The great benefit of using radar is that it can estimate rainfall over a huge area, including most places where there are no rain gauges. However, it turns out that using radar to estimate rainfall is much more difficult than one might think for a number of reasons:
1. The radar reflectivity and rainfall relationship is not unique, it changes constantly within a storm, among different types of storms, and from place to place.
2. The radar looks within the cloud while the gauge is at the ground. The difference in height between where the radar is looking and the gauge can be thousands of feet. A lot can happen to those drops as they fall out of the cloud: some might evaporate and others might be blown far downwind from where the radar is looking.
3. The radar beam might be blocked by hills, trees or building and might not be able to see the cloud producing the rain.
4. The radar volume at all ranges is very much larger than the sample volume of a rain gauge on the ground. This large volume may have a variety of reflectors - i.e. snow, hail, raindrops and cloud drops, all of which contribute to the reflectivity but which have very different water contents.