Published: 2008-06-17 10:40:18
Updated: 2008-06-17 10:40:18
Posted June 17, 2008 10:40 a.m. EDT
By Vernon Houston
MIKE MOSS SAYS: Vernon, If you were seeing "specks," that probably means there wasn't much precipitation around and you were seeing either ground clutter or other weak non-precipitation echoes that amount to "noise" on the radar display. However, those echoes use the same intensity scale and units that the more familiar shades of green and yellow, etc that show precipitation intensity do. Those intensities are given in units called dBZ, or decibels of radar reflectivity. There is a quantity known as radar reflectivity factor (z) that depends on the number, size distribution and type of precipitation particles or other radar energy scatterers in the volume being sampled by the radar beam. This quanitiy is related to the power returned to the radar and is measured in rather inconvenient units of "millimeters to the sixth power per cubic meter" which can range from a few tenths for light sprinkles to over 30,000,000 for sizable hail. To keep the units for the radar display a little more manageable, the "logarithmic radar reflectivity factor" (Z) is usually used. By doing this, the light sprinkles might produce a reflectivity in dBZ of -5 to 10 or so, while the large hail might be 60 to 75 dBZ, a much easier range to wrap your mind around or to show on a radar legend.