Weather radar is a powerful tool for predicting the potential for storms and severe weather. But it’s also confusing for many viewers, especially when it comes to interpreting the various colors and shapes used on radar images.
Radar reflectsivity is determined by the size of the precipitation target (raindrops, ice crystals, snowflakes) as well as the wavelength of the scanning wave. The targets have to be much smaller than the wavelength of the radar wave in order to reflect back the radar radiation, otherwise known as “Rayleigh scattering.”
A large, wet hailstone has a very high radar reflectivity value, typically 70 dBZ or higher. During thunderstorms, this high reflectivity is due to the fact that the ice crystals in these hailstones have started to melt.
Because these ice crystals have such high radar reflectivity, they are difficult to distinguish from raindrops and other smaller, more easily-seen precipitation targets. This can lead to inaccurate rainfall rate estimations in certain situations, especially when large hailstones are present.
During winter, sleet and partially melted snowflakes have their own unique problem. Partially melted snowflakes have very high radar reflectivity because they melt at the edges of the flakes and then distribute water across the arms of the flakes as they fall down to Earth.
Likewise, wet sleet or melting ice pellets have very high radar reflectivity because they distribute water along the edges of the sleet or ice pellet as they fall down to the ground. These high-reflectivity echoes are often seen embedded in areas of generally light rain.