Rain measurement

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Rain is measured using a rain gauge (also known as a udometer or a pluviometer [Pluviograph ] which gathers and measures the amount of liquid precipitation over a set period of time.

All rain gauges have their limitations.

  • Attempting to collect rain data in high wind (hurricane conditions) can be nearly impossible and unreliable due to wind extremes preventing rain from entering the gauge.
  • Rain gauges only indicate rainfall in a localized area.
  • 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 units (millimetres and inches). Most processing by Cumulus is based on the total rainfall count output by a weather station, see 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 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 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):

  1. 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
  2. Wettest day (equivalent to 'Highest Daily Rainfall' parameters of webtags)
  3. High rainfall rate
  4. Rain days (count of days in period with rainfall above configured threshold)
  5. Dry days (count of days in period with rainfall below configured threshold)
  6. Days in period with snow falling (from Weather_Diary)
  7. 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 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

Types of rain gauges include graduated cylinders, weighing gauges, tipping bucket gauges, optical, and simple buried pit collectors. Each type has its advantages and disadvantages for collecting rain data.

Standard (graduated cylinder)Rain Gauge

The standard rain gauge consists of a funnel attached to a graduated cylinder that fits into a larger container. If the water overflows from the graduated cylinder the outside container will catch it. So when it is measured, the cylinder will be measured and then the excess will be put in another cylinder and measured. In most cases the cylinder is marked in mm.

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.

Tipping gauges can 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.

In the more common design, 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 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 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 securely mounted causing the balance to tip resulting in a false rain measurement.

The advantage of the tipping bucket rain gauge is that the rain rate may be easily obtained. Rainfall rate is decided by counting the number of 'clicks' in a short fixed period and lets the observer decide the character of the rain.

Weighing Precipitation Gauge

A weighing-type precipitation gauge consists of a storage container, which is weighed to record the mass. Certain models measure the mass using a pen on a rotating drum, or by using a vibrating wire attached to a data logger.

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 Gauges

  1. One design has a row of collection funnels. In an enclosed space below each funnel is a laser diode and a 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.
  2. The 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 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.

Rain Rate

See FAQ#How_is_my_rain_rate_calculated.3F for Cumulus specific information.

How fast rain is falling is classified according to the rate of recording/detecting precipitation.

Very light rain precipitation rate is < 0.25 mm/hour
Light rain precipitation rate is between 0.25mm/hour and 1.0mm/hour
Moderate rain precipitation rate is between 1.0 mm/hour and 4.0 mm/hour
Heavy rain recipitation rate is between 4.0 mm/hour and 16.0 mm/hour
Very heavy rain    precipitation rate is between 16.0 mm/hour and 50 mm/hour
Extreme rain recipitation rate is > 50.0 mm/hour

The UK Met Office use a colour (CSS hex codes shown in table) coding system on their forecast maps with different split points:

Blue background:#0404fb; precipitation rate is between 0.01mm/hour and 0.5 mm/hour
Azure background:#3366fc; precipitation rate is between 0.5mm/hour and 1.0mm/hour
Olive background:#808003; precipitation rate is between 1.0 mm/hour and 2.0 mm/hour
Yellow background:#fbca04; precipitation rate is between 2.0 mm/hour and 4.0 mm/hour
Orange   background:#fc9804;   precipitation rate is between 4.0 mm/hour and 8.0 mm/hour
Fuchsia background:#fb0404; precipitation rate is between 8.0 mm/hour and 16.0 mm/hour
Red background:#fb04fc; precipitation rate is between 16.0 mm/hour and 32.0 mm/hour
Aqua background:#e2fcfc; precipitation rate is > 32.0 mm/hour

Snow, Sleet and Hail

Although this article is headed rain measurement, there are other forms of precipitation.

Dew and mist/fog may be detected and counted as rain if your sensor has a high resolution.

Sleet, snow, and hailstones may melt into water that can be counted by a rain gauge, although wind effects are perhaps more likely to distort measurements by blowing these forms of precipitation away before they get collected.

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

This Austrailian web site gives some ideas