It’s summer here in the Southern Part of Heaven and that means it’s time for hail storms. So I thought a column on the science of hail would be timely. Plus, my wife recently asked me a question about hail and my explanation was a bit disorganized, so I hope this one is an improvement.
Any discussion of hail should begin with a review of the temperature of the air as a function of altitude. As North Carolinians who head to the mountains in the summer know, the air gets cooler as you get higher above sea level. On a yearly average, the air temperature drops below freezing, 32 °F, around 8,000 feet, then drops to 2 °F at 16,000 feet, and to -30 °F at 25,000 feet. The temperature continues to drop to -70 °F at 50,000 feet, the top of the troposphere, at which point it begins to warm again. The reason it begins to warm above an altitude of 50,000 feet, while interesting, is not important to our discussion here, since what we commonly refer to as “weather,” including hail, occurs within the troposphere. The science behind why the air cools between 0 and 50,000 feet is also quite interesting. However, in order not to throw us off track, I have consigned the summary of that to the endnote below.
Now that we know the temperature profiles in the troposphere, we need to turn our attention the prevailing air currents in a thunderstorm. The first required feature for a thunderstorm is an upward flow of warm, moist air. This prerequisite is what generally limits thunderstorms to the summertime. As the warm air rises, it cools, causing its density to increase,e as well as resulting in condensation of some of the moisture into raindrops. Then the now cooler, denser air and rain begin to fall, creating a downdraft. As I explained in a previous column, Shelter from the Storm, the upward and downward flows of air, moisture, and raindrops generate static electricity that is released as lightning.
Now let’s turn to how and why hail can form in a thunderstorm. As the air in the updraft cools, the initial water droplets formed are quite small. Therefore, at least for a while, the velocity of the updraft is sufficient to continue to lift them. Over time the little water droplets coalesce into larger ones and the velocity of the updraft slows as it cools, such that the raindrops become too heavy to be held aloft and begin to fall. Generally speaking, the updrafts in all thunderstorms are sufficiently strong to propel at least some of the water droplets to altitudes of greater than 8,000 feet, where temperatures are below freezing. At these elevations and temperatures, the water droplets freeze and form hail. Since the particles of hail are “sticky” they can agglomerate together to form larger and larger particles. As you follow along with the logic in this paragraph, you will discover an interesting but generally unreported feature of thunderstorms. All thunderstorms make hail. Usually the hailstones melt back into rain before they reaches the surface. But if the thunderstorm has a strong enough updraft, the pellets of hail spend sufficient time at elevations higher than 8,000 feet to grow to a size too large to melt as they fall even when the air near the surface is hot. Your local weather service can “look” into the thunderstorm with Doppler radar to determine the size of the hailstones being formed and determine if they are sufficiently large to survive the decent.
While hailstones of up to 8 inches in diameter have been reported, the vast majority have diameters of less than 2 inches when they reach the ground. Nevertheless, with speeds ranging from 20 to 100 miles per hour depending on conditions, their impact can cause injury significant damage. Each year in the United States hail causes approximately one billion dollars of damage, primarily to crops, livestock, automobiles, and glass windows. People often receive minor injuries from hail, but generally are able to move to shelter before things get out of hand. Death from being struck by a hailstone is extremely rare. Hail can cause some damage to aircraft, but it is generally not sufficient to cause a plane crash. Further, modern radar allows pilots to avoid flying through strong storms.
So those are the basics of hail. I hope this column inspires you to consider the science next time you hear the patter of hail on a tin roof and to scurry under it as well.
Jeff Danner spoke with Aaron Keck on WCHL Monday – not just about hail, but also about the science of sagging power lines.
Have a comment or question? Use the interface below or send me an email to email@example.com. Think that this column includes important points that others should consider? Share this column on Facebook or Twitter. Want more Common Science? Follow me on Twitter on @Commonscience.
Endnote: There are two primary reasons that the air is cooler at higher altitudes. The first has to do with the interaction of the radiation from the sun and the earth and its atmosphere. The energy from the sun arrives primarily within the visible and ultraviolet energy ranges and passes through the atmosphere without being absorbed. When the sun’s rays strike dark objects on the ground, some of the energy is absorbed, warming the surface, and some of the energy is reflected back upwards as infrared radiation. Infrared radiation is readily absorbed by water vapor, carbon dioxide, and other greenhouse gases in the atmosphere. The combined effect of these various interactions is that the sun warms the atmosphere from the bottom up.
The second reason that it is cooler at the mountaintop is known as adiabatic cooling. You are likely familiar with adiabatic cooling even if you don’t know the term. When you lower the pressure of a gas by allowing it to expand its temperature falls. You can observe this anytime you dispense Reddi-Wip® from a can, inflate a bicycle tire with a CO2 cartridge, or allow propane to flow from a tank to your grill. The nozzle through which the gas is expanding will get cold and often accumulate condensation. The same thing happens to air as it moves from the high-pressure zone near the surface of the earth to the low-pressure zone at the top of a mountain, it expands and, thus, cools.http://chapelboro.com/columns/common-science/the-science-of-hail/
A Chapel Hill daycare worker who left a toddler in a van for hours on Tuesday has been fired.
The mother of the toddler told multiple television stations her daughter was left inside the Operation New Life Child Development Center daycare van for several hours during the record-breaking heat.
A police report shows the incident listed at 6:42 Tuesday evening in the parking lot of the daycare on North Estes Drive.
The mother, Kimberly Cates, says her two-year-old daughter was left in the van for more than six hours during the middle of the day on Tuesday. Cates says the toddler is now in good condition.
The daycare issued a statement saying that the individual responsible had been fired.
The incident is still under investigation.http://chapelboro.com/news/safety/report-chapel-hill-daycare-worker-leaves-toddler-in-hot-van/
When Winter Storm Remus dropped eight inches of snow on the Triangle last week, local public works crews worked around the clock to clear the roads as quickly as possible.
But who – if anyone – is responsible for the sidewalks?
If you walked around Chapel Hill or Carrboro after the snowstorm, you might have found some of the sidewalks remained snowy and icy long after the roads were clear – and the same was true for a number of apartment complex parking lots as well. Who’s in charge of those?
“Pursuant to our town code – it’s actually Section 7.6 – occupants of store buildings are responsible for clearing the sidewalk in front of their building,” says Carrboro mayor Lydia Lavelle.
The town code reads: “Every occupant of a store building, in front of which the sidewalk is paved with stone, brick, asphalt or cement, shall remove snow, ice or other similar obstruction from such sidewalk at the earliest possible time and as soon as the weather permits.”
In addition to businesses and store owners, Lavelle says residents are also encouraged to clear the sidewalks in front of their homes – and Chapel Hill Mayor Mark Kleinschmidt and Hillsborough Mayor Tom Stevens both say their towns have similar policies too.
“For the sidewalks that are in front of your property, the property owners are required – or (rather) requested to do it,” Stevens says. “That’s a policy that probably applies more to shop owners downtown than anything else.”
That’s standard procedure across the country – generally speaking, town crews are not responsible for clearing sidewalks; that responsibility typically falls on the citizens.
But the operative word there is “requested,” not “required.” All three towns have it written into their ordinances that residents and store owners are “requested” or “encouraged” to clear their sidewalks – Carrboro’s ordinance says they “shall” clear them – but neither Chapel Hill nor Carrboro nor Hillsborough appear to impose any legal sanctions on residents or businesses who fail to do so.
Lavelle says the reason for that is simple: the towns simply don’t have the authority.
“(When) this happened last winter…we talked at one point about whether we had the authority to require clearing – and we don’t,” she says. “Local authority, that kind of issue. So here we encourage it – but by authority we can’t require it, or (impose a) fine or charge.”
It all comes back, in other words, to the problem of “home rule”: by North Carolina law, towns and counties are only able to do what the state legislature allows them to do – and in this case, Lavelle says, the state has not given towns that power.
Compounding the problem, Lavelle says, is the fact that a lot of sidewalks in Carrboro and Chapel Hill simply don’t have storefronts or houses behind them at all – so there’s no one to clear them regardless.
“So in a town like Carrboro, where people are used to walking down Weaver Street between Town Hall and Weaver Street (Market), and then on down to the Hampton Inn and East Main Street project – that’s a lot of sidewalk that just doesn’t get cleared,” Lavelle says.
Facebook and Twitter were abuzz in the last two weeks with residents pointing out icy sidewalks and snow-covered driveways. Lavelle says the towns can “nudge” businesses and residents to clear them off, but beyond that there’s not much they can do.
Kleinschmidt, though, says businesses do have a strong incentive to keep their sidewalks and driveways clear – they may not be subject to fines, but they could be subject to civil suits.
“Private property owners…do risk liability when they don’t keep their walkways clear,” he says. “So it’s always particularly in a business’s best interest to do that – also just to make sure customers feel welcome to come in, when it’s open during bad weather.”
And Stevens says for the most part, people did fulfill their duty.
“Most folks do a pretty good job of trying to make the way clear,” he says. “It was several days with that really bitter cold weather, but we managed to get through.”
Kleinschmidt and Lavelle both say town officials in Chapel Hill and Carrboro are looking into ways to make the response even better next time.
And in the meantime – while there may be a little snow left in the forecast this week – we can all take solace in the fact that last week’s snow will soon be only a memory.http://chapelboro.com/news/safety/when-it-snows-who-clears-the-sidewalks/
With more than an inch of snow expected to fall, plus a quarter inch of ice, local road crews were out all night trying to keep the streets drivable.
In Hillsborough, Mayor Tom Stevens says all that work is done by a skeleton crew of just seven people.
“These guys are unsung heroes,” he says. “We have seven in the department, and they’re doing good work out there.”
Stevens says about half the crew was out during the nighttime hours, and the rest will be out today. They’ll be clearing the roads of as much snow as possible – but Stevens says it still won’t be ideal.
“There’s very little we can do about the ice, if it gets to be icy out there,” he says. “We just have to wait it out like everybody else.”
The winter storm warning is in effect until 9 am.http://chapelboro.com/news/local-government/hillsborough-just-seven-people-clear-roads/
With the threat of lingering ice on the roadways, local schools, businesses, and other agencies are announcing closures, cancellations and delays for Thursday morning.
See below for the full list in Orange, Durham and Chatham Counties – and keep checking this page throughout Wednesday night and Thursday morning for updates. No more precipitation is expected to fall in our area, though, and temperatures should reach the mid-40s by Thursday afternoon.
ORANGE COUNTY SCHOOLS: 2-hour delay
DURHAM PUBLIC SCHOOLS: 2-hour delay
BETHESDA CHRISTIAN ACADEMY: 2-hour delay
CARTER COMMUNITY SCHOOL: 2-hour delay
CRESSET CHRISTIAN ACADEMY: 1-hour delay
FELLOWSHIP BAPTIST ACADEMY OF DURHAM: Delayed until 10 am
GORMAN CHRISTIAN ACADEMY: Delayed until 9 am
IMMACULATA CATHOLIC SCHOOL: 2-hour delay
KESTREL HEIGHTS CHARTER SCHOOL: 2-hour delay
MAUREEN JOY CHARTER SCHOOL: 2-hour delay
MONTESSORI FARM SCHOOL: 1-hour delay
ORANGE CHARTER SCHOOL: 2-hour delay
PACE ACADEMY: 2-hour delay
SOUTHPOINT ACADEMY OF DURHAM: 2-hour delay
VOYAGER ACADEMY: 2-hour delay
WESTMINSTER KINDERGARTEN: Delayed until 10 am
With the threat of freezing rain in the forecast, Orange County is under a Winter Weather Advisory from 7:00 Tuesday evening through noon on Wednesday.
“Temps are going to drop through the evening, down around freezing by 9:00 or so,” says National Weather Service meteorologist Ryan Ellis. “Initially that won’t cause a problem, but as precipitation starts…you’ll start to see the first impacts around midnight or so, (with) glazing on overpasses and bridges first…
“As precipitation increases after midnight, expect to start seeing travel impacts on the roadways (through) the morning commute.”
Ellis says it’s not yet clear how much precipitation is going to fall, but forecasters expect about a tenth of an inch of ice. Ellis describes the expected impact as “low to moderate” – road conditions will be treacherous, especially during the morning commute, but there shouldn’t be many power outages or downed trees.
“The worst case scenario would probably be two-tenths of an inch (of ice),” Ellis says. “The best case scenario is that the precipitation doesn’t make it as far north – and if we don’t get a lot, we could see very limited impacts with maybe just patchy ice on bridges and overpasses.”
Still, even a small amount of ice is enough to cause havoc on the roadways. “Last week in Sampson County…they had a very brief period of very light drizzle and rain, and they had 17 cars off the road within a couple hours,” Ellis says. “It doesn’t take much with the ice.”
So when will the ice go away? Ellis says it shouldn’t be long: the precipitation should end around 10:00 Wednesday morning, and temperatures will rise above freezing around then as well.
“Once the temperatures warm back up…you get cars traveling over the road (and) things heat up a little better,” Ellis says. “So we don’t anticipate impacts beyond Wednesday morning.”
Continue to check Chapelboro.com for updates on the weather, as well as any school and business closings, cancellations or delays.http://chapelboro.com/news/weather/expect-tenth-inch-ice-tuesday-night/
The possibility of inclement weather throughout our area has led to the state Department of Transportation going into overdrive to prepare the roadways.
Mark Mueller is the Communication Officer for the division of the DOT that oversees Orange County, and he says they have been watching the forecast to most efficiently prepare.
“It’s looking like .01 – .12” of accumulation,” he says. “The potential for down tree lines usually happens at .25 – .5” – so it does not look like that’s going to be happening.”
Mueller adds crews have been preparing all day to ensure the equipment will be up to the task for a long night’s work.
“We’re looking to have crews start around 7 o’clock [Tuesday] evening,” he says. “And we’re expecting 30 – 35 people on hand.”
Mueller says the lingering rain has caused a change of plans for how they typically prepare thoroughfares.
“They’re not planning, at this point, to put down any salt brine, since there’s rain in the forecast,” he says. “But they’re likely going to be putting down the hard salt.
“[It will be put down] at a minimum on the bridges, likely on the roads – depending on how the forecast comes.”
Mueller says the DOT is advising commuters to stay off the roadways, if at all possible, and to exercise caution if you are traveling.http://chapelboro.com/news/safety/dot-preparing-long-night-inclement-weather-looms/
Winter has officially settled in across the Tar Heel state, and our area is no exception. After seeing heavy rain early Monday morning, temperatures have fallen and led to the possibility of some icing as we continue through the week.
The National Weather Service has issued alerts – including a Winter Weather Advisory for Orange County and a Winter Storm Watch for Durham and Wake Counties – that will go into effect late Tuesday and remain through mid-day Wednesday.
NWS Meteorologist Shawna Coakley says Tuesday we expect to have lingering drizzle, but the real problems may develop late Tuesday night into Wednesday morning.
“We’ll have temperatures right within a few degrees of freezing, and that brings with it a chance of freezing rain,” she says.
Coakley says we are not expected to see major accumulation, but “certainly you could get some glazing on surfaces. And you might see some difficulty with travel on roadways and walking on sidewalks.”
Coakley adds the chance of inclement weather will be rather widespread.
“We’re looking at the whole area for this, the entirety of central North Carolina,” she says.
The Wednesday morning commute may be a slippery one, if the variables of the forecast develop over the next 24 hours.
After that, Coakley says the temperatures will climb above freezing for the foreseeable future and the chance of rain will diminish to close out the week – taking any chance of inclement weather with it.http://chapelboro.com/news/weather/freezing-rain-may-lead-slippery-commute-wednesday/
The Triangle is expected to see extremely cold weather on Thursday morning, with temperatures as low as 10 and below-zero wind chills. As a result, some schools in the area will be closed or delayed:
ALAMANCE BURLINGTON SCHOOLS: 2-hour delay
CHAPEL HILL-CARRBORO CITY SCHOOLS: 2-hour delay
ORANGE COUNTY SCHOOLS: 2-hour delay
PACE ACADEMY: 2-hour delay
We’ll continue to update this website with additional information.
Temperatures are transitioning from rather mild to well below normal. National Weather Service Meteorologist Shawna Coakley says this cold weather will stick around for a few days.
“We’re going to have temperatures quite a bit below normal through the weekend,” she says. “Thursday we’re expecting the coldest day so far, probably struggling to get out of the upper-20’s.”
Coakley says there are dangerous situations that can develop with the frigid temperatures.
“We’re not expecting precipitation over that time period, but there is the possibility we could see some wind chill issues,” she says. “Lows Wednesday night will be into the teens, possibly nearing single digits.”
Last year, “Polar Vortex” seemed to be everyone’s catchphrase; Coakley says that, while it’s fun to say, the polar vortex is not an anomaly.
“The polar vortex is always there,” she says. “We frequently get pieces that break off and can influence our weather.”
Even though we may not see any snow or ice associated with this system, Coakley says it is still important to take precautions if you have to venture into the cold weather.
“The best thing to do is stay indoors as much as possible,” she says. “[If you are outside] cover any exposed skin and try to stay out of the wind because that can lead to problems with frostbite or hypothermia.”
If you or someone you know is experiencing signs of hypothermia – shivering, dizziness, fatigue, and shallow breathing – call 911 immediately.
Chapel Hill town officials are taking the opportunity to remind residents to have your heating systems checked. This time of year, residents are also reminded to not bring any outdoor heating devices – including grills or outdoor propane heaters – inside because there is an increased risk of setting fire to your home.
Leaders are also reminding residents to leave your water on a slow drip to reduce chances of a burst pipe, in cases where pipes are affixed to an outdoor wall and you do not have alternate ways to insulate the pipe.
Also, pets should either have an adequate structure with proper insulation or should be brought inside to ensure their safety.http://chapelboro.com/news/weather/frigid-weather-nearing/