MANCLIM Manual of Climatological Observations: precipitation
In meteorology, precipitation refers to water particles, whether liquid or solid, that form when water vapour changes to liquid or solid form and is deposited on the earth's surface. The amount of precipitation, expressed in millimetres (mm), refers to the depth of water which would have accumulated if the surface of the earth were horizontal and none of the water were lost as runoff, evaporation or absorbed into the ground.
Precipitation may occur in these forms:
- Liquid precipitation;
- Freezing precipitation; and
- Frozen precipitation.
2.1.1 Liquid precipitation
Precipitation composed of very small water droplets (less than 0.5 mm in diameter) which are too small to make any noticeable ripple on the surface of still water is called "drizzle." These droplets follow the slightest air currents and almost seem to float in the air as they fall to the ground.
Precipitation composed of water droplets of various sizes (most greater than 0.5 mm in diameter) is called "rain."
2.1.2 Freezing precipitation
When drizzle droplets freeze on impact with unheated objects at or near the earth's surface they are called "freezing drizzle."
Rain that freezes on impact with unheated objects at or near the earth's surface is referred to as "freezing rain."
2.1.3 Frozen precipitation
Snow, snow grains, snow pellets and ice pellets are categorized as "frozen" precipitation.
Snow is precipitation composed mainly of hexagonal ice crystals, mostly star shaped, and usually clustered together to form snowflakes.
Snow pellets are white, opaque balls of snow. They range from 2 mm to 5 mm in diameter and usually bounce or shatter when striking a hard surface.
Snow grains are very small white and opaque grains with a snow-like structure. The grains are somewhat flat or elongated. Their diameter is generally less than 1 mm. When they strike a hard surface they do not bounce or shatter. They usually fall in small quantities.
Ice pellets are pellets of ice which form when raindrops freeze before reaching the ground. Ice pellets can also form when pellets of snow are covered by a thin layer of ice before reaching the ground. Ice pellets are 5 mm or less in diameter. They usually bounce and make a noise when striking a hard surface.
Hail is precipitation of small balls or pieces of ice (hailstones) ranging in size from 5 mm to 50 mm or more in diameter. These balls of ice can form separately or fuse into irregular lumps. Hailstones can be composed of transparent layers of ice or they can be made up of a series of transparent layers alternating with translucent layers. The layers are at least 1 mm in thickness. Cut in half, the layers of a hailstone resemble those of an onion.
The equipment normally supplied for the measurement of precipitation consists of:
- One type-B rain gauge; and
- One snow ruler.
2.2.1 Rain Gauge - type-B (large capacity)
The type B rain gauge is used to measure liquid and freezing precipitation types, including hail. This rain gauge was developed to eliminate loss of data due to overflow of rainwater during heavy storms or during long periods of exposure. This gauge consists of a collecting funnel, an outer container, a graduated inner container made of a high-strength plastic, a metal mounting/leveling bracket, and a pipe stake to support the gauge.
Other features of this gauge are:
- Its large capacity--over 250 mm of rainfall;
- The collecting funnel empties directly into the attached graduate, which holds 25 mm of rain; and
- The gauge can be leveled quickly and easily by adjusting the leveling bracket.
Figure 2-1 Rain gauge - type-B (collector, funnel and graduate)
2.2.2 Snow Ruler
The snow ruler is a one-metre-long ruler in 0.2 cm increments. It is used to measure newly fallen snow and snow accumulation on the ground.
2.3 Measurement of precipitation
Precipitation measurement falls under two categories:
2.3.1 Measurement of rain
The amount of rain is determined by using the type-B rain gauge to collect and measure the amount of rain, drizzle, freezing rain, freezing drizzle and hail. The amount of rain is always measured to at least the nearest 0.2 mm.
2.3.2 Operation - rain gauge, type-B
The following general instructions must apply for the operation of the rain gauge:
- The rain gauge should be examined at each observation to ensure that it has not been damaged and that no obstructions are shielding it.
- Ensure that there are no obstructions to the funnel of the gauge such as leaves, grass, or dirt which might prevent the water from running into the collectors inside the gauge.
- The top rim of the gauge must be kept level. Check the level of the gauge frequently during spring because it might be affected by frost heaving.
- The rain gauge graduate must be kept clean so that the water level is readily seen. Because the graduate is stored within the gauge, it is likely to collect dirt and foreign objects more readily. Cleaning can be done with mild soap and water and a bottle mop or sponge. The outer container should also be cleaned as required.
- Do not allow grass to grow longer than about five centimetres (cm) for a distance of about two metres (m) around the gauge.
During periods of cold weather, the observer should try to ensure that no rainfall measurements are missed. In areas where it is likely to rain every month of the year, the rain gauge should remain exposed at all times. However, in the event that the rain gauge is also exposed to snowfall, any accumulated snow and ice should be removed from inside and around the gauge.
In areas where it is unlikely to rain for several months during the winter, the rain gauge may be removed from the mounting stake and stored indoors. When this procedure is adopted, observers must keep a close watch on the general weather conditions, as it is their responsibility to make sure that all occurrences of rain are measured. When rain or freezing rain is likely to occur, the rain gauge should be prepared to collect rain as follows:
- Return it to its outdoor position; and
- Make sure that the collecting funnel and container are free of snow, ice and dirt.
2.3.3 Rainfall measurement - rain gauge, type-B
Rainfall up to 25 mm can be read directly from the graduate. It is not necessary to remove the graduate from the collecting funnel. To empty the graduate after measurement, simply invert the funnel and the attached graduate.
The level of water in the plastic graduate is correctly taken to be the lowest part of its curved surface, or meniscus. When this lies between two scale marks, its amount is that of the nearer mark (see Figure 2-2). In the case where the level is exactly midway between two scale marks, the amount reported is the intermediate (odd) value, e.g. 0.3 mm. Precipitation amounts up to 0.2 mm are exceedingly difficult to measure. Whenever the level of the meniscus is below the 0.2 mm scale mark -- that is, less than 0.2 mm -- the amount is called a trace, which is reported as "T" in the COOLTAP or IVR application.
For rainfall amounts greater than 25 mm, the rain water overflows from the graduate into the outer container. The procedure for measuring the overflow is as follows:
- Record the amount of water in the graduate (25 mm) without removing the funnel.
- Leaving the collecting funnel attached to the graduate, empty the graduate.
- Pour the water from the outer collector into the empty graduate to an amount less than 25 mm. Pour the water slowly to prevent spilling or overflow.
- Record this amount and drain the graduate.
- Repeat this procedure, recording each measurement, until all the water has been measured.
- Total the recorded amounts.
Extremely heavy rainfalls are very important statistically; the greater the rainfall, the greater the care that should be taken in measuring the rainfall and in preventing any loss through spilling.
Occasionally, rain can freeze in the container or in the funnel. In such a case, pour a measured amount of hot water into the gauge (e.g., using a spare graduate or measuring cup) to completely melt the ice in the funnel and container. The rainfall can then be determined by subtracting the added amount of hot water from the total measurement.
- The measurement of hot water is 4.6 mm.
- The measurement of hot water and ice totals 8.4 mm.
- Subtract the hot water measurement from the total: (8.4 mm - 4.6 mm).
- Rainfall = 3.8 mm.
If, for any reason, the outer container does overflow and rainfall is lost, this fact should be noted in the "Comments" section of the COOLTAP report. In such cases, it would be helpful if the observer would check to see if the heavy rainfall might have collected in other containers (suitably exposed), such as a can or a bucket, and measure the depth of the water in the container using an ordinary ruler, then record this depth in the "Comments" section.
Should rain occur when the rain gauge is not operational -- during an unexpected winter rainfall, for example -- estimate the rainfall and record the amount in the COOLTAP report in the "Comments" section.
Figure 2-2 Reading the graduate
2.3.4 Snowfall measurement
Where snow has fallen without drifting, snowfall is measured easily with the Standard Snow Ruler, which is one metre (m) long and marked in centimetres (cm) and fifths (i.e., to the nearest 0.2 cm). In an area where the snow has fallen undisturbed by the wind, the ruler is inserted vertically to the depth of the new snow that has fallen since the last observation. Several different probes should be made. The average of all these depths of new snow will be taken as the actual snowfall and reported to the nearest 0.2 cm. A "trace" of snow is an amount measuring less than 0.2 cm, and is recorded as "T" in the COOLTAP application. Traces must be recorded, but two or more traces add up to only a trace.
After the snowfall measurement has been recorded, the area should be swept clean to ensure that the next new snowfall can be recorded accurately. The use of a snow board can be useful as a tool to keep the snowfall measurement area consistent. This may be a sheet of plywood measuring at least one square metre and is suggested for locations where the site for measuring snow is not ideal. When a snow board is used, it may be covered with white flannel to reduce melting. White paint, while not as efficient, can serve the same purpose. As an alternative, a Weaver Snow Board 2000 can be supplied by the MSC.
The measurement of snow where drifting occurs is quite difficult and no fully satisfactory method has yet been developed for taking accurate snowfall measurements. The measurement of snowfall under drifting conditions requires a great deal of judgment and careful consideration on the part of the observer. When snow has been drifted by the wind; the depth of fresh snow in the drifts and in exposed areas should be measured, and the observer should then estimate the depth of snow that would have accumulated if the fall had been undisturbed by the wind.
There will be occasions when the snow melts as it falls to the ground or into the rain gauge. Under these conditions the amount of snowfall is obtained by measuring the water in the collector and converting it to snowfall equivalent by multiplying by ten. For example, if 2.4 mm is the amount of water in the gauge and this resulted from melting snow, its snowfall equivalent is 2.4 cm. In this case, a snowfall of 2.4 cm should be recorded with an appropriate remark indicating that the snow was melting as it fell and that the snowfall was estimated.
2.3.5 Snow on ground
This is a measure of the total accumulated depth of snow on the ground regardless of whether there has been snowfall or not. It is measured once per day using the snow ruler. At sites that perform two observations per day, "snow on ground" must be included with the morning observation. "Snow on ground" is reported every day, even if there is no accumulation present.
To obtain an accurate representation of the "snow on ground," the observer should take a number of measurements in the general area of the station and average them. "snow on ground" is reported to the nearest whole centimetre (no decimal value).
Note: This differs from newly fallen snow, which is reported to the nearest 0.2 cm.
- An average depth of 10.8 cm would be reported as 11 cm.
- An average depth of 8.2 cm would be reported as 8 cm.
- An average depth of 5.5 cm would be reported as 6 cm.
An average depth of less than 0.5 cm is considered a "trace" and reported as such. If there is no snow on the ground, then zero is reported.
There are situations in which the ground is only partially covered by snow and observers must use their best judgement. An example is in the spring, when significant melting has occurred and the ground is generally exposed, with the exception of snow and/or ice in the form of drifts. As well, accumulations can persist for extended periods in shady areas. Quite often, the depth of snow in these persistent accumulations can be several centimetres, even though the ground is essentially bare. Under these conditions, the "snow on ground" should be reported as a "trace."
2.3.6 Measurement of mixed rain and snow
When both rain and snow have occurred within the observation period, it is desirable to separate the rainfall from the snowfall so that the precipitation records will be complete. The best method is to melt the snow in the gauge, including any accumulation in the funnel portion, and measure the water collected. At the same time, with the snow ruler, measure the average depth of newly fallen snow and record this for the snowfall. From the amount of water collected in the gauge, subtract the water equivalent of the snowfall measurement (using the 1 mm to 1 cm ratio). The remainder will be the rainfall.
- The rain and melted snow collected in the gauge total 3.8 mm.
- The average new snowfall measured is 2.0 cm.
- 2.0 cm of snow is equivalent to 2.0 mm of water.
- The rainfall is 3.8 mm - 2.0 mm = 1.8 mm.
When both rain and snow have occurred within the observation period, and all or part of the snow has melted, the total precipitation is that amount measured from the container of the gauge. The relative amounts of rainfall and snowfall should be estimated to the best of the observer's ability, taking into consideration the length of time the snow fell and whether the rate of fall was heavy or light. Appropriate "comments" should be made in COOLTAP or IVR.
2.3.7 Measurement of hail
The rain gauge is not a satisfactory instrument for measuring hail directly. Usually, a great deal of hail will bounce out of the gauge, so that less than the correct amount that would fall on a horizontal surface is collected in the gauge. The most practical method of measuring hail is to take the top section of the gauge and invert it over the ground where a uniform hail cover exists. Collect the hail contained within the area of the top, melt it to obtain the water equivalent, and include it with the rainfall amount that has just been measured.
Hail usually falls when the air temperature is above freezing; therefore, the water equivalent of the hail should be determined as soon as possible after the shower of hail ends.
2.4 Reporting precipitation observations
At climatological stations that are equipped with MSC instruments for measuring precipitation only or both temperature and precipitation, the official station record is reported using COOLTAP or IVR. Refer to the COOLTAP or IVR manual for instructions on how to use these applications.
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