butterflies and clouds and wind ---

[Michael Gochfeld]

Several years ago I adapted a procedure used by industrial hygienists to quantify the average exposure of workers to chemicals, for the purpose of measuring "sunshine", recognizing that butterfly censusing on partly cloudy days might yield different results from days with full sun or no sun. On partly cloudy days, one gets periods of full sun alternating with periods of partial sun or no sun, with changes occurring frequently (more than once a minutes) or slowly (every 10-20 minut) depending on the speed of cloud movement across the sun. 

Industrial hygienists use what is called a time weighted average to quantify chemical exposure.  At intervals of at least every 15 min. I score the sunshine from 0 to 100% based on the strength of my shadow. .  For example a sunny day with a slight haze would result in a score of 90%.  Or sometimes the shadow is only barely visible and gets a 10% score.  I have pretty good reproducibility at scoring 100%, 90%, 75%, 50%, 25%, 10% and 0%, and less reproducibility at some of the intervening numbers. 

If you score the sunshine at regular intervals, then a simple average will suffice.  But, when the sunshine changes more rapidly, I actually try to note each change, and compute the time weighted average as the average of the number of minutes at each percentile.  Thus a 30 min observation period  with 5 min @ 100%, 5 min @ 0%, and 20 min at 75% would result in a TWA of 67%. 

However, the average sunshine is not the only (nor necessarily the best metric).  It might turn out that the percent of time with > 50% sunshine (or "shadow" as I call it), is a better prediction of butterfly activity.  

Also, it might turn out that such detail is just a pain, and that it's adequate to simply record whether it's sunny or not for all or most of part of the afternoon. 

It will be at least another season before I work up the data to answer that question.  

As far as wind is concerned, I estimate it in MPH from aBeaufort scale (which I learned about many years ago).  I think there is a threshold below which wind has little affect on Butterfly activity, and above which it suppresses activity. 

M. Gochfeld