What is hail?
How is hail formed?
Why do we see hailstones in varied sizes?
When and where do hailstorms occur?
How can I tell the size of a hailstone?
Why should "marble" size hail be discouraged?
Why does the sky sometimes turn green during thunderstorms, especially hailstorms and tornadic storms?

What is Hail?

Hail is a form of precipitation that occurs at the beginning of thunderstorms. They consist of near spherical pellets of ice and snow, usually combined in alternating layers. 

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How is hail formed?

Hail is formed when raindrops pass through a belt of cold air on their way to earth. This belt of cold air causes the raindrops to freeze into small blocks of ice. The formation of hail requires the presence of cumulonimbus or other convective clouds with strong updrafts. The air turbulence that accompanies thunderstorms aids the formation of hailstones.  The water that goes into the formation of hailstones is supercooled water, that is to say, it is at a temperature below freezing point but still in the form of a liquid. Hailstones start falling when they become too heavy to be supported by air currents. 

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Why do we see hailstones in varied sizes?

Hailstones are not formed of singled raindrops. However the process of formation of a hailstone does start with the freezing of a single raindrop. This may be carried by a strong current to the level where rain is still falling as drops. And as this again passes through the cold air belt, new raindrops may cling to the frozen hailstone, thus increasing its size. Or in other words, hailstones grow in size by repeated collisions with supercooled water. This water is suspended in the cloud through which the particle is travelling. Those single frozen raindrops that do not get carried back to the raindrop level remain as smaller hailstones.  

Typically, the larger in size a hailstone is, the more severe the thunderstorm that produced it.

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When and where do hailstorms occur?

Hailstorms are very common in middle latitudes and a heavy shower generally lasts around 15 minutes. It has been observed that hailstorms generally occur during mid to late afternoon.  

Big hailstones falling with force are known to have caused fatal harm to human and animal life. In an attempt to reduce the destructiveness of hail, large quantities of silver iodide have been injected into a thunderstorm. What the scientists are trying to do is to overseed the cloud so that many smaller hailstones form, thereby preventing them from growing into large destructive hailstones. However, not much headway seems to have been done, for the results have been rather inconclusive.

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How can I tell the size of a hailstone?

The National Weather Service had laid out the following criteria for measurement of hail in relation to size:

It is generally not good practice to use "marble" size when referring to hail.

Why should "marble" size hail be discouraged?

There are two reasons why we do not generally use "marble" size hail:

1.  There are many different sizes of marbles.  To one person, a "marble" may be the size of a pinball machine "marble", while to another "marble" may refer to the size of a pellet, or "B B".

2.  Also, notice in the chart above that the criteria for severe weather starts at the level after "marble" size.  Therefore, if you were to spot and report hailstones the size of pinball-sized "marbles" the National Weather Service would not consider the report to be in the "Severe" category.

All other measurements on the above chart are based on objects that are unvaried in size and are widely associated with the size to which it pertains.

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Why does the sky sometimes turn green during thunderstorms, especially hailstorms and tornadic storms?

While, dangerous thunderstorms that drop hail and spin up tornadoes seem to be routinely observed as turning green by the public and even severe storm researchers when out conducting experiments, there may be a number of different reasons for the green color of storm clouds. It may be that thick, thunderstorm clouds, with their water vapor, rain and hail, scatter sunlight in such a way as to give the clouds a blue hue. When the sun is low in the sky, such as before sunset when most severe storms occur, the reddening light of the sun when shining through Earth's atmosphere makes the bluish cloud tint turn green. Or it may be that storm clouds act as a kind of canvas upon which low sunlight scattered by particles in the air paints a greenish color. These explanations may play in concert with each other to create the observed green tint.

In the spring of 1995, a University of Oklahoma graduate student used a special device called a spectrophotometer to measure the wavelength of light emitted from thunderstorm clouds during Project VORTEX. His research confirmed green, greenish-yellow, and greenish-blue colors of the clouds, while ruling out lush farmland as the source of the green color. But the researcher concluded that the actual green color observed may be different for different people. In other words, what may look strikingly green to one person, might be more greenish-blue or greenish-yellow to someone else. Maybe not even green at all. Green and other color clouds were found in thunderstorms dropping hail. This seems to discount the notion that ice within the clouds creates the green color.

All that's known for sure is that green thunderstorms do exist. It's not known whether all green thunderstorms are severe, nor whether all severe thunderstorms turn green. Our advice: don't wait to see green clouds before seeking shelter from severe thunderstorms.

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