A rain gauge (also known as an udometer, pluviometer, or an ombrometer) is an instrument to gather and measure the amount of liquid precipitation over a set period of time. It is also an important part of automatic weather station.
Before introducing the rain gauge, I would like to introduce the definition of precipitation.
Precipitation is defined as the liquid or solid products of the condensation of water vapour falling from clouds or deposited from air onto the ground.
Types of precipitation includes rain, hail, snow, dew, rime, hoar frost and fog precipitation. The total amount of precipitation which reaches the ground in a stated period is expressed in terms of the vertical depth of water to which it would cover a horizontal projection of the Earth’s surface.
Snowfall is also expressed by the depth of fresh, newly fallen snow covering an even horizontal surface .
Precipitation intensity is defined as the amount of precipitation collected per unit time interval.
According to this definition, precipitation intensity data can be derived from the measurement of precipitation amount using an ordinary precipitation gauge.
In that sense, precipitation intensity is a secondary parameter, derived from the primary parameter precipitation amount. However, precipitation intensity can also be measured directly.
Units and scales
The unit of precipitation is linear depth, usually in millimetres (volume/area), or kg m–2 (mass/area) for liquid precipitation.
Daily amounts of precipitation should be read to the nearest 0.2 mm and, if feasible, to the nearest 0.1 mm; weekly or monthly amounts should be read to the nearest 1 mm (at least).
Daily measurements of precipitation should be taken at fixed times common to the entire network or networks of interest. Less than 0.1 mm (or 0.2 mm depending on the resolution used) is generally referred to as a trace.
The measurement unit of rainfall intensity is linear depth per hour, usually in millimetres per hour (mm h–1). Rainfall intensity is normally measured or derived over one-minute time intervals due to the high variability of intensity from minute to minute.
Rain Gauges Definition & Principle
What Is Tipping Bucket Rain Gauge
The tipping-bucket raingauge is used for measuring accumulated totals, the rate of rainfall and intensity precipitation. As recommended by WMO, it is a professional rain gauge.
Tipping-bucket rain gauge can be divided into single tipping and double tipping. Double tipping rain gauge has higher precision, up to 0.1mm.
This kind of rain gauge is very suitable for meteorology, hydrology, agriculture, forest and etc, in which liquid water precipitation be measured, which is also the main part of professional meteorological stations.
The output is two wires cable of which gives on and off signals(dry reed).
Tipping Rain Gauge Principle
The principle behind the operation of this instrument is simple.
How does a rain gauge work?
A tipping-bucket raingauge uses a metallic or plastic twin bucket balance to measure the incoming water in portions of equal weight. When one bucket is full, its centre of mass is outside the pivot and the balance tips, dumping the collected water and bringing the other bucket into position to collect.
The bucket compartments are shaped in such a way that the water is emptied from the lower one. The water mass content of the bucket is constant (m [g]). Therefore, by using the density of water (r = 1 g/cm3), the corresponding volume (V [cm3]) is derived from the weight of the water and, consequently, the corresponding accumulation height (h [mm]) is retrieved by using the area of the collector (S [cm2]). The equation is:
Thus, by using the density of water, h is calculated, where 1 mm corresponds to 1 g of water over an area of 10 cm2 . To have detailed records of precipitation, the amount of rain should not exceed 0.2 mm. For a gauge area of 1 000 cm2, this corresponds to a bucket content of 20 g of water.
The raw output is a contact closure (reed switch or relay contact), so each tip produces an electrical impulse as a signal output which must be recorded by a data logger or an analogue-to-digital converter (data acquisition system equipped with reed switch reading ports). This mechanism provides a continuous measurement without manual interaction.
rain sensor windshields around the gauge reduce the error caused by deformation of the wind field above the gauge.
What Is Weighing Rain Gauge
The weighing rain gauge is an instrument for calculating rainfall accumulated totals based on the weight of precipitation.
All precipitation, both liquid and solid, is recorded as it falls. This type of gauge normally has no provision for emptying itself; the capacity (namely, the maximum accumulation between recharge) ranges from 250 to 1 500 mm depending on the model.
Weighing Rain Gauge Principle
Weighing bucket type rain gauge is most common self-recording rain gauge. It consists of a receiver bucket supported by a spring or lever balance or some other weighing mechanism. The movement of bucket due to its increasing weight is transmitted to a pen which traces record or some marking on a clock driven chart.
Weighing bucket type rain gauge instrument gives a plot of the accumulated (increased) rainfall values against the elapsed time and the curve so formed is called the mass curve.
Therefore, mechanical degradation and the resulting need for maintenance are significantly reduced.
Weighing technology combined with a self-emptying tipping-bucket allows to perform high resolution and high precision measurements at a very small construction volume. This type of instruments measures the weight of water in a tipping-bucket with a volume of up to 20 ml and can determine smaller amounts of precipitation compared to “classic” tipping-bucket gauges.
What Is Float Rain Gauge
Float Rain Gauge is a pen-mounted recorder, which writes rainfall values by accumulating rainfall volume and then using a floating pen. It is not a electronic rain gauge.
Float Rain Gauge Principle
The working of this type of rain gauge is similar to weighing bucket rain gauge. A funnel receives the water which is collected in a rectangular container. A float is provided at the bottom of container, and this float raises as the water level rises in the container. Its movement being recorded by a pen moving on a recording drum actuated by a clock work.
When water rises, this float reaches to the top floating in water, then syphon comes into operation and releases the water outwards through the connecting pipe, thus all water in box is drained out. This rain gauge is adopted as the standard recording rain gauge in India and the curve drawn using this data is known as mass curve of rain fall.
A large undercatch by unshielded heated gauges, caused by the wind and the evaporation of melting snow, has been reported in some countries, as is the case for weighing gauges
A heating device (preferably controlled by a thermostat) should be installed inside the gauge if there is a possibility that water might freeze in the float chamber during the winter. This will prevent damage to the float and float chamber and will enable rain to be recorded during that period.
The amount of heat supplied should be kept to the minimum necessary in order to prevent freezing, because the heat may reduce the accuracy of the observations by stimulating vertical air movements above the gauge and increasing evaporation losses.
Apart from the fact that calibration is performed using a known volume of water, the maintenance procedures for this gauge are similar to those of the weighing-recording gauge
Further Technology Rain Gauge
With the growth of measurement electronics technologies and smart instruments, other precipitation sensors have been developed in recent years. Their performance is approximately of the same quality as performance of conventional tipping bucket rain gauges.
Those instruments provide rain intensity with higher resolution comparing to classic methods, starting from 0.01 mm, and are particularly suitable for areas which are difficult to access due to being less maintenance demainding devices.
What Is Optical Rain Sensor
Optical rain sensors, using forward-scatter technology or transmittance measurement technology, have been used operationally for decades. In forward-scatter technology, precipitation detection and identification is based on light scattering properties of hydrometeors, while transmittance measurement technology is based on light attenuation of hydrometeors.
The advantage of optical rain sensor is that it can be used for all precipitation types in all weather conditions. Furthermore, forward-scatter precipitation sensor is usually able to report MOR as well.
What Is Capacitance Rain Sensor
rain sensors are based on measuring the dielectricity of water. They typically consist of surface capacitance measurement.
As alone, they can be used to detect the precipitation onset and end of an event. They can also be used as supportive measurement with other technologies.
What Is Haptic Rain Sensor
Haptic rain sensors are based on measuring the impact from a hydrometeor. They typically consist of a piezoelectric sensor to detect the acoustic properties of impact. These sensors are practically maintenance-free.
It is important to deal with the interference of external factors in the design of this instrument. Such as foreign matter, birds or other vibration effects.
What Is Rada Rain Sensor
How is precipitation measured by precipitation radar?
Radar rain sensors, using Doppler radar technology, have the advantage to measure the speed of most forms of condensed water, precipitation quantity and precipitation type (rain, snow, sleet, freezing rain, hail) from the first drops with a fast response time.
At the same time, we need to consider the external interference and improve the processing ability of the algorithm, such as dust storm weather, foreign body false alarm, etc.
Snow & Ice Measurement
Snowfall is the depth of freshly fallen snow deposited over a specified period (generally 24 h). Thus, snowfall does not include the deposition of drifting or blowing snow.
For the purposes of depth measurements, the term “snow” should also include ice pellets, glaze, hail, and sheet ice formed directly or indirectly from precipitation.
Snow depth usually means the total depth of snow on the ground at the time of observation.
The water equivalent of a snow cover is the vertical depth of the water that would be obtained by melting the snow cover.
What Is Ultrasonic Snow Depth Sensor
Several ultrasonic models exist on the market and are commonly used with automatic systems. This type of sensor can also be utilized to control the quality of automatic recording gauge measurements by providing additional details on the type, amount and timing of precipitation.
It is capable of an uncertainty of ±1 cm.
The temperature correction formula for ultrasonic snow depth measurement is:
where d is the snow depth in cm; dr is the raw value of snow depth in cm; T is the air temperature in K; and T = 273.15 + t, t is the air temperature in °C.
What Is Laser Snow Depth Sensor
Laser snow depth sensor detects the snow depth distance to the ground by comparing phase information.
The measurement of the distance is independent of air temperature, but may depend on the penetration of the laser beam in the snow surface according to the type of snow. The laser spot is also very small, increasing the importance of ground surface representativeness. It is strongly recommended using a laser beam classified safe for the eyes (maximum power class 2).
To get reliable snow height data between 0 and 2 cm it must be ensured by the setup that the measured distance is fixed under all conditions (due to lifting and lowering of the ground dependent of type of soil, temperature, moisture and frost sometimes more than +/- 1 cm must be considered).
An area with natural vegetation can create issues, so it may be better to use a stable and controlled surface, such as a snow plate.
What Is Ice Sensor
At meteorological stations, the observation of ice accumulation is generally more qualitative than quantitative, primarily due to the lack of a suitable sensor.
Ice accretion indicators, usually made of anodized aluminium, are used to observe and report the occurrence of freezing precipitation, frost or rime icing.
Ice On Pavement
One sensor using two electrodes embedded in the road, flush with the surface, measures the electrical conductivity of the surface and readily distinguishes between dry and wet surfaces.
There are two remote-sensing methods under development.
The first method is based on the reflection of infrared and microwave radiation at several frequencies (about 3 000 nm and 3 GHz, respectively). The microwave reflections can determine the thickness of the water layer (and hence the risk of aquaplaning), but not the ice condition. Two infrared frequencies can be used to distinguish between dry, wet and icy conditions. It has also been demonstrated that the magnitude of reflected power at wavelengths around 2 000 nm depends on the thickness of the ice layer.
The second method applies pattern recognition techniques to the reflection of laser light from the pavement, to distinguish between dry and wet surfaces, and black ice.
Rain Gauges Compare Table
Tipping Rain Gauge
Most widely used
Calibration is simple
Can’t monitor snow, ice
Unable to eliminate evapotranspiration effects
Weighing Rain Gauge
Snow & Ice measurement
Decreased evapotranspiration effect
More complex installation structure
Float Rain Gauge
Automatic Recording Function
Installation easy, Portable
Independent, not accessible to automatic observation systems
Rather old generation
Not a digital rain gauge
Optical Rain Gauge
Large monitoring range
No moving parts, no maintenance
No calibration standard
Cost higher than normal
Capacitance Rain Gauge
Qualitative but not quantitative
No rainfall output
Haptic Rain Gauge
Snow & Ice measure
No moving parts, no maintenance
Decreased evapotranspiration effect
Poor consistency in monitoring rainfall intensity
Light rain is not as accurate as optical detection
A rain gauge (also known as an udometer, pluviometer, or an ombrometer) is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a set period of time.
Provides an introduction to the different types of precipitation and explains how precipitation is related to weather.
A Treatise on Meteorology – The barometer, thermometer, hygrometer, rain-gauge, and ozonometer; with rules and regulations to be observed for their correct use is an unchanged, high-quality reprint of the original edition of 1866. Hansebooks is editor of the literature on different topic areas such as research and science, travel and expeditions, cooking and nutrition, medicine, and other genres.
This book presents current applications of remote sensing techniques for clouds and precipitation for the benefit of students, educators, and scientists. It covers ground-based systems such as weather radars and spaceborne instruments on satellites. Measurements and modeling of precipitation are at the core of weather forecasting, and long-term observations of the cloud system are vital to improving atmospheric models and climate projections.