THE EFFECT OF NOCTURNAL RAIN ON TEMPERATURES IN A FORESTby Tim Marshall
MOST MICROCLIMATE STUDIES ARE DONE UNDER PERFECT CONDITIONS WHERE THERE ARE NO CLOUDS OR WIND TO UPSET THE TEMPERATURE STRUCTURE. IN FACT CLIMATOLOGISTS PREFER SUCH CONDITIONS. SO IN RESULT FEW STUDIES HAVE BEEN DONE ON THE EFFECT OF RAIN IN A FOREST. WHAT REALLY HAPPENS TO THE SOIL AND AIR TEMPERATURES UNDER A HEAVY RAIN CAN RESULT IN SHARP TEMPERATURE FLUCTUATIONS AS MUCH AS TEN DEGREES AT NIGHT EVEN WITH STRATUS CLOUDS. SO TO BE DIFFERENT, TEMPERATURES WERE RECORDED UNDER A DELUGE OF RAIN UNDER THE WORST POSSIBLE CONDITIONS. THE DATA WAS PUT INTO MATRIX FORM AND GRAPHED FOR SIMPLIER ANALYSIS.
An eight-hour study of the distribution of temperature was done on the evening of March 25th, 1977. The area under study is located in the Sinnissippi Forest approximately 3 miles south of Oregon, Illinois. This area consists of a variety of coniferous trees in all ages. The site chosen was on top of a small hill divided by 40-year-old pines and small evergreens. The pine forest was on a level plain with only pine needles for ground cover. The evergreen forest on the other hand was covered with a layer of dead grass and slopes northward at an angle of about l0 degrees. Each station in the north section consisted of a 4-cm soil, a l0 cm soil, a ground and a hip level thermometer. Each station in the south section consisted of a 4cm soil, a l0 cm soil, a ground, 2 foot and 6 foot thermometers. A gravel road separates the evergreens from the pines.
The evergreens lie to the north and the pines lie to the south of the road. Therefore, the road was a good dividing line. We set up our camp on this northern edge of the forest and noticed that the ground there was still frozen with patches of snow in the deep grass even after a week of Indian summer. Observations began at midnight with %he downpour of rain starting at 12:08 a.m.. The rain continued throughout the night and only paused once at 2:05 a.m. for a half-hour.
A total of seven stations were set up and divided into the north-south sections just mentioned (see page 7). The north section consisted of 4 stations and was spaced in an "L" shaped position. They were placed 25 feet apart down the evergreen slope to a black dirt path. Also 3 soil thermometers were set up on the north-facing slope just bordering the plot. The slope was around l0 degrees and consisted of cut grass. The south section consisted of 3 stations placed also 25 feet apart in the pine forest. Also two soil thermometers were placed on the northern edge of the woods: one at the gravel road and the other on a north-facing slope composed of grass. All the soil thermometers were equally spaced at 14 feet apart.
RESULTS OF STUDY
In the north section, the grass meadow became cooler as you approached the top of the hill (see page 8). R. Geiger also found this to be true in the study of the distribution of temperature around unconsolidated cones. When the heavy rain occurred at 1 a.m., the soil temperatures seem to indicate that the rain was around 48 degrees. Soil A was at this temperature and remained there through most of the night but soil C was much cooler than A and it showed a gradual rise in temperature. The percolation of rain had thus effected the temperature of the soil. Geiger showed (p. 27) how the effect of rain on the soil depends primarily on the temperature of the rain.
In general, all stations showed a decrease in above ground temperatures at 1 a.m. with the onset of the heavy rain. Then when the rain let up the temperatures rose slightly. Why? Geiger states that (p. 28) since the disturbance of water has passed the temperatures will readjust themselves. Also note the time lag effect from 4 cm to 10 cm. The 10-cm soil reacts about an hour after the initial rainfall, and then the temperatures continued to fall at most stations until the dawn hours.
In the south section, the grass meadow showed an incredible rise in temperature at 8 a.m. (see page 9). This rise cannot be explained naturally so I assume that the thermometer was misread by l0 degrees. The soil temperatures in the forest were on the average l0 degrees Colder than in the evergreen plot. When the rain fell at 1 a.m., station A on the borders edge warmed up in soil temperatures but the other stations were not as significantly effected. This is because the canopy of the forest has delayed the rain in falling to the ground and also the dry pine needles absorb the water too on the ground. It was noticed that the raindrops were enormously larger in the forest than outside the forest as was pointed out in Geiger. Since at station B the air temperatures dropped to 50 degrees and at station A, the ground temperatures rose to 50 degrees once again we can get an estimate at the temperature of the rain. In many cases the graphs here are similar but differ only in the amount to which they were effected by the rain.
The effect of rain on the forest environment showed a drastic change in temperatures. Even the sharp rise of temperatures at sunrise did not occur as rapidly. Ail stations above the ground dropped in temperature and never fully regained again. This was because the rain was colder than the air temperature. However the soil temperatures showed an increase in temperature and then tapered off right after the rainfall. The soil was somewhat cooler than the rain in some places and this allowed temperatures even at 10 cm to rise as much as 8 degrees in a very short period of time.