The Relationship between Meteorological Variables and Clearness Index for Four U

Abstract: This study aimed to examine the relationship between meteorological variables and the clearness index for three sites in Cuiabá city and one site in Chapada dos Guimar?es city, Brazil during 2007. It described the microclimate of each site on the basis of constructive elements and their surroundings, considering sky coverage using a daily clearness index. The results were that micrometeorological values were influenced by the natural elements and construction within the surrounding site, with higher air temperatures in more urbanized areas and sites with high diffuse radiation. When determining the sky coverage, on average, the days were partly cloudy or cloudy due to two reasons: (a) during the wet season, rainfall created cloudy conditions and (b) during the dry season, increases of particulates in the atmosphere as a result of anthropogenic emissions of gases and aerosols in this region of the state resulted in sky conditions classified as partly cloudy and cloudy. Future research should aim to better quantify the measurements taken inside an urban area, considering the topography and vegetation cover. This will improve the models that support urban planning, therefore favoring the thermal comfort of areas already occupied or to be urbanized.

(1)

The classification of the sky coverage was based on Ref. [13]. A cloudy sky was defined in the range 0 < KT < 0.3, a partly cloudy sky between 0.3 ≤ KT ≤0.65and a clear sky between 0.65 < KT < 1.0. Irradiation in the upper atmosphere (Ro) (MJ·m-2·day-1) was calculated by Eq. (2):

(2) where, Eo is the correction factor of eccentricity of the orbit (Eq. (3)), Ws is the solar angle (degrees) (Eq. (4)), f is the local latitude (degrees) and d is the solar declination (degrees) (Eq. (6)).

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January-March and October-December.

Incoming global solar radiation (Rsg) received by the sites was higher during the wet season for the four sites (Fig. 2). The Center, Unicampo and Chapada sites had similar seasonality. At the Chapada site, the solar radiation was less because this region demonstrated higher cloudiness.

The air temperature patterns of the four locations selected for this study can be clearly seen. As expected, for both sites studied, air temperature presented a consistent seasonal variability, varying

(Fig. 2). Relative humidity was ~20% less in the dry season.

A feature of an urban center is that it is different from a nearby rural area. In general, a typical situation is that the center of a city is hotter than the surroundings. To compare differences in air temperature of the four sites, one needs to consider that the air temperature inside the site usually depends on the shading intensity of a partial shaded area, the thermal properties of the soil and the air temperature of its immediate background [14]. Because of this, it considers the Center site as the reference temperature in Fig. 3, which shows a decrease in air temperature (T) based on the Center site for each month. It is noted that there were no negative values, which means that the monthly average air temperatures at the center were the highest for the year in relation to other sites.

The Chapada site, located at an altitude of 809 m, showed a low difference in air temperature of 1.4 °C in September (Fig. 3). However, during the dry season, the CPA site behaved to the contrary of the Chapada

most dominant surface element contributing heat to the city is bare concrete cover, which is exposed to solar radiation and has no evaporative cooling effect over this kind of surface [15].

Solar global radiation was minor in the Chapada site reaching a maximum difference in July (150 W·m-2) (Fig. 3).

3.2 Distribution of Daily Clearness Index

Daily clearness index KT (the ratio of daily global horizontal radiation to daily extraterrestrial horizontal radiation) values for all the days of 2007 for the four sites have been shown in Fig. 4. For the four locations, the KT presented seasonality, with lower values during

example through fires) occurred during the dry season.

For the four sites studied, the cleanest days (or days demonstrating the cleanest conditions within the classification of partly cloudy) were often during the wet season, due to washing the particles from the atmosphere by pluvial precipitation (days 1-100; days 260-365; Fig. 4).

Fig. 5 shows the frequency of days in the year based on the classification of sky cover for the four sites. The highest percentage of sky cover was partly

zero, classifying these conditions as a cloudy sky. For 0.3 ≤ KT ≤ 0.65, diffuse and direct irradiation is similar, this condition is called partly cloudy. And finally, in clear sky conditions when KT > 0.65, the direct global irradiation is at maximum, while diffuse radiation tends to be minimized [16].

3.3 Distribution of Daily Extraterrestrial Radiation and Solar Radiation

Fig. 6 shows the daily extraterrestrial radiation (Ro) and incoming solar radiation for the four sites studied in 2007. The highest value of daily extraterrestrial radiation obtained in March was 36.57 MJ·m-2·day-1(day 71, Fig. 6) and in October was 36.00 MJ m-2·day-1 (day 275, Fig. 6). The lowest value occurred in June at 32.24 MJ·m-2·day-1 (day 174, Fig. 6).

The maximum value of daily incoming solar radiation was 26.2 MJ·m-2·day-1 (day 358, Fig. 6) at the CPA site and was 22.1 MJ·m-2·day-1 (day 60, Fig. 6) at the Chapada site. Solar radiation is characterized by short fluctuations introduced by passing clouds. An analysis of these fluctuations with regard to solar energy applications should focus on the instantaneous

the diurnal changes of urban microclimate in four types of ground cover and urban heat island of Nanjing, China, Building and Environment 43 (2008) 7-17.

[15] C. Rosenzweig, W.D. Solecki, L. Parshall, M. Chopping, G. Pope, R. Goldberg, Characterizing the urban heat island in current and future climates in New Jersey, Environmental Hazards 6 (2005) 51-62.

[16] R.P. Ricieri, Model estimation and evaluation of methods of measurement of diffuse solar radiation, Thesis, State University Paulista, Campos de Botucatu, 1998.