Long-Term Trend and Seasonal Variability of Horizontal Visibility in Nigerian Troposphere

Abstract

A study of the long-term variability; trend and characteristics of visibility in four zones of Nigeria was carried out. Visibility and other meteorological data from NOAA-NCDC and aerosol index data over Nigeria during 1984-2013 are analyzed using time series and simple regression model. There are significant decreasing trends for every region and season during the 30-years period; the fluctuations exhibited nearly similar pattern. The 30-year mean visibilities for the four zones (Sahel; North Central; Southern; and Coastal) were 13.8 ± 3.9; 14.3 ± 4.2; 13.6 ± 3.5 and 12.8 ± 3.1 km with decreasing trends at the rates of 0.08; 0.06; 0.02 and 0.02 km/year. In all the zones; visibilities were better in summer while worse in Harmattan (dry season). During summer visibility was best in Sahel and North-central; however; in Harmattan visibility was best in southern and coastal zones. It was best between May and June (17.6; 18.9; 16.6 and 15.1 km) with a second peak in September. The 30-year seasonal averages were 16.2 ± 2.1; 16.8 ± 2.4; 15.4 ± 1.8 and 14.0 ± 2.2 km in summer; and 10.2 ± 2.5; 10.9 ± 2.9; 11.0 ± 3.3 and 11.4 ± 3.0 km in Harmattan for the respective zones. Sahel and North Central had the worse visibility reduction during Harmattan compared with Southern and coastal areas. An analysis based on simple regression equation reveals a strong and negative relationship between visibility on one hand; AI; and AOD on the other hand. The analysis also discusses the variability regarding the frequency of occurrence of a dust storm; dust haze; and good visibility over the period of study.

Analysis of the Long-Term Variability of Poor Visibility Events in the UAE and the Link with Climate Dynamics

“…This is in line with the findings of [8] on improved visibility in Tehran city during spring where the highest long-term average values of wind speed over 50 years was 10.3 m/s, that is associated with high visibility [8], most likely due to improved dilution of air pollutants [13]. In contrast, visibility decreases when RH increases most likely because higher RH promotes the growth of hygroscopic particles into larger sized particles that scatter more light, as suggested by [12,13,38]. Decrease in overall wind speed, increase in RH, and decrease in temperature depression in winter periods could foster the occurrence of such events, as illustrated in Figure 6a-d.…”

“…Visibility is also known in quantitative terminology as the visual range [4]. Good visibility, the ability to see and recognize objects situated on the ground at more than 5.0 km distance, is essential for the safety of all transport sectors including ground, air, and maritime [5][6][7][8][9][10][11][12]. Moreover, visibility can reflect changes in meteorological and air quality conditions and their impact on human health [13][14][15].…”

Monthly Temporal-Spatial Variability and Estimation of Absorbing Aerosol Index Using Ground-Based Meteorological Data in Nigeria

“…These leave gaps in our long-term TOMS AI data record and can create challenges in aerosol studies associated with dust. These gaps are visible in the previous study of Balarabe et al [19]. To overcome this problem, this paper has a dual focus.…”

“…Kehinde et al [16] also found significant correlations between the TOMS AI and visibility at eight selected meteorological stations in Nigeria. The results of a long-term study of AI values from TOMS and OMI are found to correlate well with the visibility in different zones of Nigeria [19]. The relationship between the OMI AI and latitude over Greece was studied by Kaskoutis et al [15].…”

“…Some of these studies indicate that AI is inversely proportional to the visibility, assuming a uniform distribution of aerosol in an atmospheric column [3] [9] and [19]. However, it has been noted by Anuforom et al [3] and Habib et al [9] that the sensitivity of TOMS increases with the altitude, so the AI is not always highly correlated with ground observations.…”