His home phone rang around 5 a.m. yesterday and Calvert County Schools Deputy Superintendent J. Kenneth Horsmon picked it right up. After more than a week of dealing with snow, he knew who was calling.

Supervisor of Pupil Transportation Brian W. Stevens dove into a verbal roundup. The county's only north-to-south artery, Route 2/4, was in excellent condition, plowed and salted, despite a light snowfall overnight. But some of the roadway shoulders where children await their buses were snow-covered. Roads in private communities and select cul-de-sacs remained white. And the narrower county roads, about 15 in all, were 80 percent plowed.

Horsmon and Stevens hashed out the scenarios. They talked at length about salvaging the school day with a two-hour delayed opening. That would allow county workers to finish clearing the roads, and give the district's maintenance staff time to attack the school parking lots and bus loading zones.

But there was a wrinkle: The district had previously scheduled a two-hour early dismissal for teacher training. To cancel something months in the making would cause even more havoc. And so Stevens recommended that the schools close for the day. Horsmon agreed.

Another day, another snow day.

It's the one course that school superintendents with PhDs can't take -- how to decide to close school on account of snow. Tornadoes and other violent expressions of nature are more clear-cut. Snow is light, heavy, wet, flaky, slippery, and even shows up as ice or slush. It's moody. In that early morning phone call, a school superintendent needs to know, what kind of snow are we dealing with this time?

The Southern Maryland schools -- Charles, Calvert and St. Mary's counties -- decided to close school yesterday, while most other Maryland districts stayed open.

"Our main roads were clear, but our secondary roads had ice overnight," said J. Bradley Clements, St. Mary's County schools chief administrative officer, who supervises the transportation and maintenance departments. "The temperatures were around freezing and not changing a whole lot, plus the call for more snow."

To understand the St. Mary's decision, made about 10:30 p.m. Wednesday, is to understand the geography of the skinny, sprawling county. There are two state highways, routes 5 and 235, that run north to south. They are both four-lane highways in parts of the county, but not throughout.

As a result, most of the county's 185 school buses navigate secondary roads to pick up students. In fact, 75 percent of the bus travel is on secondary roads.

Some of the roads are quite rural -- windy, low-lying, with narrow or no shoulders in some cases, Clements said. "We want to go to school every day we can," he said. "I think we know what the impact is if we [cancel], but I think we're looking at the safety of the kids."

In Howard County, it was a similar story, but with a different ending.

At 3:30 a.m. yesterday, all was clear.

Glenn Johnson, transportation director for Howard schools, clicked away at his computer while his high school-age son slept peacefully, awaiting his fate. The storm was brewing, and a word from Johnson could have led to the district's 10th snow day of the school year.

He checked an online weather radar. He checked the temperature of the 17 weather sites at schools across the county. He checked his e-mail for an AccuWeather update. Nothing new; the storm was still scheduled to hit late in the afternoon. But Johnson hit the road in his four-wheel-drive Chevy Blazer anyway.

From 3:45 to 4:45 a.m., Johnson and five other school officials braved the county's icy roads. They covered 25 to 30 miles apiece, staying in touch with two-way radios and cell phones as they looked for large drifts, blocked roads and, most importantly, snow plows.

After a stop at a 24-hour gas station for a cup of coffee, Johnson met his team at the Board of Education office in Ellicott City to debrief. They conferred with police, the state highway administration, county officials and neighboring districts. County trucks had been out salting the roads since 3 a.m. The light snow that had fallen the day before had turned dirty and slushy, but not too dangerous. At 5 a.m., Johnson called Superintendent John O'Rourke, who was awake, and recommended that schools open on time.

"Every decision is a new decision," O'Rourke said. "We make it understanding that we've missed a lot of time. But we'll also make it on its own merits . . . with our bias to try to find a way to hold school."

By 6 a.m., bright yellow school buses were rolling.

For some, a snow storm means a messy walk home. For school officials, it means another long night and more agonizing decisions.Before she can continue on her way home, Mary Hundt scrapes snow off her windshield near Glebe Road and Route 50 in Arlington. Early last night, many districts were still weighing the decision about whether to hold classes today.