Weather 101 '" Another Look at Cloud Motion '" Part II

I received the following e-mail from Lance G. (Weston, WI) on July 14, 2010. "All day, we have received storm after storm. We are about to get our third line in a few minutes. I live in Weston, WI and throughout the day today, I have seen low clouds floating to the west. There was a west wind, so that was probably driving them. But, are these also fueling the storms coming from the west? And, do you know what these clouds are called?

To answer Lance's question, I first went to the Google maps web site to locate Weston, WI (Fig. 1). Weston is located just to the southeast of Wausau in east-central Wisconsin.

Then I perused archived (historical) data at the Plymouth State University web site (fourth item on the left menu). I examined surface weather maps, meteograms and upper air weather balloon profiles (known as radiosonde soundings). These and other data confirmed Lance's assessment of waves of storms coming in from the west.

Looking at the surface weather map at 1:00 pm CDT on July 14, 2010 (Fig. 2), you can see a broad fetch of southerly winds across southern Wisconsin and Minnesota. Here temperatures are in the low to mid 80's. To the north of this lies a thunderstorm-filled area (thunderstorm symbols is a letter R with a lightning arrow at the bottom of the right-most leg of the R), with temperatures (upper left value of station plotting model) at least 10 degrees cooler, and a more chaotic wind field. You can easily see this by looking at the table of Wausau, WI surface data (Fig. 3) and understanding the 360-degree compass (Fig. 4). During the two thunderstorm events (shaded), surface winds shifted from southeast (winds blowing from 100-150 degrees on a 360 degree circle) and started blowing from the northwest (300 degrees), northeast (60 degrees) and then from the west (270 degrees).

The southerly surface winds are bringing warm and humid air (the fuel, if you will) INTO the thunderstorm area (Fig. 5). Then where the thunderstorms abound, it is the storms that affect the wind pattern. You've probably experienced this many times. As the storm approaches, chillier, rain-cooled winds typically start to blow outward from the storm. Visual signs of this could be a rainshaft (Fig. 6), a curtain-like cloud that masks the sky or other clouds behind it, or a shelf cloud (Fig. 7), a horizontal (sometimes layered cloud) that arrives before the main thunderstorm.

But, these only relate to surface winds. Winds above the ground also play a significant role in the thunderstorm's evolution and movement. Just take a look at the radiosonde sounding taken at Green Bay, WI at 7:00 pm CDT on July 14, 2010 (Fig. 8). Winds vary from the southeast near the ground to mostly westerly through the remainder of the atmospheric column up to heights of 10 miles or so.

Thunderstorms on July 14, 2010 moved primarily from west to east, as Lance noted. Hence, it was the winds higher up in the atmosphere that were moving the thunderstorms.

The bottom line to this is that winds at the surface may or may not bear any semblance to thunderstorm movement. Sometimes clouds move WITH the storm; sometimes they flow INTO it; and sometimes they do both.

But, I extend a big "thank you" to Lance for his observations and for his important question.

© H. Michael Mogil, 2010

If you have a weather question, please feel free to send it to me. I'll try to answer as many as I can in online stories like this one.