How is rainfall measured?

How Is Rainfall Measured?
Rain gauge is an meteorological instrument for determing the depth of precipitation (usually in mm) that occurs over a unit area (usually one metre squared) and thus measuring rainfall amount. One millimetre of measured precipitation is the equivalent of one litre of rainfall per metre squared.

Usually a tapering funnel of copper or polyester of standard dimension allows the rain-water to collect in an enclosed bottle or cylinder for subsequent measurement.

The gauge is set in open ground with the funnel rim up to 30 cm above the ground surface.

Some gauges are calibrated to allow the amount of rainfall to be read directly; with others it must be calculated from the depth of water in the container and the dimensions of the funnel.

How Is Rainfall Measured?
Standard meteorological gauges have a funnelled aperture of 150-170 cm and are designed as a simple passiv collector. The amount of precipitation is determined by use of a graduated measuring glass.

The second type of rain-gauge is the autographic gauge which can be either of the tilting-siphon type or the tipping-bucket type. The recording chart on an autographic rain-gauge is mounted on a drum which is driven by clockwork and typically rotates round a vertical axis once per day.

For a tilting-siphon rain gauge, the rainwater in a collector displaces a float so that a marking pen attached to the float makes a continuous trace on the paper. The two buckets in a tipping-bucket rain gauge rest on a pivot so that when one bucket has received 0.2 (or 0.

5 mm) of rain it tips by gravity, empties the rainwater and allows the other bucket to start collection. During the tip, an electrical switch is closed and triggers a nearby autographic recorder to register each 'tilt', thus givi
ng a fairly continuous record of precipitation and, in a more sophisticated form, even rainfall intensity.

Rain gauges must be sited in as representative a location as possible, but the choice of location is difficult, since many precipitation events are highly localized.

Related features: Rain forecast Rain recordings

Rain gauge

Standard NOAA rain gauge

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 an area in a predefined period of time.

History

The first known rainfall records were kept by the Ancient Greeks, about 500 B.C.

People living in India began to record rainfall in 400 B.C.[1] The readings were correlated against expected growth. In the Arthashastra, used for example in Magadha, precise standards were set as to grain production.

Each of the state storehouses were equipped with a rain gauge to classify land for taxation purposes.

[2] In 1247, the Song Chinese mathematician and inventor Qin Jiushao invented Tianchi basin rain and snow gauges to reference rain, snowfall measurements, as well as other forms of meteorological data.[3][4]

In 1441, the Cheugugi was invented during the reign of Sejong the Great of the Joseon Dynasty of Korea as the first standardized rain gauge.[5][6][7] In 1662, Christopher Wren created the first tipping-bucket rain gauge in Britain in collaboration with Robert Hooke.[5] Hooke also designed a manual gauge with a funnel that made measurements throughout 1695.

It was Richard Towneley who was the first to make systematic rainfall measurements over a period of 15 years from 1677 to 1694, publishing his records in the Philosophical Transactions of the Royal Society.

Towneley called for more measurements elsewhere in the country to compare the rainfall in different regions,[8] although only William Derham appears to have taken up Towneley's challenge.

They jointly published the rainfall measurements for Towneley Park and Upminster in Essex for the years 1697 to 1704.[9]

The naturalist Gilbert White took measurements to determine the mean rainfall from 1779 to 1786, although it was his brother-in-law, Thomas Barker, who made regular and meticulous measurements for 59 years, recording temperature, wind, barometric pressure, rainfall and clouds. His meteorological records are a valuable resource for knowledge of the 18th century British climate. He was able to demonstrate that the average rainfall varied greatly from year to year with little discernible pattern.[10]

National coverage and modern gauges

Symons in 1900

The meteorologist George James Symons published the first annual volume of British Rainfall in 1860. This pioneering work contained rainfall records from 168 land stations in England and Wales. He was elected to the council of the British meteorological society in 1863 and made it his life's work to investigate rainfall within the British Isles. He set up a voluntary network of observers, who collected data which were returned to him for analysis. So successful was he in this endeavour that by 1866 he was able to show results that gave a fair representation of the distribution of rainfall, and the number of recorders gradually increased until the last volume of British Rainfall which he lived to edit, for 1899, contained figures from 3,528 stations — 2,894 in England and Wales, 446 in Scotland, and 188 in Ireland. He also collected old rainfall records going back over a hundred years. In 1870 he produced an account of rainfall in the British Isles starting in 1725.

Due to the ever-increasing numbers of observers, standardisation of the gauges became necessary. Symons began experimenting on new gauges in his own garden. He tried different models with variations in size, shape, and height.

In 1863 he began collaboration with Colonel Michael Foster Ward from Calne, Wiltshire, who undertook more extensive investigations. By including Ward and various others around Britain, the investigations continued until 1890.

The experiments were remarkable for their planning, execution, and drawing of conclusions. The results of these experiments led to the progressive adoption of the well-known standard gauge, still used by the UK Meteorological Office today, namely, one made of “…

 copper, with a five-inch funnel having its brass rim one foot above the ground …”[11]

Most modern rain gauges generally measure the precipitation in millimetres in height collected on each square meter during a certain period, equivalent to litres per square metre. Previously rain was recorded as inches or points, where one point is equal to 0.254 mm or 0.01 of an inch.[12]

Rain gauge amounts are read either manually or by automatic weather station (AWS). The frequency of readings will depend on the requirements of the collection agency. Some countries will supplement the paid weather observer with a network of volunteers to obtain precipitation data (and other types of weather) for sparsely populated areas.

In most cases the precipitation is not retained, but some stations do submit rainfall and snowfall for testing, which is done to obtain levels of pollutants.

Rain gauges have their limitations. Attempting to collect rain data in a tropical cyclone can be nearly impossible and unreliable (even if the equipment survives) due to wind extremes.

Also, rain gauges only indicate rainfall in a localized area. For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .

01 inches or .25 mm may be recorded as a “trace”.

Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel and ice or snow may collect in the gauge, blocking subsequent rain. To alleviate this, a gauge may be equipped with an automatic electric heater to keep its moisture-collecting surfaces and sensor slightly above freezing.

Rain gauges should be placed in an open area where there are no buildings, trees, or other obstacles to block the rain. This is also to prevent the water collected on the roofs of buildings or the leaves of trees from dripping into the rain gauge after a rain, resulting in inaccurate readings.

Types

A self-recording rain gauge (interior)

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

Standard rain gauge

The standard United States National Weather Service rain gauge, developed at the start of the 20th century, consists of an 8-inch diameter (203 mm) funnel emptying into a graduated cylinder, 1.17 inches (29.7 mm) in diameter, which fits inside a larger container that is 8 inches in diameter and 20 inches (508 mm) tall.

If the rainwater overflows the graduated inner cylinder, the larger outer container will catch it. When measurements are taken, the height of the water in the small graduated cylinder is measured, and the excess overflow in the large container is carefully poured into another graduated cylinder and measured to give the total rainfall.

A cone meter is sometimes used to prevent leakage that can result in alteration of the data. In locations using the metric system, the cylinder is usually marked in mm and will measure up to 250 millimetres (9.8 in) of rainfall. Each horizontal line on the cylinder is 0.5 millimetres (0.02 in).

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In areas still using Imperial units, each horizontal line represents 0.01 inch.

Pluviometer of intensities

Pluviometer of intensities (1921)

The pluviometer of intensities (or Jardi's pluviometer) is a tool that measures the average intensity of rainfall in a certain interval of time. It was initially designed to record the rainfall regime in Catalonia, but eventually spread throughout the world.[13]

It employs the principle of feedback … the incoming water pushes the buoy upwards, making the lower “adjusting conic needle” to let pass the same amount of water that enters into the container, this way … the needle records on the drum the amount of water flowing through it at every moment—in mm of rainfall per square meter.

It consists of a rotating drum that rotates at constant speed, this drum drags a graduated sheet of cardboard, which has the time at the abscissa while the y-axis indicates the height of rainfall in mm of rain. This height is recorded with a pen that moves vertically, driven by a buoy, marking on the paper the rainfall over time. Each cardboard sheet is usually used for one day.

While the rain falls, the water collected by the funnel falls into the container and raises the buoy  that makes the pen arm raising in the vertical axis marking the cardboard accordingly.

If the rainfall does not vary, the water level in the container remains constant, and while the drum rotates, the pen's mark it is more or less a horizontal line, proportional to the amount of water that has fallen.

When the pen reaches the top edge of the recording paper, it means that the buoy is “up high in the tank” leaving the tip of the conical needle in a way that uncovers the regulating hole, i.e., the maximum flow that the apparatus is able to record.

If the rain suddenly decreases, making the container (as it empties) to quickly lower the buoy, that movement corresponds to a steep slope line that can reach the bottom of the recorded cardboard, if it stops raining.

The rain gauge of intensities allowed precipitation to be recorded over many years, particularly in Barcelona (95 years), apart from many other places around the world, such as Hong Kong.[13][14]

Weighing precipitation gauge

A weighing-type precipitation gauge consists of a storage bin, 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.

[6] 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.

The weighing-type recording gauge may also contain a device to measure the number of chemicals contained in the location's atmosphere.

This is extremely helpful for scientists studying the effects of greenhouse gases released into the atmosphere and their effects on the levels of the acid rain.

Some Automated Surface Observing System (ASOS) units use an automated weighing gauge called the AWPAG (All Weather Precipitation Accumulation Gauge).

Tipping bucket rain gauge

How we measure rainfall

The entrance to the gauge through the funnel is narrow to avoid debris clogging the mechanism and undesirable evaporation in hot weather.

However, the gauge rapidly becomes blocked in snow and any readings at the time, and during thawing events when melted snow gradually trickles into the gauge, should be treated with caution. Where an observer is present to make a daily precipitation reading, the water equivalent of freshly fallen snow is reported.

For many years the Met Office has used a tipping-bucket rain-gauge for the automatic measurement of rainfall rate.

The collecting funnel has a sampling area of 750 cm2, the rim is set 450 mm above the surrounding ground level and a mechanism records an event each time a rainfall increment of 0.2 mm has been detected.

Storage rain-gauge

Since the earliest years of weather records, the de facto standard for the measurement of daily rainfall has been the 0900 UTC reading made by an observer from a 5 inch storage rain-gauge. The gauge has a sharp brass or steel rim of diameter 5 inches (127 mm), sited 30 cm above ground level with a funnel that collects rain in a narrow necked bottle placed in a removable can.

To make the rainfall measurement, the observer empties the collected rain into a graduated glass rain measure. Versions of the 5 inch gauge with greater storage capacity are used at sites where readings may be taken infrequently.

Automating rain gauges

As automated instruments were introduced across the synoptic network in the 1980s and 1990s the 5 inch gauge was still deployed alongside the tipping bucket gauge to continue a long consistent record of measurements for climate purposes.

In recent years this practice has proved impractical and many automatic sites now only report rainfall amount from a tipping bucket gauge. Storage gauges are still used widely at non automated climate stations and rainfall-only stations. Where an observer is not available to provide daily rainfall, readings may be made at weekly or monthly intervals.

Weather: Measuring the Rain

Cite

Rain gauges are thought to be the most ancient weather instruments, and they're believed to have been used in India more than 2,000 years ago. A rain gauge is really just a cylinder that catches rain. If an inch collects in the cylinder, it means an inch of rain has fallen. It's that simple.

Most standard rain gauges have a wide funnel leading into the cylinder and are calibrated so that one-tenth of an inch of rain measures one inch when it collects inside. The funnel is 10 times the cross-sectional area of the tube. Rainfall as low as .01 inches can be measured with this instrument. Anything under .01 inches is considered a trace.

This standard rain gauge is shown in the following figure.

Rain gauge—rainfall measurements.

A rain gauge is an instrument that measures the amount of rainfall at a given time interval.

In the more modern era, a common rain gauge is called the tipping bucket type. A bucket doesn't really tip—a pair of small receiving funnels alternate in the collection of the rain.

When one fills up with water, it tips and spills out, and the other comes into place to do the collecting. These little funnels tip each time rainfall amounts to .01 inches.

The tip triggers a signal that is transmitted and recorded.

Of course, these rain gauges have a problem when the temperature drops below freezing, so the standard versions are heated for the occasion.

What about snowfall? When snow falls on these heated rain gauges, it melts, and a water equivalent is determined. The recorded precipitation is always expressed in terms of rainfall or melted snow.

The snow depth doesn't count—unless, of course, you have to shovel it! Sometimes a foot of snow amounts to just a half-inch of water, other times it amounts to three inches of water.

It really depends on the water equivalent of the snow, which varies widely.

On the average, 10 inches of snow is equivalent to one inch of rain, but that's only an average. If a rain gauge measures one inch of water during a snowstorm, an observer can't automatically assume that 10 inches of snow has fallen. The snow depth can only be determined the old-fashioned way—by measuring it.

That depth is determined by taking an average of three or more representative spots. A ruler is stuck into the snow, and its depth is recorded. Because of blowing and drifting, the determination of three or more representative locations is not always easy. You would think that there would be a better way, but there really isn't.

Most recently, Doppler radar has been used to estimate rainfall. We'll take a look at this newest technology in the next section.

Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

  • Weather: Types of Precipitation

News from Mathnasium of Pflugerville

Original Article: http://www.quickanddirtytips.com/education/math/how-is-rainfall-measured?page=1

What does it mean when it rains an inch? How is rainfall measured? How can you measure it yourself? And how much rain falls on your roof in a big storm? Those are exactly the questions we'll be talking about today..

See also:  How to multiply numbers with exponents

How Is Rainfall Measured?

As you probably know, rainfall amounts in the United States are typically measured in inches. Actually, although we usually just say “inches,” we really mean “inches in the storm” or “inches in the last 24 hours” or “inches in some time period.

” Why does that matter? Well, obviously 1 inch of rain in a 15 minute period is a lot more water than 1 inch of rain in the last month. So we really don't know how much it's rained if we don't know the time period we're talking about.

So what does 1 inch, 2 inches, or whatever number of inches of rain in some time period mean? Well, if all the rain that falls stays right where it lands—meaning it doesn't run off and accumulate in streams and rivers and eventually in lakes and oceans, and it isn't absorbed into the ground—then 1 inch of rain in an area is enough to evenly cover the ground in that area with a layer of water 1 inch deep. Of course, water typically does run into streams and is absorbed into the ground, so 1 inch of rain rarely means an inch of standing water. 

But whether or not we actually see 1 inch of water on the ground for each inch of rain, we can use this definition to construct a device that measures how much it has rained—a so-called rain gauge.

How Do Rain Gauges Work?

But, you say, what size should the can be? In other words, does its diameter matter? Nope, any can will do.

How to Measure Rain

The capability to measure rainfall is important to many industries, so it’s no surprise that rain gauges were one of the first weather-related instruments our ancestors invented. They’re believed to have been used in India 2,000 years ago.

[1] Their measurements help farmers make decisions about planting, harvesting, and irrigating crops; they also enable engineers to design effective storm drains, bridges, and other structures.

While most professional devices for this purpose use electronic systems nowadays, anybody can assemble their own gauge to measure rainfall at home.

  1. 1

    Find a clear, cylindrical container. This cylinder can be either glass or plastic, and should be at least 12 inches tall. The shape is important: if the top is wider than the bottom (or narrower) it will require much more calculation and measurement.

    • It doesn’t actually matter how wide container is, so long as it’s the same diameter all the way through. As the volume of the container gets bigger—from say, a coke can to a mop bucket—so does the area which collects the rain. Because of this, one inch of rainfall will be recorded consistently between cylinders of varying sizes.[2]
  2. 2

    Make a container. If you don’t have a cylinder on hand, you can make an equally effective gauge with an empty 2-liter soda bottle and little work. Using scissors or a knife, cut the top 4 inches of the bottle off. Don't worry about the uneven bottom of the bottle. That will be taken care of in the next step.

  3. 3

    Weigh your gauge down with pebbles. Because rain will often be accompanied by wind, you’ll want to steady your gauge so that it can stay upright through a storm.

    Fill the bottom with pebbles or marbles, but don’t go higher than an inch or so. Once that’s been done, you’ll want to fill your container with water, to provide a level starting point for your scale.

    Your weights will be taking up volume, and so we don’t want them included in the measurable area.[3]

    • Rocks, stones, marbles: any small, relatively heavy objects will do, as long as it won’t absorb any of the water.
    • If you’ve created your own gauge with a soda bottle, make sure the entire bottom (the four separated points of the base) is filled with water and stones, to provide a flat starting point for your scale.
    • As an alternative to placing pebbles in your gauge, you could place it within a sturdy container, such as a heavy bucket or flowerpot.
  4. 4

    Inscribe a scale upon your container. This can be done with a waterproof sharpie. Hold a ruler or measuring tape up against your bottle, and line its zero up with the current water level of your gauge. Your scale’s zero should also be at this water level.

    • If you’ve opted to go pebble-free and are going to place your rain gauge inside a flower pot, you won’t have any water in your gauge yet. In this case, zero will be at the bottom of your container.
  5. 5

    Place it under the open sky, on level ground. You need level ground to lessen the chance of your gauge tipping over. Ensure your gauge has no obstructions above it, e..g trees or eaves, as these will disrupt your measurements.[4]

  1. 1

    Check your gauge every day.

    To determine how much rain has fallen in the previous 24 hours, you’ll need to regularly check it every 24 hours! Read the gauge by looking at the waterline straight on, at eye level.

    The water line's surface will be curved; this is the meniscus, formed as the water comes in contact with the container and creates surface tension.[5] Your reading needs to be from the bottom of the curve.

    • It’s important to check it every day even if there hasn’t been any rain. You can lose water from evaporation, and water mysteriously showing up in your rain gauge without the accompanying rain clouds might mean your rain gauge needs a new spot (sprinklers are a common culprit).
  2. 2

    Mark the amount of rainfall on a graph or chart. For example, you could make a 7 x 7 chart, marking the days of the week along the x-axis and 1 to 7 inches (2.5 to 17.8 cm) along the y-axis.

    After filling in a dot at each appropriate intersection of rainfall (in inches) and the day of the week, you can use a ruler to connect the dots and see the fluctuations in the rain measurement for that week.

  3. 3

    Empty the rain gauge. After each recording, you’ll want to empty the rain gauge to ensure an accurate reading. Ensure you keep the same stones in your gauge, and refill water up to the zero on your scale. If you ever add or subtract stones from your gauge, ensure the water is again filled to the zero point before setting your rain gauge back in place.

  4. 4

    Calculate the averages. Once you’ve recorded data for a month, you can analyze your data and see overall rainfall trends. Adding up the rainwater of all 7 days in a week, then dividing it by 7, will give you the average rainfall of that week. Over a longer period of time, you could do this for months (or even years, if you’re particularly dedicated).

    • The formula for finding an average is easy to apply. The average equals the sum of all the items (in this case, rainfall measurements for a day, week, or month) divided by the number of items (however many days, weeks, or months you’ve added up).[6] If you're looking for average weekly rainfall over 4 weeks, with recorded weekly rainfall totals of 20 inches, 12 inches, 6 inches, and 25 inches, we would say 20 + 12 + 6 + 25 = 63 (the sum of the items) / by 4 (the number of weeks) = 15.75 inches average weekly rainfall.

Add New Question

  • Question What are precautions needed while using rain gauges? A gauge should be unprotected from the elements in all directions, it should be kept clean inside, and it should be emptied every day at the same time.
  • Question Should I measure in mm or ml? Use mm, cm, or inches.
  • Question How does rainfall measure in centimeters?
  • Question Why is the rain gauge put in 30 meters above the ground? There could some some deviation on the ground level due to wind direction, buildings, tree, etc. Putting the rain gauge 30 meters above the ground can negate some of these factors.
  • Question Is daily rainfall measured from 4 am to 4 pm? Daily rainfall is measured over 24-hour periods beginning at local midnight (so, 12 am to 12 am).
  • Question Why do the measurements on a tapered rain gauge not match exactly with a ruler's measurements? The surface area increases as the rain gets higher in the gauge, therefore the gradation is smaller as the gauge fills.
  • Question What units can I use to measure rain? Inches, centimeters, liters, gallons, feet, etc., it just depends on what you're going to use the data for.
  • Question Our new rain gauge measures in 1/8 inches. How to I know how many tenths is in 1/8 inch? 1/10″ = 0.100″. 1/8″ = 0.125″. So for casual measurement, there is not a lot of difference. But if you want to be more specific, there is 1¼ tenths in an eighth of an inch.

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Annex II: Rainfall measurement*

* The information in this Annex has been obtained from FAO irrigation and Drainage Paper 27, Agrometeorological Field Stations, Chapter 6.

The total amount of rainfall over a given period is expressed as the depth of water which would cover a horizontal area if there is no runoff, infiltration and evaporation. This depth is generally expressed in millimetres.

Accuracy of rainfall measurement is mainly affected by wind, by the height of the gauge and exposure. Wind and exposure errors can be very large, even more than 50 percent. The catch of rainfall is a function of the height of the gauge; the more open the location the greater will be the difference in catch with height.

RAINGAUGES

Raingauges have a cylindrical form. The leakproof collector rim is placed above a funnel which leads to a receiver. The receiver should have a narrow neck into which the funnel fits to reduce evaporation loss.

The collector should have a receiving area of 200 to 500 cm2. The rim of the collector should have a sharp edge which falls away vertically inside. The collector is designed so that rain cannot splash out; the walls should therefore be sufficiently deep and the slope of the funnel sufficiently steep (more than 45°).

Raingauges are made of non-corrosive metal, fibreglass or plastic.

Since type, diameter of the collector, height and manner in which the gauge is exposed vary considerably from country to country, it is important that the type selected and method of installation should be similar to any other raingauge in the area in order to obtain comparable data. Normal height of exposure is usually 30 cm above ground level. At greater height wind affects the accuracy of measurement. Where the raingauge placement and particularly the siting are very different from local practice, a side by side comparison between the two raingauges may be needed. The graduation of the gauging device (jar or rod) must, however, always be consistent with the size of the collecting area of the raingauge. A number of raingauge types are shown below.

SITING

The site must be level and the surrounding ground should be uniform. The ground should preferably be grassed or loose earth. No object such as another instrument, building or trees should be closer than four times their height.

Very exposed sites, such as on the top of a hill, should be avoided. For very exposed sites without any natural shelter raingauge shields are sometimes used. The raingauge should be firmly mounted on a concrete base.

The rim of the raingauge must always be horizontal.

MEASUREMENTS

Measurements should be taken at the same time each morning. A graduated measuring cylinder or a graduated dipstick or rod should be used. The former is preferred. If it is raining at time of observation, measurements should be taken quickly to avoid loss of catch.

A measuring cylinder, standard for the instrument in use, should be of clear glass or plastic. The diameter of the measuring cylinder should not be more than one third of the diameter of the rim of the gauge. Graduations should be clearly engraved in 0.2 mm graduations.

The measuring procedure is to pour the rain water from the storage contained into the measure and to read the value from the graduations. If there has been considerable rainfall, this may have to be done in two or more stages. The bottom of the water meniscus should be taken as the defining line. When reading, the cylinder should be held vertically.

The empty storage vessel is then returned to the gauge and the collector replaced.

If no special graduated measure adapted to the raingauge in use is available, a measure graduated in cm3 can be used. The procedure is the same but the observed volume should be divided by the surface area of the collector of the gauge in cm2 to find the cm of rainfall. Errors using this type of measurement can be greater.

MEASURING CYLINDERS

OBSERVATIONS

Rainfall should be observed in units of 0.1 mm. Readings of less than 0.05 mm should be recorded as a “trace”. A “trace” is also recorded when there is no sign of precipitation in the gauge but it is known for certain that slight rainfall has occurred since the last raingauge reading.

It is conventional to allocate the 24 hours catch observed in the raingauge before or at 09.00 hours to the previous day. For example, the catch measured at 08.00 hours on 1 December will be shown in the record dated 30 November and be included in the November totals. The hour of observation should, however, still coincide with local practice.

MAINTENANCE

Raingauges should be checked for leakage; dust and leaves should be removed from the collector. The inside should be cleaned but should not be polished. The measuring cylinder should be clean, and should not be dented. A spare measuring cylinder should be available. Plant growth around and above the raingauge should be kept out.

SWOP Rainfall Measuring Tips

SWOP Rainfall Measurement Tips

Rainfall data received from the SWOP network is used in a variety of ways at the National Weather Service in Lincoln.  Your reports can be utilized to enhance routine short-term forecast products, and can even serve as the impetus for issuing a number of flood watch and warning products.

  In addition, rainfall reports provide excellent ground truth, allowing forecasters to assess the accuracy of Doppler radar rainfall estimates.  Given the wide range of uses for your rainfall data, accurate readings are extremely important.

  This guide will provide useful tips for properly measuing rainfall with your National Weather Service issued 4-inch rain gauge.

Tip #1: Place the gauge in an open area

The rain gauge must be placed in an open area away from obstructions such as trees, buildings, garages, etc.  It also needs to be far enough from your house to prevent wind currents flowing around/over the structure to distort your measurement.  An ideal location would be on a fence post well away from your house or any trees.

  • Tip #2: The gauge must be level
  • To obtain a proper rainfall reading, please ensure your gauge is level.
  • Tip #3: Proper gauge height

Try to place your gauge at a height of no greater than 5 feet.  This allows minimum impact from nearby structures.

Tip #4: Empty your gauge regularly

You can check your gauge periodically during a rainfall event, but once the rain is over, be sure to take one final measurement, then empty your gauge.  This will ensure an accurate reading the next time it rains.

  1. Tip #5: Take your gauge inside for the winter
  2. The plastic inner-tube of your gauge is particularly sensitive to freezing conditions, and can easily crack if left outside with water in it during subfreezing conditions.
  3. Tip #6: Reading your gauge correctly

Your NWS 4-inch rain gauge will hold exactly 1 inch in the inner cylinder.  If rainfall exceeds this amount, it will overflow into the outer container.

  To properly measure the rain, first dump out the inner cylinder, then carefully pour the contents of the outer portion of the gauge into the inner tube.  Add the amounts together for your final total.

  The entire rain gauge has a capacity of 11 inches.

Tip #7: Proper rainfall reporting  

After taking a careful rainfall measurement, please relay the data to us as soon as possible.  The preferred method is via our online reporting form, but reports are also welcome through the SWOP e-mail account at: [email protected]  Rainfall should be reported in hundredths of an inch (i.e 0.08, 0.27, 0.50, 2.61)

Within the SWOP program, we encourage observers to take rainfall readings periodically through an event…then provide a storm-total once the rain has stopped.

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