TEMPERATURE VARIATIONS IN A CLIFF ENVIRONMENTby Tim Marshall (April 1977)
THIS STUDY IS DEVOTED TOWARDS LEARNING THE BEHAVIOR OF TEMPERATURE AND WINDS SURROUNDING A CLIFF ENVIRONMENT. THIS STUDY IS UNIQUE IN THAT IT HAS NEVER BEEN DONE BEFORE. THE AREA WAS DIVIDED INT0 FOUR SMALLER AREAS AND A SECTION ASSIGNED TO EACH PERSON BECAUSE OF THE TIME FACTOR INVOLVED. THESE AREAS INCLUDE THE TOP OF CASTLE ROCK SLOPING SOUTH- WARD WITH LITTLE VEGATATION, A FLAT BRUSHY PLAIN WITH LOOSE LEAF COVER, A BARE GRAYISH SANDSTONE CLIFF WITH A SLOPE OF 72 DEGREES, AND A GRASSY FORESTED AREA SLOPING FROM THE BASE OF THE CLIFF TO THE ROCK RIVER.
EACH ENVIRONMENT YIELDED CERTAIN TEMPERATURES THAT WERE INDICTATIVE TO THEIR MICROCLIMATES. A STUDY OF MORNING INVERSIONS, SHADE AREAS ON THE CLIFF, AND EVEN THE HEAT GENERATED BY OUR CAMPFIRE SHOWED INTERESTING RESULTS. A GRAPHICAL ANALYSIS OF THE MATRIX DATA SHOWED HOW THE GENERAL TRENDS VARIES SURROUNDING THE CLIFF.
A twelve-hour study of the distribution of temperature was done on the evening of April 22nd, 1977. The area is located approximately five miles south of Oregon, Illinois on Route 2. The place is known by the name of Castle Rock and it sits on the west bank of the Rock River (see fig 1.). What makes Castle Rock much different than any other hill in the area is that it is fully exposed with no trees or ground cover? The gleam of the weathered, grayish sandstone appearance gives it a Castle shape looking from the river. This particular area sparked my interest and it was decided to base our microclimate study there. The critical time period was selected between 3 a.m. and 3 p.m. since the main area of study was on the east side of the cliff. Sunrise temperatures would be of utmost importance here and so ropes were flung over the cliff for rappelling purposes.
We arrived about 6 p.m. on a Friday afternoon and took sample observations at 1:30 a.m. Thirty two stations were set up and the amount of time required took nearly 6 hours. Observations were planned every hour with the exception of the nocturnal hours on the cliff where observations were taken every two hours. This is because rappelling at night takes longer than in the daytime. Also the intervention of altostratus reduced the temperature variation to isothermal conditions.
The area divided into four regions consisted of the northern area, the southern area, the cliff area, and the river area. A map was drawn incorporating the last two areas for easier identification (see Fig. B). The north area consisted of seven stations spaced on the south slope of Castle Rock where noticeable steps in height occur. Each station has a ground thermometer facing south and a ten-foot thermometer. A rope was hung from the flagpole atop Castle Rock down to a tree in the valley (see Fig. 1). A piece of elastic was tied to the rope with the thermometer hanging from the other end. Then a piece of string was attached to the thermometer so as to pull it down for each observation (see Fig. 2 and 3). The angle from where these pictures are taken from -to the top of Castle Rock is 15 degrees. Stations 1 through 5 are all exposed and can be seen on the pictures. Stations 6 and 7 are on top of Castle Rock which opens towards the north. Here the vegetation is completely lacking and the color of the rock is darker because of the lack of sun in various places. Station 6 is located on a south-facing wall completely shielded from the strength of the north wind and open to direct radiation. The wall rises approximately 12 feet and has a slope of nearly 90 degrees. Station 7 is located in a corridor between two sandstone walls. As the wind blows from the north it funnels through the corridor at high speeds effecting the temperature~. A thermometer was placed on the north wall facing south exposed to the daytime sun while another thermometer was placed on the south wall permanently shielded from direct radiation. Wind measurements and psychrometer readings were taken at the two extreme stations, station 1 and station 7. Here it was anticipated that the valley and mountain relationships would vastly differ.
The south area consisted of 4 stations and gave a representative view of temperature with a vegetation-covered surface. . The area is dotted with small bushes just budding and the ground cover consists of grass and dead leaves (see Figs. 4 and 5)· Station 1 is located on the north slope of the middle plateau region and for the most part does not receive the direct radiation of the sun. Here dead bushes prevail over a thin green layer of moss. Three thermometers were placed over this terrain at 2, 5,and 10-foot levels. Figure 1 shows this area in the distance. Station 2 is located on the west side of a tree and only receives the direct radiation in the late afternoon. Three thermometers were placed on the ground, at 2 feet, and at 6 feet. An abrupt change in soil material occurs here with a transition from a layer of sand approximately 2 feet in depth to a sandstone outcropping just to the west of the tree. To the east, small bushes slope down to the forest below at an angle of around 11 degrees. This tree is the tallest in the area and it stands alone among small bushes. The leaf growth has not started yet but buds are present. Station 3 is located at the 2-foot level in the middle of the bushy plain on a branch-facing southwest. Here the bushes are packed together and only grass is allowed to grow below. The soil is also covered with a layer of dead leaves from last fall. 0nly small buds are present and the effect of these bushes on the surrounding microclimate must be greatly enhanced when the leaves are fully exposed. Station 4 is located on the southern edge of the bushy plain next to an outcrop of sandstone. This area is the top section of the cliff area and is composed of 4 different types of ground cover in a one-foot square area. Primarily green grass, dead grass, leaf mulch and loose white sand are prominent. Thermometers were placed at the one foot level and on the ground surface which was a minimum thermometer facing eastward.
The cliff area includes a 51-foot sandstone cliff with an assorted array of heavy mosses and ferns scattered about, to around 25 feet (see Figs. 6 and 7). The light and dark areas in the picture indicate the layering of sandstone interrupted by a large crack which extends vertically at about an angle of 45 degrees up the entire length of the cliff where it broadens. At the top of the cliff a small rope holds 11 thermometers spaced 5 feet apart. At the intersection of the crevasse and the thermometer, at 20 feet, a small ledge of sandstone supports an entire colony of ferns. With the effect of this plant environment this thermometer should behave somewhat differently than the rest, Underneath the ferns lie four thermometers which are protected by the forest adjacent to the cliff. The leaves in the forest are partially exposed as indicated by the pictures. The 15-foot thermometer is located under the fern growth in a small underhang while the rest of the thermometers are open and exposed. The thermometers which hang between 50 feet and 25 feet lie upon barren rock with no mosses or ferns present. This is because it is much sunnier at this level and is only effected by the morning sun. Since the cliff faces east at an angle of 72 degrees the maximum intensity of direct radiation occurs much earlier in the day as compared to a relatively flat surface so at l0 a.m. the sun here is perpendicular to the cliff. The sunset also occurs much earlier than normal at about 2 p.m. The river area is dominated by at least two environments:
The forest area which is a scattered arrangement of poplar trees and the floodplain area is composed of grass, dirt and gravel. The forest area consists of five thermometers spaced 4 feet, 8 feet, 16 feet, 26 feet, and 36 feet away from the cliff sloping at 21 degrees towards the river. The thermometers are facing west to the cliff and are approximately 6 inches from the surface attached to sticks (see figs. 8 and 9). They are located next to a dirt path leading down to the river and border along a transitional boundary of dense foliage. Small plant life exists on the forest floor quite uniformly but the underbrush near the cliff has not yet developed buds. The wall of foliage at the forest boundary blocks most of the wind movement in the forest when the wind is coming down the river. Also the incoming radiation is blocked by the trees leaving only some radiation to penetrate to the forest floor where certain areas are heated intensely. These effects on the forest environment caused the forest to be hot and humid during the day. Trees are spaced about 30 feet apart and enough sun penetrates to supply the necessary radiation for plant growth.
The floodplain area consists of 6 thermometers 47 feet, 76 feet, 106 feet, 136 feet, 166 feet and 176 feet away from the cliff sloping 8 degrees towards the river. All the thermometers face westward and are 6 inches above the surface. The first three thermometers are located over an area of mowed grass whereas the last three thermometers are located over a dirt and gravel river basin. This was found to be important later in the study because it was found that this difference in ground cover was significant enough to separate these areas into two different environments.
RESULTS OF STUDY
In the north section differences in slopes, direct radiation, and wind speeds were analyzed. But inversions were found at every station at sunrise though they were not very strong due to the influence of cirrus and cirrostratus clouds (see page2~). However some inversions lasted up to 6 hours long. Geiger states (p. 74)
that the strongest inversions were to be found in clear skies just before sunrise and usually break down from below about an hour later. But why would an inversion over open rock last six hours which was the case at station 4? The ground thermometers were facing a south-southwest direction and were protected by the wind while the 10-foot thermometers were exposed to the wind. What physical principles could account for such a variation till mid afternoon? It was found that as the sun arced across the sky certain other sandstone outcroppings would temporarily obscure the ground thermometer causing it to read lower and thus making the inversion really stand out.
During the morning hours, the temperature on Castle Rock would skyrocket due the intense heating of the white sandstone. At 11 o'clock it was getting quite Uncomfortable so I took off my shirt and tried to lie on the sandstone rock. It was extremely warm and uncomfortable. One could get a tan within a half an hour. But to my surprise a troop of red ants were enjoying such a climate. No other place could you find them.
Because of the warm temperatures "heat frills~ were common. They were rising up from the rock and really were noticeable when there was no wind. Geiger says (p.90) as you approach the ground you limit the source of austausch and increase the amount of heat conduction. He also says (p.72) that this is the result of the small amount of mixing that takes place giving rise to a larger temperature gradient while at higher levels there is more mixing and thus a lower temperature gradient.
There was an increase in ground temperatures with slope from stations 2 to 5. This could be the result of the direction in which the thermometer is facing. Station 2 was facing due south while station 3 was facing about 170 degrees and station 4 and 5 were facing about 150 degrees. Geiger says (p.422) that the warmest area in temperature moves around to the southeast corner in summer because of the diurnal variation in cloud cover. Also noticed was the state of isothermal conditions which occurred around 6 a.m., Geiger points out (p.83) that the minimum occurs at sunrise near the ground and occurs about an hour or two later at higher levels with the state of isothermal conditions occurring simultaneously.
At station 6 temperatures of a 12 feet south facing wall were measured (see page26). The high contrast in temperatures were the result of a few conditions, First no wind was allowed to penetrate the intense heating in the morning and secondly and more importantly there was another wall perpendicular to the first wall and it was around 8 feet high. This allowed the corner area to heat up greatly during the morning hours. Note the ideal inversion which occurred from isothermal conditions around 6 a.m.
At station 7 the contrast in temperatures on both sides of a 4-foot wall at a height of 3 feet were measured. The thermometers were only I foot apart and the temperature difference amounted sometimes too as much as 12 degrees. Though the results occurred as expected this station was set up mainly to get an idea of the contrasts in temperature to see how important shade areas are in effecting temperature. The north side of the wall was in permanent shade while the south side was fully exposed to the sun.
Wind speeds were taken at the top of Castle Rock and at the bottom in the valley separating Castle Rock and the middle plateau region. They were measured by a calibrated anemometer. Sometimes the wind was to light to measure but the string we hung the l0 foot thermometers served a double purpose. So it was possible to take wind measurements even when the wind was light. The wind at the top of the rock started picking up around 6 a.m. where the first sign of wind in the valley was not until an hour later. The wind gradually picked up and leveled off in the valley where on top of the rock the wind was quite gusty. The effect of the wind on temperatures was noticed. Looking at the relation between the wind speeds and station 7 temperatures as the wind decreases around noon the temperatures rise. Then when the wind picks up an hour later the temperatures fall in both shade and sunny areas. Geiger states (p. 122) that the temperature structure is influenced significantly by the wind.
The south section includes the area of the middle plateau region and is bordered by outcroppings of sandstone with a bushy plain located in the center of the region. Temperatures were measured in relation to their surrounding vegetation. The environments included an area of permanent shade, the west side of a tree, and the central and border region of bushes with grass and dead leaf ground cover. First the temperatures of a north facing slope (11 degrees) with lichen and moss covering was measured (see Page 27). Three thermometers were placed at 2 feet, 5 feet, and at l0 feet. A limited amount of sun in the morning restricted the rise of afternoon temperatures. All three thermometers were reading relatively the same in the afternoon. However in the early hours of the morning an inversion still occurred. It was also noticed that the lichen and moss covering had a greenish color to it. Geiger points out (p. 266) that the color of lichens plays an important role in the surrounding temperature.
Next the temperature on the west side of a tree was measured at three levels, on the ground, at 2 feet, and at 6 feet. The temperature at the 6-foot level is warmer than the temperature at the 2-foot level. Why? Once again it was found that the direction in which the thermometer faces and the amount of exposure to the sun is extremely important. Note that with the cardboard backing to these thermometers, if you were to measure the temperature of a certain environment rotated through 360° you would get different readings because of the shade effects. This problem accounted for most of the variations in temperature in this study. The 6-foot thermometer was hanging from a branch about 3 feet away from the trunk of the tree. Here it was exposed by direct radiation almost the entire day. The 2-foot thermometer was attached to the trunk of the tree and only was exposed in the late afternoon. Geiger illustrates (p. 382) a diagram which shows the amount of heat derived from direct solar radiation on a tree trunk. The helix shape diagram faces due south with the maximum amount of heat occurring there. Geiger also says (p. 383) that the temperature maximum follows the sun from SE via S to SW. The parts receiving radiation later are drier and the maximum temperature would therefore occur in the SW quadrant. This accounted for the variation in the temperatures. The ground thermometer was the highest in temperature throughout the day. Since eddy diffusion is small near the surface the temperature of the ground becomes warmer.
The temperatures at various south section locations were compared at the two-foot level (see page 28). Here it is interesting to note the changes in maximum temperature. During the time of outgoing radiation, station 1 is the warmest on the north side of the rock. The sandstone retains much of the heat from the previous day and is a better conductor than loose sand. Geiger shows (p. 145) that Granite rock heats up to great depths and retains the heat more than sand. During the time of incoming radiation, station 3 is the warmest located over the bushy plain. The ground cover over this region consists of thick grasses. Geiger states (p. 281) that grass is a heat protector and the maximum temperatures increase above the grass in relation to the height of growth.
The temperature on the edge of the bushy plateau showed the greatest diurnal fluctuation at the ground level. This temperature, however, is not the true temperature of the surface. The thermometer here had a different backing than all of the others and it was made of metal. Since metal is a much better conductor to heat than the ground it gains heat rapidly. Geiger also experimented (p. 165) and showed that aluminum foil reflected 3 times as strongly as the natural ground. In the daytime, the strong reflecting foil gives rise to a maximum temperature in the afternoon. At night the radiation from the foil allowed temperatures to cool to become the minimum.
Contrasts in wind speeds were measured from the north side of the middle plateau at station 2 which was open to the north and from the south side of the bushy plain at station 4. As the wind came unobstructed from the north it encountered the bushes and was slowed down considerably. Geiger showed (p. 273) how vegetation effects the wind profile such that there is an absence of wind movement within the vegetation. This effect plays an important role in the heat budgets of plants.
The cliff area includes all the temperatures in the vertical on a 51-foot high escarpment of sandstone rock. The effect on temperature increases were limited to the morning hours and when the cliff was cooling it did so isothermally. The forest adjacent to the cliff had a marked influence on the temperature structure (see page 29). Geiger states (p. 369) that topographic influences on temperature are much more noticeable during the because of the uneven distribution of heating that takes place. The development of shade areas on the cliff was due to the effect of the forest canopy and this produced some interesting results. The temperatures were analyzed in the vertical using tautochrones. At 6 a.m. the cliff was nearly isothermal with a small inversion occurring at 5 feet. Then the cliff underwent intense heating and a cool pocket of air was noticed at around 35 feet at 7 a.m.
This pool of air moved downward with the increasing angle of the sun so that by 9 a.m. the coldest area was at the 15 foot level. At this time the sun was almost perpendicular to the cliff and a small peak of temperatures occurred here (see page 30). As noon approaches the sun is straight overhead and the entire cliff is exposed with the exception of the 15-foot thermometer which is located under an overhang. At this time the shade areas are only prevalent in the forest region and the temperatures rise on the cliff again to a higher maximum. Then the shade areas return with the oncoming sunset. Therefore the temperature graph has two peaks.
The river area as stated before is composed of the forest area, a grass meadow, and a dirt and stone river basin. The temperature reactions from shade areas were first measured in the forest. At sunrise an isothermal state occurred with the center of the forest to heat up the most. Then between 8 and 9 a.m. a total reversal in temperatures took place and the forest became cooler. Why did this occur? Two things mainly the angle of the sun and the start of the convective wind process. Geiger says (p. 312) that the amount of eddy diffusion in a forest will depend on the wind strength which will determine whether the radiative heat will remain in the forest or be transported elsewhere. Well at 8 a.m. there was no wind movement in the forest but at 9 a.m. there was a slight breeze. This light breeze never got any stronger because the forest border was blocked with intense underbrush. However, advective influences still played an important role in the reversal of the temperatures.
The light and dark areas in the forest also produced noticeable changes in temperature. Note the sun peaks that occur at various times. This shows the movement of the mosaic pattern the Geiger talks about. Since there was little movement of wind, when a thermometer is exposed it can warm up as much as l0 degrees in some areas. As the sun began to set the temperatures in the forest became isothermal and remained this way till the next morning.
Temperatures were then measured at 6 inches above a grass meadow. This area lies between the forest and the river basin (see page 31). Here the presence of the river had an influence on the temperatures. Note that as you approach the river the air becomes cooler and as much as l0 degrees on the river bank. Geiger states (p. 246) that a river and the land adjacent to it have a mutual effect on one another. As the land is heated the air over it rises and is replaced by the cool air over the river. So a river breeze was quite common in the daytime over the floodplain. Geiger noted (p. 246) that the greater the distance inland the larger the austausch coefficient and the convective processes can operate only after a certain time after the cool air mass moves in. This then can account for the variation of decreasing temperature towards the shore. The temperatures of the floodplain where the dirt and loose gravel are found are enormously restricted. Geiger states (p. 157) that a black surface would yield the highest temperatures but in our case the influence of the river was responsible for a small temperature fluctuation.
GENERAL WEATHER CONDITIONS
The cloud conditions varied in the early morning but then stabilized by mid afternoon. Altostratus dominated the evening with overcast skies and then near sunrise they broke up. A thin layer of cirrus and cirrostratus took over (see Fig. 1). No dew occurred because of the overcast conditions due to a low-pressure system about 300 miles to the southeast. The relative humidity was found to decrease uniformly during the day and did not vary significantly from place to place.
The variations in temperatures and their environment showed how many effects can occur within a small area and left me with a lot to talk about. The basic principles outlined in Geiger could be directly related to this study and did explain some of the variations due to the many influences on the environments.
In the north section, temperatures were found to be the warmest in the southwest quadrant of exposed rock. Also the wind influences on temperature in shade and sunny regions showed an inverse relationship.
In the south section, it was found that the heating of a tree trunk varies in direction and the experimental analysis with metal behind a thermometer showed the greatest diurnal variation. Wind speeds slowed down over the bushy plain and caused daytime temperatures to be higher.
In the cliff section, the angle of the sun and shade areas showed the development of cold pools that moved down the cliff with time. Also this occurrence led to a double peak temperature curve.
In the forest region, the relation of shade areas and temperatures were inversely proportional and the advective wind influences were responsible for a daytime cooling effect.
In the river basin area, the river influenced temperatures from normal curves and showed a decrease in temperature as the river is approached.
These variations led to a better understanding of the different microclimates surrounding a cliff environment.