they’re actually the result of smoothing and interpolation — the
algorithm’s attempt to “fill in the blanks.” Don’t get too attached
to the exact location of the hail size zones or the estimated hail
sizes — they are best guesses not absolutes.
Hail aloft does not always mean hail at the ground. Powerful hail
storms require warm, moist air at the surface and cold air aloft to
form and sustain themselves. Hail is produced in the cold, high
regions of the storm, but once it falls beneath the freezing level
(typically between 7,000' and 15,000' AGL, depending on loca-
tion and season), it starts to melt. The amount of melting hail
experiences once it falls beneath view of the radar depends on:
• Distance from the radar site — as previously mentioned, the ra-
dar beam gains in altitude with distance from the radar dish. At
a range of 90 miles, the lowest radar beam is already at 10,000 ft
altitude — the radar cannot “see” any lower than that. A lot can
happen below 10,000 ft (like substantial melting of hail).
• The initial size of the hailstones — small hail melts proportionally faster than large hail, so the smaller the hail is initially, the
more it will melt before reaching the ground. Hail that is initially 0.5" diameter or less often melts completely before reaching the ground.
No two storms are alike
Finally, no two storms are alike, yet the thresholds used for classification and size estimation within hail detection algorithms are
fixed. They are designed to describe most, but not all, conditions.
Dual-pol radar is a powerful tool, one that literally allows us to
peer into the heart of storms. However, interpretation of what we
see is by its nature complex, and no single algorithm can capture
that complexity. An algorithm that provides an accurate estimate
of hail size in one storm is not guaranteed to provide similar performance with all storms.
The bottom line
Hail detection and hail size algorithms are useful first-alerts —
they reliably show where hail is found aloft within a storm and
can generally distinguish between large vs. small hail. They are
less reliable at estimating exact hail size on the ground or providing street-level detail of the location of hail swaths.
Claims adjusters who use such products for their own investi-
gations should understand and consider the capabilities, limita-
tions and inherent uncertainty. Uncertainty is highest when:
• Only small hail is indicated aloft — hail can melt significantly
or completely before reaching the ground
• The location of interest is a long distance from the nearest
radar site — the radar cannot “see” what’s happening close to
• The atmosphere is warm with a high freezing level — most
common during the peak summer months, this increases the
probability of significant melting
Take advantage of dual-pol radar data and the insight offered
by algorithm-based products, but do so with a healthy degree of
caution. And if you have any doubts about the reliability of the
algorithm output, work with a forensic meteorologist who will
investigate the data not with a fixed formula but with rigorous
training, years of experience, and an understanding of local con-
ditions and storm types.
A forensic meteorologist will provide a more nuanced analysis of dual-polarization data, taking into account those factors
that automated hail track products typically don’t: distance to the
radar, melting considerations, comparison with on-the-ground
hail reports, data reliability and the like.
When the stakes are high (i.e., when litigation is involved or
a claim is particularly costly), working with a certified meteorologist who will rigorously reconstruct and verify the weather
conditions at the insured property — and who can provide expert testimony should litigation proceed — can ultimately save
thousands to tens of thousands of dollars.
Don’t get taken by the dark side of dual-pol — understand the
capabilities and limitations of this valuable tool, and don’t hesitate to seek an expert opinion when needed.
Megan D. Walker-Radtke, CCM is the chief meteorologist
at Blue Skies Meteorological Services. Contact her at
National Weather Service radar coverage across the U.S. regions marked
in gold, gray, or white do not have radar coverage below 6,000 ft AGL.
In these regions, it is more difficult to determine whether hail is melting
significantly before reaching the ground. Source: NOAA
Below 10,000 Feet AGL