The COVID-19 disease is a global pandemic since the onset of which the world has altered. This pandemic has negatively affected economic activities and human health (Lin et al. 2020). In the present study, the correlation of five main weather variables, including daily average temperature, wind speed, humidity, maximum temperature, and minimum temperature, with the number of daily active, recovered, and death cases of COVID-19 is investigated. Among weather elements, the effect of temperature has been the most studied (Liu et al. 2020; Tosepu et al. 2020). A significant negative correlation between COVID-19 cases and temperature is found in this study. This result seems to be consistent with other researches. Adekunle et al. indicated temperature average to be related inversely to COVID-19 growth in Africa (Adekunle et al. 2020). A moderate negative correlation between average temperature, maximum temperature, and the number of COVID-19 infections in the State of Rio de Janeiro, Brazil, was reported by Rosario et al. (Rosario et al. 2020). According to the study by Alkhowailed et al., as the average and maximum temperature increased, the number of COVID-19 positive cases decreased in Saudi Arabia (Alkhowailed et al. 2020). In a study from eight South American locations, Zhu et al. observed a negative association between average temperature and COVID-19 confirmed cases (Zhu et al. 2020). In a comprehensive study in 166 countries, the authors reported the number of daily positive cases and new deaths of COVID-19 to be reduced for every1°C rise in temperature (Wu et al. 2020). In another study in Brazil, Prata et al. reported that when the temperature was below 25.8°C, every 1°C increase in temperature led to a 4.9% decrease in COVID-19 cases (Prata et al. 2020). According to Wu et al., with increasing 1°C temperature, the daily new cases and dead were decreased by 3.08% and 1.19% (Wu et al. 2020). Other studies have reported conflicting results. Bashir et al. pointed out a positive correlation between minimum and average temperature with COVID-19 cases in New York City (Bashir et al. 2020). In another study from Wuhan, Ma et al. suggested diurnal temperature ranges be significantly associated with the number of COVID-19 daily deaths (Ma et al. 2020). Xie and Zhu illustrated that when the average temperature was below 3°C, the number of daily confirmed cases of COVID-19 was increased for every 1°C increase in temperature in 122 cities in China (Xie and Zhu 2020). A significant positive correlation for both minimum temperature and mean temperature with daily new COVID-19 cases was reported in Singapore by Pani et al. (Pani et al. 2020). The possible explanation for our results might be that an elevated temperature has the potential to damage the virus lipid membrane and consequently diminishes the stability and transmission rate of viruses (de Ángel Solá et al. 2020; Moriyama et al. 2020). Also, in hot weather, viral load in the air and surfaces can be decreased due to exposure of pathogens to ultraviolet (UV-A) and infrared (IR) rays of the sun; as a result, they are damaged, inactived, or degraded, thus decreasing the transmission (Eslami and Jalili 2020). Cold weather in winter can reduce blood supply, thus decreasing immune cells in the nasal mucosa and innate human immunity (Sun et al. 2020). However, differences in minimum and maximum temperatures, characteristics of the participants in different area, and other factors such as overcrowding and non-compliance with hygiene tips, recommended by WHO, can also cause differences in study results (Tosepu et al. 2020).
The wind is considered a crucial climatic factor in transmitting infectious respiratory ailments since it may modulate the dynamics of various vectors and pathogens (Ellwanger and Chies 2018). The data of the present study demonstrated that the number of COVID-19 positive, recovered, and dead cases elevated due to the decrease of wind speed; studies on this element are scarce. Pani et al. founded that wind speed decrease was associated with an elevated incidence of COVID-19 (Pani et al. 2020). Alkhowailed et al. also reported a significant reverse correlation between daily confirmed cases and wind speed (Alkhowailed et al. 2020). Moreover, another study from Iran indicated that in provinces such as Golestan, Mazandaran, Gilan, and Tehran, where the wind speed was low, the outbreak rate of COVID-19 disease was much higher (Ahmadi et al. 2020). Research by Rendana revealed that wind speed was negatively correlated with the number of total cases of COVID-19 (new and dead cases) (Rendana 2020). A significant negative correlation between confirmed cases and wind speed was reported when its speed was below 7m/s in 127 countries (Yuan et al. 2021). These results are in line with those of the present research. However, the current study does not support the earlier researches reporting a significant positive association between wind speed and COVID-19 cases (Adekunle et al. 2020; de Paula Correa et al. 2021). One of the main factors to explain the negative correlation between daily confirmed cases and wind speed is that the wind speed has the potential to spread suspended particles in the air, and higher wind speed results in less particle density and contamination (Afiq et al. 2012; Sun et al. 2019). Due to the small size of SARS-COV-2 (50-200 nm), it is transmitted via droplets and aerosols; Therefore, higher and stronger wind speed reduces the concentration of the SARS-COV-2 virus in the air, thus reducing the transmission potential of the virus (Lai et al. 2020).
In this study, daily average humidity was found to be positively but insignificantly related to daily new confirmed cases and daily recovered cases, while it was positively and significantly associated with daily dead cases. These results are in line with those of other studies, which reported no significant association between humidity and COVID-19 (Adekunle et al. 2020; Bashir et al. 2020; Meyer et al. 2020; Tosepu et al. 2020). A significant positive correlation between the number of COVID-19 cases and relative humidity was reported in the US (Chien and Chen 2020), China (Pirouz et al. 2020), and Saudi Arabia (Alkhowailed et al. 2020). Experimental studies showed that viral inactivation could occur due to viral capsids accumulation at the air-water surface, leading to viral cell structural damage (Alkhowailed et al. 2020). However, the studies from China (Qi et al. 2020; Wu and McGoogan 2020) and Spain (Paez et al. 2020) found a significant negative relationship between humidity and COVID-19 incidence. Currently, the COVID-19 pandemic shows a non-stable trend in different areas of the world, and it is expanding rapidly; thus, the effect of meteorological factors, such as temperature, wind speared, and humidity, on COVID-19 is not enough to control it (Leung et al. 2020).
This study had few limitations: firstly, the number of daily confirmed cases was based on the results of PCR test, and also, patients infected by the SARS-COV-2 virus but did not go to health-care centers or hospitals were not reported. Secondly, the effect of important factors, including medical resources and government interventions, on the COVID-19 mortality were ignored; thus, future studies should focus on their effect. Finally, due to the effect of multiple factors such as medical resources and demographic variations on the spread of the virus, they should be analyzed in future researches. Also, this study data and results related to ambient air and therefore, indoor air needs to future studies for this relation.