Weather Hazards


Weather is the measure of short-term conditions of the atmosphere such as temperature, wind speed, and humidity. The average weather of a region over decades is its climate. Although weather can vary day-to-day or year-to-year, the climate of a region is relatively stable because it represents the average weather over a long period of time. Extra-warm summers and extra-cold winters, when combined with typical seasonal change, result in a moderate average temperature over long periods of decades or centuries. Proximity to a large body of water can also decrease the temperature range of a geographic region. While the Midwest is far from an ocean, it is in close proximity to the Great Lakes;; nevertheless, states in this area experience a considerable range of temperatures over the course of a year. The greatest temperature ranges are found during the winter: The average winter temperature of northern Minnesota is -13°C (8°F) while that of areas around the Ohio River is 2°C (35°F). Weather hazards can occur fairly frequently, such as several times a year, or relatively infrequently, such as once every century.

Extreme Temperature

Extreme temperatures can create dangerous conditions for people and may lead to property damage. Heat waves are periods of excessively hot weather that may also accompany high humidity. Temperatures of just 3°C (6°F) to 6°C (11°F) above normal are enough to reclassify a warm period as a heat wave. Under these conditions, the mechanism of sweating does little to cool people down because the humidity prevents sweat from evaporating and cooling off the skin. Heat waves have different impacts on rural and urban settings. In rural settings, agriculture and livestock can be greatly affected. Heat stress recommendations are issued to help farmers protect their animals, particularly pigs and poultry, which, unlike cattle, do not have sweat glands.

The impacts of heat waves on urban settings include a combination of the natural conditions of excessive heat and the social conditions of living in a densely populated space. Cities contain a considerable amount of pavement, which absorbs and gives off more heat than vegetation-covered land does. Air conditioning units that cool down the inside of buildings produce heat that is released outside. Pollution from cars and industrial manufacturing also elevate the outdoor temperatures in cities. This phenomenon, in which cities experience higher temperatures than surrounding rural communities do, is known as the heat island effect. Other social conditions can cause an increase in the hazards associated with heat waves in urban areas. People who are in poor health, live in apartment buildings with no air conditioning, or are unable to leave their houses are at greatest risk of death during heat waves. In 1995, a heat wave impacted the Midwest, leading to nearly 740 heat-related deaths in Chicago alone. In addition to causing widespread illness from dehydration and exposure to extreme heat, the high temperatures buckled road pavement and warped train rails.

Recently, a different extreme temperature phenomenon has made the news: the polar vortex. As the name implies, a polar vortex is a regularly occurring area of low pressure that circulates in the highest levels of the upper atmosphere. Typically, the polar vortex hovers above Canada. However, a pocket of the counter-clockwise rotating low-pressure center can break off and shift southward at a lower altitude, covering the Midwest with frigid air. The jet stream then shifts to a more southward flow than usual, chilling the Midwest and even the southern states. A polar vortex can lock the jet stream in this new pattern for several days to more than a week. Extreme low temperatures can endanger livestock, and precautions should be taken regarding travel on roadways. Although the cold temperatures of a polar vortex can be uncomfortable and make traveling dangerous in the winter, the Midwest has not yet experienced any major economic or health-related impacts from this extreme weather event.

Seasonal Severe Storms

Several types of severe storms present challenges to people living in the Midwest. Summer brings severe thunderstorms associated with cold fronts. Fall and spring can bring ice storms, and winter brings the challenge of snow and, in some cases, blizzard conditions. Although rare, hurricanes moving north from the Gulf of Mexico can impact the weather in the Midwest as well. Severe thunderstorms are a common occurrence for people living in the Midwest because the conditions over the Great Plains are perfect for the development of severe weather. The flat, open fields are warmed by the summer sun, which sits high in the sky during this time of year. This results in large temperature differences when cold air masses move across the country. The boundary between the warm air and the cold air moving into a region creates a cold front.

At this boundary, denser, colder air moves in, making the less dense, warm air rise. This displaced warm air cools as it rises because air pressure decreases with increasing height in the atmosphere. As the air cools, it becomes saturated with water vapor, and condensation (the shift from a vapor [gas] state to a liquid state) begins to occur. This phase shift takes place because the cooler air contains less thermal energy than warmer air does, and this reduction in energy allows the water molecules to “link” together faster than they are torn apart. At frontal boundaries, warm air quickly rises and condenses, and clouds form. Because liquid water droplets in the clouds must be very small to remain suspended in the air, when there is a significant amount of condensation, the small water droplets come together, eventually becoming too large to remain suspended. This process leads to dramatic rainstorms.

Air pressure plays a key role in the formation and severity of these storms. Warm air has a lower pressure relative to cold air, and the movement of air from areas of high pressure to areas of low pressure generates wind. Therefore, when a cold front moves into an area that is very warm, the significant difference in air pressure will generate strong winds. The greater the temperature difference, the greater the air pressure difference and, consequently, the greater the speed at which the air will move. Wind is very common in the Midwest, and the topography of the area plays an important role in wind formation, allowing for warm air to heat up over large expanses of flat cropland without hills or mountains to influence the direction of air movement. Therefore, the Midwest has the perfect ingredients for severe weather: flat topography and large temperature differences on a day-to-day basis.

While severe thunderstorms are often a weekly occurrence in much of the Midwest, two less common storm hazards have the potential to cause serious property damage and endanger lives: derechos and tornados. Both storm events are associated with wind shear, which occurs when the wind speed or direction changes with increasing height in the atmosphere. Wind shear can happen when a cold front moves rapidly into an area with very warm air. There, the condensing water droplets mix with the cooler, drier air in the upper atmosphere to cause a downdraft.

When these downdrafts are very powerful, they can cause a derecho, or a set of powerful straight-line winds that exceed 94 kilometers per hour (kph) (58 miles per hour [mph]) and can often approach 160 kph (100 mph). These powerful windstorms can travel over 400 kilometers (250 miles) and cause substantial wind damage, knocking down trees and causing widespread power outages. The lightning associated with these intense storms can cause both forest fires and house fires. Approximately one derecho every year or two will occur in much of the Midwest (Figure 10.1). They are less frequent in the upper Midwest states, which remain cooler throughout the summers.

Figure 10.1: Derecho frequency in the continental US.

Figure 10.1: Derecho frequency in the continental US.

The differences between tornadoes and derechos are indicated in their names: the word derecho is the Spanish word for straight ahead, while the word tornado has its roots in the Spanish word tonar, which means to turn. Both types of storm events can be associated with the same major cold front boundary because they require similar ingredients to get started. However, tornado formation is a more complicated process. At the frontal boundary, warm, moist air rapidly rises as cooler, dry air descends. In the meantime, the pressure differences between the warm and cold air masses cause strong winds. As conditions in the atmosphere develop to cause a tornado, clouds with a visible horizontal rotation can appear. The clouds seem to roll like waves crashing on the shore of a beach. This horizontal motion can tilt, lifting the rotating cloud vertically, and the rolling cloud will form a tornado. Most tornados will last a few seconds to several minutes. During that time, many tornado-prone areas will use tornado sirens to alert residents of the danger. A smaller tornado might generate flying debris that can cause injury or damage to buildings, while larger tornados can cause buildings and houses to be completely broken apart. Tornados are classified by their ranking on the Enhanced Fujita scale, or EF scale. The classifications are estimates of the wind speeds based on the type of damage that is observed following the storm.

Although specific tornado paths are not predictable, the conditions that produce them are used to alert people so that they will seek shelter. The National Weather Service issues a watch, if the conditions are right for a type of storm event, or a warning, if the conditions are occurring or imminent for the storm event. The National Weather Service is part of the National Oceanographic and Atmospheric Administration, which maintains a US map of all current watches and warnings. Since the atmospheric conditions can change very quickly, an important factor in preventing loss of human life is getting the public to act upon the severe weather alerts. One way in which severe weather expert Dr. Greg Forbes has sought to improve public response to warnings is through a tornado alert index that helps people evaluate the risk of a local tornado. The Tor:Con index used by the Weather Channel provides a number from 1 to 10 that represents the probability of a tornado occurring. Meteorologists evaluate the atmospheric conditions associated with a storm and assign a score. For example, a 4 on the Tor:Con index would indicate a 40%, or moderate, chance of a tornado forming in a particular area. The hope is that by representing risk as a number from 1 to 10, people will be more likely to heed warnings and seek shelter.

Other severe weather events are more loosely associated with seasonal weather. Hurricanes occur when a warm and moist tropical low-pressure air mass forms over portions of the Atlantic Ocean south and east of Florida. These storms gather strength because the warm summer ocean water evaporates, causing very humid, low-pressure air. The air rises and condenses into water droplets that form clouds and release latent heat. The latent heat provides energy for even greater evaporation of warm ocean water, and thus the cycle continues until the low-pressure center moves over land. These storms are considered tropical depressions when wind speeds are below 63 kph (39 mph). As the storm develops a more organized structure, however, with more concentrated rising warm air in the center and bands of rain, it will officially become a tropical storm when its wind speeds reach the 63 to 117 kph (39 to 73 mph) range. Once winds have reached 119 kph (74 mph), the storm is classified as a hurricane.

Hurricanes are not common in the Midwest. However, if a hurricane is particularly strong, it can move far enough northward and inland to cause a significant rain event for areas in the Midwest. The impact on the Midwest is usually less serious than the property damage experienced along the southern and eastern seaboards of the United States. Natural hazards experienced during a hurricane are similar to those experienced during a severe thunderstorm that is accompanied by flooding.