RAINDROP SIZE MEASUREMENT TECHNIQUES

 Manual Raindrop Measurement Techniques:

i) Stain Method

This method involves the use of chemically treated paper to measure the size of raindrops. For a short period of time rain drops are allowed to land on a sheet of absorbent paper covered with a water-soluble dye. A variety of absorbent papers have previously been used including filter paper, blotting paper, blueprint paper, paper towelling, photographic paper,and adding machine tape. Upon impact, the embedded dye reacts with the rain drops and this leaves permanent marks on the paper. The marks are then carefully measured and counted to provide information about the rain drops. One of the limitations of this method is that during prolonged sampling, the rain drop stains can overlap, which can make it difficult to accurately measure and count individual drops.

ii) Flour Pellet Method

A tray (0.05 m2) of flour was exposed to simulated rainfall for a period of 1 s. The flour was then dried for 24 h at ambient temperature (28–30 °C) and the pellets formed were passed through a series of sieves (4.75, 3.35, 2.36, 1.18 and 0.85 mm). The pellets were then dried for 24 h at 105 °C, weighed and measured. The size of raindrop was calculated from the size of pellet.

iii) Oil Immersion Method

The oil immersion method involves the collection of drops on a glass trough containing a fresh mixture of lightly viscose liquids, such as Vaseline and light mineral oil which prevents evaporation and

condensation. Using a camera and microscope, this technique does not require calibration or special equipment.

The low viscosity and hydrophobic nature of the oil causes rain drops to form discreet spherical shapes, allowing drop counting and measurement by microscope or via photograph.Generally any low viscosity oil can be used, and several alternative liquids have been utilised in a range of studies, including vacuum pump oil, paraffin oil and hydraulic fluid mixture, hydro carbon solvent, silicone oil, anisole mineral oil mixture, cold hexane, and grease. Courshee and Byass found that the use of two oils of different densities improved drop shape measurement. Using a microscope or a photograph, they found it easier to identify the drops trapped at the liquid interface (two liquids) rather than one.

iv) Photographic Method

The photographic method has been used extensively to measure rain drop size and velocity, and undergone many iterative improvements since its development by Mache in 1904. Initially, Laws measured drop sizes using a 9 cm * 12 cm still camera mounted behind a chopper-disc driven by a small synchronous motor. Light infiltration problems have restricted some use of the photographic method to night time sampling. Use of the Illinois camera resulted in drop count errors due to superimposition of multiple drops. Digital pixilation also limited the accuracy of several photographic techniques. In addition, photographic techniques are subject to environmental influences such as wind which may cause drop drift and measurement errors. 

Automated Raindrop Size measurement Techniques :

i) Impact Disdrometers

The kinetic energy of rain drops is critical to soil erosion and stormwater pollutant wash off studies because it is indicative of the potential of drops to displace particles normally bound to a surface, causing to soil particles to enter surface water flows. The combination of drop size distribution and drop velocity can provide an estimation of kinetic energy, however there have been several previous attempts to take measurements directly. This has been done using either acoustic or displacement methods.

ii) Acoustic Disdrometers

Acoustic disdrometers involve the generation and recording of an electric signal via a piezoelectric sensor when drops fall on a specialized diaphragm. Based on the relationship between kinetic energy and drop size calculations, this electrical signal is converted to kinetic energy via the measured acoustic energy in obtaining a uniform acoustic response over the entire diaphragm. Difficulties in the accurate measurement of smaller drop sizes also remain because of insensitive diaphragms, and splash effects.

iii) Displacement Disdrometer

Energy generated by drops falling on the top surface of a displacement disdrometer is translated via magnetic induction, and converted via electrical pulse to estimate the size of a rain drop.

iv) Optical Disdrometers

Optical technologies (optical imaging or optical scattering) are non-intrusive rain drop measurement techniques. These methods do not influence drop behaviour during measurement,and have successfully resolved drop break up, and drop splatter problems experienced by other measurement methods.

~ Optical Imaging

Recent imaging techniques developed have involved two motion cameras (2DVD) to show raindrop microstructure, including front and side drop contours, fall velocity, drop cant and horizontal velocity. General rainfall parameters such as rain intensity and drop size distributions have also been accurately measured. Two motion cameras record images of drops which have been used to accurately measure drop velocity, diameter, and shape.

~ Optical Scattering 

Optical scattering techniques involve the generation of a horizontal light beam which travels to a receiver where electrical measurements are taken. Drops that pass through the light beam cause the light to scatter. The attenuation of the light caused by each drop is converted to an electrical pulse by the receiver which is then successfully converted to accurate drop velocity measurement. Performance evaluations have suggested that optical disdrometers may be limited to measuring larger drop sizes and that the rainfall intensity measurements were inaccurate.


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