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Clear Air Mode Precipitation Mode

NEXRAD operates in one of two "modes": Clear Air and Precipitation modes. These basically determine the sensitivity of the NEXRAD image. When there is no substantially strong radar signals, NEXRAD operators will place the radar in "Clean Air" mode. This mode is more sensitive and is susceptible to more anomalies due to its increased sensitivity. In this mode it is easier for NEXRAD to detect meteorological phenomenon as they are just forming. When NEXRAD detects strong radar signals it automatically switches to "Precipitation" mode. In this mode, the weakest signals are essentially ignored in favor of displaying only the strongest. Thus when NEXRAD is tracking a large, well-formed thunderhead you will not see weak returns that might simply be dust or smoke.

Following a precipitation mode, a NEXRAD operator must manually switch it back to "Clear Air" mode. It will not switch back to Clear Air mode automatically. Additionally, operators will sometimes monitor a snow storm in Clear Air mode to see more details of the storm due to the increased sensitivity.

When NEXRAD is in "Clear Air Mode" it produces one updated image every 10 or 11 minutes. When it is in "Precipitation Mode" it produces an updated image every 6 minutes.

To the left we have two images from the same NEXRAD site (click for full-sized version). The one at the top is in "Clear Air" mode at 19:31. The image below is in "Precipitation Mode" 11 minutes later at 19:42. In this case, NEXRAD detected sufficiently strong returns in the "Clear Air" mode in the top image to switch to the "Precipitation Mode" for the next image.

As you can see, the images show basically the same radar image but the colors seem to have changed. This is because Clear Air and Precipitation modes use different colors to represent the same return strengths. It is important that one look at the "scale" that is on the left side of each reflecitivty image. A side-by-side comparison of the two scales is shown in the graphic to the right. In the image to the right, the scale on the left side is "Precipitation" mode and the scale to the right is "Clear Air Mode". The colors look the same: what changes is the numbers they represent.

In the "Clear Air" image above you will see that the cloud formation is basically surrounded by an "orange" which corresponds (according to the "Clear Air" scale) to a DBZ strength of +4. The "Precipitation" image below shows the same formation surrounded by a light blue which corresponds (according to the "Precipitation" scale) to a DBZ strength of 5. So the two images represent the same cloud formation, their color-coding has simply changed due to the fact that the radar has switched to "Precipitation mode."

You can always tell whether the radar is in Clear Air mode or Precipitation mode by looking at the scale. If the scale goes as high as 75 then it is in "Precipitation " mode. If the scale goes only as high as +28 then it is in "Clear Air" mode.

REFLECTIVITY INTENSITY

Whether in Clear Air mode or Precipitation mode, what is important to look at is the level of reflectivity measured in dBZ. The stronger the return, the higher the dBZ. In Clear Air mode, the scale goes as low as -28 which is why it is very possible to occasionally detect dust or pollen in the air. When in Precipitation mode, the scale goes as high as 75 which usually indicates an abnormally high return or extremely severe weather.

Virtually anything in "Clear Air" mode (maximum dBZ of +28) is, at worst, a light rain or snow. If the meteorological activity were more severe, the NEXRAD site would switch to Precipitation mode. Thus whenever you see a NEXRAD image in "Clear Air" mode you can be failry certain that no significant precipitation is occurring.

For this reason, interpreting precipitation usually is a matter of interpeting an image in Precipitation Mode.

INTERPRETING INTENSITY IN PRECIPITATION MODE

dBZ	Condition
10-20	Very light rain or snow
20-30	Light rain or moderate snow
30-40	Moderate rain or sleet
40-50	Heave rain or sleet
50-60	Heavy thunderstorms
60-70	Extreme thunderstorms, possibly with hail
70+	Extreme weather or other RF interference

When interpreting reflectivity intensity in Precipitation mode, the table to the right can be used to approximate meteorological conditions.

Note that NEXRAD cannot differentiate between different types of meteorological phenomenon. For example, it can't determine whether a given echo was caused by rain, snow, or hail. However, the meteorologist (or us) who reviews the data can often make an educated guess based on knowledge of prevailing conditions and the general nature of the image.