Introduction
Solar eclipses have long fascinated scientists and brought
about essential scientific knowledge on the meteorological effects of the
phenomenon. The most commonly studied effect is that on temperature at
Earth's surface Table ;. Less,
however, is known about the effects on local and larger-scale wind directions
at places where only partial occultation is observed. Here we test the
hypothesis that even a partial occultation of a solar eclipse may have
short-term influences on wind direction. During a solar eclipse, the new
moon passes in front of the sun's disk and thus reduces incoming solar
radiation. This astronomic event is typically described with four phases. It
begins with the first contact between the moon and the sun as seen by an
observer on the Earth. At the first contact, the penumbral shading begins,
i.e., the partial shading where sun rays from one part of the sun's disk
are blocked by the moon and sun rays from the opposite part of the sun's
disk still reach the observer. This phase ends with the second contact, when
the moon completely obscures the sun and the observer is in the shadow
of the moon. During this second phase of totality, only diffuse sunlight
reaches Earth's surface. This phase ends with the maximum occultation, when
the solar corona can be seen in the case of a total eclipse. With annular and
partial eclipses, the maximum occultation simply means the darkest
conditions. After the maximum, light levels increase again until the third
contact, when the transition from the umbral shadow to the less dark
penumbral shadow takes place. The astronomical event ends with the fourth
contact, after which meteorological conditions should no longer depend on the
moon's position. The second and third contacts are observed only in the
narrow band of the umbra during annular or total eclipses.
Compilation of literature reports on temperature drops during
maximum occultation of an eclipse since 1834. Most reports are from total or
annular eclipses but a few studies also report values from partial eclipses
or partial occultation at a given locality. More observations are tabulated
in but not all these reports allow the calculation of the
temperature drop.
Date
Location and additional information
Temperature drop
Reference
30 Nov 1834
Boston, Mass., USA
0.3 K
09 Aug 1896
Vadsö, Norway (70∘04′ N)
1.0–1.6 K
09 Aug 1896
Vadsö, Norway (70∘04′ N)
3.1 K
30 Aug 1905
Burgos
8.3 K
08 Jun 1918
Goldendale, Washington, USA
3.6 K
08 Apr 1921
Bexley Heath
1.1 K
08 Apr 1921
Bristol
4.2 K
08 Apr 1921
Nottingham (Lenton Fields)
3.0 K
08 Apr 1921
Prestou (Hoghton)
1.7 K
29 Jun 1927
Bangor, UK
0.5 K
29 Jun 1927
English eclipse, cloudy
nothing remarkable
29 Jun 1927
Southport
0.5 K
31 Aug 1932
Canadian eclipse, cloudy
very small fall
19 Jun 1936
Chios
1 K
19 Jun 1936
Omsk
5 K
19 Jun 1936
on steam ship Strathaird
1.5 K
19 Jun 1936
Portugal
2.7–3.3 K
13 Dec 1936
New Plymouth, New Zealand
4.2 K
07 Mar 1970
Lee, Florida, USA
3.2 K
30 Jun 1973
Chinguetti, Mauritania
3.5 K
30 Jun 1973
Chinguetti, Mauritania
2.5 K
30 Jun 1973
Chinguetti, Mauritania
2.5 K
26 Feb 1979
Hecla, Manitoba, Canada
2.0 K
30 May 1984
“in Georgia”
7.8 K
18 Mar 1988
Ship, coast of Karimata island
2.2 K
11 Jul 1991
Agriculture/golf, wet fraction 1.00, albedo 0.20–0.25
1.40 K
11 Jul 1991
Costa Rica
no info
11 Jul 1991
Costa Rica, Damas
4.7 K
11 Jul 1991
Costa Rica, Fabio Baudrit Experimental Station
5.5 K
11 Jul 1991
Costa Rica, Liberia, Alajuela and Palmar Sur
3.0–3.5 K
11 Jul 1991
Costa Rica, Limón
3.0 K
11 Jul 1991
Costa Rica, Puntarenas
2.7 K
11 Jul 1991
Costa Rica, Santa Cruz and Filadelfia
2.0–2.5 K
11 Jul 1991
Costa Rica, Tárcoles
8.5 K
11 Jul 1991
Desert, wet fraction 0.00, albedo 0.27
2.65 K
11 Jul 1991
Fresno, California, USA, cotton field
ca. 4.5 K
11 Jul 1991
Industrial/airport, wet fraction 0.07, albedo 0.1
1.38 K
11 Jul 1991
Residential/commercial, wet fraction 0.47, albedo 0.20–0.25
1.93 K
10 May 1994
Ames, IA, USA
2.3 K
10 May 1994
Boulder, CO, USA
2.2 K
10 May 1994
Chicago, IL, USA
6.1 K
10 May 1994
Estes Park, CO, USA
3.6 K
10 May 1994
Ft. Collins, CO, USA
2.2 K
10 May 1994
Keenesburg, CO, USA
3.0 K
10 May 1994
Lakewood, CO, USA
2.7 K
10 May 1994
Lamberton, MN, USA
3.1 K
10 May 1994
Longmont, CO, USA
2.8 K
10 May 1994
Loveland, CO, USA
3.3 K
10 May 1994
Morris, MN, USA
2.3 K
10 May 1994
Norman, OK, USA
3.6 K
10 May 1994
Nowata, OK, USA
3.0 K
Continued.
Date
Location and additional information
Temperature drop
Reference
10 May 1994
Nunn, CO, USA
1.9 K
10 May 1994
Rollinsville, CO, USA
2.3 K
10 May 1994
Sedalia, MO, USA
4.2 K
10 May 1994
Springfield, IL, USA
6.1 K
10 May 1994
St. Paul, MN, USA
1.5 K
10 May 1994
Waseca, MN, USA
3.7 K
10 May 1994
White Sands, NM, USA
5.5 K
10 May 1994
White Sands, NM, USA
0.4 K
03 Nov 1994
Coronel Oviedo, Paraguay
3.3 K
24 Oct 1995
Neem ka Thana, India
3 K
24 Oct 1995
New Delhi, India
1.5 K
24 Oct 1995
New Delhi, India
6–8 K
25 Oct 1995
Hyderabad, India
9–10 K
26 Feb 1998
Sinamaica, Venezuela
5 K
11 Aug 1999
Akola, central India
1–2 K
11 Aug 1999
Kharkiv, Ukraine, max. occultation 0.73
7.3 K
11 Aug 1999
Modeling study, central Europe
average 3.5 K
11 Aug 1999
Modeling study, central Europe
peak up to 5 K
11 Aug 1999
Silsoe, Bedfordshire, UK, soil temperature at 10 mm depth
1.6 K
11 Aug 1999
Silsoe, Bedfordshire, UK, under grass temperature
0.5 K
11 Aug 1999
southern UK
up to 3 K
11 Aug 1999
Szczawnica, Poland
11 K
21 Jun 2001
Lusaka, Zambia
5.38±0.04 K
31 May 2003
Kharkiv, Ukraine, max. occultation 0.64
2.1 K
03 Oct 2005
Kharkiv, Ukraine, max. occultation 0.24
1.3 K
29 Mar 2006
central Greece
2.7 K
29 Mar 2006
Finokalia, Greece
2.3 K
29 Mar 2006
Ibadan, Nigeria, 1 m
1.6 K
29 Mar 2006
Ibadan, Nigeria, 12 m
0.8 K
29 Mar 2006
Ibadan, Nigeria, 6 m
1 K
29 Mar 2006
Kastelorizo, Greece
2.3 K
29 Mar 2006
Kharkiv, Ukraine, max. occultation 0.77
2.3 K
29 Mar 2006
Kislovodsk, Russia
3 K
29 Mar 2006
Kislovodsk, Russia, 600 ma.s.l.
2 K
29 Mar 2006
Kislovodsk, Russia, surface atmospheric layer
3.4±0.5 K
29 Mar 2006
Manavgat, Turkey
5 K
29 Mar 2006
Markopoulo (Athens), Greece
2.7 K
29 Mar 2006
northern Greece
3.9 K
29 Mar 2006
Palini (Athens), Greece
1.6 K
29 Mar 2006
Penteli (Athens), Greece
2.7 K
29 Mar 2006
Side, Turkey
5 K
29 Mar 2006
southern Greece
2.3 K
29 Mar 2006
Thessaloniki, Greece
3.9 K
29 Mar 2006
Thission (Athens), Greece
2.6 K
29 Mar 2006
Athens, Greece
0.7 K
29 Mar 2006
Ibadan, Nigeria
2.2 K
01 Aug 2008
Svalbard, Norway
0.3–1.5 K
15 Jan 2010
Gadanki, India, -0.10 m
3.0 K
15 Jan 2010
Gadanki, India, -0.20 m
1.3 K
15 Jan 2010
Gadanki, India, 0.00 m
5.4 K
15 Jan 2010
Gadanki, India, 0.05 m
5.0 K
15 Jan 2010
Gadanki, India, 12 m
2.5 K
15 Jan 2010
Gadanki, India, 4 m
5 K
15 Jan 2010
Gadanki, India, 8 m
3 K
Continued.
Date
Location and additional information
Temperature drop
Reference
15 Jan 2010
Gadanki, India, surface
5.8 K
15 Jan 2010
Kanyakumari, India
4 K
15 Jan 2010
Ramanathapuram, India
1 K
15 Jan 2010
Thiruvananthapuram, India
1.2 K
15 Jan 2010
Thiruvananthapuram, India, over cassava
4 K
15 Jan 2010
Thrissur, India
2 K
15 Jan 2010
Thumba, India
2 K
15 Jan 2010
Thumba, India
1.2 K
15 Jan 2010
Tirunelveli, India
3.2 K
20 Mar 2015
mainland UK, maximum drop (of 266 sites)
4.23 K
20 Mar 2015
mainland UK, median drop (of 266 sites)
1.02 K
20 Mar 2015
mainland UK, minimum drop (of 266 sites)
0.03 K
20 Mar 2015
mainland UK, mean drop (of 76 sites)
0.83 ± 0.63 K
20 Mar 2015
mainland UK, mean drop, clear sky (14 sites)
0.91 ± 0.78 K
20 Mar 2015
mainland UK, mean drop, cloudy sky (16 sites)
0.31 ± 0.40 K
20 Mar 2015
Svalbard, Norway
0.3–1.5 K
20 Mar 2015
Switzerland, 184 stations
1.5 K
This study
20 Mar 2015
Sorniot–Lac Inférier (Switzerland, most extreme drop)
5.8 K
This study
During the total vernal equinox eclipse of 20 March 2015, a Saros 120 eclipse
(https://eclipse.gsfc.nasa.gov/5MCSE/5MCSE-Maps-10.pdf), which
produced a partial occultation over the Swiss Alps, the weather conditions were excellent with mostly clear skies
due to a persistent high-pressure band between the UK and Russia, with
a rather weak pressure gradient over the continent
. On normal days under weak larger-scale
pressure gradients, local thermo-topographic wind systems develop that
are driven by small-scale pressure and temperature gradients, which are
strongest in mountainous areas such as the Swiss Alps. It could hence be
expected that during penumbral shading, these thermo-topographic winds would
be subject to modifications superimposed by the larger-scale circulation
generated around the umbra of the solar eclipse. The meteorological
conditions during an eclipse are expected to be the same as that of a
“cyclone with a cold centre” as described by
pp. 337–342. In such a cyclone, the vertical motion is
reversed as compared to a typical warm centered cyclone. This leads to
a (narrow) core with cyclonic rotation in the cold center of the cyclone and
an anticyclonic counterflow around this core. Due to its reversed
structure, the cyclonic rotation in a cold-centered cyclone according to
is strongest in the upper troposphere and weakest near the
surface, whereas the anticyclonic rotation is weakest in the upper
troposphere and strongest near Earth's surface.
Theoretical effects of penumbral shading on wind direction. Wind
directions around the center of the umbra (east of Iceland) according to the
theory of (a) and
(b), drawn for the time of maximum occultation over the Swiss Alps
(yellow area). The eclipse trajectory is shown with a blue arrow, and the
umbral path (100 % occultation) is shown with a blueish band. The diagram
shows the position of greatest eclipse with its center
indicated by a gray circle.
Based on 's () concept,
empirically determined the direct influence of the
occultation on wind direction within the shaded area during the 28 May 1900
total eclipse (Saros 126). According to 's
() theory, the cyclonic rotation in the center of the
umbra is not detectable, but an anticyclonic outflow should be generated in
the inner zone of the penumbral shadow. This zone with anticyclonic rotation
is expected to extend at least 2400–3200 km 1500–2000
miles; from the center of the umbra. In contrast, the outer
part of the penumbra is subject to a cyclonic wind direction rotation
(a further ≈1600 km; Fig. a). Although
concluded that this rotational pattern “confirms so well
Ferrel's theory of the cold-air cyclone”, he did not provide an estimate of
the dimension or strength of the cold-air cyclone in the center of the umbra
because no cyclonic effects could be seen in his analyses. Following
, carefully assessed the penumbral winds at
Reading and Camborne (UK) during the 11 August 1999 total eclipse (Saros 145)
of which a maximum occultation of 97 % was observed at Reading. Their
1 Hz ultrasonic anemometer wind speed and wind direction measurements
showed a pronounced drop in wind speed during the eclipse. The wind then
changed in a cyclonic direction on first contact, and later returned via an
anticyclonic rotation to the synoptic wind direction after maximum
occultation. Thus, their finding conformed to what is expected for the inner
core around the umbra, where expected the cold-air
cyclone but did not find it in his own analysis of synoptic-scale weather
maps. thus postulated a revised model with an inner cold
core of ca. 160 km around the center of the umbra containing cyclonic
flow (Fig. b). They expected an anticyclonic rotation outside
this zone that extends to ca. 1600 km (1000 mi), with a further
outermost zone of cyclonic rotation then extending up to ca. 4800 km
(3000 mi).
was the first to empirically confirm an indirect effect of
the cooling during occultation on wind direction. He reported that “the wind
showed a marked tendency to back” (as expected from cyclonic influence of
the thermal wind), consistent with the cold-core cyclone in the umbral zone.
There is observational evidence of thermal wind effects superimposed on
near-surface winds during occultation, but no regional-scale weather
prediction model has been able to reproduce this effect successfully.
simulated the 11 August 1999 eclipse in central Europe
using a hydrostatic regional weather prediction model. This model was able to
produce a slight cyclonic circulation in the surface winds, but it only
lasted for some minutes and thus challenges the idea that such a weak effect
can be observed in field measurements.
During the vernal equinox eclipse of 2015, we expected a clear difference in
wind direction for the Swiss Alps dependent on the or the
theories. Thus, we investigated whether the onset of
penumbral shading leads to an anticyclonic (following
's theory; Fig. a) or a cyclonic
(following 's theory; Fig. b) influence
on near-surface wind directions. We hypothesized that during maximum
occultation (66–70 % across Switzerland, ≈2000 km away
from the umbral center), Switzerland was in the ideal position to determine
which of the two theories is closer to reality. Our aim in this paper is thus
to extend the analysis of temperature drops during occultation to assess
whether available wind direction data can support one of the two theories
about air mass circulation inside the penumbra during the eclipse.
Material and methods
The vernal equinox eclipse of 20 March 2015 was a Saros 120 total eclipse (partial in Switzerland), with its maximum at 09:45:39 UTC
10:45:39 CET;. In Switzerland, occultation started with the first contact of the sun and moon disks at
09:21:58 CET in Geneva (western border) and ended with the fourth contact at 11:47:49 CET in Martina (eastern border). Maximum
occultation ranged from 65.8 % in Chiasso (southern border) to 70.1 % in Bargen/Schaffhausen (northern border). The timing of
maximum occultation varied from 10:29:26 to 10:35:55 CET across Switzerland . The second and third contacts of a total
eclipse are the entering into and exiting out of the umbra, respectively. In areas with only partial occulation, the second and third
contacts do not occur.
This eclipse has been thoroughly investigated with a focus on the mainland UK
in a themed issue with 16 papers introduced by .
Although all contributions are relevant, we specifically refer only to the
articles with a direct link to our own study
.
Sites and data
We used six Swiss FluxNet sites (www.swissfluxnet.ch; Table S1 in the
Supplement) with 20 Hz ultrasonic anemometer–thermometer data and
184 conventional MeteoSwiss weather stations (Table S2) across Switzerland
and Liechtenstein , of which 165 not only provided
temperature data but also wind speed and wind direction measurements
(Table S3). We used data from 20 March 2015 and – where possible – the
previous one or two days for reference. All three days were nearly clear,
except for occasional scattered high-level clouds.
Sensors used at the MeteoSwiss station are Pt-100 thermistors for temperature measurements and Lamprecht L14512 cup anemometers with
a wind vane for wind speed and wind direction measurements. Some sites alternatively use 2-D ultrasonic anemometers
. At the Swiss FluxNet sites, the specific instruments included in this study were Gill HS or R3 ultrasonic
anemometers (Gill Ltd., Lymington, UK) and Kipp and Zonen CNR-1 four-way net-radiometers (Delft, the Netherlands) with active ventilation
(Markasub, Olten, Switzerland). Only at the CH-OE2 site, a Delta-T BF5 sunshine sensor (Cambridge, UK) was available for measurements
of diffuse and total photosynthetic photon flux density (PPFD). PPFD is the quantum flux in the visible range 400–700 nm that
plants use for photosynthesis.
For all these variables except temperature (see Sect. ) and wind direction (see Sect. ), the difference
between 20 March and either 18 or 19 March was calculated, depending on which of the previous two days had closer to clear-sky
conditions. We used two different concepts to determine the drop in temperature (see Sect. ).
Calculation of temperature drop
All analyses were done with R version 3.3.1 . The local
temperature effect at each site was estimated by fitting a local polynomial
regression with a span parameter of 0.1 (“loess”, a locally weighted least
squares regression function in R) to each time series from 20 March. The
measurements during the penumbral shading and the adjacent 12 min on both
sides were excluded from the fit. The maximum difference between the
measurements during penumbral shading and these fitted values was then
determined. This approach closely followed the method used by
, or the linear approach used by . In a few
cases (sites GRH, ROG, ULR, VAD, VSBLI; see Table S2), however, this approach
failed (e.g., because of instationarity shortly before, shortly after, or
during the time period of the eclipse, which can lead to erratic
interpolations) and thus the simple temperature difference with respect to 19
March was used. We did not use this latter approach for all sites because
there were substantial temperature differences between 18, 19, and 20
March 2015, despite persistent and very similar fair-weather conditions. The
period of interest coincided with the peak spring snowmelt. For example, at
the CH-FRU mountain grassland flux site (1000 ma.s.l.) where four
phenological camera images were taken per day, the snow cover in the morning
of 18 March 09:30 CET was still around 80 %, but declined strongly
during 18 and 19 March and had less than 10 % coverage by the evening of
19 March at 18:30 CET. On 20 March, the site was basically free of snow,
similar to other mountainous stations that were subject to
snowmelt.
Radiation effects at the CH-OE2 Swiss FluxNet site. (a)
Incoming short-wave radiation, (b) fraction of diffuse radiation,
(c) ratio between diffuse and direct radiation, (d)
evolution of brightness of the vegetation during the eclipse, and
(e) long-wave radiation components. The four vertical lines indicate
first contact (1), maximum occultation (M), fourth contact (4),
and local noon (LN). Bold lines show the conditions during the day of the
eclipse (black or darker color) in comparison with the preceding day
(brighter color). The dashed reference curve in (a) is the
measurement from 19 March 2015 multiplied by 0.6996, the theoretical maximum
occultation at the site (± 10 % shown with a gray band). The
fraction of diffuse radiation in (b) was measured with a quantum
sensor measuring diffuse photosynthetic photon flux density (PPFD,
400–700 nm wavelength) in relation to total PPFD. The red model
curve in (c) shows an empirical parabolic fit to the three periods
with blue data points to obtain a reference for the correction of
(a) for cloud passage during the eclipse (red line in a).
The assumption made here is that the ratio of diffuse to direct PPFD is valid
also for the entire wavelength spectrum of short-wave radiation shown in
(a). During the penumbral shading phase, the phenological camera
recorded images every 2 min, from which relative surface brightness was
computed in the footprint area of the radiation measurements (gray symbols in
d). The local polynomial regression fit (black line in d)
shows a marked decrease in brightness after first contact in comparison to
the theoretical cosine response (blue line in d), anchored at the
pictures marked with vertical blue bars), followed by an almost constant
brightness during the occultation phase. LWin and
LWout in (e) are incoming and outgoing
long-wave radiation, respectively.
Long-wave back-radiation effect during the eclipse (black symbols and regression line) in comparison to conditions before and
after the eclipse (gray symbols and regression line). The differences ΔLW between 1 min averages from the day
before the eclipse and the same time during the day of the eclipse are shown (ΔLWin in relation to
ΔLWout using the regression approach ΔLWx=LWx(20 March)-LWx(19 March) with x representing index in or out). Measurements made during a period
with cloud passage on 19 March 2015 (blue symbols) were excluded from analysis.
Oblique view of the surroundings of the Sorniot–Lac Inférieur de Fully weather station. The red arrow marks the location
of the station, the red line marks the drainage area up to the lowest pass, and the orange line marks the full drainage area. The
area was completely covered with snow and the lake was frozen during the eclipse. Imagery from Atlas of Switzerland V3
, © 2016 swisstopo (JD100042).
Temperature reduction maximum during the solar eclipse at 184 weather stations in Switzerland and Liechtenstein (open circles)
that record 10 min averages. The elevation profile (bold line showing the moving mean over a 350 m elevational window, equal to
10 % of the entire elevation range) and its 95 % confidence interval (blue band) were estimated using nonparametric
bootstrapping.
Fitting parameters of the gamma distribution (Eq. ) fitted
to empirical histograms of temperature drops ΔT during the eclipses
and mean ΔT. All values are best estimates ± standard error (SE) of
the estimate. Values in italics indicate that the parameter estimates were
not significantly different from zero (p>0.05).
Offset
Shape
Scale
mean ΔT
(K)
(K)
This study
-1.1±0.3
06.6±1.9
0.4±0.1
1.5±1.0
Literature data
-3.5±3.1
12.8±13.1
0.5±0.3
2.6±1.7
Combined
-0.7±0.1
03.4±0.4
0.8±0.1
1.9±1.4
Results and discussion
Short-wave radiation effects
Incoming short-wave radiation can be used as a control for correct timing and
magnitude of the occultation. Standard MeteoSwiss weather stations, however,
only record 10 min averages, which are too coarse for an in-depth
assessment. The Swiss FluxNet sites use averaging times ranging from 10 to
30 min (Table S1), with the exception of the CH-OE2 cropland and CH-DAV
forest sites where 1 min averages of all four radiation components were
available. Diffuse and total PPFD were also measured with the same resolution
at CH-OE2, and thus we used data from CH-OE2 as an example here. The expected
reduction of incoming short-wave radiation was ≈70 %
(Fig. a) during maximum occultation of the sun's disk (≈70 %). The difference between measured and expected incoming short-wave
radiation was -9.8±0.7 Wm-2 (mean ± SE). For a second
class pyranometer such as the CM3 model used in the CNR-1 radiometer, this is
the order of magnitude of the accuracy. However, the timing of the radiation
minimum in Fig. a was not exactly as expected assuming that
minimum radiation should be observed at the time of maximum occultation: the
theoretical radiation level during maximum occultation was reached 19 min
before the eclipse's maximum. Most likely, this was a confounding effect due
to minor high-level Cirrostratus clouds passing at that time; the fraction of
diffuse radiation already started to increase shortly after the first contact
on 20 March (Fig. b), whereas on the previous day, a curvilinear
decrease was observed as expected during this early morning period with
rising solar elevation. Cirrostratus clouds are the likely cause since images
taken every 2 min during the occultation phase at the site do not
indicate any signs of medium and low-level clouds. This coincidence of
shading by cirrostratus clouds and occultation of the sun may have led to the
stronger than expected decrease in short-wave radiation levels and the
earlier than expected radiation minimum.
Histogram of temperature reduction at all sites included in this study and those reported in the literature. The stacked bars
show number of sites of this study (green bars) on top of those for literature reports (violet bars). The solid lines show the best
fit of the scaled probability distributions (Eq. , Table ) of values reported in the literature
(violet line) and the combination of literature data with values reported in this study (dark green line).
Wind direction as a function of time at (a) CH-DAV and (b) CH-AWS on 18, 19, and 20 March, showing
a delayed wind direction reversal at CH-DAV on 20 March (a), and a suppression of the typical wind reversal at CH-AWS on 20
March (b) determined from 1 min average data. The bold lines are local polynomial regression (loess) fits.
Deviation of wind direction changes (range -180∘ to 180∘) during the eclipse expressed as Δfrequency
with respect to a random uniform distribution. The wind direction change is the difference between the first half of the eclipse in
comparison with the corresponding time period on 19 March.
Elevation dependence of cyclonic and anticyclonic influences during penumbral shading. Using MeteoSwiss sites the percentage
of sites showing cyclonic or anticyclonic effects was determined and elevational best estimates (bold line) and uncertainty of the
estimate (90 % confidence interval in blue) were estimated using nonparametric bootstrapping. The vertical dashed line at
50 % indicates the insignificant random outcome. Each horizontal mark near the elevation axis represents one weather station.
Spatial distribution of wind direction changes over Switzerland during the 20 March 2015 eclipse. Yellowish to reddish colors
indicate anticyclonic rotation, while greenish to bluish colors indicate cyclonic rotation. The bold and broken isolines show lines
of equal rotation angle at 20∘ intervals. The bold line separates areas with cyclonic from anticyclonic wind direction
changes. Base map from Atlas of Switzerland V3 , © 2016 swisstopo (JD100042).
To test this hypothesis, we made an attempt to empirically correct measured
short-wave incoming radiation for potential concurrent cloud shading that
leaves a trace in the fraction of diffuse radiation (Fig. c).
The assumption we made for such a correction was that no change in the ratio
of diffuse vs. direct radiation would occur due to the occultation of the
sun's disk alone. Thus, if we assume that the diffuse radiation (expressed as
absolute radiation flux density) remains unaffected by the cirrostratus
clouds, then we can correct this effect with
SWin, corr=ααfit⋅SWin,
where SWin and SWin, corr are measured
and corrected incoming short-wave radiation, and α and
αfit are ratios of diffuse and direct radiation for the
measurements and the model, where the empirical best fit for
αfit used in the model (Fig. c) is
αfit=diffuse radiationdirect radiation=(0.674±0.002)+(0.0452±0.0002)⋅(Δt)2,
with Δt being the time difference to local noon (12:36:45 CET on 20
March 2015) in hours. This empirical fit was used because α was not
a simple function of the solar elevation angle (fit not shown). The resulting
SWin, corr (red line in Fig. a) still did not
show a symmetric effect before and after the short-wave radiation minimum. An
analysis of images during that period (Fig. d) also indicated
that the shading effect was not symmetric during occultation: the image
brightness decreased very quickly after first contact, but then remained
almost constant, irrespective of the fraction of occultation of the sun's
disk. This potential effect of cloudiness during the eclipse was investigated
by analyzing the brightness of the vegetation in a sequence of phenological
camera images that were recorded every 2 min during the eclipse (4 times per
day otherwise). For this, we used the ImageJ software as implemented in the
Fiji image processing package version 2.0.0 (http://imagej.net/Fiji).
The vegetation brightness is simply the normalized gray value of the image
region that was manually defined as “vegetation”. A brightness of 100 %
corresponds to a white image, and 0 % is black.
Both observations – diffuse/direct radiation measurements and camera images – indicated a confounding effect of cirrostratus cloud
passage.
Long-wave radiation effects
To quantify the eclipse effect on long-wave back-radiation from the sky
(sometimes referred to as “longwave albedo”) we determined the difference
between the two long-wave flux components from 20 March 2015 and the
reference day before,
ΔLWx=LWx(20 March)-LWx(19 March),
where x is the incoming (in) or the outgoing (out) long-wave radiation
component. Then, two linear regressions between ΔLWin and ΔLWout were
calculated with the 1 min averages of the CH-OE2 site, (a) for the period of
the eclipse (09:26–11:42 CET), and (b) for the times of day not including
the period of the eclipse.
The reduction in short-wave radiation also reduced energy dissipation at
Earth's surface, which in turn reduced long-wave emitted radiation
(LWout; Fig. e, blue line). The reduction in
LWout was symmetric during the penumbral shadow passage,
supporting our interpretation that if a cloud passage confounded the
SWin term, then it most likely was a cloud type that
affected short-wave radiation more than the long-wave radiation components.
Cirrostratus may have this quality; however, the role of high clouds on
Earth's radiation budget is difficult to quantify .
The reduction in LWout reduced the re-emitted sky
radiation LWin (Fig. e, black line). The
regression between ΔLWin and
ΔLWout (Fig. ), i.e., the differences
between the respective radiation components measured on the day of the eclipse
and the day before the eclipse, showed an order of magnitude difference
between the penumbral shading (ΔLWin≈0.24ΔLWout) and the unshaded conditions
(ΔLWin≈2.84ΔLWout).
Direct effects on air temperature
Although radiation effects could only be investigated at the two sites having
1 min measurements, similar conditions were observed in 10 min data at all
radiation measurement sites. All sites showed a reduction in 2 m air
temperatures (Table S2). The strongest effect of –5.8 K was seen at
an Alpine site (Fig. ; VSSOR in Table S2) at
1987 ma.s.l., which was still completely snow covered during the
eclipse. The topographic setting (Fig. ) is a small basin
surrounded by a larger catchment area of 7.5 km2. The most important
effect thus appeared to be the position of the weather station. It was
located in a closed topographical basin ca. 66 m below the mountain
ridge. A cold-air pool building up during the eclipse could be drained
towards the Rhône valley over this ridge. Had we taken
the difference between 19 and 20 March for estimating the temperature effect,
then this site would have yielded a difference of -8.8 K.
To quantify the uncertainty of the temperature drop (and wind direction
effects) as a function of elevation, we employed nonparametric bootstrapping
with the “Bootstrap Resampling” package of R in combination with the “loess”
function with a span of 0.5 to describe the temperature drop or wind
direction effect as a function of elevation. Elevations were binned in
10 m intervals for the bootstrap procedure, which was repeated 9999
times. Statistical confidence intervals were then determined from the output
at the 95 % level. Bootstrapping is an efficient computer-based method to
quantify uncertainty intervals e.g.,.
Nonparametric bootstrapping means that the uncertainty calculations are done
on randomly selected subsets of all data points available in such a way that
the variation in the results obtained from many repetitions (9999 repetitions
in our case) represents the uncertainty of the estimate.
The mean effect over the entire elevation range covered by MeteoSwiss weather
stations (Table S2) as determined by nonparametric bootstrapping was
a reduction of 1.51±0.02 K (mean ± SE; see
Fig. ). The weakest effects were found at the lowest elevation
sites (<350 m a.s.l., reduction of 0.62±0.06 K), and the highest
elevation sites, where data coverage was poor (>3150 m a.s.l., reduction
of 0.69±0.03 K).
Although the 20 March 2015 eclipse featured a partial occultation of
66–70 % throughout Switzerland, the temperature effects
(Fig. , Table S2) were comparable to temperature reductions
previously reported in the literature for all eclipse types
(Fig. and Table ). We found no clear
dependence of temperature reductions on eclipse type, geographic location, or
other factors in the literature reports. Therefore, we summarized the data
set by fitting a gamma probability distribution to the data as shown in
Fig. . The gamma probability density function
f(ΔT)=1sa⋅Γ(a)⋅ΔT-T0a-1⋅eT0-ΔTs
was fit to the histogram of the maximum cooling ΔT during the
eclipse, where ΔT is the maximum temperature drop during an eclipse
event (a positive value), and a and s are the shape and scale parameters
of the probability density function f(ΔT), respectively. Γ is
the gamma function, and T0 is the reference value (or offset) of ΔT to fit the peak of the probability density function f(ΔT) to the
data. The distribution developed in this study will allow researchers to
quickly assess the probability that the existing literature contains values
that exceed a given measured temperature drop during an eclipse. The
parameter estimates for the probability distribution (Eq. ) are
given in Table . The average temperature drop reported in
the literature thus far was 2.6±1.7 K (based on Eq. ,
Table ), while the mean drop calculated for our study was
1.5±1.0 K. Already, temperature effects of this (smaller) magnitude
during occultation have the potential to induce thermal winds.
Wind direction effects
The effect of penumbral shading on wind direction was determined by comparing
(a) the 1 h reference period that ended 12 min before the first contact
with (b) the first half of penumbral shading (from first contact to maximum
occultation, which was roughly 1.1 h). For both periods, the vector-averaged
mean wind direction was computed, and then the rotation angle was determined.
The same procedure was repeated for the same time periods of 19 March, and
the difference in rotation angle was calculated as the net effect of
penumbral shading used in this study.
The most striking effect on wind direction was found at two Swiss FluxNet
sites with high-resolution 3-D wind velocity measurements. The timing of the
solar eclipse between 09:22 and 11:48 CET across Switzerland (see
Sect. ) coincided with the hours when the mountain valley wind
system typically changes direction by 180∘ under normal conditions.
A mountain valley wind system is characterized by down-valley winds at night
that contrast with up-valley winds during the day when solar irradiation on
the mountain slopes leads to convective uplifting of air masses, thereby
leading to up-slope and up-valley winds. At night, the radiative cooling on
the same valley slopes leads to the production of cold air, which is denser
than the surrounding air, and hence moves down-slope and down-valley (also
known as katabatic drainage flow). At the CH-DAV subalpine forest site
(1639 ma.s.l.), the penumbral shading resulted in a delay of the
onset of the daytime wind direction by roughly one hour
(Fig. a), whereas at the CH-AWS alpine grassland site
(1978 ma.s.l.) the shading even prevented the establishment of the
diurnal up-valley wind altogether (Fig. b). From 09:26:32 CET
(first contact) to 11:46:37 CET (fourth contact), the short-wave
radiation decreased by up to 68 % (-447 W m-2; 10 min
average) with respect to perfect clear-sky conditions two days before (18
March 2015). This delayed the down-valley to up-valley wind direction
transition that had been pronounced on both preceding days. Further, the
penumbral shading hindered the onset of up-valley winds in such a way that
the down-valley winds persisted even after the shading had ended. This means
that the valley wind blew in the opposite direction to what we would have
predicted for comparable conditions without penumbral shading. The lack of
reversal of wind direction could have been an effect of the cyclonic rotation
in the outer circle of the penumbra as predicted by for
this distance of ≈2000 km from the umbra. This lack of
reversal of wind direction at CH-AWS also occurred at a conventional
agrometeorological weather station ca. 1 km further up-valley
.
Although the cyclonic effect appeared to be rather pronounced at CH-DAV and
even more so at CH-AWS, most conventional weather stations did not show
a clear signal (Figs. S1–S4 in the Supplement). In principle, only sites
located on a valley bottom (Fig. S2) are expected to respond in a similar way
as CH-DAV and CH-AWS. In fair weather conditions, winds at slope sites
(Fig. S4) typically rotate clockwise when on the right sidewall of a valley
(i.e., facing down-valley) and counterclockwise on the left sidewall, as winds
undergo their diurnal transitions from along-valley to along-slope wind
systems . Thus, the inclusion of slope sites in our
analysis would confound our analysis of the wind turning associated with the
eclipse passage. The sites classified as “slope sites” (Table S3) turned
out to be embedded in rather complex terrain for which it was impossible to
make a reasonable prediction of which rotation should be expected. Consequently,
we focus primarily on the sites not located on slopes in the following
analysis.
The wind direction effect during the eclipse observed at the 165 MeteoSwiss
sites included both anticyclonic and cyclonic changes
(Fig. , green bars). If slope sites were excluded from the
analysis, then a rather clear dominance of the cyclonic effect was seen
(68.7 % of the remaining 112 sites; Fig. , black bars).
In comparison to conditions during the same time of day on the day before the
eclipse, the directional changes were mostly in the range -30 to
-45∘ during the period from the first contact to maximum
occultation (Fig. ). Large direction changes exceeding
-75∘ were less frequent than on the reference day before the
eclipse.
The effect of the penumbral shading on wind direction at 10 m a.g.l. showed
a strong dependence on site elevation. Figure showed an
elevational integration of the percentage of sites showing cyclonic influence
during the penumbral shading as expected according to
(Fig. b). There was only one elevation zone
(1708–2730 m a.s.l., 14 sites) in which a significant cyclonic rather than
anticyclonic influence (p<0.05 according to bootstrapped uncertainty
bounds) was found. The eclipse effect seen at the sites in this elevation
zone was clearly in support of the revised theory by , which
reduced the extent of the anticyclonic outflow around the umbra from ≈2400 km to ≈1600 km
(Fig. ). Interestingly, however, did not
find any discernible effect of this eclipse on wind directions in the UK and
Iceland, and thus deduced that there was no evidence of an eclipse cyclone.
In contrast to Switzerland, the UK observed a fair share of cloudiness
see satellite imagery in which may have muted some
meteorological responses to the occultation, as noted. But
when the analysis was constrained to sites with clear-sky conditions,
found a clear cyclonic effect of approximately
20–40∘ in the comparison of surface measurements with forecast model
simulations which were ignorant of the eclipse. While such a cyclonic effect
is what would be expected for Switzerland following the theory of
, it contradicts this theory for the geographic location of
the northeast Atlantic islands. Here, anticyclonic modification
of the wind direction should have been observed under both
and theories. Hence,
offer a new interpretation similar to the nocturnal low-level jet. This
interpretation is, however, not likely to explain conditions in the complex
terrain for the Swiss Alps because low-level jets develop primarily on the
Swiss Plateau , not in the Alpine valleys. Hence we did not
further adopt this interpretation here.
Spatial patterns of wind direction effects
Wind direction effects were spatially interpolated with ordinary kriging
using the krige.conv function of the geoR package of R. The partial sill
parameter was set to 300∘ and the range parameter was set to
1000 km. We tested the range 90–300∘ for the partial sill
setting, and 10–10 000 km for the range setting. The results were
similar due to the optimization method used in kriging and differed only in
very minor details (see examples in our response,
10.5194/acp-2017-321-AC2). The selection for the final computations was
thus based only on the facts that 300∘ (or -60∘) corresponded to the typical deviation
angle of wind directions under cyclonic influence and that 1000 km
covered the entire domain of Switzerland.
To test if there existed a geographically consistent pattern of wind
direction changes across Switzerland and Liechtenstein we performed a spatial
interpolation (Fig. ). For the reasons given above, we again
focused on sites that were not located on slopes (Fig. b).
The resulting map, however, is relatively similar to the one using all sites
(Fig. a).
If all stations were considered irrespective of their individual topographic
environment (Fig. a), then a complex pattern emerged that
did not show a clear spatial structure that could be related to the passing
of the penumbral shadow. Valley bottom sites (Fig. e) also
showed a mixture of anticyclonic effects in the west, south of the Alps, and
eastern Switzerland, with cyclonic effects seen between these three
anticyclonic areas. Sites on flat ground without clear topographic influences
(Fig. c) showed cyclonic effects in the center of
Switzerland, surrounded by anticyclonic effects namely in the Valais
(southwestern Switzerland) and the northeast. Mountain top and hilltop sites
(Fig. d) showed yet another pattern, with anticyclonic
influences seen mostly in the western half of Switzerland and cyclonic
effects in the eastern half. The only group of sites showing a consistent
spatial pattern were those in the elevation range 1708–2730 m a.s.l.
(Fig. f), the ranges with a statistically significant
preference for cyclonic effects in Fig. . This cyclonic
effect was seen across most of Switzerland, with the exception of a few
high-mountain sites in the Grisons (eastern Switzerland), the part of the
study domain that was farthest away from the trajectory of the eclipse
(Fig. ). This high-elevation range corresponded to the level
where the influence of the topographical roughness of the Swiss mountains
vanished in vertical radiosonde profiles at ≈2500 m;.
Taken together, these findings support the hypothesis that Switzerland was in
the cyclonic part of the penumbral shading as expected from the
theory (Fig. b), and not within the
anticyclonic part as would be expected from the theory
(Fig. a). Although no theory exists for how the transition
zone between the inner anticyclonic and the outer cyclonic ring around the
umbra should affect local wind directions, our analysis indicated that the
effect was most likely a combination of distance from the center combined
with the meso- and micro-scale topography around the site. This is not
unexpected if the net effect of the shading is weak. The fact that there was
a significant preference for cyclonic effects at sites in the elevation range
1708–2730 m a.s.l., whereas there was no significant difference between
anticyclonic and cyclonic effects at other elevations, suggested that the
reduced dimensions of the anticyclonic band around the umbra as proposed by
are more likely to be correct than the original
model, in which Switzerland should have experienced
a shift from cyclonic to anticyclonic influence as the occultation progressed
towards its local maximum.
Comparison with findings from other eclipses
The present study expands previously published analyses of the eclipse effect
using multiple stations in a given region. For example
performed a similar analysis of temperature effects using 16 weather stations
in the Phoenix, Arizona, metropolitan area, but were unable to analyze wind
direction effects. They mentioned two reasons why this was not feasible: (1)
the eclipse occurred in the morning when the wind flow tends to reverse due to
topographic heating in the Salt River Valley and (2) several of the
available stations did not record wind direction but only wind speed. The
first point given with reference to is typical for any
topographically varying region on the globe and exactly matches the
conditions experienced in this study. This effect was also emphasized by
, and : surface
cooling can trigger downslope and katabatic winds in mountainous regions,
such as the Alps, the Arctic islands, and Antarctica.
On 18 to 20 March 2015, synoptic pressure conditions over the European Alps
showed a persistent high-pressure band between the UK and Russia, providing
an excellent basis for comparing conditions during the solar eclipse with
previous days. Although such comparisons are one of the most used approaches
to quantify the effects of solar eclipses
,
in most cases weather conditions are rather variable and nonideal for direct
comparisons. In Switzerland, the 20 March 2015 eclipse occurred when snow
cover was disappearing at mountain locations around 1000 ma.s.l.,
and thus air temperatures varied more strongly from day to day than what
would be desirable for estimating temperature effects. Therefore, we employed
the less widely used approach by to fit a curvilinear
interpolation over the period of occultation from first to fourth contact.
This approach is typical for assessing effects on atmospheric constituents
such as ozone . A third approach to deal with the
variability of measurements during reference days is to take the 30 day
hourly median values for comparison . All three
approaches have advantages and disadvantages. In our case, the direct
comparison with the day before the eclipse during which similar synoptic
weather conditions persisted would have led to a more pronounced temperature
drop by an additional 1.3 K on average. The relationship between the
temperature drop of all sites reported in Table S2 as used in our study and
the absolute difference to the day before the eclipse was
ΔT19=-2.06±0.17+(0.48±0.09)ΔT,
with ΔT being the temperature drop used in this study and ΔT19 being the alternative calculation of ΔT as
the difference between the measurements made on 20 March and the same time of day on 19 March (the day before the eclipse; p<0.001,
adj. R2= 0.124).
Empirical probabilities Pr(≤ΔT) to relate
a future temperature drop during an eclipse to values previously published in
the literature and in this paper. Bold figures are above the median, and
figures in italics are below the 10 % percentile of the empirical
probability distribution. The sign convention uses positive ΔT if
temperature gets colder during the occulation phase.
ΔT (K)
All
Literature
This study
11.0
<0.001
<0.001
<0.001
10.5
0.003
0.008
<0.001
10.0
0.003
0.008
<0.001
9.5
0.003
0.008
<0.001
9.0
0.007
0.017
<0.001
8.5
0.007
0.017
<0.001
8.0
0.013
0.033
<0.001
7.5
0.016
0.041
<0.001
7.0
0.020
0.050
<0.001
6.5
0.023
0.058
<0.001
6.0
0.030
0.074
<0.001
5.5
0.036
0.083
0.005
5.0
0.056
0.116
0.016
4.5
0.085
0.182
0.022
4.0
0.108
0.223
0.033
3.5
0.151
0.289
0.060
3.0
0.203
0.372
0.092
2.5
0.292
0.521
0.141
2.0
0.420
0.653
0.266
1.5
0.567
0.736
0.457
1.0
0.721
0.835
0.647
0.5
0.872
0.917
0.842
0.0
0.970
0.967
0.973
-0.5
>0.999
>0.999
>0.999
-1.0
>0.999
>0.999
>0.999
-1.5
>0.999
>0.999
>0.999
-2.0
>0.999
>0.999
>0.999
The choice of method to determine the temperature drop – and any other
variable of interest affected by the penumbral shading – has a substantial
effect on the result, not only in our study but also in other published
results (e.g., Table ). Only 9 % of the published
studies reported stronger temperature effects than the most extreme
5.8 K drop at the Sorniot–Lac Inférier site reported here,
although most temperature drop reports in the literature relate to maximum
occultation during total and annular eclipses. During future eclipses it may
now become possible to engage citizen scientists to determine the temperature
drop during an eclipse and relate it to the probability distribution
presented in Table or to the gamma distribution of Eq.
() and Table , which would put their
measurements into context see also.
Differences in temperature drops among different eclipse events depend on the
solar elevation at the time of the eclipse . Here we
showed that although the eclipse happened in the morning hours and although
occultation was only partial across Switzerland, substantial drops in air
temperatures at several Swiss sites (Table S2) exceeded those observed at
other locations with higher solar angles (Table ). Thus,
our results suggest that the topographic setting, not the geographic position
on the globe, may actually be the most important determinant of the
temperature drop for individual sites.
With respect to wind direction effects, few studies are available for
comparison, with still being a classical reference after
more than one century. Regional weather forecast models are now run at
a spatial resolution that should theoretically allow simulations of the
effect of penumbral shading on wind speed (not assessed here due to
inconsistent variability among the sites) and wind direction (as presented in
Fig. ). followed this approach for the
southern UK during the 11 August 1999 eclipse, but found that the umbral
shading did not produce clear effects on wind vectors. They concluded that
the primary response of the model was restricted to temperature. Similarly,
's () simulations for central
Europe showed model responses for temperature and humidity, but not for
near-surface winds. Few wind observations are available for testing of such
models, and concluded that special observation campaigns
with very accurate sensors would be required to make progress. The six Swiss
FluxNet sites provided detailed measurements, and many more sites are
available globally via FluxNet . However, the
FluxNet sites provide only aggregated 30 min averages, not the raw data at
original resolution. Of our six sites with available high-frequency
measurements, two sites (CH-DAV and CH-AWS, Fig. b) showed
a clear wind direction effect during the penumbral shading, even though the
distance to the center of the eclipse was on the order of 2000 km.