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.

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. 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.

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.

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.

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

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

These have 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.

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

How fast rain is falling is classified according to the rate or 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