Heights and heat

Ex­ces­sive opac­ity of high-rise struc­tures re­sults in cre­ation of ur­ban heat is­lands in cities

HT Estates - - HTESTATES - Subhankar Mi­tra

In­ter­na­tion­ally, a build­ing that reaches or ex­ceeds the height of 150 me­tres is con­sid­ered a sky­scraper. Un­til re­cently, Mumbai was the only In­dian city with high-rise build­ings. The fi­nan­cial cap­i­tal con­tin­ues to see the high­est de­mand for sky­scrapers, as the only op­tion to grow there is ver­ti­cally. It now seems that in the com­ing decade, Max­i­mum City will re­ceive an even more co­he­sive sky­line, with a host of projects in the race to touch the sky be­ing con­structed. The de­mand for high-rise build­ings is cer­tainly grow­ing, and other cities are catch­ing up.

Mumbai con­tin­ues to have the max­i­mum num­ber of tall build­ings ap­proved or un­der con­struc­tion. Devel­op­ment of In­dia One - the tallest in the coun­try - has al­ready be­gun in Max­i­mum City. It spans 126 floors and stretches up to a height of 720 me­tres. Apart from this, Mumbai has more than 30 such su­per-tall build­ings rang­ing be­tween the heights of 150 me­tres and 450 me­tres ei­ther at the ap­proval stage or al­ready un­der con­struc­tion.

New Delhi has around a dozen of such build­ings com­ing up. They range be­tween heights of 150 me­tres and 300 me­tres. Kolkata ,too, is catch­ing up with nine such res­i­den­tial build­ings ex­tend­ing to the height of 245 me­tres ei­ther ap­proved or un­der con­struc­tion.

Ahmed­abad has about 13 tall build­ings which are un­der con­struc­tion range be­tween 200 me­tres to 410 me­tres. Hy­der­abad and Ban­ga­lore, too, are wit­ness­ing some devel­op­ment in con­struc­tion of tall build­ings for res­i­den­tial-com­mer­cial pur­pose with two or three ap­proved projects.

All i n all, t his amounts t o around 60 sky­scrapers. De­vel­op­ers see such ed­i­fices as a good way to at­tract po­ten­tial buy­ers – high- rise build­ings are a good gam­bit to dif­fer­en­ti­ate their of­fer­ings from the rest of the pack. How­ever, this coin has two sides – high-rise devel­op­ment has its own share of de­mer­its, too:

In­creased ur­ban tem­per­a­ture

Size and den­sity of the built-up ar­eas af­fect tem­per­a­tures of ur­ban ar­eas. In the con­gested cen­tres of large cities, tem­per­a­ture lev­els are gen­er­ally higher than in the sub­urbs. The largest el­e­va­tions of ur­ban tem­per­a­ture oc­cur dur­ing clear and still-air nights, also called ur­ban heat is­land. Ex­ces­sive opac­ity of high-rise build­ings in city cen­tres re­sults in con­cen­trated heat gen­er­a­tion by high-den­sity land use (traf­fic, light­ing, heat ex­haust) and con­trib­utes to the cre­ation of ur­ban heat is­lands.

Ef­fect on night-time cool­ing

Noc­tur­nal ra­di­a­tion is a ma­jor cli­matic fac­tor that re­duces at­mo­spheric heat in ur­ban ar­eas lo­cated in hot, dry re­gions. Noc­tur­nal ra­di­a­tion de­creases when the den­sity and the height of builtup ur­ban masses in­crease.

High- rise build­ings store so­lar en­ergy dur­ing the day time and re­lease it slowly into low-speed lo­cal wind, es­pe­cially at night. The ver­ti­cal dis­tance be­tween cool winds above build­ing roofs and the ground sur­face is long, and this re­sults in de­creased ra­di­ant cool­ing dur­ing the nights.

Low- ri se build­ings t hat match the heights of trees –1215 me­tres, on the other hand, pen­e­trate night-time ven­ti­lated cool­ing at the ground level and also store cool ra­di­a­tion through built-up ur­ban ar­eas.

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