dBZ stands for decibel relative to Z. It is a logarithmic dimensionlesstechnical unit used in radar, mostly in weather radar, to compare the equivalent reflectivity factor of a radar signal reflected off a remoteobject to the return of a droplet of rain with a diameter of 1 mm. It is proportional to the number of drops per unit volume and the sixth power of drops' diameter and is thus used to estimate the rain or snow intensity. With other variables analyzed from the radar returns it helps to determine the type of precipitation. Both the radar reflectivity factor and its logarithmic version are commonly referred to as reflectivity when the context is clear. In short, the higher the dBZ value, the more likely it is for severe weather to occur in the form of precipitation. Values above 20 dBZ usually indicate falling precipitation.
Principle
The radar reflectivity factor of precipitation is dependent on the number and size of reflectors, which includes rain, snow, graupel, and hail. Very sensitive radars can also measure the reflectivity of cloud drops and ice. For an exponential distribution of reflectors, Z is expressed by: As rain droplets have a diameter on the order of1 millimetre, Z is in mm6m−3. By dividing Z with the equivalent return of a 1 mm drop in a volume of a meter cube and using the logarithm of the result, one obtains the logarithmic reflectivity LZ, in dBZ: dBZ values can be converted to rainfall rates in millimetres per hour using the Marshall-Palmer formula:
LZ
R
R
Intensity
5
0.07
< 0.01
Hardly noticeable
10
0.15
< 0.01
Light mist
15
0.3
0.01
Mist
20
0.6
0.02
Very light
25
1.3
0.05
Light
30
2.7
0.10
Light to moderate
35
5.6
0.22
Moderate rain
40
11.53
0.45
Moderate rain
45
23.7
0.92
Moderate to heavy
50
48.6
1.90
Heavy
55
100
4
Very heavy/small hail
60
205
8
Extreme/moderate hail
65
421
16.6
Extreme/large hail
Other quantities
The definition of Z above shows that a large number of small hydrometeors will reflect as one large hydrometeor. The signal returned to the radar will be equivalent in both situations, so a group of small hydrometeors is virtually indistinguishable from one large hydrometeor on the resulting radar image. The reflectivity image is just one type of image produced by a radar. Using it alone, a meteorologist could not tell with certainty the type of precipitation and distinguish any artifactsaffecting the radar return. In combination with other information gathered by the radar during the same scan, meteorologists can distinguish between hail, rain, snow, biologicals, and other atmospheric phenomena.
