AUDITORÍA DE MERCADO

The cal cu lated PET val ues for a clear day are shown in Fig. 6.9. In the cal cu la tions, the wind speeds from this day (3 May) have been as sumed to be con stant through out the day, al though mea sure - ments were taken on only a sin gle oc ca sion at each site (and not si - mul ta neously). Fig. 6.9a shows PET for a per son ex posed to so lar ra - di a tion (i.e. choos ing to walk on the sunny side of the street). At all sites, val ues are above the as sumed up per com fort zone limit be - tween 11:00 h and 17:00 h. In the early af ter noon, they far ex ceed this thresh old, reach ing 40-50°C. Fig. 6.9b shows the cal cu lated PET val ues for some one choos ing the shady part of the street (when shade ex ists). At sev eral sites, this im proves ther mal com fort con di - tions con sid er ably.

The main rea son for ex tremely high day time PET is the ex po sure to di rect so lar ra di a tion. In tense so lar ra di a tion also re sults in tem - per a tures of sun lit can yon sur faces be ing con sid er ably higher than those of the sur round ing air dur ing clear and partly cloudy days. The sum of these two ef fects leads to high MRT, reach ing val ues above 60°C in the early af ter noon dur ing clear weather con di tions. MRT is linked to the H/W ra tios of the ur ban can yons, but also to the po si - tion of the per son within the can yon, i.e. whether ex posed to the sun or in the shade. At most of the sites, the ef fect of shade on clear days can be seen as a sharp fall in PET (Fig. 6.9b). At the DMP and GRB sites, for ex am ple, PET drops by more than 10°C when shade is avail - able. The ef fect is es pe cially no tice able at the UoM site, where hor i - zon tal shad ing de vices along the north ern façade fa cil i tate shade through out the day. How ever, note that, around noon, none of the other sites pro vide suf fi cient shade be cause of the high so lar al ti - tude. At the NUG site, where the avail abil ity of shade is neg li gi ble, con di tions are very poor through out the day. Here, shade is available only in the early morning and late afternoon.

In a hot hu mid cli mate where weather con di tions vary greatly, the mag ni tude of MRT is strongly re lated to sky con di tions. Fig. 6.10

shows PET on partly cloudy days for some one ex posed to the sun. On this day (7 May), only a few wind speed mea sure ments were taken, but the rel a tive dif fer ence be tween the sites was as sumed to be the same as on 3 May. For the partly cloudy day, the dif fer ences in PET be tween the sites were smaller than un der clear sky con di - tions (Fig. 6.9a). How ever, al though the so lar ra di a tion was con sid er - ably less, the PET val ues were above the as sumed dis com fort thresh old be tween 11:00 and 16:00 h. Con se quently, shad ing will also have a pos i tive ef fect dur ing partly cloudy days. When the sky was com pletely over cast, and so lar ra di a tion weak, PET did not ex - ceed the dis com fort level and intra-ur ban vari a tions were neg li gi ble, al though areas with higher wind speeds tended to be more

comfortable.

The sea breeze has a mod er at ing ef fect on PET in that it low ers the air tem per a ture. The ef fect of the sea breeze can be seen by com par ing the BoC site, which is open to the sea breeze, with the GRB and DMP sites. The lat ter sites are also sit u ated close to the sea, al though the breeze is blocked by un in ter rupted street front ages, re - sult ing in poorer ther mal com fort. The ef fect of wind speed can also be dis cerned in the PET re sults. For ex am ple, the com pact ur ban form of the DMP site, with the high est H/W ra tio, re duces wind speed, resulting in high PET values.

Chapter 6 Re sults

Fig. 6.9 Cal cu lated PET on a clear day in Co lombo (3 May 2003) for a per son (a) ex posed to so lar ra di a tion and (b) in shade, where avail able. The as sumed up per com fort limit is in cluded as a ref er ence (dashed lines). (For site keys, see Ta ble 5.1.).

Fig. 6.10

Cal cu lated PET on a partly cloudy day (7 May 2003) for a per son ex posed to so lar ra di a tion. The as - sumed up per com fort limit is in cluded as a ref er ence (dashed lines). (For site keys,

At night, PET is fairly low (Figs. 6.9 and 6.10). The low est val ues were ob served in the shal lower can yons, which have the high est wind speeds and the high est lev els of ra di a tive cooling.

6.3 Sim u la tions of mi cro cli mate and

out door ther mal com fort

This sec tion pres ents the re sults from mi cro cli mate sim u la tions us - ing the ENVI-met soft ware and sub se quent cal cu la tions of ther mal com fort. Only day time con di tions are con sid ered. The re sults are de - scribed in de tail in Pa per IV.