The top column in the storm attribute table corresponds to the highest
vertical height of a 30 dBZ echo for a certain thunderstorm cell.
The National Weather Service radars originally display a height in
composite reflectivity as a one-digit or two-digit number with a
one-digit decimal. Multiply this number by one thousand to correctly
format the output. For example, if a radar displays the height as 15.6,
the actual height is 15,600.
Depending on the distance from the radar site, and the tilt of the
beams, the echo top can be greatly overestimated or underestimated to
the order of five thousand to ten thousand feet.
The radar may not always be able to resolve the 30 dBZ echo top. When
this error occurs, the output reading will have a "<" or
">" in front of it, meaning the top is less than or greater
than, respectively, the number that follows. For example, if the
reading <26.8 is shown, then the top is less than 26,800 feet.
The storm attribute table's top column does not produce the same
readings that are on the "Echo Tops" product because the product is not
reliant on the 30 dBZ echo, but the column's output is. In addition,
the product is generally less precise than the column. The reason for
this lies on the fact that the algorithm associated with the "Echo
Tops" product measures the echo top with the radar beam that is closest
to it. At far distances, it can appear on the product that the height
of the clouds sharply varies between two levels five thousand feet
apart, whereas the variance between tops is more gradual.
A radar cannot detect the highest vertical 30 dBZ height well if a
thunderstorm is close to the radar site. Beams are unable to extend
into the updraft at such a degree from the radar's transmitter.