Beall Woods Nature Preserve, IL Weather Radar Map

The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan. "Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used.

The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rainrate. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values.

• Less than 20 dBZ: Very Light Rain, Snow or Fog.
• 20 - 30 dBZ: Light Rain or Moderate Snow.
• 30 - 45 dBZ: Moderate Rain or Heavy Snow.
• 45 - 55 dBZ: Heavy Rain.
• Greater than 55 dBZ: Very Heavy Rain, Hail.

These values are estimates of the precipitation per hour, updated each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause precipitation estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.