If it’s summer, that cold stuff is hail
The last few rounds of thunderstorms that moved through the Inland Northwest over the past month were notable not only for producing some gusty winds, but quite a bit of hail as well.
Storms Aug. 22 produced pea-sized, accumulating hail across both Spokane and Kootenai counties. Earlier in the month, larger marble-sized hail pelted the Palouse, damaging thousands of acres of wheat fields.
Hail is mainly a summertime occurrence in our region, associated with some of the stronger thunderstorms that move through during the heat of the season. Little balls of ice, hail is often confused with other types of frozen precipitation such as graupel or sleet. Each of these, however, form under different atmospheric temperature profiles particular to the different seasons that we experience here.
You will never see sleet in the summer. It occurs almost exclusively during the winter months when cold air blankets the region. From time to time, a layer of above freezing air will get sandwiched in between two sub-freezing layers. Snow from clouds, falling into the “warm layer,” will melt and briefly change into rain before re-freezing in the bottom layer of cold air near the surface. The little pellets of ice we call sleet are the result of that scenario.
Graupel and hail are the two types of precipitation that often get confused with each other. Graupel is primarily a spring and fall phenomenon, and often occurs when surface temperatures are well above freezing. When it’s not mistaken for hail, some folks lump it into the snow category and claim the first snow of the season sometime in October.
While graupel is sometimes called “soft hail,” it is not technically hail. It can also be referred to as “snow pellets,” though it differs from snow. Confused yet? Graupel forms when super cooled water droplets (water that remains a liquid below 32 degrees Fahrenheit) merge with ice crystals during a cool season storm. The strong rising motion within the storm keeps the graupel suspended until it grows heavy enough to fall.
Unlike snowflakes which fall gently to the ground, graupel falls harder and often smashes into the ground. It is more dense than a snowflake, but less dense than hail. It will also have a milkier appearance than a hail stone.
Finally, we come to hail, those bouncing balls of ice that can be small as peas or as large as grapefruits. They occur in more intense thunderstorms which have stronger updrafts (areas of rising air). Falling water droplets get caught up in the updrafts and are carried high into the clouds where temperatures are below freezing. They freeze and collect more layers by merging with other super cooled water droplets. These “embryonic” hail stones begin to fall, but may get caught up in many more updrafts depending on the strength of the storm, continuing to grow in size.
Multiple cycles of rising and falling – partial melting and refreezing –lead to hail stones that when cut in half, look like a onion with its many layers. Up until 2003, the largest hailstone ever recorded in the U.S. was one that fell in Coffeyville, Kan., in 1970. It weighed more than 1 1/2 pounds and had a diameter of 5.5 inches. A hailstone which fell in Aurora, Neb., in June 2003, however, broke that record with a diameter measurement of a whopping 7 inches.