INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 24: 1777–1783 (2004) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1109
SUNSHINE DURATION AND DAILY COURSES OF ILLUMINANCE IN BRATISLAVA STANISLAV DARULA* and RICHARD KITTLER Institute of Construction and Architecture, Slovak Academy of Sciences, 9, Dubravska Road, Sk-845 03 Bratislava, Slovakia Received 23 January 2003 Accepted 5 January 2004
ABSTRACT The occurrences of typical sky conditions during overcast days are close to the CIE Overcast Sky Standard in the Bratislava area; their variation is specified by the diffuse coefficient kdd = Dv /Ev where Dv is the horizontal diffuse illuminance and Ev is the extraterrestrial illuminance. Luminance distributions on cloudless days are close to the CIE Clear Sky Standard. Complex daylight conditions can be expected during cloudy days when moving clouds cause frequent sun disk shading with abrupt changes in illuminance levels. Since sunshine duration is one of the basic climatic parameters that describes sunny situations, the present study attempts to investigate its relation to daily illuminance levels. However, the average daily value of sunshine duration does not exactly correspond to the same illuminance course, as it depends on the temporal distribution of sunny periods within the day. Anyhow, the estimation of daytime illuminance levels is mainly influenced by the presence or absence of sunshine with frequent sky luminance patterns. Copyright 2004 Royal Meteorological Society. KEY WORDS:
central European daylight climate; natural light; sunshine duration; daily illuminance changes; DCC method
1. INTRODUCTION To utilize natural light more effectively it is important to know not only its availability, but also its changes with time. These can be predictable and stable (depending mainly on sun elevation) or very unstable due to cloudiness characterized by more or less high differences in consecutive values. The latter conditions are typical for cloudy days with moving clouds resulting in the shading of the sun disk for particular time intervals. Results of recent research (Darula and Kittler, 2001) show that daily variations in illuminance also depend on the general weather conditions, which can be stable during consecutive sunny, overcast or foggy days or only within parts thereof. Weather changes can cause unstable variations of illuminances with random combinations of different conditions as atmospheric turbidity, cloud cover, cloud type, etc. These changes are also reflected in the relation of sunshine duration to instantaneous availability of global and diffuse illuminances (Gopinathan, 1992; Soler, 1992). Sequences of irradiances in shorter or longer periods have been published in several studies using meteorological parameters that are commonly stored as hourly averages. In these studies, an hour is presented as being a quite short available interval (Craggs et al., 2000). Additionally, different reference years have been generated for simulation purposes based on irradiance, temperature, wind speed and humidity, e.g. typical meteorological year (TMY; Hall, 1978), typical weather data (TWD; Klein et al., 1976), test reference year (TRY; Commission of the EC, 1985), ten year average (TYA; Bahadori and Chamberlain, 1986), or average year of temperature (AYT; Bilbao et al., 2000), typical meteorological day (TMD; Feuermann et al., 1985), etc. All were produced to describe characteristic annual changes in the selected parameters using different statistical * Correspondence to: Stanislav Darula, Institute of Construction and Architecture, Slovak Academy of Sciences, 9, Dubravska Road, Sk-845 03 Bratislava, Slovakia; e-mail:
[email protected]
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methods with respect to energy conservation measures or slower thermal trade-off processes characteristic in building enveloping constructions. Very few authors, however, have discussed short-term irradiance or illuminance changes (Delaunay et al., 1994; Morf, 2002; Walkenhorst et al., 2002). The application of hourly irradiance time-series for illuminance reference years would seem to be incorrect. Whereas irradiance energy can be accumulated in the building’s interior and materials, illuminance changes affect the luminous environment as perceived by the eye differently and instantly. Human sensitivity to temperature and illumination changes are, thus, fundamentally different. Problems can occur when evaluating illuminance time series from long-term measured data. However, nowadays, the higher performance of computers yields opportunities to study typical changes in very short intervals, e.g. data recorded in minute steps measured regularly over several years. To describe the daily courses, while simulating year-round illuminance changes, it seems that typical daily or half-day periods τ are sufficient within which very short time changes can be anticipated. It is evident that all rapid illuminance variations are caused by random cloud movements around the sun’s position, resulting in momentary sunshading. The total duration of sunny periods are recorded by ˚ meteorological services as daily sunshine duration. In 1924, Angstr¨ om showed that sunshine duration is one of the most important parameters linked to the frequency of direct radiation, i.e. he expressed changes of situations with or without sun influences corresponding to high or low illuminance levels respectively in analytical form. The aim of this paper is to show an alternative method for selecting and classifying daily courses of instantaneous illuminances collected over several years from the Bratislava database, where instantaneous illuminances here are meant as 1 s recordings taken in 1 min steps. 2. DATA COLLECTION AND ANALYSIS The climate in central Europe is characterized by the conventional four seasons with typical weather conditions. The climate in the Bratislava region is temperate, approaching continental, with seasonal changes, i.e. colder winter, warm summer, and temperate spring and autumn. Monthly averages of relative sunshine duration vary from 0.09 in winter up to 0.7 in summer. However, daylight climate can be discussed in three annual periods with characteristic illuminance levels, i.e. winter (November–February), summer (May–August) and the transitional months of March, April, September and October (Darula and Kittler, 2000, 2001; Bartzokas et al., 2003). The present study is based on instantaneous measurements of illuminance and irradiance data collected at the CIE IDMP station in Bratislava during a 8 year period (1994–2001). Monthly average relative sunshine durations are shown in Table I, calculated after CIE 108 (1994) and WMO (1983) recommendations with a threshold direct irradiance of 120 W m−2 . Detailed analysis of exterior illuminance availabilities and daily time courses for Bratislava’s climatic conditions document quite high variability (Darula and Kittler, 2001). It was impossible to spot two courses in illuminance levels that were exactly the same within the database used. However, some similarities during clear, overcast and cloudy days were observed when small differences in consecutive measurements occurred and illuminance courses were stable, depending mainly on the sun altitude. Therefore, the term ‘dynamic day’ is introduced, which characterizes rather unstable weather conditions that influence illuminance levels when moving clouds obstruct the sun disk in short intervals. Various solutions for modelling such trends and forecasting such time series are available in the literature. Most of them promote the idea of using statistical methods, which tend to generate peak smoothing or prefer to fit harmonic functions that express averaged variations or estimated conditions (e.g. Montgomery et al., 1990; Boland, 1995; Mora-L´opez and Sidrach-de-Cardona, 1998; Bilbao et al., 2000). To predict real daily variations of illuminances it is also important to model their time oscillations. The first step in doing so is to define types of daily illuminance level variation. Figure 1 shows the four proposed typical daily illuminance courses. The first type (code 1) represents illuminance/irradiance during a clear day; code 2 is for a cloudy day, and overcast conditions are described by code 3. Code 4 is reserved for complex dynamic illuminance changes. Copyright 2004 Royal Meteorological Society
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Table I. Monthly averages of relative sunshine duration from the Bratislava 1 min data set (1994–2001). Figures in bold are winter months; figures in italics are for transition months Year
Month
Mean
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1994 1995 1996 1997 1998 1999 2000 2001
0.295 0.186 0.158 0.104 0.307 0.165 0.217 0.202
0.354 0.343 0.431 0.441 0.562 0.237 0.364 0.499
0.306 0.310 0.330 0.473 0.457 0.442 0.337 0.279
0.424 0.385 0.480 0.445 0.416 0.493 0.634 0.448
0.437 0.530 0.416 0.549 0.539 0.521 0.694 0.641
0.509 0.408 0.575 0.488 0.521 0.493 0.714 0.466
0.669 0.700 0.520 0.412 0.476 0.548 0.383 0.489
0.583 0.530 0.482 0.689 0.596 0.536 0.720 0.734
0.497 0.426 0.196 0.695 0.342 0.551 0.468 0.236
0.387 0.478 0.424 0.524 0.298 0.394 0.456 0.401
0.218 0.115 0.292 0.273 0.279 0.150 0.314 0.314
0.244 0.091 0.244 0.113 0.138 0.253 0.172 0.284
0.410 0.375 0.379 0.434 0.411 0.399 0.456 0.416
Mean
0.204
0.404
0.367
0.466
0.541
0.522
0.525
0.609
0.426
0.420
0.244
0.192
0.410
Figure 1. Example of the four basic illuminance day course classification (DCC method)
If the daytime length (from sunrise to sunset) is divided into two identical parts (morning and afternoon half-days), this also provides an opportunity to study the transition between these basic types within the same day. Therefore, a sub-code is created for such occasions by the combination of numerals 1, 2, 3 and 4; e.g. code 14 represents a day with a clear morning and dynamic afternoon, code 33 expresses overcast conditions during the whole day, etc. The CIE IDMP station at the Institute of Construction and Architecture of the Slovak Academy of Science in Bratislava has been in operation since 1994. Horizontal global and diffuse illuminances and irradiances are measured in 1 min intervals. These measured values are necessary for calculation of relative sunshine duration in short time periods according to which half-days with prevailing sunny, cloudy or intermediate conditions can be sorted. The relative sunshine duration is defined as the ratio of the measured time to the theoretical day length. However, the only value of the relative sunshine duration is not quite reliable for the classification of half-day conditions, because this parameter allocated for each half-day does not describe abrupt changes. Dynamic cloudy conditions need the additional U parameter. This is based on a statistic of the difference between two consecutive illuminance values, xi and xi+1 : 1 U = ln |xi − xi+1 | (1) n − 1 i=1 Copyright 2004 Royal Meteorological Society
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Therefore, the value of the code during half-day periods is estimated according to the following notation: 1 2 codei = 3 4
with
s ≥ 0.75 and U < 8.4 0.03 ≤ s < 0.75 and U < 10 − 6s s < 0.03 U ≥ 10 − 6s ⊂ (1 ∪ 3)
(2)
If s < 0.03, then overcast sky conditions occur (code 3). Clear days are characterized by U < 8.4 and s ≥ 0.75 (code 1). The expression U = 10 − 6s defines the remaining situations as dynamic (code 4). Using step-bystep selection/classification, the rest of the data describe cloudy conditions (code 2). The half-day course classification (DCC) is documented in the flow chart of Figure 2 linking s and U parameters after the criteria in Equation (2). 3. DISCUSSION AND RESULTS Figure 3 gives an example of a sequence of consecutive days where sub-code numbers for half-days are shown. The solid line represents global illuminance and the dashed line shows diffuse illuminances. The clear, overcast and dynamic situations are reliably classified using the DCC method. The method is less sensitive when referring to cloudy situations with longer sunshine duration than comparison to dynamic illuminance conditions. Therefore, a reliable database is required; if errors or missing data occur then they have to be filled in using an appropriate method (Darula et al., 2003). As summarized in Table I, there are quite a few differences in relative sunshine duration during a particular year; e.g. extremes can be seen in 1995, with December having the lowest value (0.091) and July the highest (0.7). The highest s value, however, occurs in August 2001 (0.734), which is still the sunniest month in the Bratislava area. The standard deviation SD for four typical situations of s was investigated and the results are documented in Table II. Symbol s is used for the relative sunshine duration for the whole day, while s1 is for the morning period when true solar time is less than 12 and s2 for afternoon with true solar time greater than 12.
Figure 2. Flow chart for the selection criteria of illuminance DCC
Figure 3. Classification example of global illuminance half-day variations in relation to the DCC method for days between 17 and 24 May 1999 Copyright 2004 Royal Meteorological Society
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Table II. Statistical parameters of typical half-day sunshine duration in Bratislava during the period 1994–99 Code
11 12 13 14 21 22 23 24 31 32 33 34 41 42 43 44 Sum
Cases
175 87 4 70 9 128 201 38 0 53 311 34 90 160 72 750
s
s1
Description of cloud conditions
s2
Average
SD
Average
SD
Average
SD
Morning
Afternoon
0.92 0.80 0.51 0.82 0.65 0.26 0.05 0.20 — 0.06 0.00 0.10 0.75 0.38 0.15 0.50
0.03 0.09 0.03 0.09 0.15 0.22 0.07 0.12 — 0.06 0.00 0.07 0.15 0.21 0.14 0.22
0.97 0.94 0.87 0.94 0.51 0.27 0.08 0.11 — 0.00 0.00 0.00 0.69 0.47 0.26 0.54
0.03 0.05 0.03 0.04 0.19 0.22 0.08 0.10 — 0.00 0.00 0.00 0.22 0.23 0.20 0.25
0.85 0.59 0.00 0.66 0.84 0.25 0.00 0.32 — 0.13 0.00 0.22 0.84 0.27 0.00 0.46
0.03 0.13 0.00 0.17 0.04 0.21 0.00 0.15 — 0.11 0.00 0.13 0.04 0.21 0.00 0.21
Clear Clear Clear Clear Cloudy Cloudy Cloudy Cloudy Overcast Overcast Overcast Overcast Dynamic Dynamic Dynamic Dynamic
Clear Cloudy Overcast Dynamic Clear Cloudy Overcast Dynamic Clear Cloudy Overcast Dynamic Clear Cloudy Overcast Dynamic
2182
Prevailing situations with dynamic illuminance changes during the whole day (code 44) were found for Bratislava in 750 cases. In the second place, prevailing conditions can be expected during fully overcast days (code 33) with 311 cases; stable cloudy days (codes 22 and 23) occurred in 329 cases, and 175 cases with clear days (code 11) were also significant. Generally, morning periods can give higher sunshine durations than afternoon period. Note that type 31, i.e. overcast to clear conditions, was not found in the whole of the Bratislava data set used. As documented in Table III the influence in the sequence of typical conditions is interesting. The high values of relative sunshine duration during clear half-days do not depend on the other half of the day. The same effect is valid under overcast conditions with low s values. In contrast, the sunshine duration during cloudy and dynamic half-days is dependent on the illuminance levels in the previous or following half day.
4. CONCLUSIONS A novel approach to simulating daylight variations that is closer to the real situations under typical sky conditions is based on four primary day shapes of global illuminance levels, further sub-divided into 12 half-day combinations. This approach relates prevailing daylight situations (parameters s and U ) to half-day sunshine durations s1 and s2 . The analysis shows that clear situations affect higher values of sunshine duration and overcast situations affect lower values of sunshine duration without any dependence on weather conditions during the previous half-day. Different results were obtained when cloudy and dynamic situations are compared. If the sunshine duration is higher in the first half-day, then higher values of s were also found in the consecutive half-day characterized by either cloudy or dynamic conditions. This method is considered as the first step for the formulation of days with similar daylight conditions influenced by weather fronts and circulation of atmospheric masses in central Europe. Considering the fact that illuminance courses are primarily dependent on time or solar altitude during a typical day, it is feasible Copyright 2004 Royal Meteorological Society
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Table III. Relative sunshine duration selected during half-day periods Code
Half-day
Code
Morning
Afternoon
11
Clear 0.94–1.00
Clear 0.82–0.88
22
Cloudy 0.05–0.49
14
Half-day Morning
Afternoon
44
Dynamic 0.28–0.79
Dynamic 0.26–0.67
Cloudy 0.05–0.46
33
Overcast 0–0.03
Overcast 0–0.03
Clear 0.90–0.98
Dynamic 0.49–0.82
41
Dynamic 0.47–0.91
Clear 0.81–0.88
12
Clear 0.89–1.00
Cloudy 0.46–0.72
21
Cloudy 0.33–0.70
Clear 0.80–0.88
13
Clear 0.84–0.90
Overcast 0–0.03
31
Overcast
24
Cloudy 0.01–0.21
Dynamic 0.17–0.48
42
Dynamic 0.24–0.69
Cloudy 0.06–0.48
23
Cloudy 0–0.16
Overcast 0–0.03
32
Overcast 0–0.03
Cloudy 0.03–0.24
34
Overcast 0–0.03
Dynamic 0.09–0.35
43
Dynamic 0.06–0.46
Overcast 0–0.03
Clear No cases
to base the daylight reference year on computer calculations determined by selected typical half-day periods proportional to monthly/daily sunshine duration. ACKNOWLEDGEMENTS
We are grateful for the financial support obtained from the Slovak Ministry of Education received under the Slovak–Greek grant SK-GR 4/2001 (Darula et al., 2001) as well as from the Slovak VEGA grant 2/2067/22. REFERENCES ˚ Angstr¨ om A. 1924. Solar and terrestrial radiation. Quarterly Journal of the Royal Meteorological Society 50: 121–125. Bahadori MN, Chamberlain MJ. 1986. A simplification of weather data to evaluate daily and monthly energy needs of residential buildings. Solar Energy 36: 499–507. Bartzokas A, Darula S, Kambezidis H, Kittler R. 2003. Sky luminance distribution in central Europe and the Mediterranean area during the winter period. Journal of Atmospheric and Solar-Terrestrial Physics 65: 113–119. Bilbao J, Miguel A, Franco JA. 2000. Generation of a test reference year of global solar radiation from a test reference year of temperature. In Development and Application of Computer Techniques to Environmental Studies VIII, WIT Press: Southampton. Boland J. 1995. Time-series analysis of climatic variables. Solar Energy 55: 377–388. CIE 108. 1994. Guide to recommended practice of daylight measurements. CIE Central Bureau, Vienna. Craggs C, Conway EM, Pearsall NM. 2000. Statistical investigation of the optimal averaging time for solar irradiance on horizontal and vertical surfaces in the UK. Solar Energy 68: 179–187. Darula S, Kittler R. 2000. The influence of sky type on daylight climate conditions. In CD Proceedings of XIII Bioclimatology Conference on Bioclimatology and Environment, Slovak Bioclimatology Society SAS, Kosice. Darula S, Kittler R. 2001. Research of the typical exterior illuminance situations based on long-term measurements. Daily courses of global and diffuse illuminance in Bratislava for period of 1994–2000. Research Report VEGA 2/60050/99 R 2000.1, ICA SAS Bratislava, June. Darula S, Kittler R, Kambezidis H, Bartzokas A. 2001. Reference daylight conditions for energy-saving building design. Final Research Report of the GR-SK project, ICA SAS Bratislava, NOA Athens, University of Ioannina, April. Darula S, Kittler R, Kambezidis H, Bartzokas A. 2003. Reconstruction of missing measured illuminance values in regular daylight data recordings. In Proceedings of 14th International Conference LIGHT 2003, DT ZSVTS Bratislava; 62–70. Delaunay JJ, Rommel M, Geisler J. 1994. The importance of the sampling frequency in determining short-time-averaged irradiance and illuminance for rapidly changing cloud cover. Solar Energy 52: 541–545. Feuermann D, Gordon JM, Zarmi YA. 1985. A typical meteorological day (TMD) approach for predicting the long-term performance of solar energy systems. Solar Energy 35: 63–69. Copyright 2004 Royal Meteorological Society
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