Climate types in the US: Phoenix vs. Chicago

Two popular U.S. cities that couldn't be more different, Phoenix and Chicago. But the Köppen climate system helps us understand the hot desert and humid continent climate types

It's no secret that Phoenix is hot, but late summer and the start of fall were scorchers, even by Valley of the Sun standards. Between Sept. 23 and Oct. 14, Phoenix set daily record high temperatures for 21 consecutive days. This is more than any other location since Burlington, Iowa, set record highs for 14 days during the Dust Bowl in the 1930s. Phoenix's signature misters were definitely working in overdrive!

But such is desert life. Under the Köppen climate classification system, the most popular climate classification system in the world, Phoenix is in a hot desert climate (BWh), like many other cities and towns in the Sonoran Desert.

For those who missed our last Köppen rundown, here's a refresher: In 1900, German botanist and climatologist Wladimir Köppen created a system that provides insight into seasonal rainfall patterns and temperatures that define climates.

The Köppen classification scheme divides climates into five main groups: A (tropical), B (arid), C (temperate), D (continental) and E (polar). The United States sees all types of climates, and two major cities, Phoenix and Chicago, couldn't be more different, regardless of the time of year.

Phoenix

Now the fifth-most populated city in the country with around 1.65 million people in 2023, Phoenix continues to grow as people seem undaunted by the heat, further exacerbated by the urban heat island effect.

Phoenix is also marked by a lack of precipitation, with only about 7-8 inches of rain falling yearly. But what causes this signature dryness and heat in hot deserts? Deserts form in multiple ways, including rain shadows, or when rain falls on the side of a mountain that faces the wind and dry, sinking air leaves downwind areas with sparse rainfall.

In Phoenix and the rest of the Sonoran desert, however, this isn't the main driver of dry weather. Instead, this desert climate is formed by circulating air currents called the Hadley Cell. This circulation takes place as air rises at the equator and sinks at medium latitudes, typically in a global belt about 30 degrees north or south. Phoenix is at about 33 degrees north. The sinking air from the Hadley cell is dry and can't produce rain, resulting in desert climates. High pressure also plays a role, leading to a very stable atmosphere, with many cloudless days and little rain.

"This circulation pattern, though there are short-term and seasonal interruptions that can allow for limited wet periods, is also the main producer of the Saharan and Arabian deserts in the Northern Hemisphere," said AccuWeather Meteorologist Dave Houk.

A hot desert climate tends to have two seasons: a long, hot summer and a shorter, mild winter. Phoenix has four months (June-September) when the historical average high temperature is 100 degrees or more. Even in October, the typical high is close to 90 but drops to the mid- to upper 60s in December and January, with slightly lower average highs than Los Angeles.

But unlike Los Angeles, Phoenix doesn't get almost all of its annual rainfall from winter Pacific storms. It also is impacted by the summer monsoon season, leading to flash floods and thunderstorms. "The North American monsoon typically starts in northwestern Mexico in June and expands northward into the southwestern U.S. in July before settling down in September," Houk said.

Phoenix, like many other deserts, is also marked by a high diurnal temperature range because of generally low humidity and lack of cloud cover. This means it might be warm during the day but surprisingly cool at night, particularly in the winter.

Vegetation has learned to adapt to harsh desert environments where it might not rain for months. Cacti have evolved to hold on to moisture for years. Deserts lay bare the exposed earth, with rocks and pebbles instead of green grass. It almost feels like another planet, even though it's very much our Earth.

Chicago

Chicago is known for its blustery winters and humid summers with severe thunderstorms. Under Köppen, Chicago is classified as a humid continental climate (Dfa). Deep within continents, cities like Chicago are defined by huge temperature swings from cold, snowy winters to warm summers.

Continental climates are found only in the Northern Hemisphere because south of the equator, there isn't as much landmass leading to a deep interior. Chicago's weather is affected by the warm, moist southern air clashing with colder, drier arctic air, which results in extreme weather events like tornadoes. And let's not forget about Chicago's notorious lake-effect snow.

Chicago has four distinct seasons. You'll get the fall colors, winter's snow, spring green and the lush summer vegetation. And though Chicago gets plenty warm in the summer, with heat waves well into the 90s and even occasionally triple digits, it pales in comparison to the extreme heat of the Desert Southwest. Chicago averages 21 days of 90 degrees or higher at Midway Airport. In Phoenix, it's around 160.

Dfa is the most common humid continental climate, referring to an even distribution of annual rainfall, around 40 inches per year. Humid continental climates are drier than their cousin humid subtropical climates, with New York City receiving nearly 50 inches annually.

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