A Photograhpic Record of Leaf Freezing Events in the Field Produced Atomatically With Inexpensive Cameras and a Datalogger

L.S. Baker and C.D. Upper, Plant Pathology, and ARS, USDA, University of Wisconsin - Madison.

Two Minolta Disk-7 cameras were positioned 40 cm. above a row of shap beans, aimed downwards with the same group of leaves in view of both cameras.

A 0.076 mm. thermocouple (ie. electronic leaf temperature sensor) was attached to the underside of a leaflet at the top of the camopy and in view of both cameras.

Air temperature, dew-point depression and leaf wetness were measured just above the canopy near this leaf.
These sensors were monitored by a Campbell 21X data logger which also controlled the camera shutters.
The data logger was programmed to monitor these sensors each half hour and to begin a sequence of pictures when either the leaf or the air temperature fell below 0 C.
Both cameras took the first picture at the same time. Thereafter one camera took a picture every 15 minutes and the other every half hour.
As long as the temperature of either the air or the leaflet remained below 0 C, temperatures were recorded every 10 seconds.

The first killing frost of the 1987 growing season, in southern Wisconsin, occurred during the night of October 2nd. The afternoon was cold, cloudy, and dry with strong to very strong winds. The skies cleared just after dark, and the winds decreased throughout the night.

At 2100 ( 9:00 PM) the leaf temperature went below zero degrees Celsius (32 F.) and the photographic sequence was automatically initiated.

The pictures below represent the period between 2200 and 0130. Within this period all the leaves in view froze. The progression of leaf freezing can be seen as a darkening of the leaf tissue. Can you tell, from the pictures, when the freezing event occured?

(Click on any picture to enlarge frame)

2200 2230
2300 2330

2400 0030
0100 0130