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Article

Spatiotemporal Variation Characteristics and Influencing Factors of Karst Cave Microclimate Environments: A Case Study in Shuanghe Cave, Guizhou Province, China

by
Yong Xiong
1,2,3,
Zhongfa Zhou
1,2,3,*,
Shengjun Ding
1,2,3,
Heng Zhang
1,2,3,
Jing Huang
1,2,3,
Xiaohuan Gong
1,2,3 and
Dan Su
1,2,3
1
School of Geography and Environmental Science/School of Karst Science, Guizhou Normal University, Guiyang 550025, China
2
State Key Laboratory Incubation Base for Karst Mountain Ecology Environment of Guizhou Province, Guiyang 550025, China
3
State Engineering Technology Institute for Karst Desertification Control, Guiyang 550001, China
*
Author to whom correspondence should be addressed.
Atmosphere 2023, 14(5), 813; https://doi.org/10.3390/atmos14050813
Submission received: 7 March 2023 / Revised: 17 April 2023 / Accepted: 27 April 2023 / Published: 29 April 2023
(This article belongs to the Section Climatology)
Browse Figures
Versions Notes

Abstract

:
To systematically analyze the spatiotemporal heterogeneity, diurnal variation characteristics, and influencing factors of karst cave microclimate environments in Mahuang Cave, a cave in the Shuanghe Cave National Geological Park in Guizhou Province, China, was investigated. Monthly monitoring of meteorological and environmental parameters, such as wind speed, air pressure, humidity, and temperature indicators inside the cave and atmospheric temperature and precipitation outside the cave, was conducted from 2019 to 2021, as well as encrypted monitoring in August and December 2019. The results showed that: (1) The meteorological parameters of Mahuang Cave exhibited seasonal characteristics and cyclical interannual variation. Cave wind speed, relative humidity, and temperature were high in summer and autumn and lowest in winter, whereas cave air pressure was high in winter and low in summer. The atmospheric temperature outside the cave was the main controlling factor. (2) On a short time scale, the dewpoint and temperature of Mahuang Cave did not change significantly, and an abrupt change phenomenon occurred mostly around noon. The warm season was more sensitive than the cold season, and the closer to the entrance of the cave, the stronger the response. (3) In terms of spatial distribution, the overall microclimate factors of Mahuang Cave became increasingly stable and entered a constant state with the increasing depth of the cave passage. The related effects of cave morphology and structure, the physical environment of the cave passage, air movement, and groundwater dynamics were important factors leading to an abrupt change phenomenon in cave microclimates. Local meteorological conditions and cave geometry controlled the temporal variability and spatial heterogeneity of the cave microclimate environment.

1. Introduction

In karst regions, caves are natural underground spaces formed by the dissolution of soluble rocks, primarily carbonates. They are a natural window to the subsurface systems of the Earth’s Critical Zone [1] and an important place where the karst water and carbon cycles interact [2,3]. The cave system is a unique environmental structure that connects the closed system inside a cave with an open system outside, playing an important role in the karst process.
Cave climatology is a branch of mountain climatology that scholars define as transient processes in the cave atmosphere. Cave climate refers to the average state of the cave atmosphere; that is, “the total content of the gas composition” [4] or “the spatiotemporal distribution of the hydrothermal characteristics of the cave air” [5]. Recently, the physics of cave atmospheres was reviewed by Badino [6], who divided underground meteorology into “cave climatology”, the study of the average cave atmospheric conditions that vary slowly in time, and “cave meteorology”, the study of how the cave fluctuates around this average condition over relatively short timescales. The division and ideas have been widely adopted by many scholars [7,8,9,10,11,12] in the field of cave climatology. They involve meteorological and environmental parameters, such as cave wind, air pressure, relative humidity, and temperature, which are the subjects of cave climatology studies [13]. However, cave climate is often neglected in microclimatology. The unique microclimate environment plays an important role in the formation of the cave landscape, especially in chemical deposition [5,14]. Conducting modern environmental monitoring studies on cave microclimates is not only beneficial for exploration, subject investigation, analysis and providing further insight into the development of karst cavernology [15] but also for the development of optimal cave resource use (e.g., cave therapy) [16,17,18], ecosystem management, and conservation [19,20,21,22].
Scholars have conducted many monitoring studies on the internal and external climatic environments of natural and tourist show caves. The magnitude of and the variation in the wind direction are characteristics of cave air convection. Studies have focused on the factors of cave wind formation [23,24], the discovery of the “cave breathing” phenomenon, the mechanism of the “ventilation (chimney) effect” [25,26], and the modeling of cave air environment parameters [27]. Air pressure inside and outside the cave is generally balanced, and fluctuations on either side cause a pressure gradient that creates a corresponding cave airflow that varies seasonally and over the short term. When the cave openings are approximately the same height, the pressure on the cool wood or shaded side of a mountain is higher than the warm, sunny slope. Pressure drops slightly more in wide cave passages than in narrow ones. In through-caves, the pressure is higher on the windward side and lower on the lee side [4]. The difference in air pressure or temperature between the inside and outside of the cave accelerates air circulation. It drives ventilation patterns and transition mechanisms [28,29,30,31], controlling cave microclimate variables that affect various karst processes [32]. Relative humidity is an important factor affecting secondary calcium carbonate deposits in caves. Determining the wet luster and dehydration weathering of cave sediment landscape surfaces is critical [33], with relative humidity in most caves ranging from 80% to 100% [4,12]. Air temperature is an important indicator of the cave’s climate environment and has a significant influence on the migration and transformation of materials and energy inside the cave [34]. Cave atmospheres are not truly constant; the average local temperature on the surface exerts a first-order control on cave temperature [7]. Cave temperature throughout the day and year is not constant, with variations usually being small [35], and is equilibrated to an average temperature at the surface at the same altitude [6,7] or remains largely consistent with the annual average temperature of the local atmosphere [33,34]. However, other factors such as air flows, water flows, and percolating water can cause deviations [7]. A multi-year study at Postojna Cave, Slovenia, showed that small increases in cave temperature are not related to the number of visitors but to external meteorological conditions [36]. The cave temperature is primarily controlled by the average temperature of the fluids that flow through the aquifer, both air and water [6,7], and is mainly affected by external seasonal oscillations [8]. Time lags between exterior and cave temperatures [7,8] are often weeks to months [11]. Thus, the temperature deep in the cave will be approximately equal to the mean annual temperature of the air outside the cave. With theoretical breakthroughs and research paradigm changes in earth system science and the continuous development of high-resolution automated monitoring instruments, concepts of the Earth’s Critical Zone [37] and Karst Critical Zone [38] have gradually been introduced into cave research, making the means and ideas of cave microclimate environment research more systematic and integrated.
However, cave microclimate studies still have shortcomings, one of which is between long-term observations from fewer monitoring sites and short-term observations mainly based on one hydrological year. In addition, multi-year long-time series scale monitoring is less available. Second, cave climatic environments are usually expected to remain constant, and the mechanisms of temporal variability and spatial heterogeneity of cave microclimate environmental parameters have been less explored. Third, the cave climatic environment in open systems has exchange functions and dynamic change characteristics with the external atmospheric environment; however, quantitative analysis of the relationship between internal climatic environment parameters and external climatic factors is rare at present.
The varying conditions in the caves investigated are reflected in the different hypotheses. We chose to test possible expectations in a relatively simple open cave system. (1) Caves connect variable external atmospheric properties to the inner cave environment, which represents mean atmospheric (and perhaps groundwater) properties at depth. The cave microclimates are assumed to be predominantly stable, but they do reflect seasonal variations and average values of meteorological conditions outside the cave. (2) Since air density is mainly a function of air temperature, the latter can be used as the main indicator of airflow, with the difference in air temperature between the inside and outside of the cave driving “winter” and “summer” flow. Assuming that the effects from air movement are controlled, holistic, and multiple, the effect of flowing water on the air are limited to underground rivers and waterfalls and are, therefore, only local. (3) Atmospheric precipitation from any event can take a long time to transport through the epikarst and bedrock and then infiltrate and drip down to the cave ceiling. At short time scales, it is assumed that precipitation should have no direct effect on the cave environment, and the cave climate indicator values are expected to reflect atmospheric pressure. (4) Assuming a relatively constant atmospheric pressure, wind speed occasionally rises in narrow passages to reflect the strength of a storm. Where there is commonly a thermal contrast, temperature, and relative humidity both increase deeper in the cave, with abrupt changes or slight deviations in different cave sections, possibly limited by cave geometry (e.g., tortuosity, cross-sectional area, roughness) or regulated by some effects such as the ventilation effect.
This study collected monitoring data on the climate environment inside and outside the Shuanghe Cave from 2019 to 2021, analyzed the characteristics of the spatiotemporal variability of cave microclimate environmental elements and the factors influencing them, explored the occurrence mechanism of the abrupt change phenomenon, and revealed the response relationship between the inside and outside cave climate environment.

2. Materials and Methods

2.1. Study Area

The study area was located in the Shuanghe Cave National Geological Park, Wenquan Town, Suiyang County, Zunyi City, Guizhou Province, China (107°02′30″–107°25′00″ E, 28°08′00″–28°20′00″ N). The area is 600–1700 m above sea level, with a mid-subtropical monsoon climate. The cave system is in the Northern Guizhou rolling-box anticline wing. Owing to the intermittent uplift in the area caused by tectonic stress in different directions (NE, NW, and SN), three groups of folded fault zones have formed, resulting in a relatively independent geological body that is slightly triangular in shape, mainly for the upper-middle Cambrian series Loushanguan Group (∈2-3ls) [39]. The cave area lithology is mainly dolomite and dolomitic limestone [40]. The groundwater type is primarily carbonate-fractured water.
Mahuang Cave, a primary branch of the Shuanghe Cave System, which is a typical horizontal single-entry natural cave, was selected as the monitoring study subject (Figure 1). The cave entrance is 720 m above sea level, with a southwesterly development of the cave passage, with a length, height, and width of approximately 1100, 32, and 15 m, respectively, and a cave roof thickness of approximately 100 m. The entrance of the cave is in the shape of a “lock hole” (a height, upper and bottom width of approximately 15, 8, and 5 m, respectively). The near-cave section is labyrinth-shaped, with several collapsed rock blocks. After passing through a maze-type cave section, the branch caves become fewer, leading into a 400 m straight cave passage (#12–#14) and two branch caves on the southeast side near the cave exit. A sandy, >10 m thick, gray-brown soil and wet, muddy, yellow-brown clay layers are at the far end. These sediments were mainly deposited by early underground river transport. Many secondary calcium carbonate deposits have developed in the cave, such as stalactites, stalagmites, columns, soda straws, helictites, rimstones, and cave pearls. Two small underground rivers (#9–#12) converge and flow into the shaft. The overlying surface of the cave is covered by vegetation, mainly subtropical evergreen and deciduous broad-leaved mixed forest. The soil types are mainly yellow and limestone, and the main crops are corn and sorghum.

2.2. Research Methods

According to the development direction, cave structure, ventilation, and sediment landscape of Mahuang Cave, 15 monitoring points were set up (Figure 1). The inside and outside of the cave were monitored monthly by manual readings from January 2019 to December 2021. To ensure comparative accuracy, the monitoring time was fixed on the same day or adjacent dates in the middle and second half of each month, and the times of entrance and exit were 10:30 and 14:30, respectively, on the same day. February and March 2020 and February and October 2021 were affected by the COVID-19 outbreak prevention and control. Because the data were not monitored and sampled, lines were used to connect the samples when the data were added to the figures. For encrypted monitoring, we collected data with 1 min time intervals on 17 August 2019 at 12:00 to 22 August 2019 at 12:00 (warm season) and on 28 December 2019 at 12:00 to 2 January 2020 at 12:00 (cold season). No rainfall occurred before or after the monitoring. The temperature and relative humidity of the cave were measured using a Telaire-7001 portable infrared meter (Telaire Corporation, Goleta, CA, USA) and an external HOBO data logger (Onset Computer Corporation, Cape Cod, MA, USA). The wind speed, temperature, relative humidity, air pressure, and altitude inside and outside the cave were monitored in real-time using the American Kestrel-4500 portable weather station at resolutions of 0.1 m/s, 0.1 °C, 0.1%, 0.1 kPa, and 1 m, and accuracies of ±3%, ±1.0 °C, ±3%, ±0.15 kPa (25 °C), and ±15 m, respectively. The monitoring order was from outside to inside to avoid human influence and ensure measurement accuracy. The distance between the instrument and the researcher was maintained at >3 m during the operation. Meteorological data, such as temperature and precipitation data, were downloaded from the China Meteorological Data Network at the Tongzi County Meteorological Observatory, which is the closest to the Shuanghe Cave National Geopark.
Based on the characteristics of the monthly mean temperature and precipitation changes in the study area, we divided the seasons into spring (March–May), summer (June–August), autumn (September–November), and winter (December–February). By examining the cave and sediment morphology, water flow, and other environmental characteristics, the Mahuang Cave passage was divided into three cave sections: near-cave (#1–#4); transition zone (#5–#11); and deep zone (#12–#15) (Table 1). The climatic elements of the 15 monitoring points were averaged over four seasons (spring, summer, autumn, and winter) for the three hydrological years to investigate the spatial variation characteristics and influencing factors of the climatic environment of each cave section zone and monitoring point in Mahuang Cave. Dewpoint calculation formulas were from references [41]. Relevant data analysis was performed using Origin and SPSS software.

3. Results

3.1. Characteristics of Regional Atmospheric Environment Changes

As shown in Figure 2, the monthly average atmospheric temperature and cumulative rainfall statistics for outside the cave from 2019 to 2021 revealed that annual average temperatures were 15.48, 15.57, and 16.06 °C, respectively. Each year, the temperature began to rise significantly in March, with the highest temperatures occurring in August, and then gradually decreased, with the lowest temperatures occurring in January. Annual precipitation totals were 1379.6, 1289.9, and 1071 mm, respectively. Precipitation was primarily concentrated from May to October, accounting for 86.85% of total annual precipitation, and was relatively low from November to April.
During the monitoring period, the atmospheric pressure outside the cave was higher in winter than in summer. The temperature was high in summer and low in winter. The atmospheric dewpoint was high in spring and early summer. Because the air outside the cave was highly mobile, the measured values only represented a specific period and were primarily used as background values for comparing the climatic environment inside the cave. The weather outside the cave had the same overall climatic characteristics of rain and heat as the central subtropics, and its obvious seasonal changes had a significant influence on the corresponding changes in conditions inside the cave.

3.2. Seasonal and Interannual Variation Characteristics of the Karst Cave Microclimate Environment

The wind speed in Mahuang Cave was generally low, following a trend of becoming high in summer and low in winter (Figure 3a). The average wind speed in spring, summer, autumn, and winter was 0.23, 0.54, 0.29, and 0.19 m/s, respectively, with the highest value occurring in summer and autumn. A maximum wind speed of 3.8 m/s was recorded at #6 on September 2019, whereas in winter, the wind speed was noted to be “small, weak, and absent”; all monitoring points from November 2020 to March 2021 recorded a windless state. The wind direction-change monitoring showed that except for the six monitoring points in transition zones #5 (January–February, December 2019, January 2020), #6, #8, #9, #10, and #11 (January 2019), the wind direction in winter was outward to inward, while the wind direction at the rest of the monitoring points was mostly inward to outward.
The cave air pressure was clearly characterized as high in winter and low in summer (Figure 3b); this trend was nearly opposite to that of the cave relative humidity, cave air temperature, and outside atmospheric temperature. The average pressure recorded was in the order of winter > autumn > spring > summer. Overall, the cave pressure fluctuated slightly with seasonal changes in atmospheric motion and temperature, mostly at 920–930 Pa in summer, concentrated at 935–950 Pa in winter, and stable at approximately 935 Pa in spring and autumn, which was close to the mean value of atmospheric pressure outside the cave.
The relative humidity of the cave was high in spring, summer, and autumn and low in winter (Figure 3c). The average values were as follows: summer > spring > autumn > winter. In spring, summer, and autumn, the relative humidity was above 96%, condensation of water droplets on the cave wall was common, and a fog-like phenomenon was observed in the cave passages. In winter, the relative humidity was approximately 86%. The two deep “V” shaped valleys recorded the lowest values in December and January, with the relative humidity at #3 being extremely low in January 2021. In addition, the dry air caused the cave walls to become visible.
The cave air temperature was characterized as high in summer and autumn and low in winter and spring (Figure 3d), generally consistent with the monthly average temperature outside the cave (Figure 3e). The average temperatures in spring, summer, autumn, and winter were 13.98, 15.51, 15.51, and 10.84 °C, respectively. The annual average value was approximately 13.96 °C, close to the annual average temperature of the atmosphere outside the cave. The highest temperature occurred on August #2 with a value of 18.6 °C, and the lowest value occurred on January #2 with a value of 3.6 °C. The cave air temperature in January 2021 was slightly lower than in January 2020.

3.3. Spatial Variation Characteristics of the Karst Cave Microclimate Environment

The spatial variation in wind in Mahuang Cave generally showed a shape consistent with the elevation trend in the cave passage (Figure 4a). The near-cave section (#1–#4) had a weak four-season average wind speed of 0.07 m/s. The transition zone (#5–#11) had a four-season average wind speed of 0.33 m/s, with fluctuations of up to 2.19 m/s in summer, forming strong winds at #5 and #6 and resembling a peak barrier and gradually decreasing at #7 to #9, representing a similar buffer zone. The sudden fluctuation of wind speed in winter at #11 was slightly higher than at the adjacent monitoring points. The deep zone (#12–#15) had a low average wind speed of 0.05 m/s during all seasons, except for #12, with an average wind speed of 0.37 m/s in summer, which was a relatively high-value point.
The cave pressure at the near-cave section (small slight decrease), transition zone (fluctuating slight increase), and deep zone (tending to stabilize) had little overall variation. It was at constant pressure (Figure 4b). Among them, transition zones #5 and #6 had the lowest relative pressure values, opposite to the highest relative altitude (Table 1, Figure 4e). The overall spatial variation in air pressure in the cave varied weakly with the depth and degree of cave closure.
The relative humidity inside the cave was more constant than outside. The variation range and SD (standard deviation) were much lower (Table 2), increased with distance, and tended to be stable throughout the year (Figure 4c). The near-cave section gradually fluctuated and increased from the cave entrance inward, with an average value of 92.32% in spring, summer, and autumn and 79.71% in winter. The average values in the transition and deep zones reached 97.95% in spring, summer, and autumn, and 88.43% in winter. The rate of rising humidity was faster than that of the near-cave section and tended to be saturated and maintained high humidity throughout the year. Abrupt change fluctuation was greater in transition zones #5 and #6, with the average of spring, summer, and autumn reaching 98.55%, except for #11 in spring, which fell to 95.94%.
The cave air temperature showed a gradual decrease in the temperature difference with increasing cave length that tended to be constant (Figure 4d). In summer and autumn, the air temperature in the near-cave section, transition zone, and deep zone roughly showed an abrupt change, decreasing and then slightly increasing before entering a more stable state. In spring, the air temperature did not change significantly with cave distance. In winter, the temperature increased from 7.99 °C at the entrance of cave #1 to 13.58 °C at the end of hole #15, with an increasing linear trend of gradual change inward. The temperatures of transition zones #5 and #6 decreased in spring, summer, and autumn, while #11 showed a slightly higher temperature “rebound” in spring and autumn and a slightly lower temperature in summer.

3.4. Characteristics of Short-Term (Diurnal) Variation in the Karst Cave Microclimate Environment

During the warm season (Figure 5a), the ranges of cave air temperature variation at monitoring points #2, #4, and #9 were 14.55 to 16.35 °C, 14.6 to 15.29 °C, and 14.51 to 14.8 °C, respectively, indicating that the closer to the cave entrance, the higher the cave temperature. Their dewpoint variation ranges were 14.28 to 16.21 °C, 13.8 to 14.96 °C, and 14.43 to 14.8 °C, respectively, with little variation.
During the cold season (Figure 5b), the monitoring point indicator values of #2, #4, and #9 were significantly lower than in the warm season. The cave temperatures varied from 9.88 to 11.09 °C, 10.32 to 10.96 °C, and 10.81 to 11.35 °C, respectively, showing that the cave temperature increased with distance from the cave entrance. Their dewpoint ranged from 5.78 to 8.56 °C, 7.54 to 9.29 °C, and 8.61 to 9.98 °C, respectively, indicating that the dewpoint gradually increases closer to the cave’s deeper zone.
Overall, the cave temperature and dewpoint in Mahuang Cave varied diurnally, with insignificant periodic peaks and valleys. The spatial variation magnitudes were in the order of #2 > #4 > #9 during both warm and cool seasons. This shows a characteristic of becoming more stable with increasing cave distance. During the monitoring period, a total of three to five abrupt changes of different degrees occurred, all at noon, with a more sensitive response in the warm season than in the cold season and a stronger response in the near-cave section than in the transition zone; the closer to the cave entrance, the more obvious the fluctuation response.

4. Discussion

4.1. Influence among Elements of the Microclimate Environment of Krast Caves

The analysis of the Pearson correlation heatmap (Figure 6) for dewpoint, air temperature, relative humidity, pressure, and wind speed at 15 monitoring points in Mahuang Cave from 2019 to 2021 showed that all elements passed the significance test (p < 0.01), except for cave temperature and wind speed, which had no correlation (r = 0, p = 0.994 > 0.05).
The cave dewpoint was positively correlated with air temperature (r = 0.93) and relative humidity (r = 0.75) because the atmosphere is usually unsaturated, did not correlate and the dewpoint is usually lower than the air temperature. The dewpoint is equal to the air temperature only when the air reaches a saturated state. The higher the relative humidity at the same temperature, the higher the water vapor pressure and the higher the dewpoint.
Cave air pressure correlated negatively with cave dewpoint (r = −0.61) and air temperature (r = −0.55). This is due to the fact that the pressure is communicated rapidly as waves in relatively open systems. Since the pressure inside is mainly a function of pressure outside the cave, the pressure changes outside the cave should be reflected rapidly by the inside environment. Atmospheric pressure is generally lower during the summer when it is warmer, hence the negative correlation.
Cave relative humidity and cave air pressure had a negative correlation (r = −0.50) due to the increase in relative humidity in the cave, which increased the water vapor content in the air. Because the molecular weight of water vapor is smaller than that of air, the air density and pressure decreased, resulting in a negative correlation [40].
The weak correlation (r = ±0.20) or no correlation (r = 0) among cave wind speed and dewpoint, air temperature, relative humidity, and pressure elements may be caused by the small value of the monitored wind speed (or windless state value is 0). However, from the spatial variation at the monitoring points (Figure 4a) and the seasonal variation of “high summer and low winter” (Figure 3a), it can be seen that cave wind must be driven by pressure gradients, while in the mixing zone near the front of the cave, the temperature gradient enhanced cave wind.
In conclusion, the relationship among cave climate elements showed that the cave system is a coupled system that is both closed and relatively open, and a change in any one climate element will cause a change in another.

4.2. Influence of External Climate on the Microclimatic Environment of Karst Caves

As shown in Figure 7, a linear relationship existed between each climate element inside Mahuang Cave and the atmospheric temperature and atmospheric pressure outside the cave from 2019 to 2021.
Cave wind is air movement caused by pressure and temperature differences. The stronger the cave wind, the greater the pressure gradient, the greater the temperature gradient, and the greater the horizontal pressure gradient force. It was positively correlated with atmospheric temperature (Figure 7a1). When combined with Figure 3a,b,d, and Figure 4a, the wind speed in the mixing area near the front of the cave was significant, especially at #5–#7, which was primarily influenced by the large temperature difference between inside and outside the cave. Based on the principle that in a flowing gas, the higher the gas flow rate, the lower the pressure, atmospheric pressure was negatively correlated with cave wind speed (Figure 7a2).
Cave air pressure was negatively correlated with atmospheric temperature (Figure 7b1) (R2 = 0.55 to R2 = 0.61). Generally, the atmospheric pressure decreases as the temperature rises and also drops as elevation rises. Due to the fact that the pressure inside the cave is a function of the pressure outside the cave and that a change in the pressure outside the cave should be quickly reflected in the environment inside the cave, there was a substantial and positive correlation between cave air pressure and atmospheric pressure (R2 = 0.76 to R2 = 0.81) (Figure 7b2).
Cave relative humidity correlated favorably with atmospheric temperature (R2 = 0.09 to R2 = 0.61) (Figure 7c1). Whether variations were seasonal or over the short term, the cave temperature and dewpoint fluctuated significantly near the cave entrance. Perhaps this represents the short-term blocking of the cave entrance, where the sun’s altitude moves with the seasons, and the light intensity gradually decreases from the entrance to the inside, and the nearly constant values near the back of the cave, where the temperature and moisture are controlled by average, long-term values. The underground cave is a relatively closed system and represents the ratio of the absolute humidity in the cave air to the saturated absolute humidity at the same temperature [42]. The more water vapor in the cave air at the same temperature, the higher the relative humidity value and the higher the dewpoint value. In addition, cave water vapor pressure generally always increases from the near-cave section to the deeper zone, and the corresponding cave air temperature and relative humidity fluctuate in a generally consistent manner until reaching a constant temperature and humidity. This has been confirmed to be the same type as that of caves in the subtropical monsoonal humid zone, Baxian Cave [43], Guizhou Province, and Quanshan Cave [44], Zhejiang Province, China.
The overall linear relationship between atmospheric temperature and cave temperature was R2 = 0.76 at the cave entrance and R2 = 0.51 at the end of the cave, showing an order of near-cave section > transition zone > deep zone (Figure 7d1), which was generally consistent with the trend of cave air temperature (Figure 4d). The closer to the cave entrance, the more solar radiation heat is received. Because the activity of cave organisms is mainly controlled by temperature [45], most cave organisms are distributed at the entrance to the cave or in the low-light zone. The linear connection (R2 = 0.14 to R2 = 0.41) between atmospheric pressure and cave temperature was negatively correlated (Figure 7d2); that is, the higher the temperature, the lower the air pressure.
In conclusion, the atmospheric temperature and pressure outside the cave both affected the environment inside the cave. Since the pressure gradient between the inside and outside of the cave was small (Figure 3b) and the temperature gradient was significant (Figure 3d), it is reasonable to infer that atmospheric temperature is the primary factor controlling the changes in the microclimatic environment in Mahuang Cave.

4.3. Influence of Cave Structure and Air Movement on the Karst Cave Microclimate Environment

Cave air conditions are a result of the degree to which the effects of the advection of heat and moisture from outside the cave are modified by internal heat and moisture transfer processes. The spatiotemporal variations in the climatic and environmental parameters of Mahuang Cave and the abrupt change phenomenon were inseparable from the multiple effects produced by cave structure and air movement.
The cave wind speed (Figure 4a) gradually became windless from the entrance to the deep zone as the cave distance increased and the ventilation conditions worsened. At #5 and #6 (Figure 4e), the cave was steep, high, and narrow enough for only one person to pass through, prompting the strong wind formed by the violent disturbance of airflow. According to the principle of mass conservation and fluid continuity [13], the flow velocity is inversely proportional to the cross-sectional area of the pipe; air flowing through a rough and uneven pipe is affected by friction, which hinders the speed of airflow. When the strong airflow in the narrow cave channels from #5 to #6 was forced to flow forward to the relatively open channels of #7–#9, the airflow cross-section increased, and the “narrow tube effect” of cave wind speed slowed [46]. Therefore, points #5–#9 highlighted the “wind barrier–buffer zone” that forms. The wind velocity was slightly convex at #11 in winter when the underground river and shaft were developed. On the one hand, the wind flow was accelerated by the dragging and carrying effect of the water in the underground river, also called the “carriage effect” [47]. On the other hand, the “siphon effect” [4] was generated between the air chambers connected to the cave and the shaft according to the principle of pressure difference in fluid mechanics, which also causes wind flow. Monitoring point #12 had an average summer wind speed of 0.37 m/s because of the approximately 5 m-high seasonal fissure waterfall near the location (Table 1). Because water is abundant in summer, the waterfall water flowed from the wall cavity to produce an obvious air movement and formed a closed loop called the “waterfall effect” [4]. Without a pool of water under the waterfall, local airflow was displaced in all directions to form a rapid flow of higher wind speed (Figure 8b).
The cave relative humidity (Figure 4c) near the cave entrance was lower than the transition and deep zone. Water vapor evaporation and condensation exchange were frequent because of the weather conditions outside the cave near the entrance and were also related to the “ventilation effect”. Generally, the strength of air mobility is inversely related to high relative humidity; the greater the air mobility, the more the relative humidity is biased toward lower values, as confirmed in Jiutian Cave [48], Loufang Cave [49], and Baojinggong Cave [50], China. The average value reached 98.55% in spring, summer, and autumn at monitoring points #5 and #6, presumed to be related to the perennial water flow in the cave wall and the water pool environment at this location (Table 1). The transition and deep zones were weakly influenced by the atmosphere outside the cave and tended to saturate more rapidly than the near-cave section to maintain constantly high humidity throughout the year. In spring, humidity decreased to 95.94% at #11, possibly due to the evaporation of the underground river, heat exchange in the form of radiation, and sensible, latent heat of the water body to regulate the cave temperature; thus, the absolute humidity of the air increased, while the relative humidity decreased.
The cave temperature (Figure 4d) was higher than the annual average cave temperature by 2–3 °C in summer and autumn and lower by 4–6 °C in winter at the near-cave section, mainly because of the better connectivity with the outside atmosphere near the cave entrance (the light of the cave entrance is faintly visible at #3), which enhanced the exchange of heat with the outside of the cave. The decrease in the transition zones in #5 and #6 was mainly due to the evaporation of moist water from the cave wall to absorb heat from the air, leading to an enhanced air-cooling effect and a decrease in cave temperature. It increased in spring and autumn and decreased in summer at #11. In addition to the important control provided by the temperature outside the cave, this phenomenon was potentially also related to the rise and fall of the water level of the underground river caused by atmospheric precipitation in different seasons and the cooling effect of the water temperature. In the deep zone, the cave halls are wide and well-sealed (Table 1) and were less disturbed by the external climate, which was conducive to heat reaching equilibrium and entering the temperature stabilization zone [43].
In terms of seasonal (Figure 3) and diurnal variations (Figure 5a,b), both temperature and dewpoint in Mahuang Cave were higher in the warm season, mainly because airflow can increase condensation rates by transporting warmer and moister air from outside the cave to inside the cave in the summer. The cave controls the fluids that flow through the aquifer, both air and water, whether by conduction or convection [6,7]. Assuming that the cave rock surface temperature is relatively stable, condensation is a function of the vapor gradient between the rock surface and the air in the cave. Condensation occurs when the dewpoint of the cave air is higher than the rock surface temperature [5]. In summer, the increase in heat and moisture caused the rock surface temperature to rise, adding moisture to the cave in the form of condensation. In winter, the air flowed through the cave, constantly transferring heat and moisture. Evaporation led to moisture loss, and the cave walls were cool and dry.
In the cold season (Figure 8a), the temperature inside (T inside) and outside (T outside) the cave and atmospheric density inside the cave (ρ inside) were all less than the atmospheric density outside the cave (ρ outside). The low-temperature, high-density dry, and cold air outside the cave flowed into the cave through the bottom of the passage below the cave entrance. As Mahuang Cave is a single-entry cave, the cave “piston effect” [51] continuously guided airflow from outside the cave into the cave, and the fluid continuity principle [13] further strengthened the airflow to the depth of the cave. The air inside the cave rose or fell along the space with a large slope to enhance convection and formed the “ventilation (chimney) effect” [52]. The upward airflow entered the pore spaces, dissolution spaces, and fissures of air pockets to block the relatively high soil airflow (compared with that inside the cave) from entering the cave. This airflow effect is called the “uplift effect” [53]. Under the interaction of advection and convection, the cool, dry surface air entering the cave remained near the floor of the cave passage, while warmer air roses toward the ceiling and a thin layer of condensation fog formed at the interface between these two masses of air. Condensation releases heat which warms the overlying air mass, and condensation droplets fall into the cooler, drier air near the floor. The droplets evaporate, further cooling the incoming air while raising the relative humidity of that air [54].
In the warm season (Figure 8b), T inside < T outside, ρ inside > ρ outside, warm air outside the cave flowed into the cave from the cave roof, moist air frequently flowed out of the cave and was cooled as it approached the entrance. This created a pocket of cool, dense air that partially blocked the airflow out of the cave entrance. Because the deep interior cave temperature was near-constant, air from the interior maintained this uniform temperature in the entrance when the air was blowing out of the cave. Likewise, the high relative humidity resulted from air moving from the interior of the cave [54]. Diurnal monitoring (Figure 5a,b) also showed that the cave temperature and relative humidity were greater in the warm season, and the abrupt change phenomenon occurred more strongly in the warm season than in the cold season. Owing to the limitation of the cave structure, the frequent intersection of airflow inside and outside the cave mainly occurred in the near-cave section; therefore, the closer the cave entrance, the more apparent the sudden change fluctuation (#2 > #4 > #9).
In summary, atmospheric temperature can be considered as the main indicator of airflow and a key control factor for microclimate fluctuation in the cave. Cave distance, elevation, cave structure, physical characteristics of the surrounding rock cave wall, and the presence of groundwater have significant influences on the material and energy exchange inside and outside the cave. The related effects of air movement and hydrodynamic action have a moderating effect on the abrupt change phenomenon in cave microclimates.

5. Conclusions

Although the study concerned a single cave, the purpose of the work was to provide greater insight into the temporal variability, spatial heterogeneity, and mechanisms of a cave microclimate environment in karst regions. The results of monthly monitoring and diurnal encrypted monitoring of meteorological indicators inside and outside Mahuang Cave from 2019 to 2021 showed that the wind speed, relative humidity, and temperature exhibited cyclical seasonal and interannual patterns of becoming high in summer and autumn and lowest in winter. In contrast, cave pressure was high in winter and low in summer. The diurnal monitoring indicators did not vary significantly from day to night, but an abrupt change phenomenon occurred around noon during the day, and the warm season responded more strongly than the cold season. This temporal variation was mainly influenced by local meteorological conditions such as atmospheric temperature and pressure. Quantitative analysis of the linear relationship between meteorological parameters inside and outside the cave revealed that atmospheric temperature was the main controlling factor influencing the microclimatic environment of caves. The spatial variation at 15 monitoring points showed that microclimate indices became more stable and approached a constant state farther away from the entrance of the cave. The short-time (diurnal) variations also indicated that fluctuations in the microclimate index were more evident closer to the cave entrance. The spatiotemporal variability of the cave microclimate and abrupt change phenomena were also influenced by air movement, cave structure, and the local physical environment. Does atmospheric precipitation outside the cave or underground rivers inside the cave directly or indirectly affect the cave microclimate? More research is still required. In future cave microclimate studies, the cave system should be viewed as a dynamic and complex coupled system. Focusing on both the interactions between environmental elements inside the cave and the response relationship between climatic elements inside and outside the cave is essential. Simultaneously, the relevant environmental effects triggered by differences in cave geometry, air movement, and groundwater dynamics are also important factors that cannot be ignored in cave monitoring.

Author Contributions

Conceptualization, Y.X. and Z.Z.; methodology, Y.X. and Z.Z.; formal analysis, Y.X. and Z.Z.; investigation, Y.X., Z.Z., S.D., H.Z., X.G., D.S. and J.H.; data curation, Y.X.; writing—original draft preparation, Y.X.; writing—review and editing, Y.X., Z.Z., S.D., H.Z., X.G., D.S. and J.H.; visualization, Y.X.; supervision, Z.Z.; project administration, Z.Z. and Y.X.; funding acquisition, Z.Z. and Y.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 42161048; the Academic New Seedling Fund Project of Guizhou Normal University (Qian Shi Xinmiao [2021] B02), and the Science and Technology Plan Project of Guizhou Province (Qiankehe Jichu [2020]1Y154).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We sincerely thank the editor and anonymous reviewers for their valuable comments and suggestions. We also want to thank the leaders and staff of the Shuanghe Cave National Geopark for their permission and support in monitoring sampling.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Schematic diagram of the location of Shuanghe Cave in Guizhou Province, China, (b) hydrological sketch of the study area (Revised from references [39]), (c) map location of Mahuang Cave, and (d) Mahuang Cave Plan and distribution of monitoring points.
Figure 1. (a) Schematic diagram of the location of Shuanghe Cave in Guizhou Province, China, (b) hydrological sketch of the study area (Revised from references [39]), (c) map location of Mahuang Cave, and (d) Mahuang Cave Plan and distribution of monitoring points.
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Figure 2. Atmospheric environment parameters in the study area. The range of atmospheric pressure, dewpoint, and temperature outside the cave was 923 to 950 Pa, −3.22 to 25.82 °C, and 4.3 to 31.8 °C, with an average value of 934 Pa, 14.69 °C, and 18.32 °C, respectively; n = 32 months. The monthly atmospheric precipitation and temperature ranged from 7.5 to 331.2 mm and 4.2 to 26 °C, with an average value of 103.9 mm and 15.7 °C, respectively; n = 36 months. The cyan bars indicate that precipitation is mainly concentrated from May to October each year.
Figure 2. Atmospheric environment parameters in the study area. The range of atmospheric pressure, dewpoint, and temperature outside the cave was 923 to 950 Pa, −3.22 to 25.82 °C, and 4.3 to 31.8 °C, with an average value of 934 Pa, 14.69 °C, and 18.32 °C, respectively; n = 32 months. The monthly atmospheric precipitation and temperature ranged from 7.5 to 331.2 mm and 4.2 to 26 °C, with an average value of 103.9 mm and 15.7 °C, respectively; n = 36 months. The cyan bars indicate that precipitation is mainly concentrated from May to October each year.
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Figure 3. Interannual and seasonal variations of meteorological parameters inside and outside Mahuang Cave. Solid line of (a–d) indicates cave wind speed, pressure, relative humidity, and temperature, black dashed line indicates the value of atmospheric parameters outside the cave, respectively. Wind speed outside the cave was not recorded. (e) Indicates monthly atmospheric temperature and precipitation. Two vertical lines indicate the interannual dividing line.
Figure 3. Interannual and seasonal variations of meteorological parameters inside and outside Mahuang Cave. Solid line of (a–d) indicates cave wind speed, pressure, relative humidity, and temperature, black dashed line indicates the value of atmospheric parameters outside the cave, respectively. Wind speed outside the cave was not recorded. (e) Indicates monthly atmospheric temperature and precipitation. Two vertical lines indicate the interannual dividing line.
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Figure 4. Spatial variation characteristics of altitude, cave distance, and climatic parameters of monitoring points in Mahuang Cave (a–e).
Figure 4. Spatial variation characteristics of altitude, cave distance, and climatic parameters of monitoring points in Mahuang Cave (a–e).
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Figure 5. Diurnal variations of temperature and dewpoint in Mahuang Cave for the (a) warm season and (b) cold season. Monitoring time interval was 1 min, n = 120 h.
Figure 5. Diurnal variations of temperature and dewpoint in Mahuang Cave for the (a) warm season and (b) cold season. Monitoring time interval was 1 min, n = 120 h.
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Figure 6. Correlation heatmap of cave climate factors.
Figure 6. Correlation heatmap of cave climate factors.
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Figure 7. Linear relationship between cave climatic factors with (a1–d1) atmosphere temperature and (a2–d2) pressure monitored at Mahuang Cave from 2019 to 2021. Asterisk at the top right of R2 indicates significance test p-value (**: p < 0.01, *: p < 0.05, none*: p > 0.05); “—” indicates no correlation.
Figure 7. Linear relationship between cave climatic factors with (a1–d1) atmosphere temperature and (a2–d2) pressure monitored at Mahuang Cave from 2019 to 2021. Asterisk at the top right of R2 indicates significance test p-value (**: p < 0.01, *: p < 0.05, none*: p > 0.05); “—” indicates no correlation.
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Figure 8. Schematic diagram of cave structure and air movement in Mahuang Cave. Red, purple, and black dashed arrow lines indicate upward airflow, soil airflow, karst fissures, and lacunae, respectively. Small blue dots indicate dripping points, blue straight line indicates the underground river flow direction, and a blue oval indicates a water pool (a) Cold season (b) Warm season.
Figure 8. Schematic diagram of cave structure and air movement in Mahuang Cave. Red, purple, and black dashed arrow lines indicate upward airflow, soil airflow, karst fissures, and lacunae, respectively. Small blue dots indicate dripping points, blue straight line indicates the underground river flow direction, and a blue oval indicates a water pool (a) Cold season (b) Warm season.
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Table
Table 1. Brief description of the monitoring points at Mahuang Cave.
Table 1. Brief description of the monitoring points at Mahuang Cave.
Cave SectionMonitoring PointsAltitude
(m)		Distance from Cave Entrance (m)Cave Morphology, Sediment Morphology, and Water Flow
Near
Cave Section		#172010.72Lock-hole shaped cave entrance, wide and flat roof, cave wall always has flowing water, villager-built cistern
#2721107.27Labyrinth-type cave hall, mammoth stalactites, crumbling blocks, perennial and more rapid stalactite drip
#3720173.47Labyrinthine cave passages, granular concretions, sky pots, side channels, perennial and slow stalactite drips
#4724217.5Labyrinthine corridors, mammillary stalactites, spiral twisted stalagmite weathering, perennial and slow stalactite dripping
Transi-tion zone#5725250.33Cave-like aisles, mound-like stalagmites, stalagmites, thin sheets of diffuse water, small pools of water, significant wind sensation
#6729282.54Narrow rift passage and rising steep scarps, curved branching stalactites, thin sheets of diffuse water, airflow wind sense is obvious
#7724335.89Rift narrow drop, stone waterfall, cone stalactite, side stone dam, cave beads, splash drip, diffuse water
#8720427.4Wide cave passages, mammillary, conical stalactites, goose tubes, spinous twisted stone columns, perennial fissure flowing water
#9718523.75Narrow fissures, avalanche accumulation of rock masses, mantle, hanging rocks, shell nests in groups, perennial fissure flow,
#10710681.05wide and high cave passage, triangular fissure, developed cave wall grooves, moist bottom clay, developed underground river,
#11708720.68wide and high cave roof, developed grooves, moist clay, two underground river tributaries converge into a small pool of water flowing into the shaft
Deep
zone		#12710799.82Straight cave passages, mammoth, flag-like stalactites, nitrate turtles, boiled nitrate sites, seasonal fissure waterfalls
#137111007.9Straight cave passage, closed cave cavity, flow marks, side grooves, sandy clay layer mostly interspersed with cobbles and crumbling materials
#147171106.38Large cave cavity, large avalanche rocks, and weathered stalagmites, inaccessible west branch cave, low southeast-facing cave passage
#157171169.73Wide cave hall, stalagmites more weathered, sand layer thick, clay mud, perennial and very slow stalactite drip
Table
Table 2. The variation range in relative humidity inside and outside Mahuang Cave from 2019 to 2021. RHoutside atmospheric relative humidity outside the cave, SD standard deviation.
Table 2. The variation range in relative humidity inside and outside Mahuang Cave from 2019 to 2021. RHoutside atmospheric relative humidity outside the cave, SD standard deviation.
#1
(%)		#2
(%)		#3
(%)		#4
(%)		#5
(%)		#6
(%)		#7
(%)		#8
(%)		#9
(%)		#10
(%)		#11
(%)		#12
(%)		#13
(%)		#14
(%)		#15
(%)		RHoutside
(%)
Max100100100100100100100100100100100100100100100100
Min70.668.467.273.170.871.771.17277.582.278.683.781.38283.740.2
Average89.6089.9989.2890.8795.7795.3294.2595.9596.6796.8295.8897.5995.9396.4695.3581.87
SD8.139.177.616.766.846.717.066.245.094.445.344.114.764.604.8814.54
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Xiong, Y.; Zhou, Z.; Ding, S.; Zhang, H.; Huang, J.; Gong, X.; Su, D. Spatiotemporal Variation Characteristics and Influencing Factors of Karst Cave Microclimate Environments: A Case Study in Shuanghe Cave, Guizhou Province, China. Atmosphere 2023, 14, 813. https://doi.org/10.3390/atmos14050813

AMA Style

Xiong Y, Zhou Z, Ding S, Zhang H, Huang J, Gong X, Su D. Spatiotemporal Variation Characteristics and Influencing Factors of Karst Cave Microclimate Environments: A Case Study in Shuanghe Cave, Guizhou Province, China. Atmosphere. 2023; 14(5):813. https://doi.org/10.3390/atmos14050813

Chicago/Turabian Style

Xiong, Yong, Zhongfa Zhou, Shengjun Ding, Heng Zhang, Jing Huang, Xiaohuan Gong, and Dan Su. 2023. "Spatiotemporal Variation Characteristics and Influencing Factors of Karst Cave Microclimate Environments: A Case Study in Shuanghe Cave, Guizhou Province, China" Atmosphere 14, no. 5: 813. https://doi.org/10.3390/atmos14050813

APA Style

Xiong, Y., Zhou, Z., Ding, S., Zhang, H., Huang, J., Gong, X., & Su, D. (2023). Spatiotemporal Variation Characteristics and Influencing Factors of Karst Cave Microclimate Environments: A Case Study in Shuanghe Cave, Guizhou Province, China. Atmosphere, 14(5), 813. https://doi.org/10.3390/atmos14050813

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