- hold spectacles horizontally at arm’s length, front of lenses pointing upwards
- remember where the door is (this bit is to a large extent dependent on your prescription strength)
- open door and quickly thrust arm out as far as you can reach – note no upward sweeping movements of arm
- have independent timekeeper count to five seconds – recommend the one thousand and one, one thousand and two etc method
- outstretched arm must be free of all interference – ensure no collateral precipitation occurs from guttering, pub sign above door etc. artificial drips play havoc with the measurements and lessen the accuracy of results. if necessary take one or more steps out of the door taking care not to be run over by traffic (public safety bit in litigious times)
- after exactly five seconds immediately withdraw arm and retreat to seat for statistical analysis (a sip may be taken from pint at this point)
- count number of drops on both lenses and compare with calibration chart compiled over many wet visits to pub
To help first time rainfall measurers we have put together an example sweepstake that can make rainfall measurement fun. Also there is a Q&A section at the end of this post.
The Victoria pub rainfall measurement sweepstake entries
- Rob 12 drops
- Sheila 20
- Tel 11
- Tref 25
- Chrissy 15
- Tony 5
- Albert 10
Result 19 drops – winner is Sheila – congrats. Also thanks to Albert for the very precise and measured timekeeping. Albert says “it feels absolutely tremendous“.
Q&A
Is there a minimum drop size?
This is a topic that has been the subject of heated academic debate. In theory there should be no minimum drop size provided that drop represents an Actual Precipitation Event (APE – these technical scientific terms may sometimes confuse but often there is no way around this).
At the end of the day it comes down to the quality of the instrument being used to count the drops, ie your eyesight. The better the tools the smaller the drops that can be counted. Be careful to obtain agreement of all stakeholders here or it can lead to disputes – especially from those that are almost blind or simply blind drunk.
Does the colour of the lens matter?
No although you might find it harder to count the drops on a darker lens. Probably safer to use clear lenses though if you are using coloured for consistent results you should use the same colour each time you perform the measurement.
Can I use Rainex coated lenses?
No Rainex coating smears the drops evenly across the lens and makes counting impossible.
My lenses have some scratches – will that affect the result?
Unfortuately scratches on a lens can materially affect that lens’ Drop Countability (DR – another technical term) because of the effect they can have on surface tension. It is best to avoid using such lenses although if you have no choice then a note should be made against these results indicating that a scratched lens was used. Analysts will then have the option to apply a wider statistical error rate to these particular results.
If you have scratched lenses you should consider visiting your optician to have them replaced.
Does the age of my lenses matter?
There is no evidence to suggest that the age of the lense has any effect on the accuracy of the measurements provided the lenses are not scratched or coated in Rainex.
What is the best method for counting the drops?
Experience shows that the best way to count the drops is to hold the lenses up to the natural light provided by a window. Make sure that you count the drops on each lens in turn – it is a lot easier to do it this way than to go back and forth from one lense to another and provides more accurate and consistent results.
There will of course be circumstances where natural light is not available, especially during very heavy rainfall and at night. On these occasions you might find it easier to use a light bulb, prefereably one of those “natural light” ones. There will be times later in the evening where accurate counting is to all practical intents and purposes impossible. On these occasions you should consider abandoning the measurements. The occasional missing data set does not affect the overall reliability of the system though if the Lost Data Statistic (LDS) rises above 7.8% you should consider finding alternative methods of rainfall measurement because you are clearly drinking too much for this one to be effective.
Happy counting.
I question the choice of time-keeping method. The 4-word “One-thousand-and-one” technique is regarded as outmoded as the high syllable count produces too much variability of result. Recommend re-running the test cycle using the “Elephant” method (one elehpant, two elephants etc…).
[…] Editor’s footnote: for authoritative post on rainfall measurement techniques see here. […]