Montana's Earth Science Picture of the Week

Classic Squall Line of July 8, 2002

RADAR can "see" the storm . . .
This radar image shows a squall line that swept through central and eastern portion of Montana on the evening of July 8, 2002. Blue areas indicate light precipitation, green areas indicate moderate precipitation, and the red areas show where precipitation is intense. The image was captured by the National Weather Service's RADAR device located in Glasgow. The N.W.S. has RADAR in Glasgow, Great Falls, Billings, and Missoula. These cities were selected because they are far enough from each other to give the N.W.S. good coverage of the entire state.

Somewhat unusual in these parts . . .
A "squall line" is a line of thunderstorms that forms along a cold front as cooler air pushes into very humid, warmer air. Since air in the Midwest and southeastern United States tends to be more humid than in Montana, squall lines are much more common in these regions.

Rising humid air is the key . . .
Where cooler air is pushing into warmer air along a cold front, the warmer air rises because it is lighter. As this warmer air is forced upward, it cools by expansion. Eventually the cooling causes the vapor (humidity) in the rising air to condense, forming cloud droplets or ice crystals. The changing of vapor to liquid or solid releases heat which helps the air continue to rise, and the cycle continues.

It just kept going . . .
If the warmer air is especially humid as it was on July 8, a line of dangerous thunderstorms may sweep through an area. Since this cold front moved eastward, this squall line also moved eastward, causing lightning, hail, strong winds and even a threat of tornadoes across much of Montana.

Below: This is a G.O.E.S. East Satellite view of the squall line at 5 pm MDT on July 8, 2002. The line of thunderstorms extends from southwest Saskatchewan toward northcentral Wyoming.

Image courtesy of N.O.A.A.



*Squall line explained (USA TODAY)
*Understanding weather radar (USA TODAY)
The Weather Book
Very impressive collection of weather images (Penn State)
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