Measuring Precipitation

Precipitation is the name given for any moisture that falls from the air above us. This term includes rain but also drizzle, hail, sleet and snow. Air is never totally dry, even in arid landscapes such as the centre of the Sahara Desert. The heaviest raindrops can fall at about nine metres per second compared to lighter precipitation such as snow, which being very much lighter, only fall at around half metre per second.

A variety of instruments are available to measure the depth of water that would be collected from a level area in one day. Naturally, this is an indication of the precipitation close to the instrument, as it does not fall uniformly over a greater area. The Science Museum exhibits the first rain gauge that was invented in 1442, named the Korean. In 1639 an Italian called Castelli invented the first European gauge. In the UK there are over 5,000 weather-recording stations, both amateur and professional, that routinely measure precipitation.

To make regular readings a storage container and measuring device are necessary. The daily readings can be added together to provide monthly and annual totals. From such analysis comparisons can be made against previous months and years and ultimately, after a longer period of time, trends can be deduced. Rainfall is usually measured in millimetres. It is useful to know that 1mm is equivalent to 1 litre of water per square metre.

Simple Plastic Cone or Tube

The most basic unit consists of a plastic tube or cone with a printed measuring guide along its length. A simple unit can be constructed from a plastic bottle, the top having been carefully sliced off horizontally. It is then inverted and inserted into the bottle top to act as a funnel. The markers for levels can be drawn on the side or narrow bands of adhesive strips applied using a metric ruler as guide. This equipment is unlikely to have the ability to store larger amounts of precipitation when the daily total is considerable.

Modest Readily Purchased Units

Manufactured rain gauges, constructed from a variety of materials, are available. These units are suitable for fixing on a post or into the ground and have an integral measure calibrated in millimeters or inches. Some gauges come supplied with a rainfall chart.

Professional 5” Copper Gauge

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The rain gauge for the serious amateur meteorologist is expensive but accurate. It is a 12cm copper cylinder, often referred to as a 5 inch, with a removable funnel. It has a sharp rim so that raindrops hitting it are divided to fall accurately and proportionately into or out of the gauge. Within the copper cylinder is a collecting bottle, which every day has to be emptied into a glass-measuring cylinder with a tapering end in order that small amounts of precipitation can be accurately measured. The imprinted ruler has a measure set out, usually in millimeters, to an accuracy of 0.1mm that is equivalent to the same amount falling on level ground. Using a metric system is helpful in that one millimeter of precipitation is equal to one litre per square metre in volume.
Whatever unit is used, it is essential that the funnel fits firmly into the outer container so that it is not blown around in high winds that would reduce the accuracy of the readings. There is another important reason for a good fit that is far more significant in hot, dry weather. Any gap will allow evaporation of the contents, especially during summer months, which would again lead to inaccurate measurements.
Snow is a problem that can arise in winter when attempting to measure the water content of any frozen precipitation. Any snow overlapping the top must be carefully cut vertically around the rim. The snow within the funnel and any liquid or frozen precipitation in the container is then taken indoors and very slowly melted, taking care to minimize any evaporation by covering the funnel top. The resulting total liquid can then be measured in the usual way when the snow or ice has melted.

There is a more precise way of obtaining the water equivalent of collected snow, but takes a little more work to calculate. Prepare a quantity of warm, but not hot water and place a precise 10mm in the glass-measuring jar. Slowly pour this over the snow in the copper gauge. Repeat until all snow has melted and note how many sets of 10mm warm water has been used. Finally, empty out the melted snow and added warm water, calculate the total using the glass measuring jar and deduct the quantity of warm water used in melting the snow. This will give the best rainfall equivalent of the snow collected.
An additional method to calculate the rainfall equivalent of snowfall, but which is not especially accurate, is to measure the depth of level snow in several places that are a distance apart and not affected by any drifting from nearby taller hedges or buildings. Discard the highest and lowest value and calculate the average depth by calculating an equivalent liquid amount by using a conversion ratio using the following rules of thumb. If the snow is dry, powdery and not compacted use the ratio of 10: 1. However, for wet snow the ratio can be as low as 5:1 and in extreme cold, dry weather the ratio can be 20:1. It might be useful to remember that 10cm of snow is equal to 10mm of equivalent rainfall using the 10:1 ratio.

Small Electronic Gauges

There are small electronic units that measure rainfall as well as temperature. The data is sent wirelessly to a monitor for indoor use but the equipment has limited range, especially through solid objects. The rainfall data is restricted to a minimum time period of an hour, displayed on the monitor,as are daily totals.

Advanced Automatic Rain Gauges

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For a rain gauge that continuously monitors precipitation and records it in very small increments of time, down to a minute, much more expensive equipment is available which is often part of a complete weather-recording station. The advantage is that rainfall amounts in small time intervals are measured and thus the rate of rainfall over a specific time interval, usually 10 minutes, is calculated.This is a very useful extra piece of information differentiating between torrential rainfall from a short, sharp downpour to an extended major storm. To enable the equipment to continuously store such large quantities of information a data logger is included, which is an electronic record of all data, but has limited capacity. From this source the data can be extracted over a time period extending up to a month, dependent on the sampling period. These types of unit usually have a much larger funnel and container that record the precipitation in amounts of 0.2mm and the data is wirelessly transmitted to the data logger. There is therefore no need to be on site, on a daily basis, to measure the precipitation.
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A specific amount of water is allowed to fall into a pair of scoops, which are linked by a metal axis that pivots at its centre. This type of mechanism is called a ‘tipping bucket’. As one scoop falls with the predetermined weight / quantity of water, an electronic pulse is sent to the receiving station that is equivalent to the 0.2mm. This is twice the quantity as from the copper gauge described above and is thus less accurate.

It is not uncommon for this unit to under record when torrential rainfall overwhelms the mechanism, however briefly. The black plastic material used for the large funnel on some units will absorb warmth from the sun in hot weather, as a result some moisture from any short, passing shower will lose some of its content from evaporation.
This type of instrument can function independently for a considerable length of time but users must bear in mind that the amount of data that can be stored, via the data logger is limited. As an example, if the interval time is set to a fifteen-minute period it will store information for only half the time period as if the interval period was set at half an hour. As described in chapter 4, these units are often run from an attached solar panel that utilises an included lithium battery as a back-up source of power during darkness or weak solar energy in winter months.
Finally, birds are known to sit on the rim and place their droppings into the funnel that then restricts the rain flow and could ultimately block it completely, as I have experienced on several occasions and is particularly relevant when the unit is sited in the garden of a domestic residence. I love to see the birds, especially in winter, but my local Robins and Blackbirds are the major culprits!

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