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Precipitation Measurement - Ppt rate

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All the forms of precipitation are measure on the basis of vertical depth of water that would accumulate on a level surface of precipitation remained where it fell. In metric system precipitation is measure in millimeters and tenths. Any open receptacle with vertical side can be used as a gauges for measuring rainfall. These refined receptacles with vertical side can be used as a gauges for measuring rainfall.

Rain gauges for measurement of precipitation are of two types

  1. Recording rain gauges
  2. Non-Recording rain gauges

The main difference between these rain gauges is that with the help of recording rain gauges we get the rain recorded automatically with respect to time, so intensity of rain fall is also known whereas an observer has to take readings from non recording rain gauge for rain and he has to record the time also, for calculation of intensity of rain fall.

1. Non-recording Rain gauges:

Non-recording rain gauges are commonly used. They do not record the data and collect only rain and this collected rain is then measured in a graduated cylinder.

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The standard gauge of U.S. Weather Bureau has a collector of 200 mm diameter and 600 mm height. Rain passes from a collector into a cylindrical measuring tube inside the overflow can. The measuring tube has a cross sectional area 1/10th of the collector, so that 2.5 mm rain fall will fill the tube to 25 mm depth. A measuring stick is marked in such a way that 1/10th of a cm depth can be measured. In this way net rainfall can be measured to the nearest 1 mm. The collector and tube are removed when snow is expected. The snow collected in the outer container or overflow can is melted, poured into the measuring tube and then measured. This type of rain gauge is one of the most commonly used rain gauges.

Depth of rain = volume of rain collected in cm3/area of aperture of gauges in cm3

This type of gauges measures precipitation for only a specified period. Any open receptacle with vertical sides may be used for precipitation measurement but because of varying wind and splashing effect. The measurements are not compatible unless the receptacles of same size, shape and exposed in a similar manner. The US national weather service gauges is type of non recording gauges.

It consists of:

  • Collector (receiving 8" 20.3 cm diameter)
  • Overflow can
  • Cylindrical measuring tube of area of 1/19th of collector
  • Measuring scale

Measuring tube is placed in overflow can and with measuring sticks; the depth of rainfall can be measured. As area of M-Tube is 1/10th of collector, therefore 1" depth of rain in M-Tube corresponding to 0.1" of rainfall. In case of snow the M-Tube and the collectors are removed and the snow is collected, melted and poured in M-Tube and measured.

2. Recording gauges:

Are those which automatically record rainfall without any bottle reading. The worker is not required to record the reading but instead mechanical arrangements are there by which total rainfall is recorded automatically on graph paper. A graph of total rainfall VS time which is known as mass curve of rainfall is plotted by the gauges.

Its three types commonly used are:

  1. Tipping bucket gauges
  2. Weighing type gauges
  3. Float recording gauges

Tipping Bucket1. Tipping bucket gauges:

In TBG the collector is funneled into two compartment buckets. When one compartment of bucket is filled with rain water it becomes over balanced and tips such that the other compartment takes its place beneath the funnel. As the bucket is tipped it automatically activates an electronic circuit. This type of gauges is not suitable for measuring snow (without heating the collector).

This type of gauge is equipped with a remote recorder located inside the office which is away from the actual site. The gauge has two compartments pivoted in such a way that one compartment receives rain at one time. A certain amount of rain (usually 0.25 mm fills one compartment and over balances it so that it tips, emptying into a reservoir and bringing the second compartment of the bucket into place beneath the funnel of receiver. As the bucket is tipped by each 0.25 mm of rain it actuates an electrical circuit, causing a pen to mark on a revolving drum. This type of gauge is not suitable for measuring snow without heating the collector. Plotting is similar to that of other recording rain gauges.

2. Weighing type gauges:

It consists of a storage bin, which is weighed to record the mass. It weighs rain or snow which falls into a bucket, set on a platform with a spring or lever balance. The increasing weight of the bucket and its contents are recorded on a chart. The record shows accumulation of precipitation.

3. Float recording gauges:

The rise of float with increasing catch of rainfall is recorded. Some gauges must be emptied manually while others are emptied automatically using self starting siphons. In most gauges oil or mercury is the float and is placed in the receiver, but in some cases the receiver rests on a bath of oil or mercury and the float measures the rise of oil or mercury displaced by the increasing weight of the receiver as the rainfall catch freezes. Float may get damaged by rainfall catch freezer.

This type of rain gauge also has a receiver and a float chamber along with some recording mechanism or arrangement. In this type the rain is led into a float chamber containing a light, hollow float. The vertical movement of the float as the level of water rises is recorded on a chart with the help of a pen connected to float. The chart is wrapped around a rotating clock driven drum. To provide a continuous record for 24 hours the float chamber has either to be very large, or some automatic means are provided for emptying the float chamber quickly when it becomes full, the pen then returning to the bottom of the chart. This is usually done with some sort of siphoning arrangement. This arrangement activates when the gauge records a certain fixed amount of rain (mostly 10 mm of rainfall.). Snow can not be measured by this type of rain guage.

Storage guages are used in remote areas where frequent servicing is not possible. Weighing type storage gauges operate for 1 or 2 months without any servicing required.

Weighing type storage gauges are designed to operate for entire season without attention. WTSG located in heavy snowfall areas should have collectors to prevent wet snow from clinging to the inside of walls and clogging the orifice. The orifice should be above the maximum snow depth expected. Gauges are initially dry and require moisture for funnel and inside surfaces.

During rainfall of 5" to 6"/hr (12.5 to 250 mm/hr) the bucket of tipping bucket gauges tips every 6 to 7 seconds and takes about 0.3 seconds to complete the tipping procedure, during which some water is still pouring into already filled compartments. The recorded ratio is 5% too low.

  • Error caused by wind in gauges reading
  • When gauges are installed on the ground, it is inclined against the wind and thus catches less precipitation. Towards the wind it will catch more precipitation.
  • Obstruction due to trees, buildings and un-even topography.

4. Measurement of Precipitation by Radar

This is a modern technique for measurement of rainfall rate. It can also detect local movement of areas of precipitation. The electromagnetic energy released and received back by radar is a measure of rainfall intensity. The measurement is appreciably affected by trees and buildings. However extent of rainfall can be estimated with reasonable accuracy. Use of radar is useful where number of rain gauges installed in an area is not sufficient.

Rain Gauge Network

  • The number of rain gauges and their distribution affect the nature of collected precipitation data. The larger the number of rain guages the more representative will be the data collected. But on the other hand we have to observe other factors also, like economy of the project, accessibility of certain areas and topography of the area. So, one has to look for some optimum solution. In this regard the World Meteorological Organization (WMO) has made following recommendations for minimum number of rain gauges in a catchment:
  • In comparatively flat regions of temperate, Mediterranean and Tropical Zones, the ideal is at least one station for 230 – 345 sq. miles. However one station for 345 – 1155 sq. miles is also acceptable
  • In mountainous regions of Temperate, Mediterranean and Tropical Zones, the ideal is at least one station for 35 – 95 sq. miles. However one station for 95 – 385 sq. miles is also acceptable.
  • In arid and polar zones, one station for 575 – 3860 sq. miles is acceptable.

Snowfall Measurement:

Snowfall is often measured with regular rain gauges. Snowfall is measured by the depth of snow using snow survey. Such survey is particularly useful in mountains.

Installation of Gauges

While installing rain gauges following points should be kept in mind.

  1. Flatter ground
  2. Avoid steep hill side
  3. Avoid sloping down towards prevailing wind
  4. Site should not be too closed to building or forested area.
  5. All obstruction should be avoided.
  6. Interpretation of precipitation
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