The magnitude of cooling down of air is one of the physical indexes of heat exchange between the human body and the environment. It is very useful for the assessment of a thermal sensibility of man being in motion and dressed according to season of the year and actual atmospheric conditions. The magnitude of cooling down of air is an important element for the typological elaborations, and one of the main criteria for the estimation of weather conditions for the needs of bioclimatology (Kozłowska-Szczęsna et all 1997).

In most of bioclimatic papers the authors use the magnitude of cooling down of air calculated for the observation time-limit II, motivating that it represents that part of the day in which man works most actively, takes climatotherapeutic treatments, or rests. It has turned out, however, that at this time of the day there is dinner being served in many sanatoriums, convalescent hospitals, and rest-houses in health resorts of Poland. Moreover the period between 12:00 and 15:00 is considered to be the most unfavorable due to a high level of solar radiation and a high hazard of an excessive sunburn. This is especially important in open areas such as beaches, meadows, recreational forest clearings, and areas surrounding water reservoirs. It should also be noticed that human activity is not limited to only one form of spending time out of doors. This is the case when work takes place in the morning and forenoon, while rest and recreation in the afternoon and evening. As a rule, with the exception of some climatotherapeutic treatments especially arranged for a definite group of patients with a defective thermoregulation system, man stays out of doors from sunrise to sunset. Therefore, would the magnitude of cooling down of air determined on the basis of the observation time-limit II only, be adequate to characterize the conditions of thermal sensibility of a human being during the whole day? It seems that it would not be adequate. Besides, if we have values of meteorological elements from three observation time-limits at our disposal why not to use them?

The purpose of this study was to prove that to calculate the mean diurnal magnitude of bioclimatic cooling down it is necessary to use all values of magnitude of air cooling down, or meteorological elements used to its calculation measured in three observation time-limits.

It was assumed in the research hypothesis that using the formula for calculation of the weighted arithmetic mean, with the magnitude of bioclimatic cooling down from the measuring time-limit II as the weight factor, is the most correct method of calculation of the diurnal magnitude of air cooling down H in stations measuring meteorological elements in three observation time-limits.

The series of meteorological data gathered at meteorological stations and microclimatic posts situated in southern Poland (Beskid Śląski Mts, Beskid Sądecki Mts, Beskid Niski Mts, and Kraków-Częstochowa Upland) were used in this study (Tab. 1). The meteorological measurements were carried out by the HUGER automatic meteorological station with data recording every 60 minutes. At the meteorological station in Wisła Wyrchczadeczka the measurements were taken every 30 minutes obtaining 2880 results in November 2004 (Table 12). Together, there were gathered over 160 000 data on hourly values of air temperature and wind velocity 200 cm above the ground. Moreover, 34 series of microclimatic measurements, carried out during 1995–2004 in the warm part of the year in the area mentioned above, were used. Using this method a total of 12 000 data on air temperature, wind velocity, and magnitude of air cooling down (dry bioclimatic cooling down) measured with the Hill´s cathetometer, were gathered. The manual microclimatic measurements were carried out using the psyhrometer TB91AI, hand anemometer, and Hill´s cathetometer. The microclimatic measurements were conducted from May to August, while the automatic measurements during the entire year.

H₁ = (36,5 - t) x (0,20 + 0,40 x √v)

when v Ł 1.0 m/s

H₁ = (36,5 - t) x (0,13 + 0,47 x √v)

when v > 1.0 m/s

where: t – air temperature (^oC), v – wind speed (mĚs^-1).

Table 1.Localization of meteorological station¹ and microclimatic post².

In the remaining cases the direct measurements of the biometeorological index were used. For each twenty-four hour period the following were computed:

a. mean magnitude of air cooling down (arithmetic average) on the basis of hourly measurements taken from 07:00 to 19:00 (13 hours);

b. magnitude of air cooling down on the basis of measurements from the observation time-limit II (single value);

c. mean magnitude of air cooling down (weighted average) on the basis of measurements taken at the I, II and III time-limits:

H_śr = (H_I + 2H_II + H_III)/4

It was assumed that the mean calculated from all hourly values from 07:00 to 19:00 (13 values) or 30 min values (25 values) of the same period of time represents in the best way the conditions of bioclimatic cooling down during the day. It reflects the whole day variation of this index, especially of its most variable element, the wind. In connection with this the following indexes were compared with each other:

a. value of bioclimatic cooling down from the observation time-limit II with value of the index calculated on the basis of measurements taken at 13 (25) hours, from 07:00 to 19:00;

b. value of bioclimatic cooling down calculated by the formula [3] as proposed by the author with value of the index calculated on the basis of measurements taken at 13 (25) hours. Values of index H, shown in tables, are expressed in W?m^-2.

The indexes were calculated using the program BioKlima 2.3 (Błażejczyk 1999). The significance of differences between means of individual measurement series was estimated using t-test for dependent variables. In two cases the measurement series did not have normal distribution. In those cases the Wilcoxon´s test was used as a non-parametric equivalent of t-test for dependent variables. Calculations were made using STATISTICA 6.1 (StatSoft, Inc. 2004)