Weather Systems and Severe Weather

OUR ATMOSPHERE IS DYNAMIC, featuring various types of weather systems, some relatively benign and beneficial, while others, like hurricanes and tornadoes, are dangerous and destructive. What types of weather do we experience, what causes weather, and what determines if a weather system is relatively gentle or severe? In this chapter, we explore the main types of weather systems on Earth.

Powerful tornadoes are one of nature’s most spectacular but frightening phenomena. A tornado forms beneath a thunderstorm as a swirling vortex of winds that can reach speeds of several hundred kilometers per hour. They can have a tapered, almost delicate shape, or can be thick, dark, and ominous columns filled with debris.

How do tornadoes form, how wide do they get, and how are they related to thunderstorms?

Tornadoes rampaged through the southeastern U.S. in April 2011, as part of the largest outbreak of tornadoes in history. Between April 25 and April 28, 358 tornadoes tore across Alabama and nearby states, completely destroying some houses, like the ones above, and killing more than 300 people.

How strong do tornadoes get, where do most occur, and what times of the day and year are they mostly likely to develop?

The tornadoes carved swaths of destruction, totally leveling parts of neighborhoods, while leaving nearby houses essentially undamaged, except for uprooted and toppled trees, downed power lines, and broken windows. In addition to the destruction caused by the tornadoes, deaths and damage resulted from hail, lightning, and flooding.

How do hail and lightning form, and are they related to one another?

The tornado outbreak of late April 2011 was part of a large weather system, shown in the satellite image above. A swirling storm centered over the upper Midwest (in the upper part of the image) has a distinctive comma shape, with a long tail curving to the south and southwest. The tornadoes formed in the cluster of bright clouds to the southeast — out in front — of the main system.

What is a weather system, how does one form, and why do some acquire a curved shape, like a comma, or a coil?

Weather radar systems tracked each tornado as it moved across the region, as shown by this map depicting colorful lines (red signifies a very strong tornado) of tracks for each tornado. Some tornadoes stayed on the ground for hours and crossed multiple states. Radar images are a mainstay of daily weather reports.

How do radar systems work, what do they measure, and how do such data help us anticipate a storm’s path and severity? Once a tornado or other severe storm is identified, how is the public notified?

A map of temperatures for the day of peak tornado activity shows a boundary between warm air (shown in green and yellow) in the southeast against cold air (shown in purple and blue) in the upper Midwest. A map of specific humidity for that day indicates that the warm air was also humid, but the cold air was relatively dry.

Why are some masses of air warm, while others are cold? How are boundaries between warm and cold air related to thunderstorms and tornadoes?

Regional wind directions during this time show distinct patterns, with winds blowing from the south to the north in the area affected by the tornadoes (such as Alabama and Georgia). This wind direction brought warm, moist air northward. The colors on this map indicate wind speeds, with yellow and green representing the fastest winds.

Do regional wind patterns help us predict which way a storm will move?

Heavy precipitation was concentrated in the same region as the tornadoes. In this map, purple and blue indicate the highest amounts of precipitation, which for these storms was as much as 40–50 cm (15 to 20 in.). The heavy rains, falling in a relatively short time period, produced severe flooding that destroyed roads and buildings and caused additional deaths.

Does precipitation influence the strength of tornadoes?

What Is Weather?

When we use the term weather, we are referring to conditions in the atmosphere at some specific time and place, whether it is right now, an hour ago, or sometime next week. Weather refers to the temperature, humidity, and windiness, and if it is fair or there is precipitation. It also refers to whether an area is experiencing severe storms, such as a thunderstorm, tornado, hurricane, or snow storm, or is in a time of exceptional dryness. Weather refers to the various aspects featured on these two pages, and much more! Examine the four maps on this page and figure out what the weather was like on April 27, 2011 where you live.

Weather affects all of our lives, from deciding what clothes to wear, what week to plant crops, or how soon to take shelter from some type of severe weather. In this chapter, we explore the fundamental processes that control weather and then examine the main types of weather phenomena, including rain and snowstorms, freezing rain, thunderstorms, hail, lightning, hurricanes, and, of course, tornadoes. Along the way, we explain features you can observe, like the types of clouds, to understand and predict the weather around you.