Meteorological observations during solar eclipse events date back to the 19th century, when Birt [1
] recorded changes in cloud cover during a partial solar eclipse over the United Kingdom. A comprehensive summary of historical eclipse weather observations is given by Aplin et al. [2
], who cite more than 100 articles. Some meteorological variables respond similarly across multiple solar eclipses. For example, temperature near the surface drops during the first half of an eclipse, then begins to rise again shortly after totality. However, the response of near-surface winds has been less consistent across eclipse events.
] was the first to propose that a solar eclipse could affect mesoscale wind patterns. He observed changes in wind patterns and an increase in gustiness during the 28 May 1900 eclipse and attributed these observations to the formation of a “cold-air cyclone”. Bigelow [4
] disputed this explanation, claiming that the cooling effect of the moon’s shadow was not sufficient to generate large-scale cyclonic flow. Instead, Bigelow [4
] hypothesized that the cooling altered the thermally-driven land–sea circulation near the coastal observation site.
Modeling studies have provided evidence that the effect of a solar eclipse on winds is strongest in regions characterized by spatial surface variability. Simulations of the 11 August 1999 solar eclipse over central Europe [5
] showed the largest changes in wind speed and direction near the coastlines. Gross and Hense [5
] described the modelled wind field as ‘a large scale land–sea circulation modified by orographic influences.’ Modifications to land–sea circulations were also clearly visible in simulations of this eclipse over France [7
] (their figure 7).
Anomalies in land–sea circulations during solar eclipses have also been confirmed by observational studies. During several eclipse events [8
], significant shifts in winds were recorded by weather stations in close proximity to bodies of water. In a more comprehensive study of the land–sea circulation, Bala Subrahamanyam and Anurose [10
] collected measurements at the surface and aloft at a coastal site in India during the 15 January 2010 total solar eclipse, which began in the late morning hours at the study site. The sea breeze that developed on the eclipse day was weaker at the surface and was approximately 50% of the vertical thickness compared to a non-eclipse control day. Additionally, the onset of the evening land breeze occurred two hours earlier than on the control day.
These modelling and observational studies indicate that land–sea circulations are susceptible to influence from a solar eclipse. Other circulations that result from surface spatial variability are therefore also expected to be influenced by such events. Thermally-driven orographic winds have not been studied to the same extent as land–sea circulations during solar eclipses. These orographic winds exist at spatial scales from a single slope to an entire mountain range, and are caused by differential heating between the atmosphere over the mountains and over the adjacent plains [11
]. The winds form a diurnal pattern with upslope and upvalley winds during the day and downslope and downvalley winds at night. Transition periods occur around the time the surface energy budget reverses sign (typically around sunrise and sunset). The time required for winds to reverse during the transition period depends on the dimensions of the valley [12
], and also on the spatial scale of the flow [14
]. Previous studies have observed slope winds [15
] and valley winds [16
] to reverse direction on time scales of 10–60 min. Hence, local changes to slope and valley winds are expected over the time span of a partial or total solar eclipse (∼2–3 h).
Previous studies have briefly described modifications to orographic winds as a result of solar eclipses. Fernández et al. [17
] attributed wind patterns recorded at several surface weather stations during the 11 July 1991 eclipse to local thermally-driven flows. Vogel et al. [18
] simulated the 11 August 1999 eclipse over a small domain centered on southern Germany. They found significant changes in wind speed and direction near the mountainous regions in the domain, with minimal modifications to the wind field over homogeneous terrain. Anderson [19
] suggested that rapid changes in thermally-driven orographic winds could be partially responsible for the anecdotal reports of the eclipse wind
, a sudden increase in wind speed at totality. Eugster et al. [20
] used data from 165 surface weather stations in Switzerland to examine temperature and wind changes during the 20 March 2015 solar eclipse. Even though the eclipse was only partial in Switzerland, the cooling was sufficient to delay or prevent the morning transition to upvalley flows.
While eclipse-induced changes in thermally-driven orographic flows have been observed at the surface, the vertical extent of these changes has not been investigated. Upvalley and downvalley winds typically extend vertically throughout the depth of the valley [14
] with large changes in the flow structure around the morning and evening transition times [14
]. A solar eclipse creates conditions that simulate an evening transition immediately followed by a morning transition, providing an opportunity to study the evolution of the valley boundary layer around the transition times in a quasi-laboratory setting.
On 21 August 2017, a total solar eclipse passed over the continental US from Oregon to South Carolina (see [22
] for information about the eclipse, including maps of the path of totality). The goal of the present study is to investigate thermally-driven orographic winds in a small valley during the eclipse. Specifically, we investigated the effect of eclipse-induced cooling (and subsequent warming) on changes in speed and direction of the valley wind at the surface and aloft. To this end, we collected observations both at the surface and in the boundary layer which are presented in this paper. We first provide a description of the site in Section 2
, followed by instrumentation details in Section 3
. We then present and discuss the observations collected during the 21 August eclipse in Section 4
and Section 5
. Finally, we provide a summary and conclusions in Section 6
2. Site Description
The investigation area is situated on the floor of a small valley in the foothills of the Blue Ridge Mountains, ∼25 km northwest of Charlottesville, Virginia at an altitude of 312 m above mean sea level (ASL). The topography in the valley is complex. On a larger scale, the valley is several kilometers wide and several tens of kilometers long, oriented north to south (Figure 1
a). Ridges to the east are ∼700 m ASL, and ridges to the west are ∼850 m ASL. On a smaller scale more local to the investigation area, the north–south oriented valley wind system is deflected by the presence of a small hill reaching ∼100 m above the valley floor (Figure 1
b). Hence, the valley wind system is oriented southeast (upvalley) to northwest (downvalley) in the investigation area—see figure 6 in [23
]. Previous observations have also revealed that downslope winds can reach the investigation area from both the small hill and the western valley sidewall. In the analysis of wind direction below, hill downslope (HDS) wind refers to downslope wind from the northeast, and sidewall downslope (SDS) wind refers to downslope wind from the west (Figure 1
b). Instrumentation was placed in farm fields that were mowed several weeks prior to installation. Vegetation surrounding the towers was predominantly grass at heights ranging from 10–40 cm.
2.1. Eclipse Characteristics
The investigation area was ∼500 km northeast of the path of totality of the 21 August solar eclipse. Eclipse literature separates a total solar eclipse into four phases. First, contact occurs when the moon first moves between the earth and the sun. Second contact refers to the time that totality begins (i.e., the sun is entirely obscured by the moon), and third contact is when totality ends. Fourth (or final, last) contact occurs when the moon no longer obscures the sun. During a partial solar eclipse, only first and final contact occur. A partial eclipse with a maximum solar obscuration of 85.7% occurred over the investigation area. First contact occurred at 13:14 EDT (local time; UTC-04:00), maximum eclipse at 14:41 EDT, and final contact at 16:01 EDT, for a total duration of 2 h 45 min. Local sunrise at the investigation area was 07:01 EDT and local sunset was 18:50 EDT.
2.2. Synoptic Conditions
The investigation area was under the influence of high pressure during 20 and 21 August, with westerly winds of about 5 m s
at 850 hPa (Figure 2
). In the early afternoon hours of 21 August, several small thunderstorm systems were present in the region (Figure 3
), with precipitation recorded several kilometers to the south of the investigation area during the eclipse event. The eclipse was not obstructed by thunderstorm clouds at the actual investigation area, and no precipitation was recorded over the area on 21 August. Timelapse photos of cloud cover at the investigation area during the eclipse are provided in the online Supplementary Material
3. Instrumentation and Data Collection
The platforms and instrumentation used in this study included a 10 m meteorological tower, a Doppler lidar, a tethered balloon system (or ‘blimp’), five automated weather stations (AWS), and three radiosondes. Figure 4
shows the layout of instrumentation at the investigation area. Descriptions of each instrument are provided below, with a summary of the details in Table 1
3.1. Main Tower
The 10 m main tower is centrally located in the investigation area (Figure 1
). The instrumentation includes two shielded, mechanically aspirated thermistors at 2.0 and 9.4 m above ground level (AGL); two shielded, naturally ventilated thermohygrometers at 2.0 and 9.4 m AGL (only humidity observations from these instruments were used for analysis); a four-component net radiometer; a downward-facing infrared radiometer to measure surface temperature mounted at 3.5 m AGL; and a three-dimensional, sonic anemometer at 10 m AGL. The sampling rates for these instruments during the observation period ranged from 0.2 Hz for the net radiometer to 20 Hz for the sonic anemometer (see Table 1
). The raw 20 Hz sonic anemometer data are block-averaged to 10 Hz to reduce aliasing effects [24
]. Post-processing of the raw sonic anemometer data includes removal of questionable data and associated outliers, rotation of the measured wind speed components into the direction of the mean wind using the double rotation procedure [24
] and linear detrending to obtain the perturbations of the longitudinal velocity
, lateral velocity
, vertical velocity
and sonic temperature
. The sonic anemometer data are also used to calculate sensible heat flux H
and turbulent kinetic energy (TKE). For these variables, standard block averages of 30 min do not provide the level of detail necessary to see changes during the eclipse. We present observations of H
and TKE every minute, with each observation calculated as a 30-min average that considers the 15 min before and after a given observation.
Five Automated Weather Stations (AWS) were deployed at distances ranging from 100 to 300 m from the main tower site to monitor the impact of the solar eclipse on local surface meteorological properties at a high spatial and temporal resolution. These AWS were previously described by van den Bossche and De Wekker [26
], and consisted of a 2D sonic anemometer to record wind speed and direction at 1 Hz, and sensors to monitor air temperature and humidity at 0.2 Hz and barometric pressure at 1 Hz, all at a height of 2 m AGL. 1 Hz measurements of wind speed and direction were block-averaged into 1 min periods for analysis.
3.3. Doppler Lidar
To supplement the ground-based observations, we used a scanning Doppler wind lidar (StreamLine XR, Halo Photonics Ltd., Worcestershire, UK), located approximately 20 m east of the main tower (Figure 1
b). The lidar was running from 05:39 to 22:30 EDT with only a few gaps due to some power supply issues (from 10:40 to 11:51 EDT and from 17:00 to 17:25 EDT). The lidar was configured to provide vertical profiles of horizontal wind speed and direction every 80 s at a vertical resolution of 29 m, starting at a height of 100 m AGL. We calculated the horizontal wind speed and direction by applying the Velocity Azimuth Display (VAD) algorithm to the radial velocities at each range gate [27
]. Similar to Päschke et al. [28
], we used n
= 12 azimuthal points with an elevation angle of 75 degrees. At each azimuthal point, 30,000 pulsed laser beams of 1.5 μm wavelength were fired, resulting in a dwelling time of approximately 5 s. After each VAD scan pattern, a vertical stare was obtained, which provided vertical wind speed.
3.4. Tethered Balloon Profiles
Vertical profiles of temperature, humidity, pressure, and winds from the surface to 100 m AGL were captured using a tethered balloon system during the afternoon and evening hours on 21 August. The system used in this project included a handheld weather meter (Nielsen–Kellerman Kestrel 4500 Pocket Weather Tracker), a wind vane, and a 4 m long helium blimp, similar to the setup used by McKendry et al. [29
] and Phelps [30
]. Attached to one horizontal end of the cross-shaped wind vane was a large plastic tail which allows the vane to point into the prevailing wind.
To collect the vertical profiles, 100 m of rope was unwound from the winch at a rate of 10 m every 90 s, which allowed the blimp to carry the wind vane and weather meter up from the surface. The sampling rate of the weather meter was 0.5 Hz. Each ascent lasted 15 min, and data collected during the descents were discarded. For analysis, we bin the ascending data by height, with each bin representing the average over 5 m of altitude. During some profiles, the temperature sensor was not well aspirated in the first 5–10 m. Therefore, we exclude the first 10 m of temperature measurements in all profiles from our analysis, and instead use observations from the main tower in the vertical profiles. For each profile, temperature measured at 2 m and 10 m is time-averaged over the duration of the ascent, and these values are used in the first and second bins, respectively.
Three Graw DFM-09 radiosondes were launched during the eclipse on 21 August: one close to first contact (actual launch time 13:16 EDT), the second at maximum eclipse (14:41 EDT), and the third close to final contact (15:59 EDT). The radiosondes provided vertical profiles of potential temperature and specific humidity that extend beyond the tethered balloon profiles near the surface. The first two soundings were terminated at an altitude of 5–6 km in order to prepare for subsequent launches. In the following discussion, we focus on the temperature and humidity profiles below 2 km in order to provide finer detail of the atmospheric boundary layer sampled by the lidar.
5. Wind Rotations during Transition Periods
Winds in and above the investigation area changed direction several times during and after the solar eclipse. Winds from the upvalley, downvalley, and downslope directions were all recorded at the main tower and the five AWS during the daytime hours of 21 August. However, significant differences in the timing and magnitude of wind direction changes existed between the various towers. Specifically, we note several periods of changing winds when the rotational direction of the change (i.e., a clockwise or counterclockwise rotation) varied between the towers.
] was the first to describe the daily rotation of wind direction over valley sidewalls due to the interaction of thermally-driven slope and valley winds. An observer on the valley floor looking toward the head of the valley would see a daily clockwise (CW) rotation over the left sidewall and a counterclockwise (CCW) rotation over the right sidewall (Figure 10
). Such rotations have been confirmed in previous observational and modeling studies [12
]. We now use this conceptual model as a framework to explain the wind rotations observed during and after the solar eclipse.
The idealized valley shape in Hawkes’ conceptual model [33
] results in a rotational pattern where one sidewall exactly mirrors the other, so that the same component of slope or valley wind exists simultaneously over both sidewalls. During a typical morning transition in our investigation area, the eastern face of the valley sidewall is heated for several hours after sunrise before the southwestern face of the hill receives any direct radiation. Therefore, we expect upslope winds to start over the sidewall before they begin over the hill. During the evening transition, the last rays of the afternoon sun illuminate the southwestern face of the hill. Meanwhile, the valley sidewall to the west has been shaded for some time. We expect that downslope winds will also develop earlier over the sidewall than over the hill. In addition to this temporal offset, the hill and the sidewall do not face each other directly. Downslope winds from the sidewall are westerly, whereas downslope winds from the hill are northeasterly. Hence, daily wind rotations with opposing directions are still expected over the sidewall and hill, but the rotations are not expected to be as symmetrical as those depicted in Hawkes’ model. Looking toward the head of the valley in our investigation area, the conceptual model predicts a CW rotation over the sidewall and a CCW rotation over the hill.
Observations from the main tower and five AWS provide evidence that wind rotations also exist at locations along the valley floor. These rotations occur in different directions (CW vs. CCW) and at different times of day depending on the locations of the towers. The rotational direction at the various locations is influenced by the proximity of the location to the two main topographic features in the investigation area, the sidewall and the hill. Additionally, differences in solar altitude and azimuth angles between the eclipse and the morning and evening transition periods resulted in different wind patterns during those three events. We separate the patterns of wind rotation into three groups: sidewall-influenced, hill-influenced, and hybrid-influenced rotations. In the final category, both topographic features influenced the wind patterns at various points throughout the day.
5.1. Sidewall-Influenced Towers: AWS3
AWS3 was the station installed closest to the western sidewall (Figure 1
and Figure 4
). Winds before sunrise on 21 August were steadily downvalley (Figure 11
). During the morning transition, winds rotated CCW from downvalley to upvalley in less than 10 min. This CCW rotation is in the opposite direction of what is predicted when applying Hawkes’ conceptual model [33
] to the topography in the investigation area. We note that AWS3 was installed on the valley floor near the sidewall, and not actually on the slope itself. Therefore, we expect upslope flows over the sidewall to have minimal influence on wind observations made by AWS3. Additionally, wind speeds of less than 1 m s
during the morning transition indicate that extremely localized flows, possibly influenced by microterrain features or vegetation, may have affected wind observations during this period.
Upvalley winds established at AWS3 after the morning transition remained until maximum eclipse at 14:41 EDT, then began a CW rotation toward downvalley winds. This rotation occurred in the direction predicted by the conceptual model. The decrease in radiation during the first half of the eclipse caused the atmosphere over the sidewall to cool and southeasterly upvalley winds to change to westerly downslope winds. For more than an hour after final contact, AWS3 continued to observe rotating winds in both the CW and CCW directions. Similarly to the morning transition, wind speeds during this late afternoon period were very low, sometimes falling below 0.5 m s. Again, extremely local effects contributed to these anomalous rotations.
Winds at AWS3 briefly returned to upvalley around 17:50 EDT, only a few minutes before the evening transition began. Winds then rotated CW to downvalley, again in less than 10 min. This rotation again aligns with the conceptual model, with a brief influence of sidewall downslope winds before downvalley winds dominate at the station. However, wind speeds recorded during this period and into the nighttime hours were still relatively low at less than 1 m s. Downvalley winds persisted at AWS3 throughout the rest of the night.
5.2. Hill-Influenced Towers: AWS5, AWS6
AWS5 and AWS6 were the two stations in closest proximity to the hill (Figure 4
). Nighttime winds at these two stations were northeasterly or northerly instead of the expected northwesterly downvalley direction. We suspect that this is a result of north-northeasterly downvalley winds in the larger valley moving over the hill and mixing down into the investigation area (see Figure 1
). Shortly after sunrise at 07:01 EDT, winds began a CW rotation toward upvalley winds, which were established around 09:00 EDT. This CW rotation during the morning transition is in the opposite rotational direction of what is predicted when applying Hawkes’ conceptual model [33
] to the topography in the investigation area. In theory, southwesterly upslope flows over the hill should rotate CCW and change to southeasterly upvalley flows during the morning transition (Figure 10
). However, because our stations were installed on the valley floor near the hill (instead of on the actual slopes of the hill), we expect minimal influence of upslope flows on wind observations at AWS5 and AWS6.
Similarly to observations at AWS3, upvalley winds established at AWS5 and AWS6 after the morning transition remained until maximum eclipse at 14:41 EDT. Shortly afterwards, winds near the two stations began a CCW rotation toward northeasterly winds. This rotation aligns with expectations from the conceptual model. As incoming radiation decreased during the first half of the eclipse, the atmosphere over the hill cooled at the same time as the atmosphere over the sidewall. At AWS5 and AWS6, southeasterly upvalley winds gave way to northeasterly hill downslope winds. However, both stations observed two additional CW wind rotations in the late afternoon hours. The first was a CW rotation from northerly/northeasterly to upvalley winds, similar to the rotation that occurred during the morning transition. This rotation was followed closely by a second CW rotation from upvalley back to northerly/northeasterly winds during the evening transition. This second rotation can be explained by the hill’s aspect. The last rays of the afternoon sun were incident upon the southwestern face of the hill while the valley sidewall to the west had been shaded for some time. Downslope winds developed earlier over the sidewall than over the hill, and these westerly downslope winds were able to reach AWS5 and AWS6 and cause the CW rotation to typical northerly/northeasterly nighttime winds at these stations.
5.3. Hybrid-Influenced Towers: Main Tower, AWS1, AWS2
The main 10 m tower, AWS1, and AWS2 were centrally located on the valley floor (Figure 4
) and recorded wind observations influenced by both the hill and the sidewall at different points during the day. All three stations recorded downvalley winds in the early morning hours before sunrise, which rotated CCW during the morning transition. Again, low wind speeds indicate that a strong thermally-driven component was not present during the morning transition. Shortly before 08:00 EDT, the main tower and AWS2 recorded northeasterly wind that gradually rotated CW toward upvalley. This is similar to the pattern recorded by AWS5 and AWS6, the towers in close proximity to the hill. Meanwhile, the initial CCW rotation recorded at AWS1 settled at upvalley winds without a period of northeasterly wind. This pattern is closer to the one recorded by AWS3, the tower nearest the sidewall.
The main tower, AWS1, and AWS2 all recorded CCW rotations from upvalley to northeasterly winds during the second half of the eclipse, aligning closely with the observations recorded by AWS5 and AWS6. This is a result of the relative size of the two main terrain features at the observation site. The atmosphere over the hill cooled more quickly than the atmosphere over the sidewall, resulting in an earlier onset of hill downslope winds from the northeast. Later, the main tower, AWS1, and AWS2 recorded a period of westerly winds between 17:00 and 17:45 EDT. This suggests that sidewall downslope winds from the west became strong enough to overpower the hill downslope winds from the northeast at these stations. However, during this transitional period of low wind speeds, microterrain or vegetation features can complicate the wind patterns.
As at the other stations, winds at the main tower, AWS1, and AWS2 returned to upvalley shortly before 18:00 EDT. The three stations recorded a CW rotation to downvalley winds, providing additional evidence that sidewall downslope winds were strong enough to reach all stations in the observation area. Downvalley winds remained throughout the nighttime hours.
6. Summary, Conclusions, and Future Work
The focus of this study was the observation of boundary layer structure and thermally-driven orographic winds during the 21 August 2017 solar eclipse. We observed these winds in a small valley to determine if eclipse-induced changes in radiation and temperature were sufficient to disrupt the normal diurnal cycle of slope and valley winds. Our analysis of the observations provided the following results:
Surface cooling was more than two standard deviations larger than average compared to previous solar eclipse events, but was similar to other temperature decreases recorded across the continental U.S. during the 21 August eclipse. This cooling in the presence of negative daytime sensible heat flux caused the formation of a stable layer at the surface.
Vertical temperature profiles from tethered balloons and radiosondes indicate that cooling during the first half of the eclipse extended to 800 m AGL, and warming during the second half extended above 2000 m AGL. The daytime sensible heat flux was not sufficient to explain the cooling and warming of the lower atmosphere, and temperature advection likely played an important role.
Surface weather stations recorded multiple rotations in wind direction during and after the eclipse. These rotations can be explained by the interaction between slope and valley flows. The size and direction of the rotations were strongly influenced by the proximity of the stations to local terrain features. Rotations recorded during the eclipse generally aligned with expectations from a conceptual model [33
], but rotations during the morning and evening transitions did not always occur in the expected directions. The high solar elevation angle and low azimuth angle during the eclipse caused different slope heating and cooling patterns to develop than what would typically occur during the transition periods.
Tethered balloon and lidar observations showed that wind shifts and rotations were not limited to the surface. A layer of downvalley winds formed between the surface and ∼300 m AGL during the second half of the eclipse. Winds above this layer remained upvalley or westerly. The timing of the appearance of this wind layering relative to the solar eclipse indicates a time lag between decreased radiation during the eclipse and the reversal of the valley wind system. Downvalley wind at the surface was not initiated until ∼80 min after the start of the eclipse. This downvalley wind layer existed for ∼70 min and dissipated shortly after final contact.
We conclude that the decreased radiation and associated cooling during the solar eclipse altered the expected cycle of thermally-driven winds in the valley. With regard to incoming solar radiation, the first and second halves of the eclipse are similar to the evening and morning transitions, respectively, but with important differences caused by the orientation of the topographic features relative to the position of the sun. A revised conceptual diagram summarizes the interactions between solar radiation, topography, and winds that occurred on 21 August (Figure 12
). Incoming solar radiation shortly after sunrise was incident on the east-facing sidewall, resulting in upslope winds over the sidewall while downslope winds persisted over the west-facing hill (Figure 12
a). The sidewall and hill experienced simultaneous cooling during the first half of the eclipse, and downslope winds formed first over the hill due to its smaller size relative to the sidewall (Figure 12
b). The two topographic features were simultaneously heated during the second half of the eclipse, and upslope winds formed over the hill while downslope winds were still present over the sidewall (Figure 12
c). Finally, during the evening transition, downslope winds began over the shaded sidewall while the hill continued to receive late afternoon solar radiation (Figure 12
The interaction between the phases of the slope and valley wind systems causes a daily wind direction rotation on the valley sidewalls [11
]. The observations from 21 August in the present study indicate that the explanation of the nature of rotations over sidewalls can be extended to locations at and above the valley floor. The magnitude and direction of rotation at various locations over the valley floor appear to vary between the morning and evening transitions, and are also dependent on interactions between incoming solar radiation and local terrain features. These rotations can impact e.g., the dispersion of air pollutants from emission sources in different parts of the valley, and therefore can have important implications for air quality studies.
This study focused on the observations collected only on the day of the eclipse. We were fortunate that calm synoptic conditions prevailed at the investigation area on 21 August, and that precipitation from the thunderstorm system south of the site did not disrupt our measurement campaign. Future work will include an analysis of one year of data from the main tower. We will characterize ‘typical’ wind and temperature changes during morning and evening transitions at our site, and interpret the deviation of the wind patterns during the solar eclipse in more detail. We will pay special attention to the climatology of wind rotations on the valley floor.