When Dave Carroll lists his biggest fears about chasing storms, the top one on his list has nothing to do with storms.

"Traffic," he says.

Then comes heavy rain, hail, and lightning. "The actual tornado is way down the list," he said.

Carroll, a Roanoke native and Blacksburg resident who teaches meteorology classes at Pulaski High School, has been chasing storms in the Midwest since the 1980s. Several times in the last decade, he has taken a small group of local high school and college students with him.

Carroll will be leading a group of eight students — four from Virginia Tech, four from Pulaski High School — on a week-long storm chasing trip beginning Friday.

The storm chasing Carroll and his group do is not daredevil stuff. It's serious science and a public service. The students look at each day's weather data and, as a group, decide where to go that day to have the best chance of intercepting severe thunderstorms. Also, Carroll's crew acts as storm spotters, helping local officials and the National Weather Service track severe weather.

The group will be traveling in two vans equipped with wireless Internet, two-way radios between the vehicles, CB radios, NOAA weather radio and police scanners.

Last year, Carroll recalls that the chasers were following a storm in Texas when they heard on a radio that a tornado had been spotted in it. An official asked over the radio if there was another spotter near the storm to confirm the sighting. Once Carroll and his fellow chasers got past a grove of trees — "the only trees in Texas," somebody can be heard joking on a videotape playback of the chase -- they were able to report back and confirm the sighting.

"Five seconds later, the weather radio goes off and there's a tornado warning. Ten seconds later, we hear the sirens go off in the town down the road from us," he said, referring to the tornado warning sirens many Midwestern towns use.

Last year's chase was one of Carroll's most successful, with one tornado spotted and photographed and thunderstorms pursued every day of the trip. Most years, there are some days when there are no storms to chase, and the group takes in tourist destinations like Carlsbad Caverns instead. While there are almost always thunderstorms somewhere to go after, tornado encounters are rare, and there's always a slight chance of a dull, stormless week. All part of the adventure, chasers say.

"If you catch a big fish every time you go fishing, then nobody would ever want to go," said Seth Price, a senior material sciences and engineering major at Virginia Tech who is responsible for the group's radio equipment.

Price has set up a way for you to follow the group's progress: At www. n3mra.com, the group's position will be pinpointed on a radar map, showing how close it is getting to storms. We'll set up a link on roanoke.com to help you follow the group's chase.

One hail of a storm

Sometimes, the chaser doesn't go to the storms, but the storms come to him. Carroll said that returning from Richmond on Sunday, he encountered two severe hailstorms: One in Bedford County near Montvale, the other as he approached his home in Blacksburg.

A number of you, particularly in Montgomery and Bedford counties, experienced severe weather on Sunday. Because of thunderstorms' slow movement, some places received hail for a long period of time, causing enough to accumulate to cover the ground in some areas.

The thunderstorms that developed over the area on Sunday were "pulse-type" thunderstorms. These storms form rapidly from updrafts and then collapse even more rapidly. When they collapse, they bring an enormous push of wind, heavy rain and hail. The updrafts were caused by afternoon heating and enhanced by the area's mountainous geography. When the sun went down, the heat source for the updrafts was turned off, and the storms soon died.

David Wert, chief meteorologist at the National Weather Service in Blacksburg, said that dry air aloft in the storm caused cold air to dip unusually low as moist downdrafts were pulled through it. This enabled hail to form much lower to the ground than would normally be expected in moderately intense May thunderstorms, and as a result, much more of it was able to reach the ground.

The "heat dome" high pressure area I talked about last week is still over us and will give us a summerlike week. But while this big tent high in the atmosphere will keep a lot of the big birds from swooping down on us, a few flies and mosquitoes will be able to get under it. With this kind of warmth, any small bug in the atmosphere can kick off a few "pulse-type" storms.

May 6, 2004

BUILDING HEAT

You've heard of the Blue Ridge. How about the "heat ridge?"

Between the severe weather outbreaks of spring and the brunt of hurricane season in the fall, often the only burning question in the weather world is whether a heat ridge will build.

We could see a little version of it over the next few days.

A strong high pressure ridge over the southeastern United States promises to bring us a few days of warm, dry weather. A cold front will try its hardest to sneak in from the north and trigger thunderstorms on Saturday, but as of this writing, it looks like the high will be strong enough to keep the front away.

A high pressure area is a mound of stable air that sinks toward the ground, thereby cutting off the rising air or convection that could cause showers and thunderstorms to form.

In the bigger picture, high pressure areas are boulders in the stream that air must flow around. The stormy stuff goes around highs, not through them.

In winter, high pressure systems sinking down from Canada bring some of our coldest weather. But the more vigorous winter jet stream keeps these highs on the move, and the cold air they bring moderates with each day of sunshine.

In warmer seasons, highs often develop from the south or southwest, allowing warm air to expand northward. As the jet stream begins retreating north for the summer, the highs also have more of a tendency to stay put, and this allows the air under them to heat with day after day of sunshine.

In summer, a high can get stuck for many weeks, and the air under it stagnates, becoming progressively more hot and hazy. This is often called a "heat ridge" or "heat dome." Its size, duration and location determine what kind of summer it will be for various locations around the continent.

In most summers, two or three of these "heat domes" build at different times, but they shift around and occasionally get knocked down by stronger cold fronts from the northwest.

Many summers a sizeable heat dome will build over one part of the country and stick around for at least a couple of weeks. When this happens, a portion of the nation will experience a heat wave with many days of temperatures at or above 100 degrees. Perhaps once every 20 years or so, a massive heat dome will cover more than half of the country for several weeks. These are years of massive heat waves and droughts that engulf many states.

Were it summer, with a weaker jet stream, an even higher sun angle and longer days, this weekend's high might build into a longer lasting heat dome. It will be enough for some summerish days into early next week. Questions linger about just how long this high will hold before the next cold front is able to move through and bring showers and thunderstorms and some cooler air.

It does appear that the forces arrayed against the high will be too strong for it to hold long. Or it may reposition itself west or south just enough to allow storms to start spilling over the top of it at us by the early or middle part of next week.

Still, there's a thought in the back of a weather geek's mind about whether this is a sign of an early trend, and whether we'll be seeing more and bigger heat ridges before very many more weeks have passed.

May 3, 2004

LET COOLER HEADS PREVAIL ON HEATED DEBATE

You won't see me weighing in on the global warming debate very often in this column.

That's because my focus is on the whims and wonders of day-to-day weather, not speculations over large-scale climate patterns decades and centuries from now.

There's so little rational discourse on the subject -- merely a heated exchage between those who are cocksure certain Roanoke will have the climate of Phoenix or Miami in 50 years and those who deny all possibility that there could ever be such a thing as human-affected global warming and insist that we should pump the atmosphere full of whatever pollution we see fit.

My one lasting statement on the issue is this: It's ignorant to say mankind's activities have absolutely no effect on weather and climate; it's arrogant to say mankind's activities are the primary driver of weather and climate.

What brings it to mind today is a pair of articles that appear on page 18 of Sunday's "A" section of The Roanoke Times, discussing the drought and warming that has been observed of late in the Western states. The implication in one of the Associated Press articles seems to be that five years of drought and spiking temperatures in the West are strong evidence, if not outright proof, for human-induced global warming.

The problem with suggesting this is that it's a partial glimpse into a tiny interval of time over a small portion of the planet. The article fails to mention that, while the West has fried and dried especially in the last couple of years, the East has shivered and shuddered under abnormally cool, wet conditions in some of the same time period. It was one of the coldest Januarys on record in the Northeast, according to the National Climatic Data Center, and the winters of 2002-03 and '03-04 have been the coldest consecutive winters in Virginia in nearly 30 years, according to the Virginia State Climatology Office.

But just as a short- or mid-term hot period in the West does not prove global warming, neither does a cold spell in the East disprove it. The severe heat wave last summer that killed more than 10,000 in Europe does not prove global warming, but neither does the severe cold wave in Eastern Europe and the Middle East that caused snow in Athens and Jerusalem disprove it.

Weather is variable whatever the overall climate structure. The trick is to follow large-scale temperature averages over a long period of time. What the data seem to show is an average worldwide temperature increase of about 1 degree Celsius (1.8 degrees Fahrenheit) over the course of the 20th century. Many scientists theorize that this is primarily or entirely due to greenhouse gases ejected into atmosphere by man, while others say that it is a natural climate cycle as the planet emerges from the centuries-long "Little Ice Age" that ended in the 1800s. Perhaps it is both, a natural cycle enhanced by man's pollution.

It gets more complicated when one considers that, even if global warming is occurring, its effects on day-to-day weather in given locations are uncertain. Global warming does not mean every single place on Earth gets warmer, but just that the overall planet's average temperature gets warmer. That could mean shifting ocean and air currents that make some locations get colder.

So that's my primer on global warming. When the computer models start nailing the weather perfectly 12 hours from now, I'll take them more seriously in what they show a century from now. In the meantime, can we cool the arguing and try to find some ways not to fill the air with so much bad stuff?

HOT AND COLD

Following a column about cold snaps and heat waves, I should note that we are likely to have both over the next few days. You're probably experiencing the cold snap as you read this on Tuesday, and some of you in the deep valleys to the west of Roanoke may even have frost or sub-freezing temperatures. By the weekend, though, high pressure builds in and could send our temperatures soaring into the 80s. We'll follow up on this later in the week.