Rain measurement
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
Different weather stations report different rainfall observations, see Monthly_log_files for the parameters that Cumulus tracks. Most processing by Cumulus is based on the total rainfall count output by a weather station, see FAQ for more. Cumulus supports two units (millimetres and inches).
On the Cumulus View menu This month/period/year screens you can see (excludes current meteorological day):
- Total for period/month/year (also available from Webtags)
- Wettest day (equivalent to Highest Daily Rainfall parameters of webtags)
- High rain rate
- Rain days (count of days with rainfall above configured threshold)
- Dry days (count of days with rainfall below configured threshold)
- Days with snow falling
- Days with snow lying
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
- Longest wet period (count of consecutive days with rainfall above configured threshold)
- Longest dry period (count of consecutive days with rainfall below configured threshold)
Webtags report
- For current, today, yesterday, this month, and this year; totals
- For today, yesterday, this month, this year, month-by-month, and all time; hourly/daily rainfall highs, 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)
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.
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 Gauge
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.
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 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.
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.
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.
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
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.
