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Smart videos for curious minds of all ages. Primary Menu. Search for: Search. Raindrops: What do they really look like? Watch more with these video collections:.

Save This. A full circle rainbow as seen from a construction crane Rion Nakaya July 26, July 26, Hydrophobic metal made with laser-etched microscopic patterns Rion Nakaya January 23, October 12, A detailed description of design, operation, and data reduction and processing can be found elsewhere [ 11 ].

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In order to measure PM 2. Details on this PM 2. The number concentrations of size-selective particulate matters i. The number size distribution of raindrops collected at two different rainfall intensities was estimated by the volume of melted raindrops and that of calculated single raindrop from its average size.

Water Cycle

Figure 3 shows the raindrop number size distribution at the beginning 0. Raindrop number tended to drastically decrease as the drop size goes up at both hourly rain rates i. Needless to say, it showed higher number concentration when the higher rainfall intensity. Raindrop number size distribution at the beginning 0.


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Prior to the interpretation of the actual measurement data about particle scavenging properties of size-resolved rain drops, the collection efficiency of ambient particles as a function of raindrop size was theoretically calculated. Slinn and Hales [ 13 ] proposed three particle collection efficiencies E , that is, E by Brownian diffusion Edif. Subsequently, Strauss [ 14 ] suggested a more advanced equation for particle collection efficiency Einteg. In this study, the collection efficiency of ambient particles for three kinds of raindrop sizes i.

Details for calculation like variable settings and computation processes were already described in our earlier articles [ 6 , 15 ]. Figure 4 shows the below-cloud scavenging coefficients of ambient particles by falling raindrops as a function of aerosol radius and collector rain drop size. There are several features that appear in the particle capturing efficiency curves. As the first outstanding feature, the smaller raindrop can scavenge particles more efficiently.

Another peculiarity for particle scavenging efficiency curves is that the efficiency is rapidly degraded in the central part of each curve. Although, the margin is varied from 0.


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Theoretically calculated collection efficiency of ambient particles as a function of raindrop size. Figure 5 shows time series variation of relative humidity and four-size resolved 0. As previously mentioned, it was a relatively weak rainfall with the rainfall intensity varied from 0. Particles of all sizes exhibit lower concentrations when the rain started. Although this was the expected result, an unusual thing was that particle number concentration was temporarily increased immediately after the rain.

Decreasing relative humidity increases scavenging in the Greenfield gap since the evaporating raindrops are cooler at the surface, and this sets up a thermal gradient that induces motion of the aerosols towards the cooler raindrop surface [ 17 ]. In the condition of initial rainfall, falling raindrops can be exposed to the dry air of near the surface.

Nevertheless, the reason for a temporarily increasing of particle concentration might be that new particles were created in the early stage of precipitation because small size rain droplets cloud be easily evaporated in the dried low altitude air. A continuous decrease of the particle number concentration over most of the range of particle diameter during a whole precipitation was not seen. There was a modest increase in particle number concentration from to see Figure 5. Such increases might be caused by a decline of rainfall intensity and the influx of pollutants to our measurement site.

Scavenging rate, R sca. Calculated R sca. The results show a good match to that of model calculation shown in Figure 4. Figure 6 shows the time series variation of PM 2. Here, let us pay attention to the R sca. R sca. However, that of PM 2. Time series variation of PM2.

Faith and Raindrops

Despite sulfate particles being water soluble, unexpectedly they have a low R sca. Therefore, Greenfield gap is the reason for the low Rsca. Meanwhile, there was a very high rain washing efficiency for PM 2. This result should be welcomed by people, particularly living in East Asia, who suffer from PM 2. Figure 7 shows the elemental concentration of individual raindrops classified in three steps i.

The data were the elemental concentration in the raindrops at the beginning 0. Especially, there was a marked decrease in the range of between 0. Although little is known at present, it is expected that smaller raindrops should have higher elemental concentration because they have lower falling velocities and consequently they can effectively remove pollutants during longer lifetimes than larger ones. On the other hand, in some of the elements e. Among many elements determined from individual raindrops, sulfur that showed the highest concentration was probably derived from gaseous SO 2 and particulate sulfur taken up by falling raindrops.

As another remarkable thing, unexpectedly, relatively high levels of lead were detected. Their concentrations were 0. In dealing with the health hazards of lead like neurologic and behavioral disorders, it is something that we ought to be talking about.

Single-drop fragmentation determines size distribution of raindrops | Nature Physics

In general, major sources of ambient lead are piston-engine aircraft operating on leaded aviation fuel, metals processing, waste incinerators, and lead-acid battery manufacturers. Although, because of its serious impacts on public health, leaded gasoline was permanently banned in Japan long time ago, the metal contaminations of roadside soils were included in ambient particles until now [ 19 , 20 ].

When melt and evaporation of frozen raindrops were completed, it was possible to maintain residual materials on the Ag thin film. These visualized elemental maps for several elements including oxygen enable us not only to presume the chemical mixing state of raindrop residual particles, but also to estimate their source profiles.

To obtain the source profile information for aerosol components, factor analysis, which is one of the multivariate statistical techniques, was carried out. It was possible to construct the matrix of set of 13 components by the result of SEM—EDX analysis for residual particles in a total of individual raindrops. The factor loadings Varimax with Kaiser normalization are shown in Table 1.

Why do millions of raindrops only make one rainbow?

Although typical soil component like Al and Si were excluded because of their missing data, the first factor was associated with soil components Ca and other minors. The third factor grouped F and Fe were the components originated from biomass burning or volcanoes [ 21 ] and iron industry. The fourth factor dominantly made up of typical marine components.

The cumulative variance of these three factors was This result indicates that the rainfall plays a valuable role in scavenging natural as well as artificial particles from the dirty atmosphere. Result of factor analysis for the elements detected from individual residues in raindrops with diameters between 1. Ambient particles are ultimately removed from the atmosphere by the natural processes generally referred to as wet precipitation and dry deposition. The former is the most important natural removal mechanism of ambient air pollutants including PM 2. The clear landscapes after the rain is a good proof of this wet precipitation.

In this study, the scavenging properties of ambient particles were investigated by collection of raindrops as a function of their size and clarifying their chemical nature. Particle scavenging rates based on both the actual measurement and the theoretically calculated values indicated that raindrops, especially small raindrops, played a great role to remove ambient particles including both naturally and artificially formed ones.

Therefore, we must be thankful for the role of the rain, at the same time, we should avoid getting wet to protect our health from heavy metals. Further study on a comparison of the real measured data with the calculated result for the particle scavenging by raindrops is being planned in the near future.