Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: FRIDAY, June 30, 1995                   TAG: 9506300041
SECTION: VIRGINIA                    PAGE: A-4   EDITION: METRO 
SOURCE: DWAYNE YANCEY
DATELINE:                                 LENGTH: Medium

"EVEN THE SUN IS YOUR ENEMY"

"The sunshine breaking through in spots is not necessarily a good thing," says Jerry Stenger in the state climatology office. "When the sun shines through, it heats up the ground and generates the potential for even more thunderstorms. What about that for a twist of fate? Even the sun is your enemy in these conditions."

What we need, he says, isn't a break in the clouds, it's for the clouds to go away completely. But the National Weather Service predicts that probably won't happen until Monday at the earliest.

Why do thunderstorms always seem to blow up late in the day?

The basic principle: Warm air rises.

1. During the day, the sun heats up the air, causing it to rise. If that air is wet, it rises into the upper atmosphere, cools off, and the moisture falls out as rain. The faster the warm, wet air swirls upward, the more violent the thunderstorms that erupt.

2. The ground stays warm well into the evening, generating heat that continues to boil up thunderstorms.

"That's why thunderstorms generally last on into the evening," says National Weather Service meteorologist Jan Jackson.

How come it rains so much in one place but hardly any in another spot not far away?

1. Not much wind in the upper atmosphere.

The rainclouds have been hanging over Virginia for the past week because there's not much wind in the upper atmosphere to move them along. That means when an isolated thunderstorm breaks out, it tends to linger over one spot. If there were more upper-air wind, the rain would be more evenly spread out.

2. Sometimes there's a weird pattern of lower-air winds.

That's what happened Wednesday night over the Roanoke Valley. One wind was blowing northeast from the New River Valley, another northwest from Franklin County, still a third east from Bedford County. They essentially scrunched all the rainclouds together over the Roanoke Valley, wringing them all out in one place.

Sources: National Weather Service, state climatology office