During the eclipse, I used various data loggers from Onset Computer Corporation to automatically record the ambient light levels, temperature, and relative humidity during the entire solar eclipse (from first contact through fourth contact).  Many thanks to Onset Computer, which graciously lent me these loggers at no cost.

Thumbnail pictures of the various environmental graphs are shown below.  To view a particular graph in full resolution, click on its thumbnail.  This same page complete with all environmental graphs at full resolution instead of thumbnails is also available:

Environmental Measurement Results page with full graphs

Light Intensity

One of the HOBO® Light Intensity loggers was pointed at the zenith and recorded the light intensity every eight seconds during the eclipse (see graphs above).  It was placed away from foot traffic on the balacony where I had set up the rest of my equipment.  The light intensity results (above) have been plotted on a logarithmic scale (log(Lumens/square foot)) because the light levels changed by several orders of magnitude during the eclipse. At first and fourth contact, the light level was about 1300 lumens/square foot with the sun at an altitude of 53.5° and 54.3° respectively.  During totality (sun at altitude of 58.4°), the light level dropped by a factor of 52,000 to only 0.025 lumens/square foot.  The graphs shows the effects of thin and thick scattered cloud in the area between first and second contact, causing local fluctuations in the light level by factors up to 4. It is interesting to note that most of the change in light level occurs in the twenty minutes before and after totality. The graph showing the light levels for ten minutes before and after totality illustrate the accelerated decrease in light level, dropping by a factor of 6300 in less than nine minutes.  In the last two minutes before second contact, the light level dropped by a factor of 1000.  After third contact, the light levels rose rapidly at similar rates.  The measurements are consistent with my qualitative observations.  I did not observe any noticeable difference in the light until about 20 minutes before totality, when the light started to take on a grey characteristic.  The grey/silver-like light became more and more noticeable as totality approached.  In the last ninety seconds before second contact, I could easily discern the rapid and accelerating decrease in light.  It felt like an almighty force was turning a down a dimmer switch on the sun high in the sky.
 
 

Light Intensity versus Time (Zenith)

Light Intensity versus Time (Zenith) - detail around totality

The other HOBO® Light Intensity logger was kept pointed directly at the sun.  Michael Koop, a fellow ATMoB group member, was generous enough to allow me to affix the logger to his clock-driven Great Polaris equatorial mount so that the logger's sensor was facing directly toward the sun/corona.  He was located on the ground with the others from the ATMoB group behind the Marina Hotel.   The light level measurements taken with this "equatorial sensor" are very similar in trend compared to those at the taken with the logger pointing at the zenith, but are about 1.5 times lower.  A possible explanation for this is that the lighting level for this sensor at ground level may have been decreased slightly by reduced visibility to the horizons by the trees and the hotel building nearby.  The directivity of the light sensor obeys a cosine relationship for angles between +45°, so the sensor high on the balcony could get more light from the illuminated sky while the ground level sensor would have much of this light unavailable.
 
 

Light Intensity versus Time (normal to Sun)

Light Intensity versus Time (normal to Sun) - detail around totality

The measurements from the StowAway® light intensity logger were not quite as useful.  While the StowAway® had more memory (i.e. it could collect samples), its sensor saturated for light levels greater than about 1000 lumens/sq ft.  The measurements for light levels lower than this were generally similar to those for the HOBO® light intensity loggers.  As time permits, I may also post the results from the StowAway® here as well.
 

Temperature

Temperature versus Time

Relative Humidity

Relative Humidity versus Time

Comments and Questions

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Table of Contents

Eric's 1999 Total Solar Eclipse Home Page
Eclipse Circumstances and Where to View?
Vacation in Budapest
Eclipse Day Eve
Eclipse Day
My Equipment Setup
Planned Environmental Measurements
First Contact and Moon's Shadow
Totality!
Third Contact and Beyond
Environmental Measurement Results
Additional Video Frames
Additional Eclipse Photos
Eclipse Links