Tracing the Paths of Hailstones and Explaining Precipitation How does hail form? Hail forms when water droplets in a cloud get very, very cold (supercooled) and collide with other supercooled water droplets. This happens in thunderstorms when the clouds grow so tall, they lift the water droplets into very cold parts of the atmosphere. Hail can grow very large if it is forming inside a storm that has strong updrafts pushing those droplets into the colder parts of the atmosphere.

What causes these big updrafts to begin with? When temperatures near the ground are warmer, it creates stronger updrafts because more thermal energy is being added to the air from below to produce more convection. The ground heats up quickly, which causes the air right above it to increase in temperature and expand, rising high into the atmosphere. This rising air is called an “updraft.” The more thermal energy added to the system (i.e., more energy from sunlight that is absorbed by the ground and transferred to the air), the stronger the updraft.

But not all thunderstorms are created equal—the strength of the storm’s updrafts depends on the temperature differences between the air at the ground and air higher in the atmosphere. When strong updrafts form, the movement pushes air, including water molecules in the air, into very cold parts of the atmosphere where they can freeze and begin forming hail on any cloud condensation nuclei (CCN) in the air around them. If the temperatures higher in the atmosphere are not very cold or there are few CCN or relatively little humidity in that air, hail is unlikely to form or to become big.

Many storms that produce rain at the ground rather than hail actually do produce pieces of ice near their cloud tops. But if those pieces of ice are too small when they start to fall, they will begin melting as they pass through the warmer air below them. If by the time they reach the ground all that ice has melted, then the only thing that hits the ground is raindrops.

1 Why does hail get really big? Some hail can get very big while other hailstones remain small. What is the difference? When updrafts are stronger, they have more lift force and can keep hailstones in the cloud longer. The more lift force there is means that a heavier hailstone can be lifted or suspended in the air. Look at the table that shows different sizes of hailstones. What pattern do you notice between the updraft speed and the size of the hailstone typically produced?

The longer hailstones are in the cloud, the more time there will be for water droplets to stick together, causing the hailstones to grow in size. As long as the updrafts are strong enough, the hail will get bigger and bigger. How does the path a hailstone follows explain its growth?

There is some debate about the path that hailstones follow as they grow inside a cloud. One explanation involves hailstones traveling up and down in the cloud many times as they get caught up in updrafts, fall out of the side of these, and then get caught in another updraft (via convection) on their way down. Layers are being added with each rise and fall.

A second, competing explanation, suggests that hailstones form as they continually rise within a cloud, growing the whole time, until they are so big that they fall out of the cloud and to the ground. This explanation suggests that the layers, that are a feature of hailstones, are formed as more water is attached and freezes instantly onto the hailstone upon contact as the it rises within the cloud.

New techniques for collecting evidence of where the ice is forming in clouds and new models to explain convection in fluids are always being developed.

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How long can a hailstone stay aloft? In both explanations, strong updrafts are needed to produce larger hail. If updrafts are really strong, they can push hailstones upward in a cloud, keeping the hailstones aloft for longer periods of time. In some storms, this may be as long as 30 minutes, particularly if the hail is produced near the top of the storm clouds whose tops may be greater than 10 kilometers high.

Why do some hailstones have layers? Hailstones grow by colliding with other water drops and become bigger as those droplets freeze on it. The more collisions with droplets that freeze to it, the bigger it gets. In order to have many collisions, the original hailstone nucleus must be in the cloud for a while. As it collides, it can grow in two ways: (1) colliding with other water droplets that are not supercooled and do not freeze together immediately and (2) colliding with supercooled water droplets that freeze together immediately. When the first type of growth happens, air molecules are able to escape from the slowly freezing water drops, giving the hailstone a clear color. But when the hailstone grows by supercooled drops colliding, they freeze instantly, trapping in air molecules and giving it a whitish appearance. Sometimes hailstones experience both types of growth and develop a layered look to them.

How are hail, rain, freezing rain, snow, and sleet different? Hail forms when strong updrafts push moisture high in the atmosphere where it condenses into ice crystals which collide with other supercooled water droplets and grow into a hailstone. For this to happen, there must be warm to hot air temperatures near the ground and relatively high humidity to start the convection cycles that produce strong updrafts and hailstone growth.

Rain forms when temperatures near the ground are above freezing and there is enough updraft to put moisture into the atmosphere. The water vapor condenses into ice crystals, which fall back to Earth because the updrafts and air pressure are low or are relatively weak and cannot keep the droplets aloft. The ice crystals melt before they reach the ground, forming raindrops.

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Snow, on the other hand, forms on days when temperatures near the ground are low, close to 32 degrees Fahrenheit (0°C) or lower. Moisture can still be lifted into the atmosphere on weak updrafts and the temperature is still colder as the moisture rises. Snow is not formed from rain drops freezing in the clouds, but rather occurs through deposition (a solid forming from a gas without becoming liquid first). This deposition is similar to what happens to make frost at ground level but occurs higher in the atmosphere. The size and shape of the snow ice crystals is determined by the amount of water vapor in the air and the temperature at which the snow is formed. Snow does not melt before it reaches the ground because the air temperatures are cold enough to keep it frozen.

Sleet occurs when snowflakes melt into a raindrop in a pocket of warm air well above the ground and then refreezes in a pocket of freezing air just above the surface. The result is frozen raindrops or small ice pellets known as sleet.

Freezing rain occurs when the pocket of warm air aloft is much thicker allowing the raindrop to survive as liquid until it comes in contact with a surface at or below freezing. A coating of ice forms on whatever the raindrops contact.

4 Name______Date______Hour_____Table_____ Questions: 1. In your own words, explain how hail forms. ______

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2. What causes large updrafts to occur? ______

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3. What pattern do you notice between the updraft speed and the size of the hailstone typically produced? ______

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4. Explain the difference between the two explanations for how hailstones grow. ______

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5. Explain what happens to give hail the whitish appearance instead of a clear appearance. ______

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Directions – select the correct type of precipitation based on the description given.

1. Most common type of 6. Water that freezes when it hits the precipitation surface of the road A. Hail A. Hail B. Freezing Rain B. Freezing Rain C. Rain C. Rain D. Sleet D. Sleet E. Snow E. Snow

2. Rises and falls within a cloud, 7. Water that freezes as it falls before becoming larger and larger it hits the ground A. Hail A. Hail B. Freezing Rain B. Freezing Rain C. Rain C. Rain D. Sleet D. Sleet E. Snow E. Snow

3. Frozen water vapor that contains a 8. Takes the forms of crystal flakes or mixture of H2O and air molecules clumped masses of flakes A. Hail A. Hail B. Freezing Rain B. Freezing Rain C. Rain C. Rain D. Sleet D. Sleet E. Snow E. Snow

4. Water droplets that become super- 9. Forms as raindrops frozen in a tall chilled upon contact with a surface cloud A. Hail A. Hail B. Freezing Rain B. Freezing Rain C. Rain C. Rain D. Sleet D. Sleet E. Snow E. Snow

5. Drops of liquid water fall all the 10. Can quickly damage crops, cars, way from the cloud to the surface windows, etc… of the Earth A. Hail A. Hail B. Freezing Rain B. Freezing Rain C. Rain C. Rain D. Sleet D. Sleet E. Snow E. Snow 6