Sunday, 16 February 2014

DAY OF EARTH SCIENCES IN AFRICA





A CLIMATE CHANGE MONITORING ACTIVITY TO BE HOSTED BY THE SOCIETY OF AFRICAN EARTH SCIENTISTS 0N 19-21ST MARCH 2014 AS PART OF DAY OF EARTH SCIENCES IN AFRICA 

The Society of African Earth Scientists (SAES) will on the 19th-21st March 2014 be participating in “Day of Earth Sciences in Africa “as part of the Association of African Women in the Geosciences-led, “Day of Earth Sciences in Africa and the Middle East”. The Society, as a partner in the event, will be hosting a climate change monitoring activity involving the annual measurement of the size distribution  of local raindrops using the standard flour pellet method.  All participants are invited to post their results on the SAES facebook page or to email them to saescientists@hotmail.co.uk. It is hoped to involve schools in Africa and the diaspora.

Activity Summary
   To measure the size distributions of local raindrops, using the flour pellet method.

Outcome
   The SAES hosted activity will

  •  Demonstrate a valuable earth science research method that requires little resources and is affordable and simple to perform.
  •  Potentially result in the collection of useful research data of use in monitoring of climate change.
  • Serve to illustrate the role of earth science in Africa
Background
   Raindrop sizes appear to increase with increasing rain intensity. But studies in tropical climates where rainfall is heavier have shown that this is only true for low intensities less than 100 mm/hr. Beyond this intensity range, we find that raindrops are smaller in size as rain intensity increases. Although no direct references are available, it has been posited within the scientific community that climate change affects the pattern of raindrop size distributions. We may, for instance, in the future see a predominance of raindrops falling into the larger size class, with fewer drops occupying the smaller size classes. Or we might see something unexpected which contradicts this. Our experiments would positively contribute to the current situation where there are too few studies to show that raindrop sizes or size distributions give an indication of climate change – although this may indeed be true.
   Raindrop sizes vary globally from (approx) 1-2 mm median size in temperate climates, to 3mm and above in the tropics.  Salako (2003) gives the following data for raindrop sizes determined using the flour pellet test, in Nigeria.

Table 1  Raindrop Sizes Determined by the Flour Pellet Test in Nigeria (Salako, 2003)
Agroecological Zone
Maximum Drop Size (mm)
Median Drop Size (mm)
Source
South Eastern Nigeria savanna
3.4
1.1-2.9
Obi and Salako (1995)
South Central Nigeria,  humid forest
5.1
2.3
Salako et al. (1995)
South Western Nigeria, humid forest
4.5
3.0
Aina (1980)

   Typically, tropical rainfall will achieve intensities as high as 150 mm per hour. The annual rainfall of temperate climates is typically 600mm per year (in UK for instance) whereas that of tropical climates reaches levels of 1000-1500mm, as in West Africa.
   Measuring the size of raindrops is most simply performed by the flour pellet method[i] , which is performed by catching raindrops in pans of flour and weighing the pellets that they form, after they have been both air dried and dried in an oven.
Detailed Description of the Activity

Objective:
   The Society of African Earth Scientists’ Earth Science Day activity invites us to measure the raindrop size distributions in our local area using the flour pellet method.

Apparatus:
1)   A quantity of plain baking  flour
2)   Particle size analysis sieves: 710 µm, 1.00 mm, 1.40 mm, 1.70 mm, 2.00 mm,  2.36 mm, 2.80 mm 
3)   Sampling pans (e.g. aluminium baking tray 32cmx22cm x3.5cm*)
*Although dimensions are specified, participants should not feel inhibited from using alternatives at their disposal. The main objective is to collect good samples of raindrops that are as approximately spherical and unspoiled as possible. In this case 3cm depth to prevent samples spoiling by splashing is crucial to maintain the quality of samples whilst sufficient tray  width will enhance the size of the sample


Procedure:
   In summary, we aim to capture samples of raindrops from a typical local storm in the period February – March, around the Annual Day of Earth Sciences in Africa. To catch the raindrop samples we fill a rectangular baking tray filled with plain baking flour to a depth of 3 cm. The raindrops are then sampled by taking two samples over a ten minute period within a typical storm. Figure 1 shows sampled raindrops in a pan filled with baking flour.


Figure 1   Raindrop flour pellet samples collected in a pan filled with 3cm depth of baking flour

   The pan or tray of collected drop samples is then left to air dry for 2 hours before being further dried for a further 2 hours in the oven at about 105 degrees centigrade.  In strict research procedure, a drying time of 12 hours in an oven at 105 degrees C may be required. But recognising limitations in time and resources for this exercise (power cuts could not guarantee 12 hours of  oven drying in some instances) we can be flexible, but still obtain good flour pellet samples. If no oven is available, then air drying for 5 to 12 hours (depending on the local climate) is probably sufficient to obtain hard dried pellet samples as shown in figure 2.
   Once the flour pellet samples are dried, they are  sorted into size classes by sieving through varied size meshes: 710 µm (or  0.71 mm),  1 mm, 1.4 mm, 1.7 mm, 2.0 mm, 2.36 mm, 2.8 mm. The pellets falling into each size class are weighed and then counted.
   Figure 2 shows the dried pellet samples of a London storm collected by the author. Photography gives us an opportunity to include those participants who are unable to obtain the necessary sieving equipment, but who can still participate by collecting raindrop samples as described above, drying them and sieving them with a standard tea strainer, and sending in photographs of the flour pellets displayed adjacent to a clearly graduated ruler graduated in millimetres (mm).  Although laborious, we can use photographs to estimate the size of the pellets and sort them into their respective size classes.



Figure 2   Flour pellets after air and oven drying, ready for size analysis

   Rigour requires that we calibrate the flour that we are using by capturing a sample of water drop of a known size. To simplify matters we require only that participants record the source of their baking flour stating the source or the brand name of the flour. The flour can be calibrated at a later stage using information on the source of the flour.
   The following is an example of the results table of flour pellet samples collected by the author in a test sampling simulated rain.

Table 2   Flour pellet test results table
A
B
C
D
E
SIZE CLASS
NO. OF DROPS
WT OF ALL PELLETS  (g)
WT OF MEAN PELLET (mg)
W
RAINDROP DIAMETER (mm)
Dr
2.8 mm
16
0.307
19.175
3.21
2.36 mm
14
0.133
9.514
2.63
2.0 mm
37
0.205
5.546
2.20
1.7 mm
71
0.256
3.611
1.90
1.4 mm
128
0.266
2.08
1.58
1.0 mm
396
0.447
1.129
1.29
710 µm
1050
0.395
0.376
0.89

   From the number of raindrops and weight of pellets in each class (columns B and C)  we are able to determine the mean weight of a pellet (column D) by dividing the weight of pellets in each class by the number of raindrops in that class. The diameter of raindrops in the class can then be determined by the following formula

Dr   =  [ (6/π) xW ]1/3                                ..........................   (1)
where W is the mean pellet weight  and Dr is the raindrop diameter.

Results:
   Please record the date of collection of flour pellet samples. This must accompany any results submitted.  Under the guidance of their tutor, it is sufficient for younger students to record in table form (as in table 2, columns A to D), results of the counting of the pellets and weighing of the pellets in each size class   to complete the exercise for DAY OF EARTH SCIENCES.  Older students (12-13 years old and upwards),   can use the data from the table with equation (1) to determine the raindrop diameters corresponding to their results and hence complete the results table  (as in table 2, columns A to E) to include raindrop diameters.  Please email your results to: saescientists@hotmail.co.uk  anytime during 19-21st March 2014.



Society of African Earth Scientists, February 2014


REFERENCES
  1. Bentley, W.A., 1904, Studies of raindrops and raindrop phenomena, Monthly Weather Review, 32: 450-456. 
  2. Salako, F.K., 2003, Susceptibility of coarse textured soils to soil erosion by water in the tropics, University of Agriculture, Abeokuta, Nigeria.





[i] first developed by Bentley (1904),

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