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Severe Weather Awareness Week: When Are Warnings For Straight Line Winds or Hail Issued?

Downbursts

Straight-line winds and microbursts are two of the more common types of severe weather in South Carolina. They can happen any time of the year and can cause damage that may look like a tornado. Hail is less frequent hazard than strong winds, but it does happen, particularly in the spring and early summer. How do these phenomenon develop and how can you protect yourself when they happen?

We often hear the phrase "wind shear" when a meteorologist talks about the potential for bad weather. Wind shear is simply a change in either wind direction or speed (or both) between two levels in the atmosphere. Oftentimes, it is the difference between the wind near the ground and at some level in the lower or middle atmosphere, say, 5,000 to 10,000 feet above us. Wind shear causes thunderheads (cumulonimbus clouds) to become tilted, where the top of the thunderstorm is in a different location than the base of it. This causes the thunderstorm to have greater access to warm, moist, unstable air that it needs to survive. If there is little or no wind shear, the top of the thunderhead is in about the same location as the base of it, so the cool downdraft from the rain causes the thunderstorm to die -- typically in about a half hour.

Stronger environmental shear makes it more likely for thunderstorms to produce damaging winds over long distances. If the winds in the atmosphere are strong enough and the air is unstable enough, strong winds above the ground can be transferred to the ground in a line. Meteorologists often refer to this phenomenon as a squall line and it looks exactly that way on a conventional radar image that you may pull up on your phone, computer, or see on television. If the squall line is well-developed, it is possible to receive Severe Thunderstorm Warnings 30 to 45 minutes before the storm hits a particular neighborhood.

 

Downburst
Credit Megan Borowski / SCEIN

Microbursts, which are pockets of intense winds that can exceed 100 mph, form differently. Oftentimes, they occur in the summertime when upper-level winds and wind shear are much weaker. Instead, very high humidity and moisture load the thunderstorm up with heavy rain. The heavy rain is simply too much for a thunderstorm's updraft to support, causing the air to rapidly descend to the ground with major, localized destructive winds. Meteorologists often receive signals from radar of a developing microburst, but may have a few seconds to a few minutes to get a Severe Thunderstorm Warning to the public. Anyone who has outdoor plans in the summer should monitor weather conditions and be able to reach sturdy shelter in an orderly way since microbursts can occur on many days.

Hail is less frequent in South Carolina than in the midwest, but it occurs every year. The combination of wind shear and unstable air keep moisture droplets (we'll refer to them as embryos) in a part of a thunderstorm where the air is cold enough to cause ice formation. As long as these embryos can stay high enough up in thunderstorm that has temperatures that support hail formation, the embryo will continue to grow. Eventually, the hail stone will tumble to the ground when a storm's updraft can no longer support its weight. The air mass in Texas, Oklahoma, Kansas, and Colorado is colder and drier at altitude than it is in South Carolina, so hail stones are frequently larger in that part of the country. As is often the case, there are always exceptions: a rare 4.5-inch diameter hail stone -- about the size of a softball -- fell in Florence, South Carolina last year on May 25.

 

Hail Size Chart
Credit Megan Borowski / SCEIN

Atmospheric conditions favoring large hail (the size of quarters or larger) are normally more predictable than microbursts. Recent advances in doppler radar enable meteorologists to detect whether objects, like hail, are tumbling toward the ground. Severe Thunderstorm Warnings are issued with a few minutes' notice of damaging hail, which can be enough time for someone to secure their vehicle in a garage. Smaller hail stones often do not receive warnings, but they do not cause as much damage to property.