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Rain measurement: Difference between revisions
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{{Version badge 1}}This page was written when only the legacy software was available. All cross-references are to other legacy pages, this page has not been updated for MX yet.
Rain is measured using a rain gauge (also known as a udometer or a pluviometer [Pluviograph ] which gathers and measures the amount of liquid [http://en.wikipedia.org/wiki/Precipitation_(meteorology) precipitation] over a set period of time.
= 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.▼
Cumulus processes rainfall in one of two [[cumulus.ini| units]] (millimetres and inches).
* Total for period/month/year (also available from [[Webtags]])▼
▲
* Wettest day (equivalent to Highest Daily Rainfall parameters of webtags)▼
* High rain rate▼
The rainfall observations output by different weather stations varies, see [[Standard_log_files]] for the parameters that Cumulus tracks. Cumulus can also read rainfall from a separate RG11 optical gauge.
* Rain days (count of days with rainfall above configured threshold)▼
* Dry days (count of days with rainfall below configured threshold)▼
== Rain day ==
* Days with snow falling▼
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.
== Cumulus View Menu ==
Cumulus 1 provides a number of ways to view derived rainfall.
=== 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
* Longest wet period (count of consecutive days with rainfall above configured threshold)
* Longest dry period (count of consecutive days with rainfall below configured threshold)
=== ''This month/period/year'' screens ===
[[Webtags]] report▼
* For current, today, yesterday, this month, and this year; totals▼
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):
* For today, yesterday, this month, this year, month-by-month, and all time; hourly/daily rainfall highs, rain rate high▼
# 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
# High [[#Rain_Rate|rainfall rate]]
# Days in period with snow falling (from [[Weather_Diary]])
▲==[[Webtags]] report ==
**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
** hourly rainfall highs and timestamp,
** daily rainfall highs and timestamp,
* 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>
▲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.
= Types of Rain 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
#One design 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,
#*either to the Data Set Ready (pin 4) and Data Terminal Ready (pin 6),
▲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, [[Cumulus.ini#Section:_Station| via parameters in cumulus.ini Station section]]. It should be connected via a serial port (the rain sensor's 'normally open' relay contacts should be connected to the DSR and DTR pins, or to the RTS and CTS pins). You can connect two RG-11 sensors to the same port, one to DTS/DTR and one to RTS/CTS. Use a USB to RS232 (serial) adapter on computers which don't have a COM port.
#*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.▼
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 =
It is important to note that Cumulus attributes rainfall to the date when the measuring period starts. [[meteorological day|Some nations attribute rainfall]] to the day when the measuring period ends.
[http://www.bom.gov.au/climate/how/observations/rain-measure.shtml| This Australian web site] used to explain more, but they have since reorganised their web site, can anybody identify the new link and update this page.
[[Category:Terminology]]
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