METR 104:
Our Dynamic Weather

(Lecture w/Lab)
Summary of Class Meeting

for Monday, March 6
Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Spring 2012

On Friday, March 3, several tornadoes struck southern Indiana, Northern Kentucky, southern Illinois, northern Alabama, and North Carolina. It is common nowadays for people to capture some of the events on video. It was a violent start to the spring severe storm season in the central, southern, and eastern U.S. Several small towns were leveled or badly damaged, and at least 39 people were killed.

Tornadoes are produced by a class of thunderstorms called severe thunderstorms. In contrast to ordinary thunderstorms, which might last an hour or less, severe thunderstorms rotate, which allows them to organize themselves in such a way that they last much longer (in some cases many hours) and, in a few percent of cases, produce a tornado.

Thunderstorms are tall, precipitation-producing clouds in which air enters the base of the cloud (the "inflow" in the sketch diagram below) and rises rapidly, reaching altitudes in the upper troposphere or lower stratosphere, so they can be anywhere from 10 km to as much as 15 km tall or even taller. Much of the stratosphere is basically a deep temperature inversion layer, and air has a hard time rising (and sinking) through inversion layers, so air in thunderstorms that rises into the lower stratosphere quickly loses momentum, stops rising, and begins spreading out sideways. As a result, thunderstorms sometimes have a long, flat extension downwind from the top, giving the cloud the appearance of an anvil. (The cloud extending laterally downwind from the top of a thunderstorm is called an anvil cloud.)

Severe thunderstorms can be anywhere from a couple of kilometers to 10 km (a mile to 6 miles) wide. (Ordinary thunderstorms tend to be smaller.)

Sketch diagram of a severe thunderstorm:

Air flowing in toward the base of a severe thunderstorm begins to rotate, forming a column of rotating air. As the air rises it accelerates, which stretches the column and causes it both to narrow and to rotate faster. The result can be seen as a funnel-shaped cloud below the base of the thunderstorm, or if the rotating funnel cloud reaches the ground, a tornado (as shown in the diagram). Wind speeds in the smallest, most rapidly rotating part of a tornado can reach 200 mph and more. Although a tornado is a small feature of the larger severe thunderstorm—it might be anywhere from a hundred yards to, in extreme (and rare) cases, a full mile across—its fast winds makes it potentially one of the most deadly natural phenomena.

Tornadoes are more frequent in the U.S. than anywhere else in the world because conditions in the U.S. are more often favorable for the development of severe thunderstorms than just about anywhere, especially in the midwestern U.S. (though the southeastern U.S. gets its share, too) in the spring.

Favorable conditions for Severe Thunderstorms

Although only a few percent of severe thunderstorms produce tornadoes, and it's very difficult to anticipate which severe thunderstorms might actually produce a tornado, it is possible to identify conditions that favor the development of severe thunderstorms. This makes it easier to warn people to be alert to tornado warnings, should they be issued.

Conditions favorable for severe thunderstorms include:

  1. A large environmental lapse rate in the troposphere. (In Thought Questions on Radiosonde Soundings, you learned that the environmental lapse rate is the rate at which air temperature decreases with increasing altitude, as it might be measured by a thermometer attached to a rising balloon, such as a radiosonde.)

    1. One way to make temperature decrease more rapidly with increasing altitude is to warm the bottom of the troposphere but not the middle or top. This might occur when the sun heats the ground strongly, for example, so air in the lower troposphere warms but air higher up doesn't.

      [As you know, as winter progresses toward and into spring, the sun angle at solar noon and other times of day, and the number of hours of daylight, both increase. That means that the sun is becoming more intense and shines for more hours each day, both of which tend to cause the ground to warm more and leads to greater heating of the lower troposphere. Hence, late winter and spring are times when the environmental lapse rate begins to increase as a result of greater heating of the ground by the sun.]

    2. Another way to make temperature decrease more rapidly with increasing altitude is if air in the middle and upper troposphere moves away and is replaced by colder air from elsewhere, while air in the lower troposphere remains.

  2. Forced lifting of air in the lower troposphere.

    1. There are several situations where this might happen, one of which we'll mention here. Relatively cold air in the lower troposphere in a "tongue" of cold air along the polar front is more dense than relatively warm air in an adjacent "tongue" of warm air. When a cold tongue shifts eastward and shoves against a warm tongue ahead of it, the denser, colder air forces the warmer, less dense air upward. As illustration, the figure below shows an east-west cross section across an adjacent pair of cold and warm tongues. The leading edge of the advancing cold tongue is called a cold front.

      [As we discovered earlier this semester, the polar front tends to shift northward from winter to spring and into summer, by which time it tends to be located well north of much of the U.S. (It also becomes weaker.) Hence, late winter and spring are times when alternating tongues of warm and cold air, and hence cold fronts, still sweep across the U.S., forcing air upward along the leading edges of the cold tongues.]

      The map below shows the temperature pattern at an altitude of around 10,000 ft. (where the pressure is about 700 mb), at about the time that tornadoes struck southern Indiana (early in the afternoon of March 2, 2012). Notice the large "tongue" of colder (blue) air west of, and approaching, Indiana. Notice also the narrow bands of color packed together around the edge of the cold tongue, separating it from warmer air to the west, south, and east. (The zone of narrow color bands outlining the cold and warm tongues and separating them from each other, is the polar front.) The section of the polar front along the eastern edge of the cold tongue, where the cold tongue is advancing eastward, is called a cold front. A cold front was approaching Indiana at the time shown, forcing air upward in the region and enhancing the chances of thunderstorms developing.

  3. Lots of water vapor near the earth's surface.

    1. One measure of the amount of water vapor in the air is dew point temperature. (High dew points mean more water vapor is in the air.) The main source of water vapor in the air is evaporation of liquid water, especially from the oceans, and especially from warm ocean surfaces (but also from lakes, rivers and streams, vegetation, and soil).

      The Gulf of Mexico is a relatively warm body of water off the coasts of Texas, Louisiana, Mississippi, Alabama, and Florida. In late winter and early spring, as the sun rises higher in the sky each day and the days become progressively longer, the already warm Gulf of Mexico becomes warmer, and more water evaporates from it into the air. Air carries this water vapor with it if and when it moves from the Gulf of Mexico into the southeastern and midwestern U.S., raising the dew point temperatures there and contributing to conditions favorable to the development of severe thunderstorms in those areas.

      The map below shows dew point temperatures near the earth's surface at about the time that tornadoes struck southern Indiana (early in the afternoon of March 2, 2012). On the map, red colors represent higher dew point temperatures, while blue colors represent lower dew point temperatures. Notice the "tongue" of air with relatively high dew point extending from the southeastern U.S. into Indiana.