Base Reflectivity

Typical base reflectivity radar beam cross sectionTaken from the lowest (½°) elevation scan, base reflectivity is excellent for surveying the region around the radar to look for precipitation. However, remember the radar beam increases in elevation as distance increases from the radar. This is due, in part, to the elevation angle itself but is more because the earth's surface curves away from the beam.

This can lead to underestimating the strength and intensity of distant storms. For this reason, it is wise to always check the radar images from different locations to help provide the overall picture of the weather in any particular area.  

Base reflectivity image from the Doppler radar in Frederick, OKThis image (right) is a sample base reflectivity image from the Doppler radar in Frederick, OK. The radar is located in the center of the image. The colors represent the strength of returned energy to the radar expressed in values of decibels (dBZ). The color scale is located at the lower right of each image.

 dBZ   Rain Rate 
65 16+
60 8.00
55 4.00
52 2.50
47 1.25
41 0.50
36 0.25
30 0.10
20 Trace
< 20 No rain

These dBZ values equate to approximate rainfall rates indicated in the table right.

These are hourly rainfall rates only and are not the actual amounts of rain a location receives. The total amount of rain received varies with intensity changes in a storm as well as the storm's motion over the ground.

Also, thunderstorms can contain hail which is often a good reflector of energy. Typically, a hailstone is coated with a thin layer of water as it travels through the thunderstorm cloud. This thin layer of water on the hailstone will cause a storm's reflectivity to be greater, leading to a higher dBZ and an over estimate the amount of rain received.

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 ¾" 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...

  • 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.
  • 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).
  • 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).

These are just some of the reasons to look at the weather using the adjacent radars.

Back: NWS Radar Images