In order to estimate the rainfall fields over an entire basin raingauge, pointwise measurements need to be interpolated and the small-scale variability of rainfall fields can lead to biases in the rain rate estimation over an entire basin, above all for small or medium size mountainous and urban catchments. <P style="line-height: 20px;"> For these reasons, several raingauges should be installed in different places in order to determine the spatial rainfall distribution during the evolution of the natural phenomena over the selected area. <P style="line-height: 20px;"> In technical applications, many empirical relations are used in order to deduce heavy areal rainfall, when just one raingauge is available. <P style="line-height: 20px;"> In this work, we studied the areal reduction factor (ARF) using radar reflectivity maps collected with the Polar 55C, a C-band Doppler dual polarized coherent weather radar with polarization agility and with a 0.9° beamwidth. <P style="line-height: 20px;"> The radar rainfall estimates, for an area of 1 km<sup>2</sup>, were integrated for heavy rainfall with an upscaling process, until we had rainfall estimate for an area of 900 km<sup>2</sup>. <P style="line-height: 20px;"> The results obtained for a significant amount of data by using this technique are compared with the most important relations of the areal reduction factor reported in the literature.