Monthly means, seasonal variances, and trends of a global climatology boundary layer height (BLH) over West Africa are presented based on 36 years (1979 - 2014) of six-hourly ERA-Interim reanalysis. In this region, we found that there is a link between the West Africa Monsoon (WAM) and the monthly means of BLH where largest values of BLH variances are developed in the tropics close to the Inter Tropical Convergence Zone (ITCZ). High temperatures and sufficient moisture are also available close this area. Seasonal trend magnitudes vary from -20 to 20 m per decade during the period 1979 - 2014 and characterize by negative trend over east of the Sahara region. Trends are only included in the analysis, if their probability exceeds the 95% significance level. Case study of diurnal variation was done during the African monsoon multidisciplinary analyses Special Observing Period 3 (AMMA SOP3) experiment (August 2006). We found that the lower boundary-layer appears around 875 hPa in the monsoon layer where the wind decreases at midday in interaction through exchange processes with air originating from above the boundary layer. In the same time, the dust in Saharan Air Layer (SAL) seems to modify the atmospheric boundary layer (ABL) thermodynamic attributes by altering the shortwave and longwave radiative budget.