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Reflectivity images are just as they sound as they paint a picture of the weather from the energy reflected back to the radar. Reflectivity images are the vast majority of radar images you will see on television as well. There are two types available on the web; Base Reflectivity (½° elevation) and Composite Reflectivity.
A Base Reflectivity image indicating precipitation.
Taken from the lowest (½°) elevation scan, base reflectivity is excellent for surveying the region around the radar to look for precipitation. The colors represent the strength of returned energy to the radar expressed in values of decibels (dBZ). As dBZ values increase so does the intensity of the rainfall. Value of 20 dBZ is typically the point at which light rain begins. The values of 60 to 65 dBZ is about the level where 1″ (2.5 cm) diameter hail can occur. However, a value of 60 to 65 dBZ does not mean that severe weather is occurring at that location.
Severe weather may be occurring with values less (or greater) than 60 to 65 dBZ due to:
1. Hail that is totally frozen (without a thin layer of water in the surface). “Dry hail” is a very poor reflector of energy and can lead to an underestimate of a storm’s intensity.
2. Atmospheric conditions such a ducting. When ducting occurs, the radar beam is refracted into the ground (indicating stronger storms than what are actually occurring). However a worse case is when subrefraction is occurring and the beam is overshooting the most intense regions of storms (indicating weaker storms than what are actually occurring).
3. Doppler radars that get out of calibration. The radar can become “hot” (indicating stronger storms than what are actually occurring) or “cold” (indicating weaker storms than what are actually occurring).
4. The radar beam spreads with distance meaning the most intense part of the storm’s reflected returns will be averaged with the weaker portions leading to an overall appearance of lower intensity.
5. The radar beam increases in elevation as distance increases from the radar.