A study on the cooling power on the eastern part of Adriatic
Abstract
1.1 The cooling power is showing how much heat per second is being lost into the air from a surface of one square centimeter by a body having the temperature of 36.5 degrees Centigrade and of the meteorological factors only the air temperature and the wind speed at the normal air density on the ground are acting. It is easy to imagine that body is representing the human organism but one has to keep in mind that it is not exposed to radiation, so it is for instance in a tree shadow and its skin is dry. After this the cooling power can be taken as a physical quantity measuring the common heat action of the temperature and the atmospheric flow speed, but it can be taken as biometeorological data serving for representing the human sensation of heat too. The relation between the heat sensation and the simultaneous cooling power value was shown tabulary by various authors, e. g.. D o r n o, M ö r i k o f e r, S c h m i d. The cooling power rate can be determined by an instrument or by computation using the temperature and the wind speed in H i l l’ s empirical relations. If the mean monthly temperatures and wind speeds are taken, then the mean monthly cooling power values are obtained by which the climate was classified by C o n r a d into five degrees.
1.2 The existing data on the cooling power for the eastern Adriatic area are successively the monthly values either for a square ten degrees latitude by 20 degrees longitude at φ = 45°N and λ = 10°E (7) (Lauscher, 1952) either for several locations on the Adriatic (B i e l, 1939, G o l d b e r g, 1940, M a k j a n i ć, 1956 and 1966, S l i e p č e v i č, 1960). The exception is a value not related to the mean circumstances: in Senj the cooling power on March 1, 1958 at 0°C and 43.8 mps was H = 118 mgcal cm-2 sec-1 (M a k j a- n i ć, 1966).
1.3. The purpose of this research is to get a better knowledge about the cooling power behavior on the Adriatic, not only regarding its monthly values in a major number of localities but especially regarding its momentaneous values at the different weather situations.
2.1. In this paper the cooling power on that part of the Adriatic belonging to the S. R. of Croatia is presented. All the values are determined by using H i l l’ s relation on the mean monthly air temperature and wind speed data measured at the various weather situations on ten stations. The weather situations were chosen for 1966 and 1967 on the basis of the morning ground synoptic map (D e u t s c h e r W e t t e r d i e n s t, 1966, 1967) and checked against three daily observation hour data of wind and cloudiness for Senj, Split and Dubrovnik. So only those days were chosen when the particular weather type existed along the whole Croatian coast.
2.2 For calculating the cooling power our own tables were used. They are made so that the error introduced by interpolation (when the input data are deviating from the values listed in the heading of the table) is less than ±0.25 mgcal cm-2 sec-1. It can become considerably greater if the input data (the temperature or wind speed) are not precisely measured. If for instance wind speed measurements are not existing but only the wind force estimation then for the speed the mean value of that speed interval corresponding to the concerned Beaufort scale degree is taken. The error which can arise in such a way is depending on the values of temperature and wind speed and it is decreasing by an increase of these values. Of ten stations for which the momentaneous values were calculated (Chapters 3.3 to 3.6) Lastovo and Dubrovnik did not have the wind speed measurements and this consideration about the errors is related to them. It could be related to the most stations in the Chapter 3.2 too. Meanwhile it is considered that by conversion of the mean monthly wind forces into speeds a lesser error is made than by conversion of the momentaneous forces into speeds.
3.1 The comparison between the air temperature and the cooling power was made to see how much the temperature itself is reliable measure for the heat sensation. For every month of 1966 an 1967 the cooling powers were compared for the days with the maximum and minimum mean daily temperature. Usually at the low temperature the cooling was stronger, but in 11 percent of cases the contrary was shown: the day with the highest mean temperature had the greater cooling power than the day being the coldest by its temperature. Such an inversion is occurring especially in the late fall and in the early spring when fairly strong winds are present. During the summer it is rare because the wind pattern is then more stable.
3.2.1 The monthly cooling power values were computed for a period comprising five years. Because it is a rather short series at first it was considered how great the maximum changes in the cooling power value ΔH in another five year or a longer period can be expected supposing the mean monthly temperature t or the mean wind speed v or both are taking the greatest possible change. By the extensive observation data for Crikvenica and Zagreb it was shown that the highest and the lowest five year temperature mean in a certain month are differing slightly less than 3°C, as for the wind speed the analogue difference was less than 0.8 mps. Therefore a maximum possible temperature change Δt = 3°C and for the wind speed Δv = 0.8 mps were taken. The greatest deviations ΔH proceeding are tabulated and discaussed. It was not possible to estimate the cooling power changes in advance because of the temperature and wind measurements inhomogenity. Yet it is expected that by moving the station to another location in the same town the mean temperature value will not be changed as much as 3 degrees, but the mean wind speed could be easily changed more than 0.8 mps. Therefore on the basis of previous discussion it is considered that the inhomogenities in the temperature data normally are not such that they could considerably influence the cooling power. Contrary, as this power is sensitive to even relatively small changes in wind speed, a relocation of the station to a position with a different circulation or the conversion from the wind force estimation to the speed measurements will be very well evident.
3.2.2. Up to now the monthly cooling power values were known for uneven periods at six locations in the considered area. Of them only for Senj it is positively known that the earlier measurements used for the cooling power computation (M a k j a n i ć, 1966) and the recent ones used here are homogenous mutually. In the Table 3 it is obvious that the difference between the older and more recent cooling power values is not systematic because its sign is changed irregularly during the years and its is smailler than the maximum values foreseen for the case of homogenous measurements in the diferent periods. For other places either it is known that the older and the more recent data are obtained from the measurements under different conditions (Crikvenica, Hvar, Rab) or about the relative homogenity nothing can be said. In some cases the old and the recent data are differing greatly and alreday from the steady sign of those diferences it may be concluded that they did not arise by chance.
3.2.3 The monthly cooling power values for the period from 1963 through 1967 are valid - after the previously made considerations - also for another period, but they must not be valid for another location, even in the same town, if there is another wind pattern. On these values C o n r a d’ s climate classification was applied. It shows that on the Adriatic in the low heights there are indeed diferent bioclimatic types, but those for the human organism most favorable are prevailing. During the cool season of the year the mild bracing or relaxing climate in all the places sheltered from stronger Bora or Sirocco (western coast of Istria, the foot of Učka mountain, the coast arround Zadar, parts of islands not exposed toward Southeast nor immediatly under Velebit mountain) are prevailing. The spring and the fall are pleasant everywhere. The summer is too warm only in those places being sheltered from the winds from the western quadrant and having also no bigger hill near causing fresh mountain wind .It should be kept in mind that the monthly values are showing only the level about which the cooling power during a month more or less is oscilating. So in the bracing climate the warm even hot days can be experienced as well as in the relaxing climate the nights and the mornings can be even cold.
3.3.1 For 139 undisturbed days the cooling power for 7 a. m., 2 p. m. and 9 p. m. was determined. It was shown that its values in some months were similar to each other, so after it the undisturbed days were classified into five groups. Three of them comprising the warmer part of the year are relatively great containing up to 35 or 36 days and so they are comparable between them. For the colder half of the year much less undisturbed days were available because then they were rare. The frequency distribution with the class width of 2.5 mgcal cm-2 sec-1 are made. These distributions are presented in the text and in the Fig. 2 and the Tab. 14 through 17.
3.3.2 The diurnal variation of the cooling power during the undisturbed weather can have for different places different forms as the Tables 18 and 19 and the Fig. 3 are showing.
3.3.3 From the preceding these conculsion are following:
i - During the undisturbed weather the cooling powers on the eastern coast of the Adriatic can be less than 5 mgcal cm-2 sec-1. It is occurring during the summer frequently but during the winter rarely. The highest amounts for the days being considered did not excess 35 units during the warm season but during the cold season they reached up to 60 units.
ii - The summer undisturbed days for a single place are not much differing by the cooling power. During the winter the fluctuation of the cooling is greater. Its cause should be found in the temperature and circulation conditions which are during the summer more stable than during the winter.
iii - The cooling is changing in the space and in the time depending on the air temperature and the wind what is meaning finally on the station position and the ground configuration in the environment. The influence of the temperature is more evident in the lower cooling rates for the southern than for the northern Adriatic, lower at the coast than on the open sea, lower during the summer than during the winter. The diurnal temperature variation would form the diurnal cooling power variation in such a way that it would have the form of a single wave with the maximum at dawn and the minimum in the afternoon. Meanwhile the regularly developed coastal fine weather circulation has two speed maximums: in the afternoon and in the night, and two minimums: in the morning and in the evening. At the same time the extremes in the diurnal cooling power variation would be found if that variation would be under the wind influence only. In the reality the diurnal cooling power variation curve is representing a superposition of a single and a double wave. That superposition can have a different look for different places, because the daytime and the nighttime wind in the periodic circulation frequently are not equally strong. So the coast at the foot of a mountain can have fresh nights owing to a notable nighttime wind. On the positions exposed to the open sea and in the channels open toward West or Northwest the daytime wind is causing strong cooling especially early in the afternoon. The places in the lowlands far from a slope are having calm nights and it means the small cooling power during the night at the summertime too. Again those places sheltered on the western or northwestern side from the daytime wind are experiencing an especially small cooling power during the day. E. g. in Senj the daytime coastal circulation branch is almost not existing but the nighttime wind is very significant. Its consequence is the diurnal cooling power variation in the form of a single wave with a great amplitude and a marked minimum after the noon and a flat maximum in the second part of the night (Fig. 3). In Zadar on the contrary no nighttime wind is present and the daytime wind (»Maestral«) is strong. On the average it annules the temperature action so the cooling power curve is similar to a double wave with its main maximum at the afternoon »Maestral« time and the secondary one at the lowest temperature time. If the details are not taken in it may be said that three diurnal cooling power variations during the undisturbed weather for the warm part of the year are existing:
a) a relatively strong cooling in the afternoon and faible during the night (Zadar)
b) a relatively strong cooling during the night but faible during the day time (Senj),
c) an approximately constant cooling during all 24 hours (Dubrovnik, Pula). From the available data it could be concluded that in July and in August the patterns b) and c) and in the other months the patterns a) an c) are prevailing.
3.4 For 19 winter days with Bora and 12 spring and autumn days with Bora the frequency distributions with the class width of 5 mgcal cm-2 sec-1 for all three daily observation times together were made. The Table 20 as showing that the cooling on the days with Bora can be very different. In the early spring and in the autumn the weather with Bora is only slightly milder than during the winter. From Fig. 4 it is evident how the cooling patterns on the days with Bora can significantly differ even on a distance of less than 70 kilometers. The difference results from the ground configuration in the hinterland of Zadar and Šibenik which is not suitable for the developmen of Bora near Zadar. The diurnal cooling variation in the presence of Bora is shown on Fig. 5. In all three examples the strongest cooling was before the noon and then the speed of Bora is usually the greatest.
3.5 On the days with Sirocco the observation time data could be by their similarity gathered into two groups too: the winter one containing 18 an that for the spring and the autumn containing 45 values for each station. The Table 21 is showing that the differences between the places sheltered from (e. g. Rijeka) or the exposed ones to (Split) that southeasterly wind are existing. During the winter in the Sirocco situations it is somewhat cooler than during the spring or fall. In the diurnal variation the cooling power during Sirocco is accompanying the wind speed changes because the weather in those situations is predominantly overcast and the air thoroughly mixed and the temperature amplitude is not great.
3.6 From the wind speed records on three stations the maximum gust was determined for 1966 and 1967 and the appropriate cooling power was calculated. It was shown that at the individual shortlived gusts of Bora the value can easily exceed 100 mgcal cm-2 sec-1. In the places where the maximum wind speed is belonging to Sirocco neverthless the highest values are found with Bora because the appropriate air temperatures are lower. The lowest cooling power values established in our material were less than one, but positive and they pertain to the calm period in the undisturbed summer weather. It is known that the air temperatures making the cooling power negative are coming on the Adriatic every few years and they are lasting only several hours during the warmest part of the day.
4. The Conclusion. Up to now it was not known how great in the individual weather situations the cooling power on the Adriatic was. Here it was shown that the specific winds taking change along the eastern coast on the Adriatic are influencing the cooling in the greatest rate so that the power takes very different values even during a single day. It was stated the there are also big differences of the cooling power between the places not much apart but having the different wind pattern.
Biologically it is coolest when Bora is blowing and this is occurring in the colder half of the year especially during the winter. The relatively low air temperatures associated with high wind speeds are making the cooling power during the winter sometimes greater than 120 units. On the places sheltered from the wind the cooling power can be in the time of Bora sometimes even less than 10 units.
The other characteristic wind of the cold half of the year, Sirocco, is accompanied by the lesser and not so fluctuating cooling powers because it represents a warmer air flow of a rather steady speed. With Sirocco no higher value than 60 units was found. The median at the spring and autumn Sirocco amounts in the different places about 25 to 35 units.
During Bora and Sirocco the cooling power has no regular diurnal variation.
On the contrary in the undisturbed weather of the warm half of the year such a diurnal variation is existing. It is formed by the regular diurnal temperature change and by the wind speed change in the system of land and sea breeze or mountain and valley breeze where such diurnal periodic circulations are existing.
The monthly cooling power values computed for a relatively great number of points are showing that on the Adriatic two climates are prevailing: the relaxing one with the monthly power value between 10 and 20 mgcal cm-2 sec-1 and the mild bracing one with the values between 20 and 30 mgcal cm-2 sec-1. And of all climates just these two are most suitable for tourism, resting, reconvalescence and climatic medical treatment.
