slightly underestimated measurements.
*When the temperature is close to or below freezing, rain may fall on the funnel and freeze or snow may collect in the gauge and not permit any subsequent rain to pass through.
 
= Precipitation measurement and Cumulus =
Cumulus processes rainfall in one of two [[cumulus.ini| units]] (millimetres and inches). Most processing by Cumulus is based on the total rainfall count output by a weather station, see [[FAQ#My_station_invented_some_rain_that_didn.27t_really_occur.2C_and_I_want_to_set_it_to_zero_.28or_some_other_figure.29 | FAQ]] for more. The rainfall observations output by different weather stations varies, see [[Monthly_log_files]] for the parameters that Cumulus tracks. Cumulus can also read rainfall from a separate RG11 optical gauge.
== Rain day ==
The threshold value which the daily rainfall has to equal or exceed for the day to be considered a 'rain day' is set in [[Cumulus.ini#Section:_Station | RainDayThreshold]] and has a default value of ''-1'' that means 0.2mm or 0.01in. Alternative thresholds can be used by specifying a number in the units your configuration uses.
== Cumulus View Menu ==
=== Highs and Lows ===
On the Cumulus '''View''' ''Highs and lows'' screens you can see those of the following appropriate to the chosen period:
* Highest Hourly/Daily/Monthly Rainfall
* High [[#Rain_Rate|rainfall rate]]
* Longest wet period (count of consecutive days with rainfall above configured threshold)
* Longest dry period (count of consecutive days with rainfall below configured threshold)
All of these values are also available from [[Webtags]].
 
=== ''This month/period/year'' screens ===
A number of values are shown for rainfall, these are calculated by summing field 14 (total rain today) from relevant rows in [[dayfile.txt]] (so will exclude the current meteorological day):
# Total rainfall
#* Total for this (any) month
#* Total for this (any) period you want (select start date and end date)
#* Total for this (any calendar) year is shown on 'This year' screen
# Wettest day (equivalent to 'Highest Daily Rainfall' parameters of webtags)
# High [[#Rain_Rate|rainfall rate]]
# Rain days (count of days in period with rainfall above configured threshold)
# Dry days (count of days in period with rainfall below configured threshold)
# Days in period with snow falling (from [[Weather_Diary]])
# Days in period with snow lying
 
==[[Webtags]] report ==
* For current, last 24 hours, yesterday (<#rfallY>), and this month;
**totals (<#rfall>, <#r24hour>, <#rmonth> all include rainfall so far today)
* Annual rainfall web tag <#ryear> reports total for ''rainfall season'' year i.e. starting month set on Configuration | station screen | Annual rainfall frame; includes rainfall so far today, same figure as shown for 'This year' on main Cumulus screen)
* For today, yesterday, this month, this rainfall season, month-by-month, and all time;
** hourly rainfall highs and timestamp,
** daily rainfall highs and timestamp,
**rain rate high
* Snow depth for current [[Weather_Diary#How_do_I_change_the_time_a_diary_day_begins.2Fends.3F|day]]
* Longest wet period (count of consecutive days with rainfall above configured threshold)
* Longest dry period (count of consecutive days with rainfall below configured threshold)
* Timestamp of last rain gauge tip (and, from version 1.9.4, how many days ago)
* <#IsRaining> <#RG11RainToday> <#RG11RainYest> are available for Hydreon RG11 detectors only (the first applies to rain detection mode, the others apply to the tipping bucket mode)
 
= Types of Rain Gauge =
 
==Tipping Bucket Rain Gauge ==
The tipping bucket gauge consists of a funnel that collects and channels the precipitation. The precipitation falls onto one of two small buckets or levers which are balanced in same manner as a balance scale. When the bucket fills sufficiently to "tip" the balance an electrical signal is sent to the recorder.
Modern tipping rain gauges consist of a plastic collector balanced over a pivot. When it tips, it actuates a switch (such as a [http://en.wikipedia.org/wiki/Reed_switch reed switch]) which is then electronically recorded or transmitted to a remote collection station.
 
Tipping gauges can also incorporate weighing gauges. In these gauges, a strain gauge is fixed to the collection bucket so that the exact rainfall can be read at any moment. Each time the collector tips, the strain gauge (weight sensor) is re-zeroed to null out any drift.
The tipping bucket rain gauge is not as accurate as the standard rain gauge because the rainfall may stop before the lever has tipped. When the next period of rain begins it may take no more than one or two drops to tip the lever. Tipping buckets also tend to underestimate the amount of rainfall, particularly in snowfall and heavy rainfall events. Tipping buckets can be subject to vibration if not surely mounted causing the balance to tip resulting in a over estimation of the rain measurement.
 
TheIn tippingthe more common design, when the bucket fills sufficiently to "tip" the balance an electrical signal is sent to the recorder. Modern tipping rain gaugegauges consist of a plastic collector balanced over a pivot. When it tips, it actuates a switch (such as a [http://en.wikipedia.org/wiki/Reed_switch reed switch]) which is then electronically recorded or transmitted to a remote collection station. Counting tips is not as accurate as the standard rain gauge because the rainfall may stop before the lever has tipped. When the next period of rain begins it may take no more than one or two drops to tip the lever. Tipping buckets also tend to underestimate the amount of rainfall, not collecting all of it, particularly in snowfall and heavy rainfall events. Tipping buckets can be subject to vibration if not surelysecurely mounted causing the balance to tip resulting in a over estimation of thefalse rain measurement.
The advantage of the tipping bucket rain gauge is that the rain rate (light, medium or heavy) may be easily obtained. Rainfall rate is decided by the total amount of rain that has fallen in a set period (usually 1 hour) and by counting the number of 'clicks' in a 10 minute period the observer can decide the character of the rain.
 
The advantage of the tipping bucket rain gauge is that the rain rate (light, medium or heavy) may be easily obtained. Rainfall rate is decided by the total amount of rain that has fallen in a set period (usually 1 hour) and by counting the number of 'clicks' in a 10short minutefixed period and lets the observer can decide the character of the rain.
Tipping gauges can also incorporate weighing gauges. In these gauges, a strain gauge is fixed to the collection bucket so that the exact rainfall can be read at any moment. Each time the collector tips, the strain gauge (weight sensor) is re-zeroed to null out any drift.
 
==Weighing Precipitation Gauge ==
The advantages of this type of gauge over tipping buckets are that it does not underestimate intense rain, and it can measure other forms of precipitation, including rain, hail and snow. These gauges are, however, more expensive and require more maintenance than tipping bucket gauges.
 
== Optical Rain GaugeGauges ==
These#One havedesign has a row of collection funnels. In an enclosed space below each funnel is a [http://en.wikipedia.org/wiki/Laser_diode laser diode] and a [http://en.wikipedia.org/wiki/Photodiode phototransistor detector]. When enough water has been collected to form a single drop it drips from the bottom of the funnel, falling into the laser beam's path. The detector is set at right angles to the path of the laser beam so that light scattered by the drop of water breaking the laser beam is detected as a sudden flash of light. The flashes from these photodetectors are then read and transmitted or recorded.
#The [http://www.rainsensors.com/| Hydreon Optical Rain Sensor] - Model RG-11 - uses an infrared sensor to detect rainfall with high precision. Some car windscreens have a similar sensor to automatically start wipers. Cumulus supports the RG11, via parameters in [[Cumulus.ini#Hydreon_specific:]] that are set using a RG11 screen within the configuration menu. Use a USB to RS232 (serial) adapter on computers which don't have a COM port. See Cumulus ''help'' for more information. The rain sensor's 'normally open' relay contacts should be connected via a serial port (9 way connector)
#*either to the Data Set Ready (pin 4) and Data Terminal Ready (pin 6),
#*or to the Request To Send (pin 7) and Clear To Send (pin 8)].
#*You can connect two RG-11 sensors to the same port, one to DSR/DTR and one to RTS/CTS (one has to be in "It's raining" mode and one in "tipping bucket" mode).
 
=Rain Gauge Location=
The exposure of a rain gauge is very important for obtaining accurate measurements. As a general rule, the windier the gauge location is, the greater the precipitation error will be. You can find lots of discussions online about wind effects, and rain shadows, together with research on positioning requirements in rural and urban locations. A good starting point is [http://www.rmets.org/weather-and-climate/observing/guidelines-observing-0 this UK website].
 
Gauges should not be located close to isolated obstructions such as trees or buildings, which may deflect precipitation due to erratic turbulence. To avoid wind and resulting turbulence problems, do not locate gauges in wide-open spaces or on elevated sites, such as the tops of buildings. For a separate rain gauge (e.g. Fine Offset), experiment to find whether the best protection from the wind occurs with the buckets arranged to line up with the mast or at right angles to the supporting arm between gauge and mast.
 
The WMO recommend that a rain gauge be placed away from any nearby obstacle a distance of at least twice the height of the obstacle. The best site for a gauge is one in which it is protected in all directions, such as in an opening in a grove of trees. The height of the protection should not exceed twice its distance from the gauge.
To avoid wind and resulting turbulence problems, do not locate gauges in wide-open spaces or on elevated sites, such as the tops of buildings.
 
The standard height for the rim of a rain gauge varies by country. In the UK it is 30cm above ground. In the US it is between 3 and 4 feet. The placement is a compromise; lower placement reduces loss due to wind which may occur at higher levels, but increases the chance of heavy rain splashing in off the ground, and of being buried by snow, where applicable.
The best site for a gauge is one in which it is protected in all directions, such as in an opening in a grove of trees. The height of the protection should not exceed twice its distance from the gauge.
 
=Rain Rate=
 
Snow can be recorded in Cumulus software using the [[Weather Diary]], it is a bit different to rain as snow depth is not automatically recorded; and the timing of snow falling and any detection on melting can be separated by several days.
 
= Making comparable with official sites =
[http://www.bom.gov.au/climate/how/observations/rain-measure.shtml| This Austrailian web site gives some ideas]
 
[[Category:Terminology]]
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