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Research Cite this article: Burt S. 2016 Meteorological responses in the atmospheric boundary layer over southern England to the deep partial eclipse of 20 March 2015. Phil. Trans. R. Soc. A 374: 20150214. http://dx.doi.org/10.1098/rsta.2015.0214 Accepted: 15 December 2015 One contribution of 16 to a theme issue ‘Atmospheric effects of solar eclipses stimulated by the 2015 UK eclipse’. Subject Areas: meteorology Keywords: eclipse, observations, atmospheric, temperature, radiation, stability Author for correspondence: Stephen Burt e-mail: [email protected]

Meteorological responses in the atmospheric boundary layer over southern England to the deep partial eclipse of 20 March 2015 Stephen Burt Department of Meteorology, University of Reading, Reading RG6 7BE, UK SB, 0000-0002-5125-6546 A wide range of surface and near-surface meteorological observations were made at the University of Reading’s Atmospheric Observatory in central southern England (latitude 51.441◦ N, longitude 0.938◦ W, altitude 66 m above mean sea level) during the deep partial eclipse on the morning of 20 March 2015. Observations of temperature, humidity, radiation, wind speed and direction, and atmospheric pressure were made by computerized logging equipment at 1 Hz, supplemented by an automated cloud base recorder sampling at 1 min intervals and a high-resolution (approx. 10 m vertical interval) atmospheric sounding by radiosonde launched from the same location during the eclipse. Sources and details of each instrumental measurement are described briefly, followed by a summary of observed and derived measurements by meteorological parameter. Atmospheric boundary layer responses to the solar eclipse were muted owing to the heavily overcast conditions which prevailed at the observing location, but instrumental records of the event documented a large (approx. 80%) reduction in global solar radiation, a fall in air temperature of around 0.6◦ C, a decrease in cloud base height, and a slight increase in atmospheric stability during the eclipse. Changes in surface atmospheric moisture content and barometric pressure were largely insignificant during the event. This article is part of the themed issue ‘Atmospheric effects of solar eclipses stimulated by the 2015 UK eclipse’.

2016 The Author(s) Published by the Royal Society. All rights reserved.

1. Introduction and motivation

At the observing location, first contact was at 082408 UTC, peak eclipse (84.9% of the solar disc obscured, magnitude 0.872) was at 093001 UTC and fourth contact at 103957 UTC (all times are given in Universal Time, UTC, in the format HHMM or HHMMSS) [14]. Observing conditions during the eclipse were less than ideal, with an unbroken, uniform overcast of low stratus cloud and a steady northeasterly wind throughout. As a result of the complete cloud cover, the solar disc was not visible from the observing location at any stage of the eclipse (although rather frustratingly for the participants and organizers of the associated schools outreach programme [15], the stratus cover did break shortly after the eclipse was over).

2. Data and methods Brief details on sensors, measurement methods and exposures are given below; for more details on standard meteorological instruments and measurement protocols, see [16,17].

1

An accompanying note by S. M. Bower includes thermograph traces from West Yorkshire and Shetland.

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(a) Eclipse circumstances at Reading

rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 374: 20150214

A solar eclipse presents many opportunities for examining the impact of the reduction in solar radiation upon meteorological conditions, while accurate predictions of the circumstances and extent of the eclipse well before the event enable bespoke observing programmes to be set out in advance. Accounts of measurable changes in various meteorological conditions during eclipses, particularly air temperature, can be found as far back as the Renaissance. Perhaps the earliest scientific programme of observations accompanied the total solar eclipse of 22 May 1724 in Paris [1]. On this occasion, the astronomers Maraldi and Cassini of the French Academy of Sciences made observations at the Trianon Palace with King Louis XV. The King himself made observations of a thermometer and a barometer to look for ‘the temperature and air pressure variations that might happen during the eclipse’. Unfortunately, the eclipse itself was partly obscured by cloud. There are numerous other accounts, both quantitative and qualitative, of meteorological effects observed during solar eclipses, three examples being the fall in air temperature of 1.5◦ C during a total eclipse in southern Iraq on 25 February 1952 [2], a fall of 3◦ C during the total eclipse in southern Sweden on 30 June 1954 [3]1 and a fall of 3.5◦ C accompanied by a reduction in wind speed of 4.5 m s−1 in Chinguetti, Mauritania, during the total solar eclipse of 30 June 1973 [4]. More recently, the meteorological effects of the total eclipse of 11 August 1999 in the UK and central Europe have been described in detail, and compared with modelled atmospheric conditions with and without the effects of reduced solar radiation [5–10, and references therein], while Aplin [11] reviews other related eclipse studies relating to atmospheric measurements. In advance of the deep partial eclipse of 20 March 2015, a detailed programme of frequent surface and near-surface meteorological observations was planned at the University of Reading’s Atmospheric Observatory (latitude 51.441◦ N, longitude 0.938◦ W, altitude 66 m above mean sea level). Surface measurements made before, during and immediately after the eclipse, and quantities derived from those measurements, are listed in table 1, together with their sampling frequency; more details on instruments and their exposure are given in ‘Data and methods’ section. The majority of observations were made using a Campbell Scientific CR9000 computerized logger at a sampling and logging frequency of 1 Hz. A high-resolution vertical atmospheric profile sounding was obtained during the eclipse from a ground-launched radiosonde, details of which are given in table 2.

2

unit ◦ C

abbreviation Tdry

sensor platinum resistance thermometer (PRT)

exposure Stevenson screen (at standard 1.25 m above ground)

frequency 1 Hz

logged

◦

C

Tfw

fine-wire PRTa

open air, shielded from solar radiation

1 Hz

◦

C

Twet

PRT

Stevenson screen

1 Hz

relative humidity

%

RH

capacitance sensor

Stevenson screen

1 Hz

...................................................................................................................................................................................................................................................................................

wet bulb temperature

...................................................................................................................................................................................................................................................................................

air temperature

...................................................................................................................................................................................................................................................................................

element dry-bulb temperature

grass temperature

global solar radiation

surface (grass) temperature

radiation

C

−2

Tgrass

PRT

surface, open air

1 Hz

Sg

pyranometer

open air, plane surface

1 Hz

W m−2 Sd

pyrheliometer

mounted on automated solar disc tracker

1 Hz

W m−2

Rn

albedometer

open air, plane surface

1 Hz

ground heat flux

potential gradient

wind speed at 2 m, mean and gust

ground heat flux

atmospheric electricity

wind direction and speed

Wm

G

flux plate

buried at shallow depth in soil

1 Hz

Vm

ms

−1

PG

field mill

mounted at approx. 3 m above ground

1 Hz

ms

−1

U2

cup anemometer

at 2 m above ground

1 Hz

U5

cup anemometer

at 5 m above ground

1 Hz

m s−1 U10

cup anemometer

at 10 m above ground (standard height)

1 Hz

degrees True

dd10

potentiometer wind vane

at 10 m above ground (standard height)

1 Hz

m.s.l. pressure

cloud base height

cloud base height

hPa

MSLP

solid-state pressure sensor

indoor exposure

1 Hz

CBL

infrared LIDAR

1 min

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b By meteorological convention, the highest gust in any given period is normally defined as the highest 3 s running mean wind speed in that period, although in this summary tabulated results are 1 Hz records.

a The fine-wire PRT instrument is described in [12]. With negligible mass, it has a very low time constant. By contrast, PRTs exposed in a Stevenson screen have a variable time constant of several minutes, depending upon wind speed [13].

.............................................................................................................................................................................................................................................................................................................................................

metres above ground

.............................................................................................................................................................................................................................................................................................................................................

barometric pressure

.............................................................................................................................................................................................................................................................................................................................................

wind direction at 10 m

...................................................................................................................................................................................................................................................................................

wind speed at 10 m, mean and gust b

...................................................................................................................................................................................................................................................................................

wind speed at 5 m, mean and gust

b

...................................................................................................................................................................................................................................................................................

b

.............................................................................................................................................................................................................................................................................................................................................

−1

.............................................................................................................................................................................................................................................................................................................................................

−2

.............................................................................................................................................................................................................................................................................................................................................

net radiation

...................................................................................................................................................................................................................................................................................

direct solar radiation

...................................................................................................................................................................................................................................................................................

Wm

.............................................................................................................................................................................................................................................................................................................................................

◦

.............................................................................................................................................................................................................................................................................................................................................

parameter air temperature and humidity

Table 1. Details of surface measurements made at the Reading Atmospheric Observatory during the eclipse of 20 March 2015. Those in italics are discussed in subsequent sections.

3

element ambient air temperature

unit C, K

◦

abbreviation Tdry

sensor platinum resistance thermometer (PRT)

..........................................................................................................................................................................................................

humidity

relative humidity

%

RH

capacitance sensor

...............................................................................................................................................................

humidity mixing ratio

g kg

−1

HMR

derived from Tdry and RH

...............................................................................................................................................................

dew point

◦

C

Tdew

derived from Tdry and RH

u, v

derived from GPS location

ff

derived from GPS location

..........................................................................................................................................................................................................

wind speed

u and v components

−1

ms

....................................................................................................................................................................

−1

scalar wind speed

ms

wind direction

Deg True

Dd

derived from u and v

altitude above launch point

m

z

solid-state pressure sensor

altitude above mean sea level

m

zmsl

= z + site launch altitude (66 m) offset

..........................................................................................................................................................................................................

wind direction

..........................................................................................................................................................................................................

height

....................................................................................................................................................................

..........................................................................................................................................................................................................

barometric pressure

observed ambient pressure

hPa

PPP

solid-state pressure sensor

..........................................................................................................................................................................................................

−2

radiation

global solar radiation

Wm

Sg

photodiode

global

cloud layer height

m

CBL

derived from photodiode output (figure 6)

..........................................................................................................................................................................................................

..........................................................................................................................................................................................................

(a) Solar radiation and net radiation Global solar radiation on a horizontal surface Sg was measured by a horizontally mounted Kipp & Zonen CM11 pyranometer, and net radiation using a Kipp & Zonen NR-Lite2 net radiometer. Both sensors were sampled and logged at 1 Hz, and then averaged over 60 s periods for this analysis unless otherwise stated. The time constant of both sensors is approximately 10 s.

(b) Air temperature and humidity Observations of air temperature were made using a calibrated platinum resistance sensor as a dry-bulb thermometer housed in a large-pattern Stevenson screen within the observatory, the sensor itself mounted at 1.25 m above ground level. (A Stevenson screen is a white, doublelouvred thermometer enclosure which protects the thermometers from direct and reflected solar radiation, infrared terrestrial radiation and precipitation, while allowing a measure of natural ventilation across the thermometers therein: more details are in [16, ch. 5].) Relative humidity (RH, %) was measured using a Rotronic capacitance-based humidity sensor, also housed within the Stevenson screen. Owing to the partial obstruction of the free air flow by the screen’s louvres, the response of the combined screen–sensor system to an external change in true air temperature or humidity exhibits a lag time, which increases with decreasing wind speed [12,13,18].2 With 2 m wind speeds > 2 m s−1 the lag time is approximately 2.5 min, increasing, when calm, to at least 15 min. Under the conditions prevailing, the lag time was probably between 5 and 10 min. Additional observations of air temperature were made using a fine platinum wire sensor [12], exposed at 1.25 m above ground in the open air, underneath a shade ring to avoid incident direct solar radiation. This instrument has a very small time constant (less than 1 s) and is thus useful 2

Details of the fine-wire thermometer are also given in [17, §5.3].

.........................................................

parameter temperature

4

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Table 2. Details of the vertical profile measurements made at 0845 UTC, by radiosonde launched from the Reading Atmospheric Observatory during the eclipse of 20 March 2015. Vertical resolution is approximately 10 m. Ambient temperature and dew point in the lowest vertical 1000 m of the ascent are plotted in figure 5.

for identifying not only rapid changes in temperature, but variations in the rate of change of temperature influenced by atmospheric processes including near-surface turbulence.

3. Results and discussion A brief summary of observations by meteorological parameter is given below, followed by a consolidated summary of results in the discussion section. For most elements, two graphical time series are presented—the first for the civil day (0000–2359 UTC) on 20 March 2015 to show the eclipse period in its diurnal context, and the second a more detailed plot of the period 0700–1100 UTC, spanning the eclipse period at the observing location (0824–1040 UTC); the eclipse period is shown by graphical symbols on the 4 h plots. Finally, a short summary of the vertical atmospheric profile as observed by the radiosonde launched at 0845 UTC is also presented.

(a) Solar radiation and net radiation Figure 1a presents a time series of global solar radiation on a horizontal surface (W m−2 ) and net radiation (W m−2 ) for the civil day (0000–2359 UTC) on 20 March 2015; figure 1b is a more detailed plot of the same elements for the period 0700–1100 UTC. For clarity, the points plotted are 1 min averages of the 1 Hz sampled data for both plots. Despite heavily overcast conditions, both global solar radiation and net radiation showed a slow but steady increase from sunrise (0607 UTC) until first contact at 082408 UTC. The peak global solar radiation (averaged over 1 min) prior to the eclipse was 46 W m−2 in the 60 s ending 082500 UTC, after which a slow irregular fall commenced, falling over the next hour to a minimum of 9 W m−2 between 0927 and 0930 UTC, a reduction of 80% and similar to the values recorded about 50 min after sunrise that morning. Net radiation fell from a peak 23 W m−2 at 0826 UTC to a minimum −0.4 W m−2 at 0927 UTC. The minimum 1 Hz value was −0.9 W m−2 at 093104 UTC, remaining at or below zero between 09.27 and 0931 UTC. (The ‘peak eclipse’ at the observing location was at 093001 UTC, with 84.9% obscuration.) By integrating all the daily measurements into a global solar radiation total for a period around the eclipse, and for the civil day (0000–2359 UTC), we can quantify the approximate loss of solar energy on a horizontal surface resulting from the eclipse. By comparing the measurements made during the eclipse with an assumed linear increase in observed Sg from first to fourth contact, the net effect of the eclipse was to reduce global solar radiation receipts during the period 0700 to 1100 UTC by about 27% (actual 0.70 MJ m−2 , versus the assumed linear increase case of 0.96 MJ m−2 ). (The assumption of a linear increase over the period 0824 to 1040 UTC is reasonable, when compared with similar conditions on the previous day: the choice and gradient of trendline do not greatly affect the calculated deficit.) For the civil day as a whole, the net effect was much smaller, a reduction in Sg of slightly less than 3% (9.85 MJ m−2 versus 10.14 MJ m−2 ), owing to the much higher values of Sg which prevailed in the sunny conditions after 1100 UTC (figure 1a). It was interesting to observe that the relatively slow decline in light levels coupled with the greater range in sensitivity of the human eye meant that few observers felt there had been any significant reduction in light levels, despite the clear instrumental evidence of this: it was only when automatic security lights around the university campus began to come on near the peak eclipse period that the reduction in light levels became obvious to the casual observer.

.........................................................

Within the observatory, wind speeds were measured at 2 m, 5 m and 10 m above ground level by Vector Instruments cup anemometers (the standard height for wind observations is 10 m). Wind direction was measured by a Vector Instruments potentiometer wind vane at 10 m. Both wind speed and direction were sampled and logged at 1 Hz.

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(c) Wind speed and direction

5

(a)

950

Reading, 0000–2359 UTC 20 March 2015

6

radiation (W m–2)

650 550 450 350 250 150 50 –50 0000

0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300

time (UTC) (b)

Reading, 0700–1100 UTC 20 March 2015 180 160

net radiation solar radiation

140 radiation (W m–2)

120 100 80 60 40 20 0 0700 –20

time (UTC) 0730

0800

0830

0900

0930

1000

1030

1100

Figure 1. Time series of solar and net radiation for Reading Atmospheric Observatory on 20 March 2015 (units W m−2 ). (a) The period 0000–2359 UTC, (b) 0700–1100 UTC. (a) The top of atmosphere (TOA) global solar radiation for 20 March is shown, without eclipse in the heavy outer line, and with eclipse by the dotted line; 1 min averages of 1 Hz sampled global solar radiation are shown by the lighter black line, and similarly 1 min averages of 1 Hz sampled net radiation by the lighter grey dashed line. For clarity, the TOA plots are omitted in (b). The graphics along the x-axis on (b) and following plots represent, respectively, the time of first contact (0824 UTC), peak eclipse (0930 UTC) and fourth contact (1040 UTC). Overcast conditions prevailing during the eclipse cleared away rapidly just after 1200 UTC, and the afternoon became sunny (figure 1a), with peak global solar radiation values in excess of 800 W m−2 around 1230 UTC.

(b) Air temperature and humidity The time series of air (screen dry bulb) temperature (◦ C) and RH (%) from the Reading Atmospheric Observatory for the civil day (0000–2359 UTC) on 20 March 2015 are shown in

.........................................................

750

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global solar radiation, Sg net radiation, Rn TOA Sg, 20 March, no eclipse TOA Sg, 20 March with eclipse

850

air temperature (ºC) relative humidity (%)

Reading, 0000–2359 UTC 20 March 2015

95

12

90

11

85

10

80

9

75

8

70

7

65

6

60

5

55

0000 0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300

time, UTC Reading, 0700–1100 UTC 20 March 2015

50

air temperature (ºC) relative humidity (%)

5.0

93

4.9 92

4.7 4.6

91

4.5 4.4

90

4.3 4.2

relative humidity (%)

air temperature (°C)

4.8

89

4.1

(c)

0800

0830

0900 time, UTC

0930

1000

1030

Reading, 0700–1100 UTC 20 March 2015 4.7 4.5

air temperature (°C)

0730

88 1100

12

fine-wire air temperature (ºC)

10

standard error

4.3

8

4.1

6

3.9

4

3.7

2

3.5 0700

0730

0800

0830

0900

0930

1000

1030

standard error (dimensionless)

4.0 0700

0 1100

Figure 2. Time series of air temperature (◦ C) and relative humidity (RH, %) measurements at Reading Atmospheric Observatory on 20 March 2015. (a,b) The dry-bulb temperature and relative humidity from sensors located in the Stevenson screen. (c) The air temperature record from the open-air fine-wire sensor located a few metres distant from the Stevenson screen together with the (dimensionless) standard error parameter ε, a proxy measure of ‘turbulence intensity’ within the near-surface boundary layer (see text for details). (a) The period 0000–2359 UTC, (b,c) 0700–1100 UTC. All data points are 1 min averages of 1 Hz sampled points. Note that the stepping in the relative humidity plots is due to pre-rounding of the sampled sensor data to the nearest integer prior to logging.

.........................................................

13

4

(b)

7 100

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air temperature (°C)

14

relative humidity (%)

(a)

Figure 3a shows the mean and (1 Hz) maximum gust and minimum lull wind speeds (m s−1 ) as measured at 10 m within the Reading Atmospheric Observatory for the civil day (0000–2359 UTC) on 20 March 2015; figure 3b shows the more detailed plot of the same elements for the period 0700–1100 UTC. (Note that here the ‘highest gust’ is the greatest logged 1 Hz value, rather than the standard measure of 3 s running mean.) For simplicity, mean and 1 Hz gust and lull wind speeds are plotted as 1 min values sub-sampled from the 1 Hz logged data.

.........................................................

(c) Wind speed and direction

8

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figure 2a; figure 2b is a more detailed plot of the same elements for the period 0700–1100 UTC. For both plots, the points plotted are 1 min averages of the 1 Hz sampled data owing to the relatively large time constant of the combined screen–sensor system under the conditions. As a result of the heavily overcast conditions, the fall in air temperature during the eclipse (from the highest pre-eclipse value) was slight—just 0.29◦ C (maximum 4.65◦ C at 0906 UTC, minimum 4.36◦ C at 0925 and 0926 UTC). The air temperature began to fall only when solar and net radiation values began to decrease sharply just after 0900 UTC (figure 1b), and in this shows a clear departure from the diurnal rise that would be expected even under overcast conditions; the minimum air temperature during the eclipse (1 min average) was reached at 0925–0926 UTC, shortly before peak obscuration. Comparison of the air temperature on 20 March with the corresponding value 24 h previously, a similarly overcast morning (not shown), showed that the expected diurnal rise in temperature of approximately 0.3◦ C during the period 0824–1040 UTC was suppressed, suggesting that the total reduction in temperature owing to the eclipse was approximately 0.6◦ C once the expected diurnal progression is included. Additional observations of air temperature were made using a fine platinum wire sensor [12], exposed at 1.25 m above ground in the open air, underneath a shade ring to avoid incident direct solar radiation. This instrument has a very small time constant (less than 1 s) and is thus useful for identifying not only rapid changes in temperature, but variations in the rate of change of temperature influenced by atmospheric processes including near-surface turbulence. The reduction in air temperature as measured by this fast-response sensor in the open air was slightly greater than that in the Stevenson screen, at 0.43◦ C in 23 min (maximum 4.24◦ C at 0902 UTC, minimum 3.81◦ C at 0925 UTC, both 1 min averages). Figure 2c shows 1 min mean fine-wire sensor temperatures (60 × 1 Hz samples), together with the (dimensionless) standard error parameter ε, where ε = σT /Tfine × 104 and where σT is the standard deviation (K) of the logged fine-wire temperature and Tfine is the arithmetic mean of the logged fine-wire temperature (K) during the 60 s ending at the ‘exact minute’ observation time. In this context, ε can be interpreted as a dimensionless proxy measure of ‘turbulence intensity’ within the near-surface boundary layer. There was a marked reduction in ε after 0900 UTC, corresponding with the most rapid reduction in global and net radiation (table 3 and figure 1b). The average ε for the 10 min centred on peak eclipse at 0930 UTC was 1.6, almost half that observed (2.8) in the 30 min ending 0900 UTC, and similar to values of ε in the hour preceding sunrise (average 1.2), representing night-time steady-state conditions under a heavy overcast with a steady breeze. Variability at peak eclipse (average ε 1.6 0925–0935 UTC) was also much less than half the variability in the same time period on the previous day (average ε 4.2), a similarly overcast morning. The minimum temperature observed during the eclipse event occurred at 0925 UTC on both screen and fine-wire records. The coincidence is slightly surprising, in view of the expected lag in screen response time, where a difference of several minutes was anticipated. There was a barely discernable increase in RH during the eclipse, around 1%, which is at the very limit of instrumental detectability. As a result, there was only a very slight change in atmospheric moisture content: the specific humidity q, calculated from observations of air temperature and RH (not shown), declining by approximately 0.1 g kg−1 .

20 March 2015

period (UTC)

average ε

average ε

maximum 1 min ε

minimum 1 min ε

notes

0508–0607

1.4

1.2

1.7

0.9

60 min preceding sunrise

0600–0629

1.3

1.3

1.6

1.0

0630–0659

1.3

1.4

2.0

1.0

0700–0729

1.4

1.7

3.2

1.2

0730–0759

1.8

2.1

3.6

1.4

0800–0829

2.1

2.7

3.5

1.3

0830–0859

2.9

2.8

5.3

1.5

0900–0929

4.4

2.2

5.2

1.2

0925–0935

4.2

1.6

2.2

1.2

0930–0959

5.3

2.0

4.1

1.3

1000–1029

7.8

4.3

9.5

2.2

fourth contact 1040

1030–1059

6.6

7.0

10.0

3.8

breaking cloud

average 0700–1059

4.0

3.1

.......................................................................................................................................................................................................... .......................................................................................................................................................................................................... .......................................................................................................................................................................................................... .......................................................................................................................................................................................................... ..........................................................................................................................................................................................................

first contact 0824

.......................................................................................................................................................................................................... .......................................................................................................................................................................................................... ..........................................................................................................................................................................................................

peak eclipse 0930

.......................................................................................................................................................................................................... .......................................................................................................................................................................................................... .......................................................................................................................................................................................................... ..........................................................................................................................................................................................................

..........................................................................................................................................................................................................

Throughout the civil day, there was a slow decrease in mean wind speeds, and a slow backing (anticlockwise progression) in wind direction from northeasterly through to northerly by around 1800 UTC. (For simplicity, wind directions are not plotted on figure 3a,b.) Mean and gust wind speeds were lower during the eclipse period than beforehand: table 4 shows the mean speed and mean 1 Hz gust speeds for 30 min periods leading up to, during and immediately after the eclipse period. During the 30 min immediately following the peak eclipse, the mean speed and mean 1 Hz gust wind speeds fell to the lowest of the morning (table 4 and figure 3b), although lower values were subsequently attained during the afternoon (figure 3a) as part of the slow reduction in wind speeds observed during the day. Although mean and mean 1 Hz gust wind speeds were lowest of all in the 30 min commencing 0931 UTC, i.e. during and immediately following peak eclipse, the gust ratio (gust speed/mean speed) was highest during this period, and several of the highest individual minute gust ratios during the eclipse period were centred on peak eclipse. This may be a statistical artefact itself resulting from the general reduction in wind speed observed over the preceding hours, or entirely coincidental owing to the turbulent nature of surface wind variations. Support for the latter hypothesis comes from an analysis of 1 min wind records from another site 10 km distant which showed no such increase in gust ratio at this time, suggesting a coincidental local variation was the more likely cause.

(d) Cloud base and type Anecdotal accounts abound of rapid variations in cloud amount during solar eclipses, but it can be difficult to determine whether the reported changes are due simply to more intensive examination of a small area of sky around the solar disc during the eclipse itself, or whether a genuine ‘Moses moment’ phenomenon occurs, whereby the passage of the lunar shadow and its effects upon the atmosphere result in a short-term clearance of cloud. It is for this reason

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19 March 2015

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Table 3. Turbulence intensity proxy parameter ε(σT /Tfine × 104 ) of the 1 Hz fine-wire thermometer data for various periods at Reading Atmospheric Observatory, 20 March 2015, with corresponding values for the previous day for comparison. All times UTC. Italics denote the peak eclipse.

(a)

Reading, 0000–2359 UTC 20 March 2015

10

8

10 m wind speed (m s–1)

5 4 3 2 1 0 0000 0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300

time (UTC) (b)

Reading, 0700–1100 UTC 20 March 2015 4.0

10 m wind speed (m s–1)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0700

time (UTC) 0730

0800

0830

0900

0930

1000

1030

1100

Figure 3. Time series of 10 m wind speed (m s−1 ) measurements at Reading Atmospheric Observatory on 20 March 2015. (a) The period 0000–2359 UTC, (b) 0700–1100 UTC. The grey bars show maximum and minimum 1 Hz sampled wind speed for each minute, and the central darker grey line shows each 1 min average.

that continuous objective observations of cloud base were included in our eclipse observation programme, using a Vaisala CL31 ceilometer mounted within the observatory. This instrument uses pulsed diode laser LIDAR technology (LIDAR = LIght Detection And Ranging) to determine the height of cloud layers from the time-of-flight of the backscattered laser signal. Measurements from this instrument are averaged and output over 60 s intervals. Until the very end of the eclipse morning, the sky remained heavily overcast with an even deck of low stratus. Figure 4a shows the plot (5 min data points) of the minimum cloud base between 0000 and 2359 UTC on 20 March 2015 as recorded by the LIDAR within the observatory, and figure 4b the expanded plot covering the period 0700–1100 UTC as in the previous plots.

.........................................................

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Table 4. Mean wind speeds (m s−1 ) and mean 1 Hz gust speeds (m s−1 ) for 30 min periods at Reading Atmospheric Observatory, 20 March 2015. Winds measured at 10 m, and all times UTC. mean gust ratio

0701–0730

1.69

2.49

1.47

0731–0800

1.51

2.21

1.46

0801–0830

1.82

2.51

1.38

0831–0900

1.67

2.47

1.48

0901–0930

1.43

2.12

1.48

0931–1000

1.14

1.84

1.61

1001–1030

1.51

2.27

1.50

eclipse details

.......................................................................................................................................................................................................... .......................................................................................................................................................................................................... ..........................................................................................................................................................................................................

first contact 0824

.......................................................................................................................................................................................................... ..........................................................................................................................................................................................................

peak eclipse 0930

.......................................................................................................................................................................................................... ..........................................................................................................................................................................................................

1031–1100

1.48

2.17

1.47

fourth contact 1040

1101–1130

1.66

2.30

1.39

breaking cloud

1131–1200

1.39

2.07

1.49

means 0701–1200

1.53

2.25

1.47

.......................................................................................................................................................................................................... .......................................................................................................................................................................................................... ..........................................................................................................................................................................................................

..........................................................................................................................................................................................................

The observed minimum cloud base decreased fairly steadily from 440 m at 0000 UTC to a minimum of 217 m at 0915 UTC, thereafter rising at a slightly faster rate than the decrease until the cloud cleared shortly after 1200 UTC. Figure 4b suggests a reduction in cloud base height during the eclipse commencing at first contact through to 0915 UTC, of approximately 20 m. Previous observational data and modelling studies have shown that a diurnal variation in cloud base can be expected under steady-state conditions of a persistent cloud deck below a marked anticyclonic subsidence inversion [19,20]. The variations arise from a complex interplay of processes including solar warming, buoyancy transport from the surface and upward and downward fluxes of water vapour, and result in a lifting of cloud base during the day and a decrease at night. Under the circumstances around the day of the eclipse, the minimum cloud base height could be expected to occur an hour or two after dawn. On 20 March sunrise at the observing location was at 0607 UTC; on the previous day, a very similar overcast morning, the minimum cloud base height (308 m) was reached at 0810 UTC. It is tempting (if very simplistic) to extrapolate the observed reduction in surface temperature (approx. 0.3◦ C) into a reduction in cloud base height using the saturated adiabatic lapse rate of 6 K per 1000 m; this would suggest a possible reduction in cloud base of approximately 50 m, which is of the same order of magnitude as that observed. It is, therefore, suggested that the reduction in solar radiation once the eclipse commenced acted to delay the expected diurnal lifting in cloud base height, with the result that the minimum cloud base was attained during the eclipse and just over an hour later than the previous morning. This is believed to be the first unambiguous instrumental evidence of the reduction in cloud base during a solar eclipse.

(e) Vertical atmospheric profile in the boundary layer A vertical profile of temperature, humidity, wind speed and direction was made by radiosonde during the eclipse, launched at 0845 UTC. The standard Vaisala radiosonde included an additional solar radiation sensor package [21,22]. Figure 5 shows the temperature and dew point in the lowest 1000 m above ground on an (expanded) conventional meteorological thermodynamic diagram or tephigram plot (for details of the tephigram, see [23,24]). On this ascent, 1000 m above ground level corresponded to 902 hPa. The atmospheric boundary layer was stable below a very marked inversion at 590 m above

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mean 1 Hz gust speed (m s−1 )

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period (UTC)

mean wind speed (m s−1 )

11

Reading, 0000–2359 UTC 20 March 2015 (a) 1800

12

height of lowest cloud base (m)

900

600

300

0 0000 0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300

time (UTC) Reading, 0700–1100 UTC 20 March 2015 (b) 300

height of lowest cloud base (m)

290 280 270 260 250 240 230 220 210 200 0700

0730

0800

0830

0900

0930

1000

1030

1100

time (UTC)

Figure 4. Time series of lowest cloud base height (m above ground level) at Reading Atmospheric Observatory on 20 March 2015. (a) The period 0000–2359 UTC (note that the cloud cleared shortly after 1200 UTC), (b) 0700–1100 UTC. For clarity, only 5 min samples are plotted, although the original data are available at 1 min resolution. The standard deviations of the sub-minute samples have been omitted as these are very small, typically 3 m, less than the thickness of the plot line on (a). ground level (656 m above mean sea level, 949 hPa), the temperature rising 2.6◦ C in 20 m vertical ascent. The base of the stratus layer (220 m above ground at 0845 UTC, according to the ceilometer record) is not particularly evident on the ascent, as RH rose to 100% just 100 m above ground level. The top of the stratus layer would be expected to lie at the inversion, which was also marked by a rapid fall in RH, and this was confirmed by the solar radiation sensor on the sonde as it emerged through the cloud layer (figure 6). The measurements obtained show the marked change from

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1200

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1500

800

5

13

5

–5 0 10 –1 –

10 1000 1050 –5°C

1000 hPa 5

0 0°C

+5°C

Figure 5. Air temperature (right-most plotted line) and dew point (left plot, derived from relative humidity measurements) in the lowest 1000 m of the boundary layer from the radiosonde ascent launched from Reading Atmospheric Observatory at 0845 UTC on 20 March 2015. The plot is an expanded lower section of a conventional meteorological thermodynamic diagram known as a tephigram; isobars run almost horizontally (900 hPa and 1000 hPa are marked), while isotherms run at about 45 degrees to the pressure surfaces—those for −5◦ C, 0◦ C and +5◦ C are marked. (Online version in colour.)

height (km)

1.5

1.0

0.5

0 0 100

300 S(W m–2)

500

Figure 6. Global solar radiation Sg plot of the first 1500 m vertical ascent from the radiosonde launched at 0845 UTC on 20 March 2015 from Reading Atmospheric Observatory, showing the rapid increase in solar radiation around 600 m above ground level marking the top of the stratus layer. (Figure courtesy of Prof. Giles Harrison.) diffuse isotropic radiation within cloud, increasing in magnitude as the cloud is traversed, to variable solar radiation above the cloud due to the effect of swing of the platform [21]. (A more detailed description of the instrumentation used in these balloon-carried measurements is given in Harrison et al. [22].) The stratus layer was, therefore, just under 400 m thick at the time of the ascent. Wind direction showed a steady veer with height up to the level of the inversion, from 002◦ True (northerly) at 100 m above ground level to 048◦ (northeasterly) at the inversion. There was little significant change above the inversion, with wind direction remaining mostly between north (360◦ ) and east (090◦ ) throughout the rest of the ascent, which reached 17 304 m 49 min after launch. Wind speeds increased from 2 m s−1 just above ground level to 4 m s−1 at 250 m above ground (within the stratus layer), with little change thereafter until 500 m. There was a relatively rapid increase to just under 8 m s−1 at 650–700 m above ground level, just above the inversion, and thereafter a slow decrease to around 6 m s−1 at 1000 m altitude.

(f) Other elements Observations at 1 Hz were also made of a variety of other elements (table 1) including barometric pressure, and examined closely for possible eclipse-related variations. Perhaps surprisingly, in the context of previous eclipse accounts [11], effects of this eclipse were either not evident, or the changes were below measurable limits, for the other elements listed in table 1.

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900 hPa

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–0 7

900

4. Conclusion

Data accessibility. The datasets supporting this article have been uploaded as part of the electronic supplementary material and can be accessed at http://dx.doi.org/10.17864/1947.8. This is a compressed archive, containing two metadata files describing the records and units (one document is in Microsoft WORD format, the other in Adobe PDF format—the content is identical) and four csv-format files containing, respectively, (1) 1 Hz resolution instrumental data logged at the University of Reading’s Atmospheric Observatory for 19 and 20 March 2015; (2) as above, but 1 min resolution data (averages and/or extremes of a subset of the 1 Hz data in the previous file); (3) high-resolution radiosonde data (2 s sample interval, approx. 10 m vertical interval) from the radiosonde ascent launched from the University of Reading’s Atmospheric Observatory at 0845 UTC on 20 March 2015; and (4) five-minute resolution ceilometer (LIDAR) cloud base data logged at the University of Reading’s Atmospheric Observatory for 20 March 2015. Competing interests. The author has no competing interests. Funding. The author is employed by the School of Mathematical and Physical Sciences, University of Reading. Acknowledgements. Dr Roger Brugge kindly provided access to the observatory’s 1 Hz logger data and to the radiosonde ascent record; Prof. Giles Harrison provided the TOA Sg data plotted in figure 1a together with useful comments on an earlier draft of this paper.

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— the most pronounced effect, at least in terms of the instrumental record, was the 80% reduction in global solar radiation Sg over the period from immediately after first contact at 0824 UTC (Sg 46 W m−2 in the 60 s ending at 0825 UTC) until peak eclipse at 0930 UTC when 85% of the solar disc was obscured (Sg 9 W m−2 between 0927 and 0930 UTC). The net effect of the eclipse was a reduction in the integrated receipt of Sg of around 27% over the period 0700–1100 UTC; — the reduction in inbound solar radiation resulted in a rapid decrease in net radiation Rn over the same period, which became briefly negative at the time of peak eclipse; — the reduction in solar radiation, and the consequent rapid fall in net radiation, resulted in a fall in air temperature of 0.29◦ C within 20 min as measured by thermometers in the Stevenson screen. The fall in temperature commenced about 40 min after the first reduction in solar radiation, and the pre-eclipse screen maximum was not re-attained until about 40 min after the peak eclipse. Accounting for the expected diurnal increase in air temperature during the period of the eclipse, the reduction in air temperature owing to the eclipse was around 0.6◦ C; — the reduction in air temperature as measured by a fast-response thermometer in the open air was slightly greater than that shown by conventional screen-based meteorological thermometry, i.e. 0.43◦ C in 23 min; — the slight reduction in near-surface temperature observed during the eclipse acted to intensify, slightly, the existing stability of the lower levels of the atmosphere. A reduction in surface turbulence would be expected from increasing stability, and there is proxy evidence for this in the high-resolution instrumental records made during the event, particularly the fine-wire thermometer record; — there was clear instrumental evidence of a decrease in cloud base height of approximately 20 m during the eclipse event, over and above what might be expected from diurnal changes, even though the layer of stratus cloud obscuring the eclipse was quite thin (approx. 400 m deep); and — changes in atmospheric humidity and specific humidity during the eclipse period were small, and close to the limit of instrumental resolution.

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Atmospheric boundary layer responses to the deep partial solar eclipse of 20 March 2015 at this site in southern England were muted owing to the presence of a dense, uniform stratus overcast. Nonetheless, intensive surface and vertical profile observations showed the following:

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