Chapter 8 of Jack Williams’ Weather Book is about thunderstorms and tornadoes. Here are some reflections on the chapter.
Page 115 – 116
This chapter opens with the setting for a series of related tornadoes in 1974, called the Super Outbreak which is well described in various places online, with beautiful pictures.
Williams offers a four-part setup:
First, the jet stream was very strong, 100 mph, and it was traveling from Texas towards the northwest, an unusual route which was set up to carry any weather system pretty fast.
Second, a strong storm was crossing the Rockies and its low pressure center was bound to pull winds together from every possible direction as soon as it reached the plains and could look around for more energy.
Third, the Gulf of Mexico was very warm and the air above it was unusually humid. As you know, humid air rises; then its water condenses and the air rise still more. This intensely, upwardly-mobile air was certainly going to be pulled into the low pressure system.
Fourth, air out of the dry southwest was also racing in, partly under the influence of the jet stream and partly seeking the low-pressure center. (JW says that the low pressure system “pushed” dry air across the Mississippi. Low pressure centers never push; they only pull; you know that. But because of the Coriolis effect, the same low pressure center can pull some winds east and others north and indeed every other direction. Hard to remember this fact.) Anyway, the dry southwest air naturally parked in the upper atmosphere because it was warm, but in doing so, it kept the moist Gulf air from climbing easily to storm heights and dissipating its energy. As a result, the Gulf air became more and more oppressive, more and more energetic, more and more determined to punch through.
When it got enough energy to break through the southwestern dry air, the wet stuff was bound to be violent, and so it was. The total system spawned 127 tornadoes, one of them ½ mile across. I didn’t know a tornado could be that wide, ½ a block is about right. This was the greatest outbreak in recorded history.
JW has some very nice graphics on the storm setup and the tornado tracks.
Normally, thunderstorms thrive where the air on the ground is warm and the air aloft is cold; this is called “unstable air”. It favors storms because the warm air rises, the water condenses out; the air rises more, the thunderhead forms and the rain falls. Straightforward. Remember the term unstable air.
A graphic on the formation and dissipation of a single-cell thunderstorm dominates this page. It is well worth studying. If you want a second image, go to the National Weather Service’s website and look at their drawing. Note particularly that while a thunderhead has a strong updraft from the rising of warm, moist air, it also develops a strong downdraft when rain forms. The falling rain pulls air down with it, and when this cool package reaches the ground, the rain just soaks in, but the cold air flows out away from the storm, sometimes for many miles. That’s the source of the cool breeze that you feel as a storm draws near. It is the outflowing of a cold downdraft from a storm cell in the distance.
The term “dryline” refers to the place where warm wet air meets warm dry air. JW wonders why this is a boundary that triggers storms. The reason seems to be that the dry air (out of the southwest) is already high and it puts a lid on the warm moist air so that it can’t rise easily and make normal, small clouds and storms; so when it finally does punch through, it is violent.
The graphic on page 119 shows how the downdraft of one storm cell be transformed into a bust that goes out to meet a body of rising warm air, plows under it or smashes against it, and causes a strong updraft to form, thereby building a new storm cell.
JW lists the conditions for a the worst storms, “killer storms”. This list seems contradictory (for example, it requires stable and unstable air) but the different weather conditions he describes must come together from different directions as he outlined for the Super Outbreak at the start of the chapter. The important new information is that conditions such as a jet stream above a storm cause the updraft to tilt; in this way, they keep the downdraft away from the updraft. Cold downdrafts running into the updraft of a storm cell are the death of storms; separating the two allows the storm to keep going. This is very important to keep in mind as we go on.
The top of page 121 is a graphic of a squall line. A squall line is a series of thunderstorms several miles apart, lined up along a cold front or often several miles ahead of a cold front. He does not give their distance apart, but I believe it is about 10 miles.
The individual thunderheads in a squall line are higher than usual, about 50,000 feet, which means they have pushed well above the usual limit of the troposphere. Remember that one mile is about 5,000 feet, so this is close to ten miles high; the troposphere is that high only over the equator. This means that pilots cannot fly above this storm but must go around it. Airplane require “air” to fly, and at this altitude there is not enough.
In almost a casual manner, the text now introduces an important new concept: the downdraft. You already know that when warm, humid air rises, the water vapor condenses and releases new heat into the atmosphere, causing the air to rise yet faster. But now we learn that when cold wet air (full of small droplets) falls into dry air, the droplets evaporate, causing the cold air to get colder and therefore denser, and making it fall yet faster.
This graphic about tornados is more statistical then meteorological, but look it over and see what you find interesting. Note: he shows the direction of the tornado’s spin, so look at it. Is it clockwise or counter-clockwise? Why? If you don’t know, ask Corey.
Here is a two-page diagram on the mesocyclone, the mid-size storm system, larger than a thunderstorm, smaller than a hurricane. Look over this diagram (or the one on the National Weather Service page given above) and then go to the storm chaser’s log and look at the pictures. This brings the diagram to life in a most unexpected way. How beautiful; how awesome!