Abstract The influence of meteorological factors ondaily Urticaceae pollen counts were studied in Córdoba(southwest Spain) in 1996 and 1997. The daily Urticac-eae pollen concentrations were obtained by using aHirst-type volumetric sampler, and meteorological datawere obtained from the Córdoba airport, located near thesampling site. The highest correlation between pollenconcentration and meteorological parameters was ob-tained during non-rainy seasons. Temperature was foundto be the most important meteorological parameter influ-encing pollen counts in spring, as temperature is themain reason for the increase of pollen concentration inthe atmosphere. In autumn, humidity was another impor-tant parameter influencing pollen counts. Rain, however,did not appear to be significant. The influence of the pol-len concentration of the 2 previous days and the pollenconcentration of the previous day has been studied. Dur-ing periods with low precipitation, the pollen concentra-tion of the previous day was a useful predictor of Urti-caceae pollen concentrations for the following day.

Key words Urticaceae pollen · Meteorological factors ·Southwest Spain

Introduction

In Mediterranean climates, pollen from the Urticaceaefamily can be found in the atmosphere throughout al-most the entire year. The Urticaceae family is comprisedof several species flowering in different seasons. Twospecies of the Urtica genus grow in the Córdoba area:U. membranacea Poiret in Lam. and U. urens L., bothflowering in spring. The Urtica species are ruderal

weeds that are very common in nitrified places, such ason roadsides and between cultivated areas. Two speciesof the Parietaria genus also grow in the area: P. judaicaL., which flowers from spring to autumn, and P. mauri-tanica Durieu in Duchartre, which flowers in spring.The Parietaria species are less common and are foundmostly on the walls of the old city and other nitrifiedplaces.

In aerobiological studies, it is important to take intoaccount the release of particles from their source andtheir transportation and dispersal in air. In respect of pol-len release, most plants have a passive mechanism ofpollen liberation and the pollen grains are liberated bythe energy of external agents, usually wind or fallingraindrops. However, in the case of the Urticaceae family,pollen liberation involves an explosive mechanism. Urti-caceae stamens are normally bent, but when temperatureand humidity conditions are suitable, they rise to the up-right position and release their pollen grains. The pollenrelease, transportation and dispersal in the air, are closelylinked with atmospheric phenomena. There have beenseveral studies of the influence of meteorological param-eters on the pollen concentration of the Urticaceae fami-ly. Most of these studies have indicated that the most in-fluential parameters are temperature, humidity and rain(Gonzalez Minero et al. 1997; Fornaciari et al. 1992;Corden and Millington 1991; Trigo et al. 1996).

The aim of the work reported in this paper was tostudy the influence of temperature, daylight hours, hu-midity and rain on Urticaceae pollen concentrations foreach day of 1996 and 1997. A number of similar Urti-caceae aerobiology studies have been performed, al-though these studies have assessed pollen counts in thewhole family. In our study, the two pollen types of thisfamily were assessed and the spring and autumn datawere separated. Moreover, the present study takes intoaccount the effect of the temperature on the day and theeffect of the temperature 1, 2 and 3 days prior to mea-surement. This study also assessed the capacity for thepollen concentration recorded on the previous day to pre-dict the pollen concentration of the following day.

C. Galán (✉) · P. Alcázar · P. Cariñanos · H. GarciaE. Domínguez-VilchesDepartamento de Biología Vegetal, Facultad de Ciencias,Universidad de Córdoba, San Alberto Magno s/n,E-14.004 Córdoba, SpainFax: +34-957-218598

Int J Biometeorol (2000) 43:191–195 © ISB 2000

S H O RT C O M M U N I C AT I O N

Carmen Galán · Purificación AlcázarPaloma Cariñanos · Herminia GarciaEugenio Domínguez-Vilches

Meteorological factors affecting daily urticaceae pollen countsin southwest Spain

Received: 4 January 1999 / Revised: 26 July 1999 / Accepted: 6 September 1999

Materials and methods

The study was carried out in Córdoba, a city with 300 000 inhabit-ants, situated 120 m above sea level on the fertile plain of theGuadalquivir River. The natural vegetation to the south of the cityhas been almost totally replaced by agriculture. The climate of thecity is broadly Mediterranean, with a slight continental influence.The mean annual rainfall is 670 mm and the mean annual temper-ature is 18°C.

Pollen concentrations were measured during 1996 and 1997 bya Hirst-type volumetric sampler, located about 15 m above groundlevel in the city of Córdoba. The samplers were analysed by themethod recommended by the Spanish Aerobiology Network(Domínguez 1995; Domínguez et al. 1992; Galán 1998). Meteoro-logical data were obtained from the Córdoba airport, located 4 kmfrom the sampling site, on the southwest outskirts of the city.Winds were mostly westerly and southwesterly.

Parietaria and U. urens have a pollen type different from thatof U. membranacea. The former have triporate pollen with a di-ameter of 20 µm and the latter has polipantoporate pollen with asmaller diameter of 9–12 µm. The two pollen types were separatedto identify the behaviour of the monospecific pollen types. More-over, as one Parietaria species (P. judaica) flowers in both springand autumn, we have separated the U. urens Parietaria pollentype into two further groups: those that flower in the spring (U. ur-ens, P. mauritanica and P. judaica), and the Parietaria species thatflowers in autumn, P. judaica.

Statistical analysis was undertaken to identify the influence ofmeteorological factors on the daily Urticaceae pollen counts for theyears studied. First, using the Kolgomorov-Smirnov test, we at-tempted to verify that the frequency distribution of the collecteddata approximated a normal curve. As the data did not approximatea Gaussian curve (except for the U. membranacea data from 1996),the data were logarithmically transformed. After checking that thedata had been adjusted to a normal distribution, parametric statisti-cal analyses were applied to assess whether the daily pollen con-centration and meteorological parameters were positively or nega-tively correlated. The influence of the pollen concentration of theprevious day and the pollen concentration of the preceding 2 and 3days was also studied. The meteorological factors studied were themean, maximum and minimum temperature; daylight hours; hu-midity; the mean temperature of the previous day and the sum ofthe mean temperature of the 2 previous days. Pearson’s test was ap-plied except in the case of rainfall. This parameter never adjusteditself to a normal distribution so a non-parametric test, the Spear-man correlation, was applied. Finally, using regression analysis foreach year and pollen type, a forecast model was proposed.

Results

The flowering period was earlier in 1997 than in 1996(Fig. 1). This probably resulted from the unusually hightemperatures recorded in the months of February andMarch 1997 (Fig. 2), and the absence of precipitation(Fig. 3). The mean temperature in March 1997 was2.5°C higher than that recorded in the previous 15 years.

In spring, the period covered by the U. membranaceapollen curve was shorter than that of the U. urens Pariet-aria pollen curve because the latter comprises severalspecies, which may respond differently to environmentalfactors. As P. judaica flowers in spring and also in au-tumn or early winter, the monospecific pollen counts forthis species were assessed separately during autumn. Theparticular weather conditions of 1997 could have causedthe different behaviour observed for the species in 1996and 1997. The high temperature and high rainfall in au-

tumn 1997 probably delayed the flowering peak of P. ju-daica until December.

Table 1 shows the significant results (P<0.05) for thecorrelation between the Urticaceae pollen counts and themeteorological parameters, the pollen concentration ofthe previous day and the pollen concentration of the 2and 3 previous days. The pollen concentration of the pre-vious day was very highly correlated with Urticaceaepollen counts when it was not very rainy (as in the springof 1997 when 18 mm of rain was recorded during the U.membranacea flowering period and 165 mm during theU. urens Parietaria flowering period) resulting in Pear-son coefficients of 0.76 and 0.79 respectively, P<0.001.However, when the weather was wetter (as recorded inthe spring of 1996, with 34 mm recorded during the U.membranacea flowering period and 193 mm during theU. urens Parietaria flowering period), Pearson’s coeffi-cients were lower than those obtained in the drier peri-ods.

For all the meteorological parameters assessed, tem-perature gave the best correlation with pollen counts inspring, and temperature and humidity gave the best cor-relation in autumn. The effect of humidity on the pollencounts was negative when it resulted from heavy rainfallin short bursts (as in autumn 1996 when only 9 days ofrain were recorded), and positive when it resulted fromlight rain distributed over many days (as in 1997 where39 days of rain were recorded). Rain, however, did notgive significant results in the Spearman correlation anal-ysis because of the opposing effects of rain intensity onpollen counts: decreased pollen concentrations were re-corded on days with torrential rainfalls and increasedconcentrations were recorded on days with light rainfall.

With regard to the regression analysis for the U. mem-branacea flowering period of 1996 (Tables 2, 3), only themean temperature entered into the regression equation,with a determinant coefficient of 46%. In the drier springof 1997 (when only 18 mm of rain was recorded for theU. membranacea flowering period), the mean tempera-ture and the pollen concentration of the previous day en-tered into the equation, with a determinant coefficient of60%. The U. urens Parietaria flowering period duringthe spring of 1996 was very rainy (193 mm), and the vari-ables that entered into the equation in this case were alsothe pollen concentration of the previous day and meantemperature, although the determinant coefficient wasvery low (9%). During 1997, however, with a less rainyspring (165 mm during the U. urens Parietaria floweringperiod), the determinant coefficient was very high (74%)and mean temperature, the pollen concentration of theprevious day, pollen concentration of the 2 previous daysand the minimum temperature, entered into the equation.

As we have previously reported, P. judaica flowers inspring and also in autumn. During autumn, humidity,sunlight hours and temperature are the best parametersfor determining the daily pollen count. If the autumn isnot very rainy (as in 1996 when 127 mm of rain was re-corded in 9 days), the pollen concentration of the previ-ous day may also enter into the regression equation.

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Thus, in 1996, the pollen concentration of the previousday, humidity and the mean temperature of the 3 previ-ous days, entered into the equation with a determinantcoefficient of 47%. In 1997, with a very rainy autumn(549 mm recorded in 39 days of rain), the humidity andsunlight hours entered into the equation, with a determi-nant coefficient of 25%.

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1996

0

10

20

30

40

50

60

70

80

90

1.1

15.1

29.1

12.2

26.2

12.3

26.3 9.4

23.4 7.5

21.5 4.6

18.6 2.7

16.7

30.7

13.8

27.8

10.9

24.9

8.10

22.1

0

5.11

19.1

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3.12

17.1

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Pol

len

grai

ns/m

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U.memb. U.urens-P. P. judaica

1997

0

20

40

60

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100

120

140

1.1

15.1

29.1

12.2

26.2

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26.3 9.4

23.4 7.5

21.5 4.6

18.6 2.7

16.7

30.7

13.8

27.8

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22.1

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5.11

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3.12

17.1

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len

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U. memb. U.urens-P. Parietaria judaica

Fig. 1 Daily pollen concentra-tion for Urtica membranacea,Urtica urens Parietaria andParietaria judaica during 1996and 1997

Fig. 2 Monthly mean temperature during 1996, 1997 and the av-erage of the previous 15 years

Fig. 3 Monthly rainfall during 1996 and 1997

Discussion

As described in previous studies (Emberlin and Norris-Hill 1991; Corden and Millington 1991; Fornaciari et al.1992; Trigo et al. 1996; Gonzalez Minero et al. 1997),temperature is the most influential meteorological pa-rameter for pollen counts, although strong rainfall some-times makes it difficult to find a predictive formula usingthis variable. This problem has also been commented onby Fornaciari et al. (1992), who found that the best cor-relation was obtained by comparing pollen concentra-tions and meteorological parameters on non-rainy days.

In autumn, the effect of the humidity was also shownto be important for pollen concentration. Humidity asso-ciated with light rainfall, as in 1997, is positively corre-lated with pollen counts, but negatively correlated whenassociated with heavy rain, as in 1996. This suggests thata degree of humidity is necessary for Urticaceae stamensto rise and release their pollen grains. In Málaga, Trigoet al. (1996) did not find a significant correlation be-tween pollen counts and humidity because of the gener-ally high values of relative humidity in that coastal city.Regarding rainfall, Gonzalez Minero et al. (1997) ob-served that low rainfall (less than 20 mm per week) fa-

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Table 1 Significant correlation coefficients between the Urticaceae pollen concentrations and the pollen concentration of the previousdays and meteorological variables

Variable Urtica membranacea U. urens Parietaria Parietaria judaica

Spring 1996 Spring 1997 Spring 1996 Spring 1997 Autumn 1996 Autumn 1997

Pollen of the 2 previous days 0.37* 0.23* 0.84*** 0.67***Pollen of the previous day 0.47** 0.76*** 0.26** 0.79*** 0.59*** 0.26*Mean temp. of the 2 previous days 0.59*** 0.20* –0.40*** 0.56*** –0.25*Mean temp. of the 3 previous days 0.55** 0.19* –0.43*** 0.58*** –0.31**Mean temp. of the previous day 0.60*** 0.20* –0.36*** 0.47***Maximum temperature 0.62*** 0.18* 0.45***Mean temperature 0.68*** 0.18* –0.31*** 0.50***Minimum temperature 0.50** –0.38*** –0.53*** 0.40***Sunlight hours 0.25* 0.25*Humidity –0.22* –0.17* –0.43*** 0.25*

* P<0.05; ** P<0.01; ***P<0.001

Table 2 Regression coeffi-cients between the Urticaceaepollen concentration and thepollen concentration of the pre-vious days and meteorologicalvariables

Variable r2 F Significance level

1996U. membranacea: mean temperature 0.46 28.85 0.000U. urens Parietaria: pollen of the previous day, 0.09 5.35 0.006

mean temperatureP. judaica: pollen of the previous day, humidity, 0.47 16.78 0.000

mean temperature 3 previous days

1997U. membranacea: minimum temperature, 0.60 59.58 0.000

pollen of the previous dayU. urens Parietaria: mean temperature, 0.74 103.6 0.000

pollen of the previous day,sum pollen of the 2 previous days,minimum temperature

P. judaica: sunlight hours, humidity 0.25 10.40 0.0001

Table 3 Equations for the fore-cast of Urticaceae pollen usingthe pollen concentration of theprevious days and the meteoro-logical variables during 1996and 1997

1996U. membranacea y=–71.10+7.31 (mean temperature)U. urens Parietaria sp. y=1.46+0.24 (pollen of the previous day)+0.03(mean temperature)P. judaica y=0.71+0.03 (mean temperature of the 3 previous days)–0.01(humidity)

+0.38(pollen of the previous day)

1997U. membranacea y=1.51+0.67(pollen of the previous day)–0.05(minimum temperature)U. urens Parietaria sp. y=0.71+0.28(pollen of the 2 previous days)–0.08(minimum temperature)

+0.22(pollen of the previous day)+0.05(mean temperature)P. judaica y=–5.14+0.07(humidity)+0.19(sunlight hours)

voured high concentrations of Urticaceae pollen becauseit was not enough to clear the atmosphere and increasethe humidity levels. As we have commented before, inCórdoba, the rain does not appear to be a significant pa-rameter affecting pollen counts because of the opposingeffects that rain intensity can have on the Urticaceae pol-len concentration. Fornaciari et al. (1992) concluded thatcorrelating rainfall and the pollen concentration is verydifficult and Urticaceae pollen does not seem to be influ-enced by this parameter. Corden and Millington (1991)also found that rainfall shows no appreciable correlationwith Urticaceae pollen levels.

We have found that the pollen concentration of theprevious day is an important variable in predicting theUrticaceae pollen concentration when rainfall is not veryhigh. This is because heavy rain clears the atmosphere ofpollen grains, preventing the data from the previous dayfrom being predictive. This was apparent in the veryrainy spring of 1996, where the pollen concentration ofthe previous day did not enter into the regression equa-tion for U. membranacea and entered into the equationfor U. urens Parietaria with a very low r2 value. Howev-er, the spring of 1997 was not as rainy, so the pollen con-centration of the previous day entered into the equationwith very high r2 values for U. membranacea and U. urensParietaria.

The meteorological parameters that most influencedUrticaceae pollen in the air have been obtained for eachyear studied and each pollen type, and a forecast modelfor each pollen type and year, using the pollen concen-tration of the previous days and the meteorological pa-rameters as the independent variables, was produced. Asthis is a preliminary study based on data from only 2

years, the resulting equations are only an approximateindication of which variables are useful in predicting Ur-ticaceae pollen counts. Further data collection is requiredto establish the usefulness of this model and to allow ac-curate conclusions to be drawn.

Acknowledgements The authors are grateful to the Spanish Inter-ministerial Commission of Science and Technology, National PlanI+D, for financial support granted through project AMB-97-0457-CO7, and for a grant from the Education and Science Departmentof Junta de Andalucía.

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