How Hail Forms!

Published on Monday, 18 June 2012 19:41

Jenny Brown

2 Comments

Now that we’ve had a few hail storms so far this year , I thought it would be a nice time time to share with y’all information on how hail forms. I promise I won’t get too scientific or technical!

Inside of all thunderstorms are warm updrafts and cool downdrafts. The updrafts carry water droplets high up into the top of the cloud above the freezing level. The water droplets then freeze and fall back into the bottom portion of the cloud partially melting along the way. Sometimes they are picked up by another updraft and carried back up into the cloud top along with additional water droplets, and the freezing/rising, falling/melting cycle repeats itself again and again. Eventually you have a hailstone with many layers of ice that can become quite large and heavy.

The mechanism that keeps hailstones lofted in the cloud is the strength, or speed, of the thunderstorm updraft. Once the hailstone’s weight exceeds what the thunderstorm updraft can hold aloft, the hailstone falls to the ground. Most garden variety thunderstorms, such as storms that just pop up during the heat of the day, only have updraft speeds in the range of about 15 to 30mph. Stronger storms such as the type that form into lines or clusters can have updraft speeds around 60mph. The larger storms known as supercell thunderstorms often have updraft speeds exceeding 80mph, and over 100mph is not undeard of.

The longer the hailstone can stay lofted by the strong updrafts, the larger it can grow. Also important is how much of the cloud is within the zone that is below freezing. In the case of the hail storm that hit Dallas back on June 13, 2012, the freezing level was measured at 14,811 feet that evening. That’s a little over 2 ½ miles up from the surface. The cloud tops for those storms were measuring roughly 53,000 feet high. That’s roughly 38,000 feet of hailstone bounce house! By my estimation that evening based on the readings for CAPE (Convective Available Potential Energy… a specific atmospheric reading that measures instability), those storms had updraft speeds approaching 100mph. Put all that together and you end up with one heck of a devastating hailstorm.

Here is a link to an animation showing the above processes. http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/52_Hail/A_52.html Once you open the link, click “Continue to Active Figure” then click “Play” and watch this basic animation of hail formation.

Hail can also come in some really unusual shapes that don’t include “round”. Hail can be spikey and/or lumpy in appearance if other smaller hail stones have adhered to a larger hail stone during its ascent. Condensation in the cloud releases latent heat which keeps the external surface of hail stones on the wet side making them more likely to stick together on their way towards the surface.

The largest recorded hail stone in the US was produced by a storm that struck Vivian, SD on July 23, 2010. The record-holding stone was measured at 8 inches in diameter, 18.625 inches in circumference and weighed in at nearly 2 pounds! The link below is to a summary article published by the National Weather Service office in Aberdeen, SD.

http://www.crh.noaa.gov/abr/?n=stormdamagetemplate