Urban climate and day of the week

Atmospheric Environment Pergamon Press 1971. Vol. 5, pp. 935-948. Printed in Great Britain.

URBAN

CLIMATE

AND

DAY

OF THE WEEK

E. N. LAWRENCE Me&orological Office, Bracknell, Berkshire, England (Received 30 June 1970 and in final form 2 June 1971) Abstract-A

brief survey is made of previous research on the day-of-week variation in a number of weather parameters, with special reference to rainfall amount. Further analysis suggests that the amplitude of the so-called “weekly rainfall cycle” in the London area is related to wind speed, sunshine duration and the magnitude of the urban heat-island. The present investigation on daily maximum and minimum air temperature in the London area, over the 20-yr period from April 1949 to March 1969, shows that there was a weekly cycle in the difference between the mean daily maximum temperature in central London and the surrounding rumI area, in midsummer (May, June and July). The mean weekly amplitude of the difference in daily maximum temperature was approximateIy 05°C; the lowest mean difference of daily maximum temperature (London minus surroundings) occurred on Thursday and the highest on Sunday. The pattern is sign&cant at the 0.1 per cent level. The weekly amplitude varied inversely as the wind speed. Although the weekly pattern of daily maximum temperature can be explained by the weekly pattern of air pollution, the results are generally consistent with the hypothesis that thermal convection can be an important cause of the differences between urban and rural rainfall regimes: also, results indicate that wind speed is an important factor in urban heatisland and rain-island phenomena. 1. INTRODUCTION

EXTENSIVE research has been carried out on the day-of-week variation in a number of weather parameters. A brief survey of this work was a useful preliminary to the investigator and is here summarized and discussed. Investigations have shown that air pollution in the London area is, on average, at a ~nimum on Sunday, as shown for example by Kew data for smoke (CHAY~%LER, 1965) and for sulphur dioxide (LAWRENCE, 1969); the latter data refer to summer (May-August) and showed Sunday and Monday to be similar. Also, an analysis (CHANDLER, 1965) of the average number of hours of bright sunshine at Kew during the period 1957-1962 showed that for the summer half-year (April-September), Sunday was the sunniest day of the week. Further, Sunday, on average, had better visibility than Friday during the winter half-year, 1946-1952, at Finningley, Nottinghamshire, a place very liable to industrial smoke pollution on weekdays (RATCLIFFE, 1953).

As a result of corresponding studies of rainfall, ASHWORTH (1929, 1944) found that Sunday was the day of the week with the least rain over the periods 1898-1927 (30 yr) and 1898-1942 (45 yr) in the manufacturing town of Rochdale, Lancashire. At Stonyhurst, Lancashire, with less industry, the rainfall on Sundays and weekdays was more nearly alike. Also, at Rochdale on weekdays, there were more rain-hours during wor~ng-hours (07-18 hours), and there were more rain-hours during daytime than in the night. This pattern was reversed on Sundays at Rochdale and at Stonyhurst on all days of the week, a pattern typical of stations to the west of the main industrial areas, According to a report (METEOROLOGICALOFZ=KE, 1929) of the subsequent discussion thereon, Sir Napier Shaw concluded that the evidence, taken altogether, was a strong argument for the reality of the weekly rainfall pattern. It was considered also that the 935 &E.--C

E. N. LAWRENCE

936

rainfall increase during weekdays ‘&wasprobably due to the chemical effect of the hygroscopic nuclei emitted rather than to any mechanical effect caused by the high temperature of the gases emitted”. However, Sunday was not always dry, for example: at Lympne, Kent in 1931 (CARTER,1931), at Gorway, near Walsall in Staffordshire (IMETEOROLOGICAL OFFICE, 1931), during the longer period of 1928-1942 (15 yr) at Rochdale (GL.-\~SPOOLE, 1969) and during the much longer period of eight decades from 1871 to 1950 at Kew Observatory (HARTLEY-RUSSELL, 1964). Kew Observatory is situated in fairly extensive urban parkland to the west-south-west of London’s centre. Following the publication (WALSHAW,1963; NICHOLSOX,1965, 1969) of rainfall data for Teddington, Middlesex, for the period 1954-1968 (15 yr), in which it appeared that Thursday was the wettest day of the week, SCORER(1964, 1969) and CRADDOCR (1965, 1968) concluded that the results were not due merely to random sampling and that there was a definite physical explanation. In 1965, NICHOLSONresubmitted the original Teddington data but presenting the summer (April-September) and winter (October-March) half-years separately, for the period October 1953-September 1964, and showed that Thursday was wettest in summer but that no clear weekly cycle occurred in winter. This result was confirmed by GOLD (1968) for a station in north-west London* for both 5-yr periods 1954-1958 and 1959-1963 (defining summer as May-October and winter as November-April) but for the subsequent 4-yr period of 1964-1967, Friday was the wettest day. DETTWILLER(1970) found a similar pattern for May-October (1960-1967) in Paris, Lvhere mean values of rainfall generally showed a steady increase from Saturday to Friday. These seasonal analyses suggest that the weekly rainfall pattern may be associated with changes in thermal convection. 2. DATA The dates of the periods of data used and the location and heights of relevant instruments for the measurement of rainfall amount, air maximum and minimum temperature, sunshine duration and wind speed are as follows: Teddington,

51” 26’N, 00” 2l’W October 1953-September 42 ft (13 m) above MSL

Middlesex:

Rainfall : Hampton Metropolitan Board, Middlesex :

1968

Water

Sunshine: Wind:

Kew Observatory, Surrey: Wind :

* 8 Hurst Close, London NW 11.

51” 25/N, 00” 22’W April 1954-September 1968 110 ft (34 m) above MSL, 7 1 ft (22 m) above ground 139 ft (42 m) above MSL, 101 ft (31 m) above ground, 30 ft (9 m) above building, 100 ft (30 m) effective height 51’ 28’N, 00” 19’W 1949-1970 92 ft (28 m) above MSL, 75 ft (23 m) above ground, 32 ft (10 m) above building, 50 ft (15 m) effective height

Urban Climate and Day of the Week

Kensington Palace, London:

Temperature :

Temperature: Edgbaston Observatory, Birmingham :

St. Mary’s, Sciily: Temperature:

51” 31’N, 00” 1O’W April 1949-March 1969 and May, June, July, 1969, 1970 85 ft (26 m) above MSL, 4 ft (1 m) above ground 51” 07/N, 01” 42’W April 1949-March 1969 and May, June, July, 1969, 1970 367 ft (112 m) above MSL, 4 ft (I m) above ground

Porton, Wiltshire :

Temperature: Wind:

937

52” 28’N, 01” 56’W May, June, July, 1957-1970 535 ft (163 m) above MSL, 4 ft (i m) above ground 643 ft (196 m) above MSL, 118 ft (36 m) above ground, 18 ft (5 m) above building, 73 ft (22 m) effective height 49’ 56’N, 06” 18’W May, June, July, 1957-1970 158 ft (48 m) above MSL, 4 ft (1 m) above ground

Porton, Wiltshire is an inland, rural station which is some 70 miles approximately south-west of London’s centre and about 55 miles from London’s outer suburbs. The Porton instruments enclosure has a generilly good open exposure on Salisbury Plain, with comparatively little air pollution. Edgbaston Observatory, in central Birmingham, is an urban station while the coast guard station site on the island of St Mary’s, in the Scillies, is non-urban. Daily temperature extremes for Kensington Palace and Porton are read at 09 hrs for the preceding 24-h period: the minimum temperature is assigned to the day of observation while the maximum temperature reading is “thrown back”, that is, allocated to the previous day. Daily maximum temperatures for Edgbaston and Stilly refer to the period 09-21 hours. Daily rainfall amounts are for the 24-h period from 09 hours on the day named until 09 hours on the following day. 3. FURTHER

RAINFALL

ANALYSIS

If the weekly rainfall pattern is real, it must be related to artificial or man-made factors associated with urbanization, and such rainfall variation should therefore be related to wind speed and perhaps also to cloudiness. For example, an increase in wind speed should reduce the “relative amplitude” of any weekly cycle. This hypothesis was tested using the 15 summer half-years from 1954 to 1968 of rainfall data for Teddington (NICHOLSON,1969). The mean weekly amplitude for each summer half-year was defined as the rainfall on Thursdays minus the rainfall on Saturdays (Saturday being the drier week-end day on average at Teddington over the 15 summer half-years), all divided by the rainfall over that summer half-year. The correlation coefficients between these values of the rainfall “amplitude” and the corresponding seasonal values of (i) sunshine duration and (ii) sunshine duration

E. N. LAWRENCE

938

divided by the mean wind speed, for Hampton, Middlesex, which is two miles southsouth-west of Teddington, are given in TABLE 1. Some significance is indicated, TABLE 1. CORRELATIONCOEFFICIENTSBETWEENTHEMEAN URBAN WEEKLY RAINFALL AWLITUDE,THE MFAX URBAN EXCESSDAILY WXIMUJMTEMPERATURE AhD VARIOUS OTHER METEOROLOGICAL PARA%IETERS FOR THE 15 SUMMER HALF-YEARS (APRILSEPTEMBER)OF THE PERIOD1954-1968, IN THE LONDONAREA

Urban weekly rain amplitudet

Urban excess daily max temp.?

Sunshine duration

Urban excess daily

Sunshine duration*

wind speed*

max temp.t

Correlation coefficient

0.46

050

0.55

Significance level

0.07

0.05

0.03

Correlation coefficient

0.52

0.53

-

Significance level

0.04

0*03-0~04

-

l Hampton, Middlesex (London area). t Kensington Palace, London minus Porton, Wilts.

2 Mean Thursday rainfall minus mean Saturday rainfall, all divided by the mean weekly rainfall over the summer half-year (April-September) at Teddington, Middlesex (London area).

though the relations with wind speed alone are not significant at the 10 per cent level. The hypothesis is further supported by the relationship between the rainfallamplitude and the urban heat-island effect as measured by the average excess of daily maximum temperature at Kensington Palace, London over that at the rural station at Porton. The correlation coefficients between this urban heat-island effect for each summer half-year of the period 1954-1968 inclusive and the corresponding values of (i) mean rainfall-amplitude and (ii) the previously mentioned sunshine and wind parameters, are given in TABLE 1. The coefficients for the urban heat-island effect with these sun and wind parameters are very similar to the corresponding coefficients for the rainfall-amplitude. The multiple correlation coefficient between the rainfallamplitude and (i) sunshine duration divided by the mean wind speed and (ii) the mean urban heat-island effect is O-60 (significance level O-07). The significance of the relationship between the rainfall-amplitude and the urban heat-island (TABLE 1) does not imply that there is a direct causal relationship, but, as suggested by the other correlations of TABLE 1, it could mean that both phenomena tend to be marked during spells of light wind and little cloud (or plenty of sunshine), without necessarily tending to occur together. Further analysis of the Teddington rainfall data for the period 1954-1968 shows that the weekly pattern is due chiefly to light winds of O-5 knots (at Kew Observatory, 14-15 GMT) during the quarter, May, June and July: the mean values for the rainfall on Thursday minus that on Sunday (the wettest day minus the driest day on average for the quarter) are 5.8, 4.6 and 2.3 mm for the three pentades in chronological

Urban Climateand

939

Day of the Week

order, and 3.5 mm over the 15-yr period as compared with less than O-2 mm for winds of 11 knots or more. The seven mean daily values of rainfall for light winds approximately fit the sine curve y = 2.11 + O-92 sin

(360 t - 529) 7

where t = 0 gives the Sunday value in mm, t = 1 the Monday value, etc., with a standard error of 0.9 mm. The evidence of weekly patterns of air pollution, rainfall and sunshine, and current research showing positive effects of urbanization on rainfall, sunshine, wind and temperature, taken altogether, suggest that there could well be weekly patterns of air temperature, at least for summer maximum temperatures. The present analysis is henceforth concerned primarily with variation of daily maximum and minimum temperature during the week in central London (at Kensington Palace) relative to the inland rural station at Porton, Wiltshire, and also with the weekly temperature patterns at Edgbaston Observatory, Birmingham and in the Scillies. 4. ANALYSIS

OF

DAILY

AIR

TEMPERATURE

DATA

4.1 Half-yearly data For each day of the week, the mean daily maximum and minimum temperatures for each summer (April-September) and winter (October-March) half-year, from April 1949 to March 1969, were calculated for Kensington Palace, London and Porton, Wiltshire (TABLE 2). Graphs (not shown) were prepared of the mean differences (Kensington Palace minus Porton) of daily maximum and minimum temperatures for each of the two seasons separately, for each of the two decades April 1949March 1959 and April 1959-March 1969 and for the whole 20-yr period. The winter graphs and summer minimum temperature graphs showed no clear and persistent weekly patterns but the graph for the 20 yr of summer maximum-temperature differences was remarkably smooth and each of the decadal graphs showed similar characteristics-namely, a maximum on Sunday and a minimum on Wednesday, Thursday or Friday. The mean weekly amplitude was approximately 0.2 & 0.03°C. The corresponding graphs (not shown) for the actual maximum temperatures at Kensington Palace show somewhat similar though less smooth patterns but such patterns were not evident in the Porton data (TABLE 2). The multiple correlation coefficient of the half-yearly mean weekly daily-maximumtemperature amplitude (Sunday lninus Thursday, Kensington Palace minus Pot-ton) with (i) the magnitude of the urban heat-island (Kensington Palace minus Porton) and (ii) the mean wind speed at Kew Observatory is 0.53 (significance level 5 O-06). A variance analysis (similar to the analysis discussed later and shown in TABLE 4) shows a significance level of 0.07 for the day-of-week variation and this level is “increased” to 0.01 when allowance is made for the orderly or systematic variation through the week by fitting a sine wave to the variation, with two degrees of freedom. The sine curve is (360 t - 692) y = l-238 - O*lOOsin 7 where t = 0 gives the Sunday value in degrees C, t = 1 the Monday value, etc.

19.18 09.63

17.87 09.02

a) Kensington krlace April-Scptenibct October-March

(b) Porton April-September Oclobcr-March

Max

08.00 02.02

1040 03.67

Min

Monday

17.89 08.88

19.13 09.48

Max

08.16 01.94

IO.54 03.76

Min

Tuesday

17.93 OX.95

19.12 09.54

Mnx

08.02 01.93

1044 03.69

Min

Wednesday

17.83 08.97

19m 09-68

Max

08.13 01.98

1044 03.72

Min

Thursday

17.94 09*08

19.1 I 09.73

Max

Min

08.14 02.16

to.45 03.91

Friday

17.77 09.06

IO.22 09.74

Max

08.06 02.12

to.39 03-W

Min

Saturday

17.91 08.93

19.26 09.61

Max

08-06 02.19

IO.42 03.87

Min

Sunday

F 5 F 9

’ ?:

Urban Climate and Day of the Week

941

4.2 Monthly data

Half-yearly temperature data have been used in order to facititate comparison with earlier published work on half-yearly rainfall, but for detailed study of the annual variation of the weekly temperature cycle, monthly data are necessary. Hence the mean daily maximum and minimum temperatures were caIculated for each of the 12 months separately, over the 20-yr period, for Kensington Palace and Pot-ton. Graphs (not shown) of these values and of the differences (Kensington Palace minus Pot-ton) were prepared. TABLE 3. MEAN DAILY MAXIXWTEMPERATURE DIFFERENCES,@-@ hours, IN DEGREES C, tiNSMGTON PALACE, LONDONMINUSPORTON,WILTS,FOREACHDAYOFTHEWEEKDURINGTHEMIDSU~~ER;MO~, MAY, Jm AND JULY, OF xxp PERIOD1949-1968

Year

Mon.

1949 1950 1951 1952 1953 1954 1955 1956 1957 1958

62 5.3 3.6 6.1 5.4 3.0 3.9 4.1 3.6 5.9

1959 1960 1961 1962 1963 1964 1965 1966 1967 1968

5.2 3.8 4.6 1.5 3.8 5.3 2.2 4.4 4.2 0.7

Total values for May -I- June 1. July (‘C) Tues. Wed. Thur. Fri. Sat. 3.0 48 4.4 6.4 2:;

4.2

49 5.6

2.6 0.6 2.9 6.4 1.4 4.0 3.5 3.8 5.8 4-l

4.4 2.3 2.3 25 3.9 48 3% 4-Y 4% 2.7

5.1 2-o 2.3 1.5 3.9 4.3 3.1 3.7 2.9 3.2

2% 4.2 3.6 3.4 4.4 5.5 3.2 1.7 I.0 1.3

4.2 ::;

2.1 4.2 2:; 3.5 5.3 ::;

Sun.

2.5 3.2 3.9 5.5 2.2 5.3 4.7 0.5 3.7 5.2

2.3 4.4 5.9 64 41 3.2 3.9 3.2 5.0 3.6

5.1 4.5 7.3 7.7 6.2 3.9 6.8 2.1 4.7 6.3

5.9 2.6 3.s 4.1 3.3 2.8 3.1 3.7 2.7 2.1

5.5 4-4 5.3 3.3 4.6 3.9 2.7 3.8 4.2 2.2

7.3 3% 5.3 2.4 2.7 3.6 4-3 4.5 3.2 4-s Standard error

Mean monthly values during May + June + July (“C) *

1959-1968 1949-1958

1.19 157

1.21 144

1.07 1.47

1.03 I.17

1.13 1.23

1.33 140

1.39 1.82

1949-1968t

I.38

1.32

1.27

I.10

1.18

1.37

1.61

1949-1968 First harmonic

1.48

l-34

1.18

l-13

1.22

I.38

149

>

* 0.13 i 0.09

* Mean values for the separate months are: May i-41, June l-33, July 1.22, Standard error & O-06. t 1949-1968 Mean daily maximum temperatures, May -t- June f July are as follows: Kensington Porton

20.11 18.73

19.93 18.60

19.91 1864

19.85 18.74

19.97 18.81

20.23 18.88

20-32 l&71

E. N.

942

LAWRENCE

As might be expected from the half-yearly data analysis, the weekly maximumtemperature pattern was mainly confined to the midsummer months of May, June and July and occurred at Kensington Palace but not at Pot-ton. The combined values for these three months are shown in TULE 3 which shows that the pattern occurred in both decades-with maxima on Sunday and minima on Thursday, and a mean amplitude (for Kensington Palace minus Porton) of approximately O-5 * 0*09”C. TABLE3 shows further that the daily variation through the week of the maximumtemperature differences closely follows a sine wave with one harmonic. These systematic changes through the week greatly enhance the significance of the variation. TABLE 4. VARLANCE ANALYSIS OF DAILY (09-09 hours) ,WUM

TEMPERATURE DIFFERENCE INDEGREES C: KENSINGTON PALACE, LONDON MINUS PORTON, WILTS, FOR THE SEVENDAYS OF THE WEEK, THE THREE MONTHS MAY,JUNE AND JULY SEPAIUTELYASDTHE 20 INDMDUALYEARS OFTHEPERIOD 1949-1968 MCLUSIvE

Total

Years(Y) Decades RelIUilliIlg Months (M) Days (D) 1st harmonic ReXll&liig

Sum of squares

Degrees of freedom

251.145 38W3 6.522 32.336 2.583

419 19 1 18 2

2.045 6.522 1.796 1.291

4.31 13.73 3.78 2.72

< O*OOl < O*OOl <@OOl 0.05 to 0.10

9.377 7.503’ 1.874

6 2 4

1.563 3.751 0.469

3.29 7.90 0.99

< 0.01 <0031 >O*lO > 0.10 > 0.10 ti.00 1

Mean square

Variance ratio

DxM DxY Mx Y

4,960 SO.824 36.216

12 114 38

0.413 0446 0.953

0.87 0.94 2.01

Days x decades Remaining

2.136 48.688

6 108

0.356 0.451

108.324

228

0,475

0.75 0.95 Standard error 0.69t

DxMxY

l602(_Yt

1.317)’ or 420 (0*189)‘/2, using the sine curvey = 1.317 - 0.189 sin

The calculated mean daily values, using this sine curve, are given in The generallevel of variation not accounted for.

Significance level

> 0.10 > 0.10

360 f - 769 , 7

TABLE 3.

TABLE4 gives the full results of the analysis of variance of the maximum-temperature differences for the three factors: (1) the seven days of the week, (2) the 20 individual years from 1949 to 1968 and (3) the three separate months of May, June and July. The three factors are referred to as D (days), M (months) and Y (years). The most important parts of this analysis are those for the D factor and the D x M interaction. The day of the week variation is significant at the one per cent level and when allowance is made for the systematic changes through the week, the day-of-week variation is significant at the 0.1 per cent level. The D x M interaction confirms that the “Kensington Palace minus Porton” variation through the week of daily maximum temperatures is essentially the same in each of the 3 months.

Urban

Climateand Day of the Week

943

Further interesting results of the variance analysis of TABLE 4 are as follows: (I) The Y-factor analysis shows that there are obviously differences in different years: the difference between the two decades is greater than the remaining differences but not greatly so; (2) The M-factor analysis shows that the pattern of differences between the mean daily maximum temperature at Kensington Palace and Porton is not quite the same in each of the 3 months, May, June and July but the ltri x Y interaction shows that there is no consistent pattern of difference from year to year. 4.3 Temperature-wind data Using the available machine-processed data for May, June and July for the period 1957-1970, mean daily maximum temperatures for each day of the week were calculated for Kensington Palace minus Porton, in “C, as follows: Monday 1.25

Tuesday 1.17

Wednesday 1.16

Thursday 1.13

Friday 1.18

Saturday 1.30

Sunday 1.55

Taking as a measure of the mean weekly temperature amplitude, the mean Sunday temperature minus the mean temperature for Tuesday, Wednesday and Thursday, the ampiitude is 040 + 0.07”C. The corresponding values of this amplitude for ranges of mean wind speed from I4 to 15 hours at Kew Observatory, of O-5, 6-10 and >, 11 knots are 0.62, 0.41 and 0.28”C, thus confirming that the weekly pattern becomes more marked during light winds and vice versa. 4.4 Daily air temperature data for other stations Temperatures and wind data are available for Edgbaston Observatory, situated in central Birmingham but elevated above the general level of housing in the vicinity and the terrain to the north and east of Birmingham. Analysis of temperature data for the months, May, June and July, 1957-1970, shows that the lowest average daily maximum temperature occurred during midweek with the weekly minimum on Thursday while the warmest days were Friday, Saturday and Sunday. The pattern is thus similar to that obtained for central London but contrasts with that of the “rural” station of Porton. Appropriate “control” data for Edgbaston are not available but the mean weekly amplitude of daily maximum temperature calculated from Edgbaston data, as in the previous section, is 0.33 It O*lO”Ccompared with a value of practically zero (- 0.02) at the non-urban station in Stilly. Edgbaston daily maximum temperature data, when analysed according to local 15 hour wind speeds, in the ranges O-5, 6-10 and >, 11 knots, gives temperature amplitudes of 0.51, 0.38 and 0.19”C respectively; that is, the amplitude decreases with increasing wind speed (cf. “relative” maximum temperatures at the central London site). 4.5 Reality of the weekly temperature cycle

Current results based on data for the period April 1949-March 1969 (20 yr) show a mean weekly pattern of variation in the mean daily maximum temperature for midsummer (May, June and July), for Kensington Palace (central London) minu

944

E. N. LAWRENCE

Porton, Wiltshire (an iniand rural station): on Sunday this “temperature” exceeded the value on Thursday by approximately 03’C. The evidence for the reality of this weekly temperature cycle is as follows: (i) The systematic day-of-week variation of the “20-jr” daily-maximum-temperature differences (TABLES3 and 4 etc.), (ii) The persistence of the pattern in each of the decades (TABLE3), (iii) The occurrence of the pattern, to a large degree, in the actual daily maximum temperatures at the urban stations of Kensington Palace and Edgbaston but not at the rural stations of Porton and Stilly (TABLES2 and 3>, (iv) The dependence of the weekly temperature and rainfall patterns on wind speed, (v) The evidence of the occurrence of similar summer weekly patterns in air pollution, sunshine and rainfall and no corresponding marked patterns of temperature and rainfall in winter, and no similar clear patterns in minimum temperatures (TABLE2), (vi) The relation of the weekly-cycle “amplitude” of rainfall with temperature and other meteorological parameters (TABLEI etc.), (vii) Current research on the effects of urbanization on temperature, sunshine, rainfall, wind, visibility etc. (see also section 5), (viii) The existence of plausible physical explanations (see also section 5). 5. PHYSICAL 5.1 Thermal convection and air pollution

>IODELS

The evidence shows that in the London area during summer, Thursdays are both wetter and cooler, and it is reasonable to enquire whether these two aspects of weather are connected. Whereas any precise mechanism is not clear, the negative correlation between the cycles of rainfall and daily maximum temperature suggests that reduction of daily maximum temperature might be caused by the onset of convective cloud and rainfall, early in the day, initiated well before the time of daily maximum temperature and at a time of the day near to that of maximum difference of temperature between the town and its surroundings; this temperature-difference may be greater within the week due to greater industrial heating and greater heat retention resulting from nocturnal air polIution and thermal and dynamical effects of the urban surface. The hypothesis that thermal convection is an important cause of differences between urban and rural rainfall regimes was suggested as long ago as 1921 by HORTONand more recently, for example, by ATKISSON(1968, 1969) with reference to the London area, CHANGNON(1968, 1969) for La Porte, Chicago and St. Louis, and PARRY (1956) for the Reading area. Many such investigations (for exampIe BXRRETT,1964) are often chiefly concerned with establishing the fact that rainfall can be increased by urbanization but ATKINSON(1968, 1969) and CHANGXOX (1968, 1969) studied urban effects on instability or thundery rainfall. ATQSSON (1970) concluded, from a detailed case-study, that urban storms were triggered by high urban temperatures and that turbulence and potential condensation and ice nuclei in the urban area played a negligibfe role in their initiation. It is interesting to note that CHANGXON (1969) found that urban-produced thunderstorms occurred largely in the morning hours, in the warm season, and with either stationary fronts or unstable air masses. Results of the present investigation, particuIarly that the weekly temperature cycle is mainly a midsummer phenomenon of daily maximum temperatures, and NICHOL-

Urban Climate and Day of the Week

945

SON’Sfindings that the weekly rainfall cycle is a summer phenomenon may be regarded as consistent with the conclusion of ATKINSON(1970), and also CHANGNON(1969) who emphasized the greater importance of vertical motion due to increased surface heating as compared with urban nuclei additions. It cannot be argued that the accumulative effect of steadily increasing air pollution through the week points to the importance of atmospheric nuclei as a cause of steadily increasing rainfall from say Saturday or Sunday to Thursday because (1) atmospheric instability, a characteristic of urban heat-islands (DAVIS and PEARSON,1970; CLARKE,1969; SAKURABA,1969), also might have a tendency to build-up similarly and (2) the accumulative air pollution effect operates also in winter when the weekly rainfall cycle, and the weekly maximumtemperature patterns, are much less evident. Nevertheless, one cannot exclude the possibility (further discussed later) that the reduced week-end air pollution, together with increased rain and cloud mid-week, is a contributory cause of the weekly cycle of daily maximum temperatures. In certain meteorological conditions, an increased number of very efficient cloud condensation nuclei on a weekday, especially if aided by an increased urban heat-island effect early in the day, might well lead to increased precipitation (HOBBS,RADKEand SHCSIWAY, 1970) and to daily maximum temperatures as observed. By contrast, GOLD (1968b) cites an example where rainfall decreased by 40 per cent at places in the path of drifting smoke from burning sugar cane, in Queensland, as the amount of cane increased fourfold. Reduced insolation might have been a contributory cause of the decrease of rainfall but WARNERand TWOMEY(1967) found that the smoke from sugar cane fires was a prolific source of cloud nuclei and increased very greatly the number concentration of droplets in clouds formed well downwind from the fires and they concluded that it might be anticipated that the production of rain by coalescent processes would be much retarded and that less rain would result. The matter is further discussed by WOODCOCKand JONES(1970). 5.2 Wind speed In contrast to the diurnal pattern of urban convective rainfall, the heat-island effect is largest at night (CHANDLER,1962). This latter fact is responsible for the main failure of the model described by MYRUP (1969), who ascribes the night-peak to radiation blanket effects of air pollution or to Richardson number effects on turbulence diffusivities at night: other possible causes are the greater urban-heat retention and the relatively greater heat sources of the urban area at night. The air pollution aspects of the urban heat-island together with the day-of-week variation of air pollution (CHANDLER,1965; LAWRENCE,1969) and sunshine (CHANDLER,1965) suggest that the weekly cycle of daily maximum temperature may result from the within-week variation of air pollution following from the weekly cycle of domestic and industrial habits. If indeed this is so, then the positive correlation between seasonal mean values of the amplitude of the weekly rainfall cycle and the urban heat-island can be explained by the fact that each of these phenomena is more likely to develop during seasons of light winds (SUNDBORG,1950; CHANDLER,1965; JAUREGUI,1969); the association of light winds with the rainfall cycle and excess urban temperature is suggested by the correlations of TABLE 1. That light winds are necessary for the weekly maximum temperature and rain patterns is further suggested by the results of

946

E. N. LAWREXCE

the present temperature-wind speed and rain-wind speed analyses. Any variation during the week in a locally induced circulation (FINDLAYand HIRT, 1969 ; CHAXDLER, aid both the 1961)-which normally originates in “light-wind” conditions-might within-week decrease of daily maximum temperature and theincrease of convective rain. In the heat-balance approach to urban heat-island models (MYRUP, 1969; OKE, 1969; BACH, 1970), greater attention might be given usefully to the effects of wind speed (including that caused by locally-induced circulations) and advection. All suburbs are not transition zones to the same degree and the heat-island, for example in Toronto (MUNN, HIRT and FINDLAY, 1969), London (CHANDLER,1965), Durban (PRESTON-WHYTE,1970) and Uppsala (SUNDBORG,1950), is liable to be displaced downwind; and likewise, the occurrence of a precipitation maximum on the lee side of a city seems to be a natural circumstance, at least in certain weather situations (SUNDBORG,1951; HOBBS,RADKEand SHUMWAY,1970; etc.). Thus, the urban heatisland problem might be considered as a thermal diffusion problem for an area1 source, as discussed by NEIBURGER(1970), and with emphasis on the “urban heat plume” (CLARKE, 1970) and on vertical mixing depths (and inversions) and lateral (horizontal) spread, in order to assess the dimensions of the heat-island, for example, along the lines of the PASQUILL(1961) model for atmospheric diffusion but with allowance for the effects of reactions on: (1) the temperature field by precipitation (SAWYER,1949), (2) the radiation regime and albedo by cloud cover (ROBINSON, 1970), (3) air pollution distribution by “washing-out”, deposition and absorption, and (4) the humidity field by urban surface water drainage and industrial water input. 5.3 Topography As the urban heat-island and the urban rain-island phenomena are the results of complex and interacting physical processes, the pattern of events for one particular topography and meteorological regime may not be applicable to other conditions and the ideal model may vary likewise. For example, SCHXKJCK(1967) found that for Wroclaw (Breslau), the urban rainfall maximum occurred on the windward side (and not downwind) presumably because, on a plain, windward urban obstructions together with general urban-surface roughness and urban-heating are more effective in promoting atmospheric uplift. It would seem that the weekly rain pattern depends considerably on such topographical factors as the relative dominance of the effects of air pollution in reducing insolation, on the one hand, and urban-heating and heat-retention, on the other, leading possibly in some circumstances to both week-end and midweek peaks of convective rain. These factors will, in turn, depend on the size and distribution of heat and pollution sources; furthermore, the type of air pollution may determine the efficiency of rain-producing nuclei. Again, the weekly temperature pattern must depend on orography because increased shelter from wind in urban valley sites will enhance the effects of air pollution and urban heating. Finally, it must be noted that not only does topography undergo long-term change but the effects of topography are liable to vary with long-term macroclimatic change and thus the weekly meteorological pattern of a particular location cannot be regarded as necessarily constant. Indeed, weekly meteorological patterns might be used to assess the character of an urban environment.

Urban Climateand Day of the Week

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5.4 Concluding remarks The weekly cycle of daily maximum temperature,

with a minimum on Thursday and a maximum on Sunday might be explained in terms of two processes, as follows: (i) The weekly cycle of domestic and industrial habits causes weekly patterns of air pollution (CHANDLER,1965; LAWRENCE,1969) and sunshine (CHANDLER,1965); and the variation in the blanketing effect over the city may be supposed to contribute to the observed cycle of daily maximum temperature. (ii) The same weekly pattern of urban behaviour results in a weekly cycle in the urban heat-island effect which, in light winds, might lead to a weekly pattern of influence on preferred areas of rainfall: this variation of convective rainfall might then lead to reduced daily maximum temperatures as observed. As it is reasonable to suppose that the process (ii) in particular is possible, the ideal model would allow for the coexistence of heat-island and rain-island phenomena. Acknowledgements-Thanksare due to Mr. A. F. JENKINSONfor advice on statistical techniques. This paper is publishedwith the permissionof the Director-Generalof the MeteorologicalOffice. REFERENCES ASHWORTH J. R. (1929)The influenceof smoke and hot gases from factory chimneys on rainfall. Q. J. R. met. Sot. 55.341-350. A~HWORTHJ. R. (1944) Nuture 12,213-214. ATKINSONB. W. (1968) A preliminary examination of the possible effect of London’s urban area on the distribution of thunder rainfall, 1951-60. Trans. Inst. Br. Geog. 44,97-l 18. ATKINSONB. W. (1969) A further examination of the urban maximum of thunder rainfall in London, 1951-60. Trans. Inst. Br. Geog. 48,97-119. ATKINSON B. W. (1970) The reality of the urban effect on precipitation: a case-study approach. Urban Climates, Tech. Note No. 108, WMO, pp. 342-360. BACH W. (1970) An urban circulation model. Archs Met. Geoph. Eiokf., Ser. B 18, 155-168. B~~aarr E. C. (1964) Local variations in rainfall trends in the Manchester region. Trans. Inst. Br. Geog. 35,55-72. CAR-R H. E. (1931) Week-end weather in 1931. Meteor. Mug. 66,163-164. CHANDLERT. J. (1961) Surface breeze effects of Leicester’s heat-islands. E. Midlunds Geog. 15,32-38. C~UNDLERT. J. (1962) Diurnal, seasonal and annual changes in the intensity of London’s heat-island. Meteor. Mug. 91, 146-153. CHANDLERT. J. (1965) The Climate oflondon, pp. 87,125,162-164, 167-168. Hutchinson, London. CHANGNONS. A. (1968) The La Porte weather anomaly-fact or fiction? Bull. Am. Meteor. Sot. 49,4-11. CHANGNONS. A. (1969) Urban-produced thunderstorms at St Louis and Chicago. Sixth Conf Severe Local Storms, Chicago, American Met. Sot., pp. 95-99 (Unpublished manuscript). CLARKE J. F. (1969) Nocturnal urban boundary layer over Cincinnati, Ohio. Mon. Wea. Rev. 8,

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WARNERJ. and TWOMEYS. (1967) The production of cloud nuclei by cane fires and the effect on cloud droplet concentration. J. alrnos. Sci. 24, 70-4-706. WOODCOCKA. H. and JONESR. H (1970) Rainfall trends in Hawaii. J. uppf. Meteor. 9, 690-696.