Hail Mini Study
Created and designed by Debbie Martin Hail Mini Study The Whole Word Publishing The Word, the whole Word and nothing but the Word." Copyright May 2011 by Debbie Martin 3627 D St. Bremerton, Wa 98312 All rights reserved. Permission is hereby granted to the individual purchaser to reproduce his book in whole or in part for non commercial individual or classroom use only. Permission is not granted for school wide system wide reproduction of materials. Images are are public domain.
What is hail? Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice. How does hail form? There are two ideas about hail formation. In the past, the prevailing thought was that hailstones grow by colliding with supercooled water drops. (Supercooled water is liquid water that is below 0 C, or water that stays in liquid form if undisturbed even though it has been cooled to a temperature below its normal freezing point. The smaller and purer the water droplets, the more likely they can become supercooled.) Supercooled water will freeze on contact with ice crystals, frozen rain drops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more supercooled water and continue to grow. The hail
falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow. Recent studies suggest that supercooled water may accumulate on frozen particles near the backside of the storm as they are pushed forward across and above the updraft by the prevailing winds near the top of the storm. Eventually, the hailstones encounter downdraft air and fall to the ground. Hailstones grow two ways: by wet growth or dry growth processes. In wet growth, a tiny piece of ice is in an area where the air temperature is below freezing, but not super cold. When the tiny piece of ice collides with a supercooled drop, the water does not freeze on the ice immediately. Instead, liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice.
Dry growth hailstones grow when the air temperature is well below freezing and the water droplet freezes immediately as it collides with the ice particle. The air bubbles are "frozen" in place, leaving cloudy ice. Hailstones can have layers like an onion if they travel up and down in an updraft, or they can have few or no layers if they are "balanced" in an updraft. One can tell how many times a hailstone traveled to the top of the storm by counting the layers. Hailstones can begin to melt and then re-freeze together - forming large and very irregularly shaped hail. What is the difference between hail, sleet, and graupel? The different ways precipitation is formed determines what type of precipitation it becomes. Hail is larger than sleet, and forms only in thunderstorms. Hail formation requires air moving up (thunderstorm updraft) that keep the pieces of ice from falling. Drops of supercooled water hit the ice and freeze on it, causing it to grow. When the
hailstone becomes too heavy for the updraft to keep it aloft, it encounters downdraft air, it falls. Sleet forms from raindrops that freeze on their way down through a cloud. Graupel also called soft hail or snow pellets forms when snow in the atmosphere encounters supercooled water. In a process known as accretion, ice crystals form instantly on the outside of snowflakes and accumulate until the original snowflake is no longer visible or distinguishable. The coating of these ice crystals on the outside of the snow is called a rime coating. The size of graupel is typically under 5 millimeters, but some graupel can be the size of a quarter. To tell the difference between graupel and hail, you simply have to touch a graupel ball. Graupel pellets typically fall apart when touched or when they hit the ground. Hail is formed when layers of ice accumulate and are very hard as a result. Hail Sleet Graupel
How does hail fall to the ground? Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall. Drag and friction also slow their fall, so it is a complicated question! If the winds are strong enough, they can even blow hail so that it falls at an angle. This would explain why the screens on one side of a house can be shredded by hail and the rest are unharmed! How fast does hail fall? We really only have estimates about how fast hail falls. One estimate is that a 1 centimeter hailstone falls at 9 meters per second, and an 8 centimeter hailstone, weighing.7 kilograms falls at 48 meters per second (171 kilometers per hour). However, the hailstone is not likely to reach it full speed due to friction, collisions with other hailstones or raindrops,
wind and melting. Also, the formula to calculate speed is based on the assumption that you are dealing with a perfect sphere. Hail is generally not a perfect sphere! What areas have the most hail? Though Florida has the most thunderstorms, Nebraska, Colorado, and Wyoming usually have the most hail storms. The area where these three states meet is called "hail alley," this area averages seven to nine hail days per year. The reason why this area gets so much hail is that the freezing levels (the area of the atmosphere at 32 degrees or less) in the high plains are much closer to the ground than they are at sea level, where hail has plenty of time to melt before reaching the ground. Other parts of the world that have damaging hailstorms include China, Russia, India and northern Italy. When viewed from the air, it is evident that hail falls in paths known as hail swaths. They can range in size from a few acres to an area 10 miles wide and 100 miles long. Piles of hail in hail swaths have been so deep, a snow plow was required
to remove them, and occasionally, hail drifts have been reported. How large can hail get? The largest hailstone recovered in the U.S. The largest recorded hailstone in the United States by diameter 8 inches (20 cm) and weight 1.93 pounds (0.88 kg). The hailstone fell in Vivian, South Dakota on July 23, 2010. (Shown here on the left.) Estimating Hail Size Hail size is estimated by comparing it to a known object. Most hail storms are made up of a mix of sizes, and only the very largest hail stones pose serious risk to people caught in the open. Above a large hailstone, approximately 13.3 cm (5 1/4 inches) in diameter, that fell in Harper, Kansas on May 14, 2004.
Pea = 1/4 inch diameter Marble = 1/2 inch diameter Dime/Penny = 3/4 inch diameter - hail penny size or larger is considered severe Nickel = 7/8 inch Quarter = 1 inch Ping-Pong Ball = 1 1/2 inch Golf Ball = 1 3/4 inches Tennis Ball = 2 1/2 inches Baseball = 2 3/4 inches Tea cup = 3 inches Grapefruit = 4 inches Softball = 4 1/2 inches Hail can be detected using radar. On Doppler radar, hail generally sends a return signal that looks like extremely heavy rainfall. Techniques for getting popular WSR-88D cell-based hail information from the Hail Detection Algorithm (HDA) have been incorporated into high-resolution gridded products that allow a user to diagnose which portions of storms contain large hail. Hail size data are accumulated over time to provide precise hail swath maps, showing both maximum hail size by location, and hail damage potential (a combination of hail size
and how long the hail has been falling). This lets the forecaster have a "quick look" at the direction the hail swath will likely take and how severe it will be. In addition, verification efforts can be better focused on areas where hail was most likely to be observed at the ground. Polarimetric, weather radar during construction Polarimetric radar (radar that sends horizontal and vertical pulses) can help tell the difference between hail and rain, and even determine hail size. Another important discovery is that polarimetric variables can help pinpoint the location of hail within a storm. A new algorithm, using polarimetric data, has been developed, called the HCA (hydrometer classification algorithm). Large hailstones up to 5 centimeters (2 in) in diameter with rings.
Can hail be forecast or predicted? To forecast hail three basic ingredients are required: Adequate updraft to keep the hailstone aloft for an appropriate amount of time. Sufficient supercooled water near the hailstone to enable growth as it travels through an updraft. A piece of ice or snow for it to grow upon What kinds of thunderstorms produce hail? There is no clear distinction between storms that do and do not produce hailstones. Nearly all severe thunderstorms probably produce hail aloft, though it may melt before reaching the ground. Multi-cell thunderstorms produce many hailstones, but not usually the largest hailstones. In the life cycle of the multi-cell thunderstorm, the mature stage is relatively short so there is not much time for growth of the hailstone. Supercell thunderstorms have sustained updrafts that support large hail formation by repeatedly lifting the
hailstones into the very cold air at the top of the thunderstorm cloud. In general, hail 2 inches (5 cm) or larger in diameter is associated with supercells (a little larger than golf ball size which the National Weather Service considers to be 1.75 inch.) Nonsupercell storms are capable of producing golf ball size hail. In all cases, the hail falls when the thunderstorm's updraft can no longer support the weight of the ice. The stronger the updraft the larger the hailstone can grow. Understanding Damage and Impacts Damage from hail approaches $1 billion in the US each year. Much of the damage inflicted by hail is to crops. Even relatively small hail can shred plants to ribbons in a matter of minutes. Vehicles, roofs of buildings and homes, and landscaping are the other things most commonly damaged by hail. Hail has been known to cause injury to humans, and occasionally has been fatal. The most deadly hailstorm on record occurred in India on April 30, 1988, killing 246 people and 1,600 animals.
A large hailstone can cause serious injury. A hailstone the diameter of a baseball falls at a speed comparable to that of a pitched baseball - on the order of 100 mph! It's like being hit by a "baseball" thrown by a major league pitcher.
Hail Suppression Since World War II scientists have tried many techniques to lessen the damage from hail storms. The most well-known technique involved "seeding" or cloud seeding large thunderstorms with silver iodide or dry ice. The theory was that the resulting small hail would probably melt and therefore not cause any damage. Above, cloud seeding can be done by ground generators, plane, or rocket (not shown). (Cloud seeding is the process of spreading either dry ice, or more commonly, silver iodide aerosols, into the upper part of clouds to try to stimulate the precipitation process and form rain. Since most rainfall starts through the growth of ice crystals from super-cooled cloud droplets (droplets colder than the freezing point, 32 deg. F or 0 deg. C) in the
silver iodide upper parts of clouds, the silver iodide particles are meant to encourage the growth of new ice particles. The history of cloud seeding has experienced uncertain results because it can never be known whether a cloud that rains after seeding might have rained anyway. This is because seeding is performed on clouds that look like they have some potential for producing rain. In the early 1960's, the National Hurricane Center began a series of experiments in seeding of hurricanes to reduce their intensity. Since the most destructive hurricane winds are produced from hurricanes with the smallest diameter eyes, seeding flights tried to stimulate the growth of a new eyewall of convection outside the inner, more destructive eyewall. Unfortunately, the project failed. It has since been determined that there are already an abundance of ice crystals in hurricane rain systems, so the production of new ice particles through cloud seeding probably has little if any effect. Unfortunately other experiments using this technique were not successful, and hail
suppression remains elusive.) Another technique to suppress hail was recently installed in a company parking lot by an auto manufacturer trying to protect its inventory of dry ice newly painted cars. Under certain thunderstorm conditions detected by the company's own radars, a cannon-like device will send sonic waves up to 50,000 feet in the air to keep hailstones from forming. Now they must wait for a hailstorm to pass over the parking lot to test the device!
My Hail Lapbook
Optional Lapbook Booklets What is hail? In the past, how did people think hailstones grew? Directions for above booklets. Cut out, fold in half, write inside and glue into lapbook.
What is supercooled water? Directions for above booklet. Cut out, fold in half, write inside and glue into lapbook. Hailstones grow in two ways, what are they? And describe each one. Directions for above booklet. Write one item on each blank scroll, cut out all scrolls. Stack with cover on top and staple at the left. Glue into lapbook.
Hail Sleet Graupel Directions for above booklet. What is the difference between hail, sleet, and graupel? Cut out booklet, cut on dotted lines fold in half. Write one item under each flap and glue into Lapbook. How does hail fall to the ground? Directions for above booklet. Cut out, fold in half, write inside and glue into lapbook.
What three states make up hail alley? How big is the largest hailstone recovered in the U.S.? Directions for above booklet. Cut out, fold in half, write inside and glue into lapbook. Directions for below booklet. Cut out booklet and cut on dotted lines, fold in half. Write the different sizes hail can be under each flap. Glue into lapbook.
What does HDA stand for? What is polarimetric radar? Directions for above booklet. Cut out, fold in half, write inside and glue into lapbook. To forecast hail, what three basic ingredients are required? Directions for above booklet. Write one item on each blank scroll, cut out all scrolls. Stack with cover on top and staple at the top. Glue into lapbook.
What two kinds of thunderstorms produce hail? What is some of the damage hail can cause? What two things can clouds be seeded with? Directions for above booklets. Cut out, fold in half, write inside and glue into lapbook.
Hail Optional Questions 1.) What is hail? 2.) In the past, how did people think hailstones grew? 3.) What is supercooled water?
4.) Hailstones grow in two ways, what are they? And describe each one. 5.) What is the difference between hail, sleet, and graupel?
6.) How does hail fall to the ground? 7.) What three states make up hail alley? 8.) How big is the largest hailstone recovered in the U.S.?
9.) Hail size is estimated by comparing it to a known object. Name the twelve objects and sizes hail can be.
10.) What does HDA stand for? 11.) What is polarimetric radar? 12.) To forecast hail, what three basic ingredients are required?
13.) What two kinds of thunderstorms produce hail? 14.) What is some of the damage hail can cause? 15.) What two things can clouds be seeded with?
Hail