Predicting Hail, When or Where will it Strike?
POSTED: Wednesday, April 23, 2014 - 12:06pm
UPDATED: Wednesday, April 23, 2014 - 12:49pm
There is hail in almost every storm, how do we know if it will make it to the ground
Wednesday, April 23rd, 2014 — This past Saturday, April 19th, 2014 Much of El Paso County experienced strong thunderstorms that produced large amounts of hail along with a moderate to heavy downpours. There was pea size to golf ball size hail that did some damage and it unfortunately even caused a homeless woman to lose her life. Some areas of central looked like winter scens as the hail covered the ground, cars and roff tops!
I read a couple Face Book comments that night saying “The Weather forecasters never said there was going to be hail”. I learned in my Meteorology classes and in my 20 plus years of forecasting that is as difficult to exactly predict where a large hail storm will hit as it is to exactly forecast where tornadoes will strike. There are forecasting tools that can be used say the morning of a day where the storm potential is high, to predict the possibility of hail. General areas of hail can be identified quite accurately , a few hours before storms hit. This is why I decided to write today’s “Weather Talk” on what is hail, what damage it can do and how do we forecast hail.
What is Hail?
We al know that hail is ice that falls from usually a strong thunderstorm. How does it form? Water droplets condense and form on particles, dust/dirt sea salt, pollution and other debris and form clouds. Most of these droplets freeze right away and become tiny ice pellets. These ice pellets grow in size as more water droplets condense and freeze on them in layers. The water droplets freeze as strong updraft winds carry the pellets and droplets higher into the coldest regions towards the cloud tops. The down draft winds in the thunderstorm and natural gravity pull the ice pellets back down, where they additional droplets that attach and freeze as the pellets are propelled, once again, back up into the cold upper regions of the cloud. The larger the hail stone, the stronger the updrafts and the more trips up and down through a cloud that the hail stone takes. A strong updraft allows hailstones to grow large enough to reach the ground. The hail rule is, the stronger the updraft, the larger the hail. Once hailstones grow large enough to begin falling despite the updraft that's been holding them up, plummet toward the ground at speeds as fast as 90 mph.
Small hailstones often melt before reaching the ground, but the larger ones reach the ground and can cause extensive damage to crops and vehicles caught in the storm. This is why hail is associated with strong thunderstorms.
Hail is classified as severe by the National Weather Service if it is equal to or greater than 3/4" in diameter, quarter size. ¾” inch hail can completely wipe out crops in a farmer’s in a matter of minutes.
Hail can range from pea size (½ inch in diameter) to quarter (3/4 in diameter) to golf ball size \ (1 ¾ in diameter) to softball, up to 4” in diameter! Comedian David Letterman said it hailed the “size of canned hams” in his brief career as a TV Weatherman in Indiana. The largest hailstone ever measured in the United States fell at Coffeyville, Kansas, on September 3, 1970. It weighed 1.67 pounds and measured 17.5 inches in circumference.
Hail Alley is located in the Great and High Plains of the United States. The hail bulls eye in the U.S.. northeastern Colorado and southeastern Wyoming. This area receives the most amount of hail every year than any other region of the United States. Hail is often responsible for extensive crop loss, property damage and livestock deaths. Deaths in the U.S.. are rare, but can happen. We saw this past Saturday, April 19th, a homeless women get washed into a drainage ditch by heavy rain and hail off US Highway 54. Rescuers found her buried in the three feet of hail. She passed away from hypothermia, her body temperature dropping dramatically from being buried in the ice. More deaths and injuries are more common in other parts of the world, especially places where many people live in poorly constructed homes and buildings. Damaging hail storms are also very common in countries like Russia, China and Italy.
The mountains and high elevations have a better chance of receiving hail. The mountains areas and the High Plains of the United States have the highest number of hail days per year. It makes sense that the Higher the elevation, the closer to the cold layers of the upper atmosphere. The smaller hail which would normally melt before reaching the surface in a low elevation area hits the ground in a higher elevation area. Storms do not need to be as severe in the lee of the Rockies as in lower elevation areas for hail to reach the surface.
These next sections get a little scientific but bear with me. Here are some of the factors that help determine if there will be hail in a storm:
The freezing level is the altitude at which temperature is at 32ºF (0ºC). The ideal average freezing level for hail formation is from 8,000 to 10,000 feet for lower elevations. The freezing level depends on elevation, the season, and the temperature profile of the atmosphere. High elevation areas will have relatively low freezing levels in all seasons.
The freezing level determines the depth of the atmosphere that is above freezing. If the freezing level is high in the atmosphere, hailstones will have more time to melt than if the freezing level is close to the surface. A high freezing level limits the vertical depth in which hailstone can form and grow.
The stronger the updrafts the longer the hail stones stay in the clouds and the more trips that take to the upper colder area of the clouds growing larger.
Weather.com gives a quick scale indicating how updraft strength corresponds to hail size; “To create pea-size hail (about 1/2 inch in diameter) winds within the thunderstorm updraft will generally be around 20 miles per hour. Quarter size hail (3/4 of an inch in diameter) requires updrafts of about 40 miles per hour. Golf ball size hail (1 3/4 inches in diameter) needs updrafts of around 55 miles per hour and softball size hail, approximately 100 miles per hour!”
Super cell thunderstorms have the updraft and down draft areas separated because the updraft area of the storm is tilted allowing for greater updraft velocity or speeds.
DRY MID LEVEL AIR
There is a dry slot of air, a freezing level that develops due to evaporative cooling within a storm. This is called the wet bulb zero level. The storm pulls in surface air and evaporative cooling takes place. This occurs most often most in storms forming over the High and Great Plains of the U.S..
CONVECTIVE AVAILABLE POTENTIAL ENERGY (CAPE)
This is getting a bit scientific but CAPE is the most important factor in determining hail size. The National Oceanic and Atmospheric Agency (NOAA) defines convective available potential energy or CAPE as: “A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.”
This number is determined by plotting the weather information obtained from a weather balloon on a chart called a “Skew T”. A Skew T, is defined by swett.com as: “A thermodynamic diagram is a chart on which pressure, density, temperature and water vapor are plotted for a point on the Earth up through the atmosphere. By having temperature against pressure, one can get a feel for many variables at a glance without performing lengthy calculations.” All the forecaster has to know is that the higher the CAPE number, the stronger upward vertical velocity the larger the hail. CAPES under 1000 J/kg generally produce borderline severe hail (near 3/4" or less) while CAPES over 2000 J/kg can produce very large hailstones.
The National Oceanic and Atmospheric Agency (NOAA) defines perceptible water as;
“Measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.”
Perceptible water is just how much water vapor and water is in the atmosphere. The more water/water vapor the smaller the hail and less likely it is to hit the ground as hail. The rule is, the lower amount of water vapor or lower perceptible water values. Large hail is formed most commonly in super cell thunderstorms with strong updrafts and lower perceptible water values.
While hailstones are ice, hail is mostly a spring and summer phenomena because the strong thunderstorms needed to produce hail are much more common during warm weather. Frozen rain drops like sleet and graupel are the common ice that fall from the sky during the cold months.
I think this is my longest “Weather Talk” ever! I will cover hail I am sure in future articles. Now that we know how hail forms and all the factors that come into play to produce hail. These ingredients can be detected, the difficulty is predicting when or most will come together to produce ice chunks big enough to make it through the warmer layers of the atmosphere below with out melting. The damage hail can do is scary and frustrating. We do get a hail more than a few times a year here in the Borderland. Watch for and pay attention to the National Weather Service Advisories and Warnings for hail potential when thunderstorms are near. Most hail storms can be pinpointed to a region an hour up to a few hours before they hit. Have a hail plan to protect yourself, family and find shelter for things that can get damaged, like your car.
Hail is one of many amazing and potentially damaging weather phenomena we have to watch out for.