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Daily and Hourly Change of Sunshine Duration Values on Crete Island2 4" oo' 25" oo' 26"00'

Fig. 1. Map of Crete showing he relief

ilM BAK~ON~___, A P E RAq

151

36"00"

35"30"

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perature of the environment in which humanactivity is present and helps in the growth ofanimals and plants, which are also very im-porta nt factors in ecological equilibrium.

Greece is characterized on the one hand bya large dispersion of islands in the Aegean Seain an area twice large as more than continentalGreece and a lack o f fossil fuels on the o ther, sothe use of solar energy will help us to solve vitalenergy problems. Thus, the necessity of knowl-edge, on a climatological basis, of the availablesolar ener gy is very clear.

Unfortunately, the majority of existingmeteorological stations do not measure solarradiation. Thus we try to estimate the incomingsolar energy using indirect methods and such amethod, possibly the most convenient, is to usethe sunshine duration in our calculations. Forthis purpose many other scientists have alsoworked on this topic, among whom we mustrefer to Macris (1976), Karapiperis etal. (1974),Flocas (1980 ), Helmes and Jaenicke (1984).

Fro m the who le Gre ek area we have chosento study the sunshine duration on Crete islandbecause, in our opinion , it fulfills the followingrequirements:

(a) It is the most southern and the largestisland of the Creek territory and also one ofthe largest islands in the Mediterranean area.

(b) The people on this island have mainlyagricultural jobs and specialize in the p rodu ctio nof early season vegetables.

(c) Crete is on e of the mo st well-developedGreek tourist areas.

(d) Last but not least, Crete is covered by asatisfactory network of meteorological stations

measuring the sunshine duration using the sameinstr ument (Campbell-Stokes sunshine recorder).

The locations of these meteorological stationsare shown on the map of Crete in Fig. 1.

2. Data and Method

The data on which this paper is based havebeen gathered from the archives of the NationalMeteorological Service (EMY) for the six mete-orological stations of the island which containrecords for the hourly sunshine duration forevery day. The sunshine duration at all themeteorological stations is measured usingCampbell-Stokes sunshine recorders (WMO 1971)which measure sunshine duration with an errorof -+5 min. Obstructions which could causeerrors in the measurement of sunshine durationare negligible at all the exam ined meteorologicalstations. The time period between 1971-1980will be examined because of the completenessof data during this time.

The daily sunshine duration is grouped into6 classes, 0.0, 0.1-3.0, 3.1-6.0, 6.1-9.0,9.1 - 12.0 and > 12.1 hrs per day. In thi s workwe study the annual distribution of the meanhourly values for each time interval and alsothe mean monthly values for every meteo-orological station separately. Moreover, whenwe consider as "sunless" those days for whichthe daily sunshine duration does not exceed3.0 hrs, we calculate the number of 1, 2, 3,...consecutive sunless days for the cold periodof the year, i.e. October, November, Decemberof one year and January, February and March


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152 A.A. Bloutsos and P. J. PennasTable 1. Geographical Coordinates o the Meteorologieal Stations o f Crete Island and the Mean Daily Sunshine Durationfor Every Mon th an d Corresponding Standard Deviat ion for the Period 197 1- 19 80

Khania Suda Tymbakian Iraklion Ierapetra Sitia= 3530 ' = 3533 ~0 = 3504 ' ~o = 35020 ~o = 3500 ' ~0 = 3512 'X = 2402 ' X = 24007 X = 2445 X = 2511' X = 2545 X = 26006M o M o M o M o M o M o

Jan (310) 3 .41 3.07 3.61 3.12 4.69 2.79 3.68 2.94 5.10 2.67Feb (283) 3.98 3.32 4.08 3.36 5.24 3.26 4.17 3.32 5.65 2.99Mar (310) 5. 93 3.53 6.03 3.63 6.88 3.15 6.02 3.46 6.71 3.00Apt (300) 7.49 3.92 7.63 3.94 7.81 3.65 7.47 3.90 7.62 3.64May (310) 9.82 3.26 9.79 3.30 9.74 3.10 9.81 3.13 9.95 3.06Jun (300) 11.98 1 .8 0 11. 78 1 .9 6 11.6 0 1 .8 6 11.8 6 1 .9 7 12.0 9 1 .73Jul (310) 12.1 9 1 .1 5 12.12 1 . 12 12 .1 6 0.94 12.10 1 . 15 12.3 6 0.99Aug (310) 11.41 1. 22 11.44 1. 31 11 .6 3 1. 54 11.14 1. 28 11 .9 0 0.78Sep (300) 9.28 2.43 9.29 2.54 9.85 2.04 9.37 2.22 1 1 . 0 6 1.74Oct (310) 6.02 3.53 6.35 3.50 7.31 2.94 6.22 3.40 7.65 2.79Nov (300) 4.94 3.14 5.15 3.12 5.92 2.77 5 . 10 2.88 6.68 2155Dec (310) 3.58 3.04 3.74 3.00 4.60 2.83 3.79 2.96 5.18 2.84Annual(365.3)7.52 4.32 7.65 4.77 8.13 3. 79 7.58 4.17 8.34 3.65

3.81 2~904.47 3.236.23 3.277.26 3.929.77 3.29

11.86 1 .8312.03 1.0410.90 1.059.22 2.056.42 3.135.37 2.783.89 2.947.62 4.02

of the following year for each meteorologicalstation. For the winter 1970-1971 only themonths January, February and March wereused while for the winter 1980-1981 only themonths October,, November and Decemberwere used.

We must also note he re tha t for some spellswe calculate the number of consecutive daysusing either the last days of September or thefirst days of April.

Finally we give a theoretical distribution forthe nearly sunless spells for each meteorologicalstation using a slightly modified Polya method(Arley et al. 1973) because we found thatPolya's method does not fit well to empiricaldata while the modified distribution, using theX2-test, fits bette r at the 95% confiden ce level.According to Polya's method the probabilityof having N ( x ) spells of x con secutiv e days isgiven by the equat ion

P x = P x - 1 ( m + ( x - - 1 ) d ) / x (1 +d),where Po = 1 / ( l + d ) m / a, m = ( N - - ~ n i ) / ~ n iis the expected number of days (mean), 02= ( ~ n i s 2 / ~ , n i ) - - m 2 is the variance, N is thetotal nu mbe r of days, d = (o 2/m) 1 is a pa-rameter of the influence Of an event on the

next one, E ni is the total number of the con-secutive days, S i = x - - 1 , x = 1 , 2 , 3 , . .

The number of the expected occurrence ofan event among N trials is given by the equat ion

v = P 'N .The modified Polya meth od gives the number ofspells of consecutive days using the formulas

2 m e - mN(1) = k N " m + d (1)( l + d ) a

(rn + d)N(2 ) = X N(1 ) (2)4(1 + d)N ( x ) = m + ( x - - 1 ) dx ( l + d ) N ( x 1 ) , x = 2 , 3 , 4 , . . . ( 3 )where k, X are empirical parameter. The fit oftheoretical to empirical data is tested by usingthe ordinary 2-test.

3 . D i s c u s s i o n

3 . 1 G e ne r a lTable 1 gives the name of the meteorologicalstation, its geographical coordinates, the mean


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Daily and Hourly Change of Sunshine Duration Values on Crete Island 153daily sunshine duration values for every monthwith the corresponding standard deviation aswell as the number of observations per month,common to every meteorological station, forthe period 1971- 1980. As we can see from theabove table or as has been mentioned by otherscientists, e.g. Pennas (1976), Crete island couldbe divided, regarding the geographical distri-bution of sunshine duration values, into twoindividual parts using an axis from West to Eastalong the mo unt ain crest (Fig. 1), i.e. into thenorth part and the sou th one. Thus, on anannual basis the excess in sunshine durationvalues of the south side of the island is about300 hrs/year while on a monthly basis thiscould rise up to 50 hrs/month. This differencebetween the two sides of the island is attri-buted to the more fregment formation of cloudson the slopes of the mountains exposed to thenorth winds, due to the forced upward motionof the prevalent north winds during the year inthis area (WMO ! 964).

But differences in sunshine duration valuesare also apparent between the meteorologicalstations o f the sam e zone during the coldmonths, in that the values for the meteoro-logical stations loca ted on th e east edge of eacl~zone exceed those located on the west side ofthe same zone. T he above differences during thecold period are to be attributed, in our opinion,more to the peculiarities of the atmosphericcirculation above the studied area, Machairas(1983), Karoulias (1975) and less to the groundrelief.

In fact, during the cold period of the year,cyclones of the Mediterranean Sea traverse thisarea with a general direction from SW to NEand as a result they influence first and morestrongly the west side of thee island.

All the differences mentioned above arestatistically test ed at t he 95% confiden ce leveland found to be significant.

Fro m Fig, 2 we can see that t h e sunshineduration exceeds 30 hrs /mont h fo r every hourinterval from 08:00 to 16:00 L T (06:00 to14:00 GMT) for July and Au gu st .

Low sunshine durat ion values whic h areapparent during the winter months, especiallyin the morning and late aftern oon hourly timeintervals are mainly attributed to the shortduration of the day. For the winter months,the sunshine :durat ion for each hour intervalfrom 08:00 to 16:00 LT has such values thatoften exceed 10 hrs/month.

The isoline of 25 hrs/m onth for each hour ofthe day, which correspond to a value of morethan 80% of the m axim um possible sunshineduration, is between 08:00 to 16:00 LT duringthe months from May to September for all theexamined meteorological stations, while forthe meteorological stations located on the eastside of the island, these isolines are extendedtill October.

3.3 Daily ValuesGroup ing the 3653 daily sunshine durationvalues for every meteorological station for thedecade 1 971 -19 80 into six classes according tothe limits mentioned at the beginning of thisarticle, and calculating percentages of occur-rence, we draw Fig. 3 (a to f). When we studythis figure we c an see that, if we exclude thecompletely sunless days and the days where thesunshine duration is greater than 12 hrs/day,there is not any sufficient differen ce betwee nthe east and west side of the island.

But when we study only the completely sun-less days (sunshine duration 0.0 hrs/day) wecan see that the percentage of their appearanceon the meteorological stations of the west sideis sometimes twice to three times greater thanthe percentage for the meteorological stationson t he east side of the island.

3.2 Stud y of Hourly ValuesThe mean values of sunshine duration in unitsof hours per month for each hour of the dayand for every month have been calculated fromour analytical records and are shown in Fig. 2(a to f) where for every meteorolog ical stationisolines of sunshine durati on appear.

3.4 Sunless SpellsHere we are going to examine the nearly sunless

spells only during the cold period of the year.As a nearly sunless d ay we consider a day wherethe sunshine dura tion does not exceed 3.0 hrs/day. The total number of such days for anymeteorological station are about 25% of the


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154 A.A . Bloutsos and P. J. Pennas: Daily and Hourly Change of Sunshine Duration Values on crete Islanda b

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Fig. 2. (a -f ) Mean month ly sunshine duration (hrs) for ea ch meteorological station


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