Mother Nature's Yo-yo

Hail.  That icy ball of wonder that occasionally falls from thunderstorms, covers the ground, and even causes damage.  It can be the size of peas, or as large as bowling balls.  So how does it form... and how can meteorologists detect it in time to warn the public?

How Hail Forms

Hail: Mother Nature's Yo-yo.

A hailstone begins as a frozen raindrop or ice crystal way up high near the top of a thunderstorm.  As it falls, it begins to melt some and collects more rain droplets as it collides with them.  This wet, icy mass then encounters a strong updraft of warm air, forcing it back up into the top of the thunderstorm where it refreezes.  It then continues this process until it becomes too heavy for the updraft to lift it back up into the storm and it falls to the surface and creates a strong, cold down draft.  This repeated up and down motion (which is why I called it a yo-yo!) and thawing and refreezing results in the onion-like layers that people often see when they cut a hail stone open.

How do meteorologists detect hail?

The most useful tool meteorologists use to detect
hail is radar.  Before dual polarized (dual pol) radars, the best indication on the radar of hail falling over an area was very high radar reflectivity values.  On the radar, these would appear as purple or white areas on a thunderstorm echo (you normally see greens, yellows, oranges, and reds).  The reason why the reflectivity value is so large is because the hail is bigger than the raindrops in the cloud!  Size matters!

The white and purple on this radar
image indicates the presence of hail.

With the dawn of dual pol technology, meteorologists now have more tools at their disposal.  With the Differential Reflectivity (which basically measures how spherical things are), hail shafts can be more easily identified because hail often takes a more spherical shape (compared to rain which is more elongated in one direction).  Values near zero from this product appearing in a return from a thunderstorm would likely suggest the presence of hail in the thunderstorm.






Another tool, known as the Correlation Coefficient, allows meteorologists to see the sizes of different types of radar targets (rain, hail, etc.).  This tool can be very useful in the detection of very large hail in a thunderstorm, which can be very dangerous and deadly.

With these new tools, meteorologists are now able to better detect the presence of hail in a thunderstorm so that the public can be warned in advance.  Hail is a common sight around the nation during the spring and summer, and if you do see some, collect a few pieces AFTER the storm has passed.  If you cut it open, you can satisfy your inner weather nerd by looking at the neat layers, like this:


A cross section of a hail stone.  Look at the layers!  How cool is that?