1-s2.0-S1352231097001362-main

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ergamon

Atmospheric Em ronment Vol. 31, No. 22, pp. 3821-3831, 1997

© 1997 Elsevier Science Ltd

All rights reserved. Printed in Grea t Britain

P I I : S1352-2310(97)00136-2 1352-2310/97 17.00 + 0.00

HE T ISL ND DEVELOPMENT IN MEXICO CITY

E R N E S T O JA U R E G U I

Centro de Ciencias de la Atm6sfera, UNAM, Circuito Exterior, Cd. Universitaria, M6xico D.F. 0451,

M6xico

First received 2 February

1996

and in f inal form

20

February

1997.

Published August

1997)

Abstract--The present paper describes the climatology of the near surface urban heat island of Mexico

City, using hourly data from two recently installed automatic stations at a rural and an urban site. The

results show that the nocturnal heat island was more frequent (75 of the time for the period examined)

than daytime cases (25 ). The maximum nocturnal heat island intensity, 7.8°C was observed in a dry

month (February) characterized by calm clear nights. Although less frequent, the daytime heat islands did

occur and may have been caused by differences in evaporative cooling from wet surface during the wet

season. Midday heat islands had a frequency of 13 and an intensity of 3-5°C during the wet season. The

afternoon/evening heat islands had an intensity of 4-5°C. They occurred during both dry and wet season

with a frequency of 12 of the time in a year. The average urban/rural thermal contrasts are positive

throughout the year varying from 5°C at day break in the middle of the dry season to 1-3°C around noon

during the wet months. © 1997 Elsevier Science Ltd.

Key word index: Urban heat island, tropical urban climatology, Mexico City.

l. INTRODUCTION

The presence of warmer air over cities has been

documented first in mid-latitude urba n areas and later

in the tropical environme nt.The noctu rnal heat island

results from diverging rates of cooling between the

urb an and the rural env ironmen ts which in temperate

climate cities produce a sharp increase in intensity

soon after sunset to a maxim um of abou t 4 h later

(Oke, 1982). Thereafter, slightly greater ur ban cooling

reduces the intensity until the early daytime rural

heating virtually erases the heat island (Oke, 1982).

Most mid-latitude studies have been undertaken

during summer, when pre vailing cloudless skies and

calm or light winds allow full development of the

phenomenon. Since the above conditions are not

often present dur ing winter, with some exceptions (e.g.

Chandler, 1962; Eriksen, 1964; Mu nn et al., 1969;

Unwin, 1980) rather few studies were attempted to

describe the seasonal behaviour of the heat island

durin g an annu al cycle in mid-latitude cities.

In vast regions of the tropics where all year ro und

wet or wet-dry seasons prevail, some studies have

attempted to derive overall climatologies of heat is-

lands (e.g. Sham, 1973; Jauregui, 1973; Pad man ab-

hamurty, 1986). In recent times an increasing numbe r

of works on various aspects of the heat island phe-

nomenon have appeared in the literature. Yamashita

(1988) examined geographical aspects of heat islands

and their relation to city size and some causative

factors such as roughness, sky view fraction and, soil

moisture an d temperature for several Japanese cities.

Using infra-red satellite data to display the surface

radia nt temperature heat islands of several mid-latit-

ude cities Roth et a l . (1989) have found tha t while heat

island intensities are largest in the daytime, nocturn al

heat island intensities are weaker and less correlated

with land use. This is the reverse of what has been

observed for near-surface air heat islands.

More recently, the use of satellite data has been

attempted for the detection and assessment of the

urb an heat island by means of urb an- ru ral differences

of a n ormalized vegetation index and radiative surface

temperatures (Gallo et al., 1994). They conclude that

data from DMSP satellite may provide a useful

quantitative measure of urban heat islands. Satell-

ite-derived surface temperature patterns are a good

indicator of the daytime urban heat island in Sin-

gapore according to Nichol (1996) who has compared

those patterns with field measurements of surface and

adjacent air temperature in urban ca nyons at different

orientatio ns in that equatorial city.

In the desert cities of the Un ited States the de mand

for cooling uses 3-8 of electric power to compen-

sate for the urban heat island effect (McPherson,

1992). Daytime heat island in large cities with sea-

sonally abu nda nt sunshine are likely to cont ribute to

the enhancement of some gaseous urban pollutants.

3821

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3 82 2 E . J A U R E G t ; I

T h i s c o u l d b e t h e c a s e o f M e x i c o C i t y w h e r e , a s w i ll b e

s e e n h e r e , d a y t i m e u r b a n / r u r a l t h e r m a l p o s i t i v e c o n -

t r a s ts a r e u s u a l l y o b s e r v e d i n c o n j u n c t i o n w i t h h i g h

o z o n e l e v e l s .

F r o m t h e c l i m a t o lo g i c a l p e r s pe c t iv e , k n o w l e d g e o f

u r b a n e f f e c ts i n t r o p i c a l u r b a n a r e a s i s s t il l s p a r s e a n d ,

a s n o t e d b y O k e

e t a l .

1990/91 , the re i s a s ye t l i t t l e

k n o w l e d g e r e g a r d i n g t h e t r a n s f e ra b i l i t y o f re s u l ts

f r o m m i d - l a t i t u d e s t u d i e s . R e s u l t s f r o m t h e f i r s l f i e l d

s u r v e y o f t h e p h y s i c a l c l i m a t o l o g y m a d e i n a t r o p i c a l

c i t y M e x i c o C i ty ) s h o w t h a t w h i l s t t h e re a r e s o m e

d i f f e r e n c e s , t h e o v e r a l l r e s p o n s e o f t h e s y s t e m i s e s s e n -

t i a ll y s i m i l a r t o t h a t o f t e m p e r a t e c i t i e s O k e e t a / . ,

1 9 9 2 ) . T h i s p a p e r i n t e n d s t o c o n t r i b u t e w i t h a d d i -

t i o n a l i n s i g h t s o n t h e d e s c r i p t i v e a s p e c t s o f t h e h e a t

i s l a n d p h e n o m e n o n c h a r a c t e r i z i n g a t r o p i c a l u r b a n

e n v i r o n m e n t a s r e v e a l e d b y o n e y e a r o f h o u r l y t e m -

p e r a t u r e d a t a . T h e n e w s e t o f o b s e r v a t i o n s a i m s t o

p r o v i d e a c l e a r e r p i c t u r e o f th e i n f l u e n c e o f f o r c in g

f a c t o r s s u c h a s c l o u d c o v e r a n d r a in , p a r t i c u l a r l y , i n

t h e d e v e l o p m e n t o f d a y t i m e h e a t i s la n d s . I t is w el l

r e c o g n i z e d t h a t t h e w a r m a i r m a s s o v e r c i ti e s h a s

i m p l i c a t io n s t h a t i n c l u d e a m o n g o t h e r s h u m a n d is -

c o m f o r t , c it y w a t e r d e m a n d a n d e p i s o d es o f u n h e a l t h y

a i r p o l l u t i o n l e v e l s .

2. I )ES RIPTION O1: SITES AND INSTRUM ENTATION

T w o a u t o m a t i c w e a t h e r s t a t i o l l s h a v e b e e n s e t u p

b y th e D e p a r t m e n t o f A t m o s p h e r i c S c i e n c e s o f t h e

N a t i o n a l U n i v e r s i t y i n M e x i c o C i t y s i n ce 1 9 94 . O n e

s t a t i o n i s l o c a t e d i n a t y p i c a l u r b a n a r e a S c h o o l o f

M i n e s h e r e i n a f t e r t e r m e d S M ) w h i l e t h e r u r a l c o n t r o l

s t a t i o n , P l a n T e x c o c o . P T . i s l o c a t e d 6 k m u p w i n d

E N E f r o m t h e ci t y a i r p o r t o n t h e v a l le y p la i n s s ee

l o c a t i o n i n F i g . 1 ) . I n a d d i t i o n t o t h e s e s t a t i o n s , s i -

m u l t a n e o u s h o u r l y t e m p e r a t u r e d a t a f r o m th e

T a c u b a y a O b s e r v a t o r y T O ) m e r c u r y t e m p e r a t u r e

s e n s o r a t 1 . 5 m a b o v e r o o f l e v e l i n a s t a n d a r d s c r e e n )

w e r e a l s o a v a i l a b l e i n o r d e r t o c o m p a r e r e a d i n g s w i t h

t h e S M s t a t io n . I n s t r u m e n t s a t t h e O b s e r v a t o r y a r e

l o c a t e d o n t h e f l at r o o f o f a n o l d t w o - s t o r e y b u i l d i n g

I s e e O k e e t a l . , 1 9 92 f o r d e s c r i p t i o n o f t h e s it e ). W h i l e

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Hea t island development in Mexico City 3823

t h e t w o a u t o m a t i c s t a t i o n s a r e a t a b o u t t h e s a m e

he i gh t 2250 m m . s .1 , a t t he r u r a l s t a t i on a n d 2270 m

m .s.1, f o r SM ) , t he O bs e r va t o r y l i e s on a s l ope 60 m

hig he r Fig . 1).

T h e i n s t r u m e n t s a t t h e u r b a n s t a t i o n l o c a t e d a t th e

S M b u i l d in g ) a r e m o u n t e d o n a m a s t o n t h e r o o f o f

a 1 8 th c e n t u r y t w o - s t o r e y b u i l d i n g i n th e o l d s e c t o r o f

t o w n t h e t e m p e r a t u r e s e n s o r w a s p l a c e d a t 6 m a b o v e

t h e r o o f ) . M o s t c o n s t r u c t i o n s i n t h i s a r e a a r e v e r y

s i m i l a r a n d c l o s e l y p a c k e d .

T h e m e a s u r e m e n t s a t S M a n d P T w e r e m a d e u s i n g

a n a u t o m a t e d d a t a a c q u i s i t io n s y st e m t h a t w a s p r o -

g r a m m e d t o g e n e r a t e 1 h a v e r a g e s . T h e d a t a w e r e

r e c o r d e d u si n g a C a m p b e l l R 2 1 ) M i c r o l o g g e r . T h e

f o l l o w i n g i n s t r u m e n t s w e r e u s e d f o r th e u r b a n s i te ;

f o r t e m p e r a t u r e a V a i sa l a H M P 3 1 U T h y g r o t h e r -

m o m e t e r , p r e c i p it a t i o n w a s m e a s u r e d w i th a t i p p i n g

b u c k e t r a i n g a g e T e x a s E l e c tr o n i c s , 5 2 5) a n d t h e w i n d

s p e e d w it h a T e l e d y n e W S - 2 0 1 5 a n e m o m e t e r . F o r t h e

r u r a l s it e P T ) t h e se n s o r s m o u n t e d o n a t o w e r t o

m e a s u r e t e m p e r a t u r e a t 6 m a b o v e th e g r o u n d ) , p r e -

c i p i t a ti o n a n d w i n d w e r e C l i m a t r o n i c s m o d e l s

102,090, 100,097 an d 100,075 respe ct ive ly . Th e tem -

p e r a t u r e s e n s o r w a s n o t v e n t i l a t e d .

T h e r u r a l s i te o n g r o u n d s o f P l a n T e x c o c o P r o j e c t ,

P T ) i s c h a r a c t e r i z e d b y f i a t p a r t l y s w a m p y l a n d

c o v e r e d b y p a t c h e s o f n a t i v e s a l t r e s i s ta n t ) g r a s s a n d

b u s h e s d u r i n g t h e r a i n y s e a s o n . T h e t e m p e r a t u r e

s e n s o r i s p r o b a b l y s u b j e c t , u n d e r n i g h t t i m e s t a b l e

c o n d i t i o n s , t o t h e i n f l u e n ce o f t w o u p w i n d s o u r c e s o f

m o i s t u r e : a s m a l l ~ 1 6 h a , 4 m d e e p ) r e s e rv o i r , a n d

a l a r g e r ~ 9 0 0 h a , 6 m d e e p ) a r t if i c ia l , r e c t a n g u l a r

a l l - y e a r l a k e F i g . 1 ). S i n c e th e i r c o n s t r u c t i o n i n t h e

l a t e 1 9 7 0s , a d v e c t i o n o f m o i s t u r e f r o m t h e s e l a k e s

h a s i n c r e a s e d th e f r e q u e n c y o f m o r n i n g g r o u n d f o g

l a s t in g s e v e r a l h o u r s ) d u r i n g t h e d r y s e a s o n a t t h e

n e a r b y a i r p o r t w h i c h i s l o c a t e d a b o u t 4 k m d o w n -

w i n d J a u r e g u i , 19 90 /1 99 1 ; M e n d o z a , 1 9 9 2 ) . F o g

f o r m a t i o n a t t h i s t i m e w o u l d t e n d t o p r e v e n t a n y

f u r t h e r c o o l i n g a t th e t i m e o f m i n i m u m t e m p e r a t u r e s .

T h e s i te is o f t en s u b j e c t t o w i n d e r o s i o n o f th e d r i e d

o u t s a l t y c l a y s o il s o f e x - L a k e T e x c o c o d u r i n g t h e d r y

s e a s o n g e n e r a t i n g d u s t s t o r m s t h a t u s u a l l y f o l l o w

a N E t o S W t r a j e c t o r y J a u r e g u i , 1 98 9 ) . T h u s , t h e

p h y s i c a l p r o p e r t i e s e .g . t h e t h e r m a l a d m i t t a n c e a n d

a l b e d o ) o f t h e s o i l in t h i s l a r g e ~ 8 0 0 0 h a ) e c o l o g i c a l

p r e s e r v e s e e F i g . 1 ) a r e s u b j e c t t o c o n s i d e r a b l e s e a -

s o n a l c h a n g e . I t i s r e s p o n s i b l e in r e d u c i n g t h e t h e r m a l

a d m i t t a n c e , a n d t h e r e f o r e i s l i k e l y t o p r o d u c e t h e

l a r g e n o c t u r n a l ) h e a t i s l a n d s a s s u g g e s t e d b y t h e

r e s u l ts f ro m s i m u l a t i o n s o f t h e t h e r m a l r e g i m e o f

u r b a n a n d r u r a l s u r f a c e s a t n i g h t b y O k e e t a l 1991).

T h i s p a p e r a t t e m p t s t o p r o v i d e o b s e r v a t i o n a l s u p p o r t

f o r s uc h f i nd i ngs f o r a t r o p i c a l c i t y .

3 THE CLIMATE

L o c a t e d a t l a t i t u d e 1 9 ° N a n d a t a n e l e v a t i o n o f

2 2 5 0 m , t h e b a s i n o f M e x i c o C i t y e n j o y s a t r o p i c a l

m o u n t a i n c l i m a t e w i t h a r e l a t iv e l y s m a l l a n n u a l t e m -

p e r a t u r e ra n g e . D u r i n g t h e c o o l p e r i o d N o v e m -

b e r - A p r i l ) m o d i f i e d p o l a r a i r m a s s e s c a u s e m o s t o f t h e

c o u n t r y t o h a v e s c a r c e r a i n f a ll a n d a n t i c y c l o n i c

w e a t he r e .g . c l e a r s k ie s , c a l m c o nd i t i ons ) . Th i s c a us e s

f r e q u e n t s u r f ac e r a d i a t i o n i n v e r s io n s . D u r i n g t h e w e t

s e a s on M a y - O c t o b e r ) t h e t r a d e w i n d s b r i n g u n s t a b l e

a i r a n d m o i s t c o n d i t i o n s t h a t p r o d u c e i n t e n s e c o n -

v e c t iv e r a in s . T h e s e s h o w e r s u s u a l l y o c c u r v e r y r e g u -

l a r l y i n t h e a f t e r n o o n a n d e a r l y e v e n i n g h o u r s

J a u r e g u i a n d R o m a l e s , 1 9 9 6 ). E v a p o r a t i o n a l c o o l i n g

l i n k e d t o p r e c i p i t a t i o n m a i n l y a t t h e r u r a l s i t e i s

t h o u g h t t o b e o n e o f th e c a u s e s o f t h e s h o r t l i v e d

1 - 2 h ) d a y t i m e h e a t i s l a n d s in t h e c i t y a s w i l l b e

i l l u s t r a t e d f u r t he r on .

4 RESULTS AND DISCUSSION

S i m u l a t i o n s o f u r b a n s u r f ac e h e a t i s l a n d s a t n i g h t

b y O k e

e t a l

1 99 1) r e v e a l t h a t a m o n g s t t h e v a r i o u s

p o s s i b l e c a u s e s m e n t i o n e d i n t h e l i t e r a t u r e , th e c o m -

b i ne d e f fe c t s o f ge om e t r y e .g . s ky v i e w fa c t o r ) a nd

t h e r m a l p r o p e r t i e s e .g . u r b a n / r u r a l a d m i t t a n c e c o n -

t r a s t s ) c a n p r o d u c e t h e m o s t i n t e n s e h e a t i s l a n d d e -

v e l o p m e n t a f te r s u n s et . F r o m a n e t w o r k o f e x i s t in g

r e g u l a r c l i m a t o l o g i c a l s t a t i o n s b o t h u r b a n / r u r a l ) t h e

m e a n p o s i t i o n o f t h e n o c t u r n a l h e a t i s l a n d c o r e h a s

b e e n f o u n d t o b e i n t h e v i c i n i t y o f th e S M s i te . T h i s

h a s b e e n d o n e b y d r a w i n g 1 0 y r - m e a n m o n t h l y v a l u es

o f m i n i m u m t e m p e r a t u r e J a u r e g u i , 1 9 93 ). W i t h a s i m -

i l a r p r o c e d u r e b y d r a w i n g m a x i m u m t e m p e r a t u r e

i s o t h e r m s ) t h e c o r e o f t h e d a y t i m e h e a t i s l a n d a p p e a r s

t o l i e s o m e w h a t t o t h e e a s t o f S M . T h e d i s p l a c e m e n t is

l i k e l y t o b e o r i g i n a t e d b y t h e d i f fe r e n t m a i n c a u s e s

g i v i n g r is e to t h e p h e n o m e n o n : d u r i n g t h e n i g h t , t h e

c r i t ic a l p r o p e r t i e s g o v e r n i n g t h e t h e r m a l c o n t r a s t s a r e

t h e r a d i a t i o n g e o m e t r y a n d t h e s u r f a c e t h e r m a l p r o -

pe r t i e s e .g . t he r m a l a dm i t t a nc e ) O ke , 1982) w h i l e

d u r i n g t h e d a y t h e d o m i n a n t p r o c es s e s a r e t u r b u l e n t

s e n s i b l e h e a t f l u x f r o m i n c r e a s e d a b s o r p t i o n o f s h o r t -

w a v e r a d i a t i o n a n d a n t h r o p o g e n i c h e a t s o u r c es i n-

d u s t r i a l a n d v e h i c u l a r , m a i n l y ) . A s p o i n t e d o u t b y

U n w i n 1 98 0) , th e t i m e o f o c c u r r e n c e o f t h e se t e m -

p e r a t u r e s i s n o t n e c e s sa r il y c o n t e m p o r a n e o u s a n d

t h e r e f o r e s o m e e r r o r i s i m p l i c it . H o w e v e r , a g l a n c e a t

t h e m e a n h o u r l y v a r i a t io n s o f te m p e r a t u r e a t b o t h

u r b a n a n d r u r a l s it e s F i g s 4 a n d 6 ) s h o w s t h a t a t

l e a s t i n t h i s c a s e , t h e e x t r e m e v a l u e s o c c u r a t a b o u t

t h e s a m e t i m e a n d t h e r e f o r e , t h e e r r o r i n e s t i m a t i n g

t h e r m a l c o n t r a s t s i s l i k e l y t o b e g e n e r a l l y s m a l l.

F i g u r e 2 s h o w s t h a t o n a d a y d u r i n g t h e r a i n y

s e a s o n h e a t i s l a n d i n t e n s i t y a s e s t i m a t e d b y t h e S M

m i n u s r u r a l P T i s g e n e r a l l y g r e a t e r t h a n t h a t o b t a i n e d

f r o m t h e T O m i n u s P T w h e r e T O i s l o c a t e d s o m e

6 k m a w a y f r o m t h e h e a t i s l a n d c o r e n e a r t h e w e s t e r n

o u t s k i r t s o f t h e c i t y . T h e r m a l c o n t r a s t s b e t w e e n t h e

t w o u r b a n s i te s S M m i n u s T O ) a r e p o s i t i v e a r o u n d

2 - 3 ° C ) i n d i c a t i n g t h a t a i r a t S M i s m o s t o f th e t i m e

w a r m e r t h a n a t T O , e s p e c i a l l y d u r i n g t h e n o c t u r n a l

8/10/2019 1-s2.0-S1352231097001362-main


3 82 4 E . J A U R E G U I

- - m - - T a c u b a ya u r ba n )

w e ~ - P I. T e x c o ¢ o m r M ) - - O - - S .

o f M i n u l u r b lm ) c e n te r

PI.

Texco¢o rura l )

X S

o f M i r t h u c b mn ) c e n t e r - T e c u b a y a u r ba n ) w e s t

a

f

4 . . . . . _ _ ~

2

0

2 t

0 0 0 0 0 0 0 0 0 0 0 0

o o o o o o o o o 9 o

Fig. 2. Tem peratu re differences: Tac ubay a (urban) - Plan Texc oco (rural) SM (urban) - Phm Texc oco

(rural) and SM - Tac uba ya. July 7, 1994 (local time).

p e r i o d . I t i s w o r t h n o t i n g t h a t u r b a n / r u r a l t e m p e r -

a t u r e c o n t r a s t s o b s e r v e d a t t h e t h r e e s it e s a l t h o u g h

d i f f er e n t i n a b s o l u t e v a l u e , s h o w v a r i a t i o n s t h a t a r e

v e r y m u c h i n a g r e e m e n t w i t h o n e a n o t h e r . T h i s i s

a l s o t r u e f o r w i n d s p e e d v a r i a t i o n s a t t h e t w o s i te s , a s

w i l l b e i l l u s t ra t e d f u r t h e r o n . T h i s f a c t g i v e s m o r e

c o n f i d e n c e t o t h e r e s u l t s p r e s e n t e d h e r e .

T a b l e 1 s h o w s t h e f r e q u e n c y o f d a y s w h e n a w e l l-

m a r k e d u r b a n t e m p e r a t u r e e x c e d a n c e ( of m o r e t h a n

I ° C m a i n t a i n e d o v e r s e v e r a l h o u r s , e x c e p t f o r t h e 1 h

m a x i m u m H I d e s c r i b e d i n S e c t i o n 4 . 4 ) w a s o b s e r v e d

a t d i f fe r e n t ti m e s o f th e d a y f o r M e x i c o C i t y d u r i n g

t h e y e a r f r o m M a y 1 9 9 4 t o A p r i l 1 9 9 5 . D a t a w e r e

a v a i l a b l e f o r 3 4 3 o f t h e 3 6 5 d a y s . I t i s e v i d e n t t h a t t h e

n o c t u r n a l ( 9 p m t o 1 0 a m ) h e a t i s l a n d i s t h e m o s t

d o m i n a n t c a s e c o v e r in g t h r e e f o u rt h s o f t h e t im e o f

t h e y e a r a n d p r e v a i l i n g m o s t l y d u r i n g t h e d r y s e a s o n .

D a y t i m e h e a t i s l a n d s w e r e c l a s s i f i e d b y t h e i r t i m e o f

o c c u r r e n c e a t a r o u n d n o o n ( 1 1 -1 5 h) an d a f t e r n o o n

( 1 6 - 2 0 h ) h e a t i s l a n d s. T h e y a r e m o s t l y o b s e r v e d d u r -

i n g t h e r a i n y s e a s o n . T h e a v e r a g e a n d e x t r e m e v a l u e s

o f t h e h e a t i s l a n d a l o n g t h e 1 y r p e r i o d o f o b s e r v a -

t i o n s a r e i l l u s t r a t e d i n F i g . 3 . W h i l e t h e m e a n h e a t

i s l a n d i n t e n s i t y f o r n o c t u r n a l e v e n t s ( a b o u t 5 ° C ) i s

n e a r l y c o n s t a n t t h r o u g h o u t t h e y ea r , t h e m i d d a y p h e -

n o m e n o n o c c u r s m a i n l y d u r i n g t h e w e t m o n t h s , a n d

h a s s o m e w h a t l o w e r i n t e n s i ti e s ( 3 - 5 ' C ) a n d l e s s

v a r i a b i l i t y o f e x t r e m e s . W h e n t h e y o c c u r , t h e a f t e r -

n o o n h e a t i s l a n d s h a v e a b o u t t h e s a m e i n t e n s i t i e s a s

t h e n i g h t -t i m e , b u t n a r r o w e r r a n g e o f v a r ia t i o n p r o b -

a b l y d u e t o t h e r e s u l t o f f e w e r ca s e s. T h a t i s, e x c e p t f o r

o n e c a se t h a t o c c u r r e d o n t h e a f t e r n o o n 1 9 O c t o b e r

1 9 9 4 w h e n t h e l a rg e s t u r b a n / r u r a l t h e r m a l c o n t r a s t

f o r t h e a v a i l a b l e p e r i o d o f o b s e r v a t i o n o c c u r r e d . T h i s

e v e n t w i l l b e e x a m i n e d i n d e t a i l f u r t h e r o n .

I t i s r e a d i l y s e e n f r o m F i g . 3 t h a t n o m i d d a y /

e v e n i n g h e a t i s l an d s w e r e o b s e r v e d d u r i n g t h e c o o l

s e a s o n ( D e c e m b e r - F e b r u a r y ) s u g g e s t i n g t h a t t h e

p h e n o m e n o n i s m a i n l y l i n k ed t o c l o u d c o v e r a n d

e v a p o r a t i o n a l c o o l i n g f r o m r a in f a l l e v e n t s a t b o t h

s i t e s d u r i n g t h e w e t s e a s o n .

Table 1. Frequenc y ( ) and num ber of heat island events

occur r ing durng th ree pe r iods o f the day as e s t ima ted f rom

thermal contrasts observed between an urban {School of

Mines, SM) and a rural (Plan Texcoco, PT) si te in Mexico

City from M ay 1994 to Apri l 1995. Fo r missing da ta refer to

Section 2

21 10h I1 15h 16 2(1tl

Ma y 1994 80.6 9.7 9.7

Jun e 1994 52.4 19.0 28.6

July 1994 36.7 26.7 36.7

Au gust 1994 10.3 41.4 48.3

Sep tem ber 1994 52.2 34.8 13.0

Oc tob er 1994 59.3 22.2 18.5

No vem ber 1994 96.7 0.0 3.3

De cem ber 1994 100.0 0.0 0.t)

Ja nu ary 1995 100.0 0.1) 0.0

Febru ary 1995 96.6 3.4 0.0

Ma rch 1995 96.8 0.0 3.2

Ap ril 1995 96.7 0.0 3.3

No. of events 256 42 45

Fr eq /y r ( ) 74.6 12.2 13.1

4 1 T h e n o c t u r n a l h e a t i s l a n d

F i g u r e 4 i l l u s t r a t e s t h e m e a n h o u r l y d e v e l o p m e n t o f

t h e h e a t i s l a n d f o r s i x n o n - c o n t i g u o u s d a y s w i t h c l e a r

s k i e s a n d l i g h t w i n d s in J a n u a r y 1 9 94 w h i c h w a s

c h a r a c t e r i s t i c o f a d r y m o n t h . T h e a v e r a g e n o c t u r n a l

h e a t is l a n d r e a c h e s it s m a x i m u m w d u e ( 5 C ) a t th e

e n d o f t h e c o o l i n g p e r i o d a t a b o u t s u n r is e . T h i s d e v e l -

o p m e n t d i f f e r s f r o m w h a t i s o b s e r v e d i n m i d - l a t i t u d e

c i t ie s w h e r e t h e u r b a n / r u r a l d i v e r g i n g r a t e s o f c o o l i n g

l e a d t o a m a x i m u m h e a t i s l a n d i n t e n s i t y b e f o r e m i d -

n i g h t ( O k e , 1 98 2). F o r t h e c a s e o f M e x i c o C i t y t h e

d i v e r g e n c e o f b o t h c o o l i n g r a t e s l e a d s f i r st t o a s l i g h t

i n c r e a s e i n h e a t i s l a n d i n t e n s i t y b e g i n n i n g e a r l y i n t h e

e v e n i n g t o a m e a n m a x i m u m o f 1 . 2 C a t a ro u n d 7 o r

8 p m a n d r e a c h i n g a p l a t e a u t h e r e a f t e r d e c l i n i n g to

a b o u t m i d n i g h t . I t is li k e l y t h a t t h i s e v e n i n g p e a k m

h e a t i s l a n d i n t e n s i t y , a l b e i t s m a l l o n t h e a v e r a g e ,

8/10/2019 1-s2.0-S1352231097001362-main



0

r

(0

7_-

g

fo

._>

g

:E

>.

2 1 t o 1 0 h s . n i g h t )

oc

9 . 0

6 ,0 '

7 . 0 ,

6 . 0

5 . 0 ,

4 . 0 ,

3 . 0 ,

2 . 0 ,

1 . 0 ,

0 . 0 I I I I I I I I I I I

M a y

Jun Jul Aug Sep Oct Nov

Dec Jan Feb Mar

Apt

94 '94 '94 '94 '94 '94 '94 '94 '95 '95 S§ 9S

+ a v e r a g e HII

[] maximum

[] minimum

0

-

6 . 0

- >,

¢p. ~_

m , ~ 5 . 0

~ 4 . 0

>

.=-

3 . 0

__> 2 . 0

1.0

o o

11 tO 15 h s . m i d d a y h o u r s )

oc

9 . 0

8 .0 .............................................................................................................................................

7.0 ..............................................................................

i ~ i B ave rage HII

[] maximum

m minimum

May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr

'94 '94 '94 '94 '94 '94 '94 '94 95 '95 95 '95

oc

1o

9.o

~ 8 . 0

7 . 0

®

-

6 . 0

g - ~

5 . 0

moo

4 . 0

g ._c

3 . 0

> 2 . 0

1 o

c

o 0 . 0

:Z

1 6 to 2 0 h s . a f t e r n o o n / e v e n in g )

i ~ ~ re aVerage II................................................................................

....... ..... [] maximum

[] minimum

o n e e v e n t

. . . . - - --

May Jun Jut Aug 8ep Oct Nov Dec Jan Feb Mar Apt

'94 '94 '94 '94 '94 '94 '94 '94 '95 'S5 '95 '95

Fig. 3 . A vera gea nde xt rem evalu esof heat i s landin tens i ty (Hl I ) fo r hree per iods of the day in Mexico Ci ty

for May 1994-April 1995.

m i g h t b e l i n k e d t o t h e r e l e a s e o f s e n si b l e h e a t f r o m

r u s h h o u r t r af f ic o c c u r r i n g f ro m a b o u t 6 - 9 p m .

T h e r e a f t e r , th e h e a t i s l a n d c o n t i n u e s t o d e v e l o p

t h r o u g h t h e e a r ly m o r n i n g h o u r s d u e m a i n l y to t h e

r u r a l n e t r a d i a t i v e e n e r g y l o s s t o a n u n o b s t r u c t e d s k y

a n d l e s s p o l l u t e d a t m o s p h e r e p r i o r t o s u n r i s e . A f t e r

t h i s t i m e s o l a r h e a t i n g g e n e r a t e s a t u r b u l e n t m i x e d

l a y e r o v e r b o t h t h e u r b a n s u r fa c e s a n d t h e c i t y e n -

v i r o n s , s o t h e r m a l c o n t r a s t s d e c l i n e u n t il a r o u n d t h e

e n d o f th e a f t e r n o o n a s s e e n in F i g . 4 .

H e a t i s l a n d i n t e n s i t y h a s b e e n n o t e d t o b e r e l a t e d t o

t he s t r e n g t h o f t he r u r a l i nve r s i on (e .g . Ludw i g , 1970).

I n t h e c a s e o f M e x i c o C i t y t h e h e a t i s l a n d i n t e n s i ty i s

n o t s i g n i f ic a n t l y c o r r e l a t e d e i t h e r w i t h t h e r a t e o f

c h a n g e o f te m p e r a t u r e w i t h h e i g h t ( ° C / 1 0 0 m ) o r t h e

t h e r m a l c o n t r a s t b e t w e en b o t t o m a n d t o p o f t h e

s u r f a c e i n v e r s i o n l a y e r o b s e r v e d a t t h e a i r p o r t . H o w -

e v e r , a s s h o w n i n t h e c o r r e l a t i o n m a t r i x i n T a b l e

2 c o n s t r u c t e d w i t h d a t a f o r N o v e m b e r 1 9 9 4 t o A p r i l

1995 , t he r e i s a h i gh l y i nve r s e r e l a t i on s h i p o f t he he a t

i s l a n d i n t e n s i t y w i t h t h e s u r f a c e i n v e r s i o n t h i c k n e s s.

T h i s w o u l d s u g g e s t t h a t s h a l l o w s u r f a c e i n v e r s i o n

l a y e r s ( c h a r a c t e r i z e d b y a m a r k e d r a t e o f in c r e a s e o f

t e m p e r a t u r e w i t h h e i g h t) t e n d t o b e a s s o c i a t e d t o t h e

f o r m a t i o n o f i n t e n s e u r b a n / r u r a l t h e r m a l c o n t r a s t s .

4.2. he noon heat island

D a y t i m e f e a t u re s o f m i d - l a t i t u d e h e a t i s l a n d s h a v e

b e e n l i t tl e s t u d i e d . N k e m d i r i m a n d T r u c h ( 19 78 ) h a v e

r e p o r te d m i d - m o r n i n g u r b a n / r u ra l m e a n m a x i m u m

t h e r m a l c o n t r a s t s v a r y i n g f r o m 5 ° ( in s u m m e r ) t o

10°C i n w i n t e r f o r t he c i t y o f C a l ga r y ( l a t i t ude 51° N ).

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3 82 6 E . J A U R E G U I

m e a n T S . o f M i n e s - P I . T e x . - 4 1 - m e a n te m p . f o r S , o f M i n e s r i g h t s c a le )

- - - < > - m e a n t e m p . f o r P I . T e x c o c o r i g h t s c a l e )

° C

5 - 2 5

20

3.5

3 15

°° 2.5

2 10

: t l l l l t t l t l l l l l l l l l l l l l l l l

0 5 T

0 1 0

0 0 0 0 0 0 0 0 0 0 0 0

o o o o o o o o o o o o

CQ

CN

Fig. 4. Me an urban /rural tem peratu re and temp erature di lferences at SM (urban) - Plan Texcoc o (rural}

for s ix noncontiguous days with no rain (dry seasom January 1994.

Tab le 2. Co rrelat ion m atr ix of the Mex ico ci ty urban heat is land with the therm al contrast between the top and bo ttom of

the surface inve rsion layer, the vertical gra dien t in the stable surface layer and the surface inversion thickness Iml at the

a i rpor t fo r pe r iod N ovem ber 1994 to A pr i l 1995 N = 138

He at island Vertical ther ma l Vertical grad ient Surface

intensity (C ) contrast I C) I C:1 00m l inversion

thickness (mi

Hea t i s l and in tens i ty (C)

Vert ical therm al contrast C

Ver t i ca l g rad ien t C /100 m

Surface invers ion thickness Ira)

1 0 . 1 2 0 . 0 1 0 . 5 1

1 0 . 5 1 0

I 0.28

1

T h e y a t t r ib u t e t h e g e n e s is o f t h e p h e n o m e n o n t o

a n t h r o p o g e n i c h e a t r e l e a s e s a n d t o g e o g r a p h i c a l f a c -

t o r s . O n t h e o t h e r h a n d , o t h e r m i d - l a t i t u d e a n d l e s s

c o n t i n e n t a l c i t ie s h a v e b e e n f o u n d t o b e c o o l e r a t

c e r t a i n d a y t i m e h o u r s i n s u m m e r ( C h a n d l e r , 1 9 6 2 ;

U n w i n , 1 98 0) . O k e ( 1 9 8 2) m e n t i o n s t h a t t h e e x i s t e n c e

o f a c o o l i s l a n d i n t h e m i d d l e o f t h e d a y i s g e n e r a l l y

a t t r i b u t e d t o c a n y o n s h a d i n g i n t h e c i ty c e n te r .

L u d w i g ( 19 7 0) c o n c l u d e s t h a t a i r t e m p e r a t u r e s i n

d o w n t o w n D a l l a s w i t h i t s c l o s e l y s p a c e d , v e r y t a l l

b u i l d in g s w e r e s o m e w h a t c o o l e r ( ~ 1 C ) t h a n t h e

s u r r o u n d i n g c o u n t r y s i d e d u r i n g th e d ay . C o m p a r i n g

m e a n h o u r l y t e m p e r a t u r e s b e t w e e n th e T a c u b a y a O b -

s e r v a t o r y ( T O ) a n d t h e a i r p o r t ( s u b u r b a n ) , J a u r e g u i

( 1 9 86 ) f o u n d t h a t f o r t h i s p a i r o f s t a t i o n s t h e c i t y a i r

w a s c o o l e r d u r i n g t h e a f t e r n o o n h o u r s . H o w e v e r , t h is

r e s ul t c a n n o t b e e x t e n d e d t o th e m a i n d o w n t o w n a r e a

s i nc e th e T a c u b a y a O b s e r v a t o r y w h i l e b e i n g u r b a n i n

n a t u r e , l i e s o n t h e w e s t e r n s u b u r b s o f th e c i t y

( J a u r e g u i , 1 9 9 3 ). S i n c e t h e S M u r b a n s t a t i o n i s , a s

m e n t i o n e d i n S e c t i o n 2 , u n d e r t h e d i r e c t i n f l u e n c e o f

t h e h e a t i s l a n d c o r e , t e m p e r a t u r e s t h e r e , a r e r a r e l y

l o w e r t h a n a t t h e r u r a l s i te . e v e n d u r i n g t h e a f t e r n o o n

h o u r s ,

F - i g u r e 5 i l l u s t r a t e s a c a s e o f w e t - s e a s o n d a y t i m e

m e a n h e a t i s l a n d f o r 15 n o n - c o n t i g u o u s d a y s i n

A u g u s t - S e p t e m b e r 1 99 4. I t i s p o s s i b l e t h a t t h e l ig h t

n o c t u r n a l p r e c i p i t a t i o n o c c u r r i n g a t b o t h t h e u r b a n

a n d r u r a l s i t e s w a s i n s t r u m e n t a l i n r e d u c i n g t h e m a g -

n i t u d e o f r a d i a t i o n a l c o o l i n g (a n d t h e r m a l c o n t ra s t} .

W i t h h u m i d i t y l e v e l s r e m a i n i n g h i g h e r a t th e r u r a l

s i t e d u r i n g t h e d a y i t i s l i k e l y t h a t a c o n s i d e r a b l e

p o r t i o n o f s o l a r h e a t i n g w a s s p e n t a t t h e r u r a l s i te f o r

e v a p o r a t i o n o f n e a r - s a t u r a t e d s o il , w h i l e t h e o p p o s i t e

w a s t r u e a t t h e u r b a n s i te w h e r e n e t r a d i a t i o n w a s

m a i n l y u s e d t o h e a t t h e u r b a n s u r f a c e s t h a t d r y f a s t .

T h e r e s u lt w a s a w e a k d a y t i m e h e a t i s l a n d o f 2 ,5 C

i n t e n s i t y .

D a y t i m e h e a t i s l a n d s c a n a l s o o c c u r d u r i n g t h e c o o l

s e a s o n w h e n t h e r e i s n o r a i n a t e i t h e r o f t h e t w o s i te s ,

a s i l l u s tr a t e d i n F i g . 6 . U r b a n / r u r a l c o n t r a s t s o f

4 - 5 C p e r s i s te d f r o m s u n r i s e t o 1 p m b e c a u s e u r b a n

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Heat island development in Mexico City 3827

A t °C)

1 4

3

1,2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 5

1 2

= 0 8

1 5

0 6

• 1

0 4

0 2 0 5

0

. . . . . I iml I I l l ~1 I

0

i ] i , i i i i i , i

0 ~ ~ 0

~ M e a n h o u r l y

precip, for S . of

B g ~ m

hourly predp, for

P I .

Texcoco.

- 4 - M e u t e m p . ~ J t f . S . o f M i a m - P I .

Texcoco.

Fig. 5. Mean urban/rural temperature contrasts; SM urban) - Plan Texcoco for 15 noncontiguous days

when rainfall occurred at both sites wet season) August and September 1994.

2 4

Janary1994

°C

6 2 5

5 2 0

4

1 5

lO

2

1 5

~ O O

+ : : : : : :: : T e n ' l p , t ul iP . S . o f l ~ , ' n n ~ - H . T e n t .

Temp. S . of ICl~es urban) d g h t s c a l e

- 4 1 - - T e m p . 1 ~ , T e x .

rural) r ight sode )

Fig. 6. Hourly temperature and temperature differences; SM urban) - Plan Texcoco rural), no precipita-

tion at both sites during a dry season day.

and rural heating rates were similar during the mor-

ning hours of 24 Jan uar y 1994. In this case, it is

possible that the origin of the day time positive ther-

mal anomaly was the increased absorption of short-

wave radiation by urban surfaces with lower albedo

than the dry soils at the rural site.

4.3.

he af ternoon~evening heat i s land

Effects of cloud cover or rain at either or both

urb an/rura l sites) can produce sudden changes in the

daytime heat island development giving rise to one

peak at aroun d noon an d anothe r in the late evening.

Figure 7 illustrates this case for a day in the rainy

season when afternoon showers first at the ur ban site

and then at both sites produc ed sudden changes that

led first to a one hour 4°C positive thermal contrast

and then to negative temperature differences immedi-

ately after the rain at both sites had stopped• This

perhaps as a result of a more marked evaporational

cooling rate both from falling rain drops and wet

surfaces at the u rban site. It is clear from Fig. 7 that

the grad ual decline of the heat island intensity after

midday is caused by the shading effect from deve-

loping clouds before rain started. The characteristic

evening peak before midnight observed in the dry

season is here also present on a day of a wet month

August) suggesting that it is quite a persistent feature

of heat island development throug hout the year.

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8/10/2019 1-s2.0-S1352231097001362-main



J=

E

E

.;

o

._=

(al

I i :~ i P l . T e x o o o o p r e d p .

- - I - - T e m p . o f S . o f

M i n e s u r b a n ,

d g h t I c e l e )

T e m p . $ . o f M | n u - F t . T e x c o c o .

~ T a m p . o f F t . T e x © o c o ( ru r a l,

d g h t sca le l

°C

9 3

8 2 5

5 1 5

4

5

o

9. 9. 9. 9. 9. 9. ? .o. 9. 9. 9. 9.

- I - - P I . T e x co c o - O - - S c h o o l

o f M i n e e

i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E

~ 4

w 3

~c 2

1

o I

0 O 0 0 0 O 0 0 0 0 0 0

0 0 0 0 0 0 0 ~ 0 0 0 0

Fig. 8 a) Largest evening heat is land 19 Oc tobe r 1994. b) Diur nal v ariat io n of wind speed at SM urban)

and Plan Texcoco rural ) for the day of the largest evening heat is land, 19 Octob er 1994.

s e a s o n a l i t y o f w e a t h e r c o n t r o l s : a n t i c y c l o n i c w e a t h e r

f a v o u r i n g c l e a r , c a l m n i g h t s i n t h e c o o l s e a s o n a n d

u n s t a b l e w e a t h e r c o n d i t i o n s w i t h c l o u d s a n d r a i n i n

t h e M a y - O c t o b e r p e r i o d . T h e r e fo r e , a s s e en i n F i g . 9 ,

t h e l a r g e s t m e a n n o c t u r n a l h e a t i s l a n d s ( 5 °C ) o c c u r

i n t h e d r y s e a s o n w h e n d i f fe r e n c e s i n u r b a n / r u r a l

t h e r m a l a d m i t t a n c e a r e m o r e m a r k e d , d e c l i n i n g t o

a m i n i m u m ( o f 2 ° C ) d u r i n g t h e w e t m o n t h s o f

A u g u s t - S e p t e m b e r w h e n s o i l in t h e r u r a l s i te is n e a r

s a t u r a t i o n . D a y t i m e h e a t i s l a n d i n t e n s i t y i s i n t h e

m e a n l e ss in t e n s e r e a c h i n g a m a x i m u m i n J u l y (3 °C )

( w h e n t h e h i g h s u n fa v o u r s r u r a l e v a p o r a t i o n a l c o o l -

i n g fr o m r a i n s h o w e r s ) , d e c l i n i n g t o a m i n i m u m ( I ° C )

i n D e c e m b e r . I t is w o r t h n o t i n g f r o m F i g . 9 t h a t a t

l e a s t in t h i s 1 y r s u r v e r y , t h e u r b a n - r u r a l t h e r m a l

c o n t r a s t s r e m a i n o n t h e a v e r a g e a l w a y s p o s i t i v e

t h r o u g h o u t t h e y e a r a n d a t a l l h o u rs o f t h e d a y .

5 C O N C L U D I N G R E M A R K S

T h e h o u r l y d i f fe r e n c e s i n t e m p e r a t u r e b e t w e e n t w o

r e c e n t l y i n s t a l l e d u r b a n r u r a l a u t o m a t i c s t a t i o n s h a v e

b e e n u s e d t o m e a s u r e h e a t i s l a n d d e v e l o p m e n t i n

M e x i c o C i t y . R e s u l ts f r o m 1 y r o b s e r v a t i o n s s h o w :

( a ) N i g h t t i m e h e a t i s l a n d s a r e t h e m o s t f r e q u e n t

a n d m o s t i n t e n s e . T h e y a r e m o s t f r e q u e n t d u r i n g t h e

d r y s e a s o n w h e n i t i s l i k e l y t h a t l o w r u r a l t h e r m a l

a d m i t t a n c e o f d r y s o i l s c o m b i n e s w i t h r e l a t i v e l y l o w

a i r m o i s t u r e t o c a u s e r a p i d c o o l i n g r a t e s a t t h e r u r a l

s i t e t h r o u g h o u t t h e n i g h t . T h i s c o n t r a s t s w i t h m i d -

l a t i t u d e e x p e r i e n c e , w h e r e t h e d i v e r g i n g u r b a n r u r a l

c o o l i n g r a te s p r o d u c e a m a x i m u m t h e r m a l c o n t r a s t

b e f o r e m i d n i g h t .

( b) T h e m o s t s i g n i fi c a n t f a c t o r s g o v e r n i n g h e a t

i s l a n d d e v e l o p m e n t d u r i n g t h e r a i n y s e a s o n a r e t h e

c h a n g e s i n w i n d s p e e d , c l o u d c o v e r a n d r a i n ( p a r t i -

c u l a r l y a t t h e r u r a l s i t e ) , t h a t a r e a s s o c i a t e d w i t h

u n s t a b l e t u r b u l e n t w e a t h e r . D a y t i m e h e a t i s la n d s d u r -

i ng t he w e t s e a s on a r e g e ne r a l l y l e s s i n t e ns e ( 3 - 5° C ) , o f

s h o r t d u r a t i o n , t h a t a t o t h e r t i m e s o f t h e y e a r .

( c) T h e a f t e r n o o n / e v e n i n g h e a t is l a n d s h a d a m e a n

i n t e n s i ty o f 4 - 5 ° C . T h e i r f r e q u e n c y d u r i n g t h e y e a r

w a s 1 2 o f t h e d a y s . T h e y w e r e a b o u t e q u a l l y l i k e l y

t o o c c u r d u r i n g t h e d r y o r w e t s e a s o n s .

8/10/2019 1-s2.0-S1352231097001362-main


3830 E. JAUREGUI

oc

5 5

~. 5

4.5

a .

2 4

. = ~

~ ~

2 B

6 2

ff

I

<

°C

8 3 .5

2 5

+

h-

g. 2

~ 1 . 5

.~ ~ ~

~ = 0 5 i

o

--

_ <

E

I

Fig. 9. Seasonal variation of urban heat island using mean monthly diflierences between maximum and

minimum temperatures for 1994 (except April 1995)•

(d) The develop ment of those evening heat islands

that occur during the rainy period, begins at around

sunset, reaches a maxim um near mid-evening and

declines thereafter to a minimum around midnight.

Their development is often modulated by rural rain

events.

(e) It is possible that the position of the sensor at

the urban site may play a role in increasing the day-

time heating rates since, as is well established, roof

tops act as elevated heat sources (see Ludwig, 1970).

Recently, Sakaida and Suzuki (1994) have shown that

afternoon air temperatures at 3 m above roof level are

about 2' C higher than at 2 m above street level.

While Oke (1981) has shown that geometry and

thermal admittance are capable of generating the

largest (nocturnal) heat islands, field observations in

this tropical city demonstrate that occasional day-

time heat islands may emerge as a result of advected

cool air or from downdrafts from rainshowers at the

rural site. The sometimes s hort-lived daytime thermal

contrasts may attain the highest value of the period.

It is likely that these short temperature contrasts em-

erge because the selected pair of stations are sensibly

aligned with the most frequent direction of cool gusts

from the northeast first at the rural site and leaving

the urban site temporarily unaffected. Even though

these apparently rare events are not central to the heat

island issue discussed here they canno t be completely

discarded. They might be of help for future heat island

studies when the decision of the sites location is made.

Clearly, the shortness of the period examined here

prevents determination of how representative the

above results are. The fact that maximum heat island

observed was not as large as would be anticipated

given the size and population of the city could be

attributed to:

(a) The city station (SM) does not represent ther-

mal conditions in the urban canopy layer but rather

the urban boundary layer, where nocturnal cooling

rates are likely to be greater than at street level.

(bt The rural site being affected by water vapour

advection from nearby reservoir for certain wind di-

rections restricting nocturnal radiational cooling.

(c) Mean u rban/ru ral thermal differences estimated

from mean monthly minimum temperature maps for

Mexico City have been shown to be as high as 9C

8/10/2019 1-s2.0-S1352231097001362-main


Hea t i s land develop men t in Mexico Ci ty 3831

( J a u r e g u i , 1 9 8 6 ) f o r a r u r a l s t a t i o n l o c a t e d ( t o t h e S E

o f t h e c i ty ) a w a y f r o m m o i s t u r e s o u r c e s . T h e r e f o re ,

h i g h e r m a x i m u m v a l u es o f t h e n o c t u r n a l h e a t i s l a n d

m a y b e o c c u r r i n g r e l a t i v e to o t h e r r u r a l s i te s .

T h e m a x i m u m h e a t i s la n d o b s er v e d m i g h t b e th e

r e s u l t o f th e c o n j u n c t i o n o f c r i ti c a l c o n t r o l s b e i n g

p r e s e n t o n l y o n c e d u r i n g t h e o b s e r v a t i o n p e r io d . P e r -

h a p s m o r e a m e n a b l e f o r g e n e r a l i z a t i o n w o u l d b e , t o

c o m p a r e u r b a n h e a t i s l a n d s in t h e t r o p i c s b y th e i r

m e a n m a x i m u m n o c t u r n a l th e r m a l c o n t ra s t s r a th e r

t h a n t o c h a r a c t e r i z e t h e m f r o m a n e x t r e m e c a s e , S i n c e

i n m a n y p a r t s o f t h e h u m i d t r o p ic s s h o w e r s t e n d t o

b e g i n f r e q u e n t l y a t a c e r t a i n h o u r w i t h c l o c k w is e

r e g u l a r i ty , i t is . li k e l y t h a t r a i n y - s e a s o n d a y t i m e h e a t

i s l a n d s d e s c r ib e d i n t h i s p a p e r f o r M e x i c o C i t y b e a l s o

a f e a t u r e i n o t h e r t r o p i c a l c i t i e s .

Acknowledgements--The auth or i s grateful to A. Es t rada for

doing the drawing, to E. Luyando for the da ta process ing

a nd c om pu t e r d r awi ng , t o t he I n s t r ume n t s De pa r t m e n t

(Mr. V. Zarra luqu i , M. G arc ia , J. Escalante, W. Gu t i r r rez

and A. Rodriguez) for main tenan ce of automa t ic s ta t ions .

Th i s s t udy wa s s uppo r t e d by a g r a n t f r om t he Na t i ona l

Counc i l f o r Sc i enc e a nd Te c hno l o l gy ( CONA CYT , 2130

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