Strange strip of cloud lines Alabama highway, see why
Meteorologists
Tuesday, December 18, 2018, 6:06 PM - The skies over Alabama did a decent job reflecting the ground on Tuesday. A very narrow strip of clouds lined up almost perfectly with the highway at one point during the morning near Tuscaloosa, making for some unusual pictures out of the city.
The band of clouds -- which look like altocumulus, or perhaps dissipating stratocumulus, clouds -- was visible both from the ground and on satellite images of the region as it cut across the northern half of the state.
So what caused it?
Visible satellite loop from Tuesday morning, image courtesy College of DuPage.
This strange feature formed ahead of what is known as a 'shortwave trough' -- a little 'kink' in the overall pattern of low pressure troughs and high pressure ridges.
This weather map, from the same time period as the satellite image above, shows the general height/pressure pattern, using the black lines that arc across the map. The heavy black dashed line, in the upper left of the map, shows the location of the shortwave 'kink' in the overall height/pressure pattern, with the arrows pointing out the outline of the region of uplift caused by the shortwave. Credit: Pivotal Weather/Scott Sutherland
Shortwaves are associated with areas of lift in the atmosphere. In this case, that lift allowed a line of clouds to condense in an otherwise pretty dry region of the atmosphere.
The sharp edges of the band are an indication of the overall dryness of the air, as well as the prevailing northerly winds at low levels. Winds from the north spreading out over the Southeastern U.S. are generally down-sloping winds, thanks to the tail end of the Appalachians. Down-sloping, or sinking, air has the opposite effect of our shortwave; it keeps the air from rising far enough to form clouds. Those northerly winds also cut off any humid on-shore flow from the Gulf or the Atlantic, preventing them from adding moisture to the region. Thus, the sky on either side of the shortwave trough is almost completely clear, while that small area of uplift still had just enough moisture on hand to generate these wisps of cloud.
As an aside, those vein-y looking lines of white that appear over southern Alabama and Mississippi at the beginning of the loop above, and fade as the loop goes on? That's fog in low-lying areas along river valleys, burning off as the sun rises.
While some of the sharpness of the feature dissipated as it drifted east and the ground and air warmed through the morning, the leading edge of the clouds eventually made its way as far east as Atlanta, too.
