Climate of the Midwestern U.S.

Present climate of the midwestern U.S.

Nearly all of the Midwest has a humid continental climate, describing temperatures that vary greatly from summer to winter, and appreciable precipitation year-round. Southernmost Illinois and Indiana are closer to a humid subtropical climate.

While averages are important factors in describing climate, the Midwest has unusually extreme annual variation in temperature. At an average temperature of 10°C (50°F), it seems similar to that of England, which has an average of 8°C (47°F). But England’s average high temperature of 21°C (70°F) and low of 2°C (35°F) is more indicative of how different their climate truly is. Average highs in the Midwestern states are around 29°C (85°F), with lows around -9°C (15°F), a variation fully twice as great as England’s. Furthermore, each state has record high temperatures of more than 43°C (110°F) and lows of less than -34°C (-30°F), a variation of a whopping 77°C (140°F)!

The Midwest is one of the most productive agricultural areas in the world, and the economies of its states depend on farmland. Its excellent soil, relatively flat geography, and bodies of water make it uniquely suited to cropland. Yet without a humid climate with warm summers, agriculture here would be completely different. It is one of the few places on Earth where enormous fields of corn and soybeans can be grown with little or no irrigation.

Severe weather

In part because of its extreme temperature variation and humidity, the Midwest experiences nearly every variety of severe weather. Because the states are so far from the coasts, they rarely experience hurricanes, but heat waves and cold snaps, droughts, floods, blizzards, and tornados are all fairly regular events.

Thunderstorms and tornados

The geography and climate of the Midwest are nearly ideal for their formation of thunderstorms and tornados, especially in the summer. Storms form when there is strong convection in the atmosphere. Convection occurs when buoyant warm air rises (moves up) while denser cool air sinks (moves down). Convective mixing forces the moisture in warm air to condense as it comes into contact with cool air, forming vapor (clouds) and precipitation (for example, rain or hail). A strong difference in air temperature at different heights creates instability; the warmer the air near the surface is relative to the air above it, the more potential (stored) energy the warm air has to move up, and the more potential for a storm. The final ingredient is wind. Wind moves the air, promoting mixing. The formation of precipitation also causes electrical charging of particles in the atmosphere, which in turn produces lightning.

Typically, a storm blows itself out once the warm air has moved up and the cool air has moved down. Convective mixing stops because the vertical column of air has turned over so that the cool air is at the bottom and the warm air is at the top. The directions of air currents in the Midewest, however, promote mixing. The Midwest receives warm, moist air from the Gulf of Mexico, and cold, dry air from the Rocky Mountains and Canada. The warmer, lower air from the Gulf is moving north, constantly adding more heat and moisture. The colder, higher air is moving west. Where these air masses meet, vigorous mixing causes storms. The inputs of warm, dry air from the Gulf of Mexico allow these storms to persist and strengthen.

This movement in different directions is also the reason for the Midwest's unusually high incidence of powerful tornados. Tornado Alley is a nickname for an area extending from Texas to Minnesota (including the western Midwest) that experiences a high number of exceptionally strong tornados. Correcting for size, Indiana and Iowa are the states with the third and fourth most tornados, respectively. A few other places in the world see tornados more frequently per given area, but those in central North America tend to be much more powerful. The thunderstorms discussed above can sometimes produce dozens of violent tornados, called tornado outbreaks.

Polar jet stream and winter storms

Harsh winter storms are a fact of life in the midwestern U.S. These storms are carried in by the polar jet stream, which typically falls near or over the area, especially in the winter. Jet streams—more than one exists—are narrow bands of fast moving air high above Earth's surface. The Earth’s rotation drives these rivers of air and causes them to blow from west to east. Jet streams are typically found between 6 and 13 kilometers (4 and 8 miles) above the surface and can move at speeds tens to hundreds of kilometers (miles) per hour.

Because jet streams separate warm and cold air masses, their movements influence the weather. Polar jet streams are typically found between 50° and 60° North and South latitude, and subtropical jet streams are typically found around 30° North and South latitude. As the boundaries between hot and cold air are sharpest in the winter months, jet streams are stronger in the winter.

In the midwestern states, the polar jet stream combined with lake effect snow (discussed below) strongly impacts the area’s weather. All of the states of the Midwest experience winter storms to some extent, during which several inches of snow falls. While inconvenient and damaging to infrastructure, these storms do not frequently cause widespread disruption, as residents and governments are usually prepared to clear roads and repair damage. Additionally, schools have snow days calculated into their schedules. Ice storms can be more dangerous; as rain freezes to trees, power lines, and rooftops, they may collapse under the weight. If you are able to free your car, the icy roads are even more dangerous to drive on than snowy ones.

The polar vortex

The polar vortex is a pattern of winds around the North Pole, including the polar jet stream. In the winters of 2013–2014 and 2014–2015, the polar vortex shifted southward, bringing unusual weather patterns to much of North America. Weaker polar vortices can occur when weather near the pole is warmer than usual, and a weak polar vortex allows for a wandering jet stream. Some climate scientists believe the unusual winters of recent years are explained by natural variations, while others suggest that they could be driven due to decreases in sea ice and faster increases in arctic temperatures when compared to areas at lower latitudes.

Lake effect snow

The Great Lakes create an interesting phenomenon primarily on their eastern shores, mainly affecting Michigan as well as parts of the Northeast, known as the lake effect. During the winter, the large volume of water in the Great Lakes acts as a reservoir of heat, making the air above it relatively warm and humid. When cold air moves across the warmer lake, convection begins, and, as described above, a storm can form. The moisture from the lake begins to precipitate soon after the air cools. The Upper Peninsula of Michigan usually receives over five meters (200 inches) of snow per year, second only to Tug Hill Plateau in New York, which also gets lake effect snow. The highest recorded winter snowfall total was 9.9 meters (390.4 inches) in 1978 to 1979 in Keweenaw County.

Past climate of the midwestern U.S.

This section covers the climate of the midwestern U.S. through the Phanerozoic, from about 541 million years ago to the recent. For the climate on early Earth, see the Intro to Climate section.

Paleozoic (541 to 252 million years ago)

Cambrian to Ordovician (541 to 444 million years ago)

At the beginning of the Paleozoic era, climates across the world were warm, and North America was located in the low and warmer latitudes of the Southern Hemisphere. As the Cambrian progressed, North America moved northward; by about 480 million years ago, what would become much of the midwestern U.S. was located between the Tropic of Capricorn and the Equator. Marine invertebrate fossils reveal that most of the area was probably covered by warm, shallow seas from the Cambrian into the Ordovician.

During the middle Ordovician, much of the Midwest was covered by very pure, quartz-rich sand, which is today makes up the St. Peter Sandstone. Quartz is extremely resistant to weathering, and it is often the last mineral left when other minerals have weathered away. The purity of the St. Peter Sandstone suggests that the climate was intensely wet and warm when the sand was deposited. Further, it suggests that the sand was heavily weathered, perhaps by being washed or blown (or both!) back and forth for a long time before being buried. 

Fossil evidence in Wisconsin and elsewhere in the Midwest shows that at least some of the Midwest was covered by a warm inland sea in the late Ordovician, following deposition of the St. Peter Sandstone. Limestone formed from the innumerable remains of living creatures was the predominate sediment in this sea. The sea persisted in some form for several hundred million years.

Silurian to Devonian (444 to 359 million years ago)

An ice age in the late Ordovician caused a global dip in temperature from 460 to 430 million years ago. Despite cooler global temperatures, the midwestern sea remained warm enough to sustain tropical reef ecosystems. One of the largest reefs the world has ever seen existed in the Midwest during this time. The reef grew around the shallow edges of a wide basin centered on Michigan. Today, the limestone reef deposits (as limestone) can be found in much of the Midwest. They are thickest in Indiana and Illinois, where they can reach thicknesses of 300 meters (1000 feet)! Although much of this limestone is under the surface, limestone quarries throughout the area reveal the richness of the fauna. 

Silurian deposits of salt centered on Michigan indicate that the North American climate experienced little precipitation during the warm period beginning 430 million years ago. Eventually, the salinity in the shallow seas of the ancient Midwest returned to normal in the Devonian, and a rich and diverse fauna occupied the sea floor, including reefs and other habitats. At the end of the Devonian, however, the fauna suffered a mass extinction that eliminated many of the important reef-builders and other animals that occupied the shallow seas. The causes of this mass extinction, which actually occurred in a series of steps, are still uncertain.

Carboniferous to Permian (359 to 252 million years ago)

As the continent continued across the Tropic of Capricorn and the Equator, the cycle of warming and cooling repeated yet again. By 360 million years ago, glaciers formed near the South Pole. Although the Earth’s temperature was falling during this time, the Midwest remained relatively warm.

Shallow, tropical seas continued to cover most of the Midwest until sea level dropped in the middle of the Carboniferous. By this time, complex land plants had evolved and diversified; these plants rapidly colonized the newly exposed landscape. Terrestrial fossils from this time show that the climate was humid and supported swamp forests. These swamps eventually became the coal deposits of the southern Midwest, especially in southern Illinois, Indiana, and Ohio. Farther north, the land was exposed and no rock record exists. It is likely that the area was traversed by rivers.

The ice age that had started near the end of the Devonian around 360 million years ago intensified during the later part of the Carboniferous. Deposits in the southern part of the Midwest, in particular, show a cyclicity of rising and falling sea level that was caused by advance and retreat of the large ice cap in the Southern Hemisphere. This ice age lasted well into the Permian period, ending about 260 million years ago, when warm temperatures again became the norm.

Mesozoic (252 to 66 million years ago)

By the beginning of the Mesozoic, tectonic forces had by then pushed the Midwest above sea level, and erosional forces tended to dominate, so little direct evidence of the climate during this time is preserved. Evidence from adjacent regions indicates that climate in the Midwest was probably very warm and relatively arid.

Worldwide temperatures began to dip again around 150 million years ago, though perhaps not enough for ice sheets to form. The tropics, presumably including the Midwest, became more humid. Pangaea, a supercontinent composed of nearly all the landmass on Earth, broke up into continents that would drift into increasingly familiar positions. By the Cretaceous, the world was heating up again. A new body of water, the Western Interior Seaway, covered parts of North America, leaving fossils of tropical marine animals in Iowa and Minnesota.

At the very end of the Cretaceous, the Gulf Coast experienced an enormous disruption when an asteroid or comet collided with Earth in what is now the northern Yucatán Peninsula in Mexico. The impact vaporized both water and rock, blocking out sunlight for weeks to years, leading to a collapse of photosynthesis and food webs on land and in the oceans. After this event, the climate may have cooled briefly, but it soon rebounded to a warmer state.

Cenozoic (66 million years ago to present)

Paleogene to Neogene (66 to 2.6 million years ago)

With the end of the Cretaceous and the extinction of the dinosaurs 65 million years ago, the world was cooling again. Antarctica moved south, and by 30 million years ago temperatures were low enough that glaciers were growing on its mountains. About 15 million years ago, ice covered much of that continent and had begun to form on Greenland. Eventually, by about 2 million years ago, a sheet of sea-ice formed over the Arctic, and other sheets spread over northern Asia, Europe, and North America and then pushed their way south. This is where the geologic record of climate in the Midwest picks up again.

Quaternary (2.6 million years ago to present)

Eventually, a sheet of sea ice formed over the Arctic, and ice sheets spread over northern Asia, Europe, and North America, signaling the start of the most recent ice age. Since 800,000 years ago, an equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces (i.e., the combined gravitational effects of the Earth, Sun, moon, and planets). The ice sheets in the Northern Hemisphere covered much of the Midwest.

During the ice’s maximum extent, it reached from the North Pole to where Chicago is now located, covering the northern half of the Midwest. The southern half of the Midwest was far colder than it is today. The temperatures in areas not covered in ice were moderated by the presence of ice. While the summers were much cooler, the winters were only a little cooler than they are today. The region was also somewhat wetter than it is today, with wetlands and forests covering much of what would later become grassland.

The glaciers last retreated from the Midwest around 10,000 years ago, leaving behind the Great Lakes and many geologic features that define the landscape of the Upper Midwest today, including eskers, kettles, and thick deposits of sand and gravel. The climate grew warmer and slightly drier, much like that we experience today.