Variance in Climate by US State

The State Similarity Index attempts to quantify how similar states are to each other relative to other states. The index is a statistically-based way to measure this. 20% of the index is based on geography. 25% of a state’s geography score (5% of the overall State Similarity Index score) is allocated for the state’s climate. The following factors were considered:

Average Temperature

One major factor that contributes to a state’s climate is its temperature. Interestingly enough, Florida is the hottest state on average, not Hawaii, despite it being located further to the south. Cool ocean currents in the Pacific Ocean help moderate Hawaii’s climate. In contrast, Florida is located by the Gulf of Mexico, which has a warmer current. Another reason is that Hawaii has several high mountain peaks, including Mauna Kea and Mauna Loa, which have significantly colder temperatures than the rest of the state.

It is no coincidence that most northern state, Alaska, has the lowest average temperature. In fact it is typically below freezing. Approximately 30% of Alaska is located above the Arctic Circle. Two other states that share a border with Canada, North Dakota and Maine, are also among the coldest in the contiguous United States.

Average temperature data source:
National Oceanic and Atmospheric Administration

States were categorized into the following average temperatures in Fahrenheit:
26.5-30.5, 30.5-34.5, 34.5-38.5, 38.5-42.5, 42.5-46.5, 46.5-50.5, 50.5-54.5, 54.5-58.5, 58.5-62.5, 62.5-66.5, 66.5-70.5

Average Precipitation

The average precipitation also varies greatly across the United States. The three states that tend to get the most rainfall are Louisiana, Florida, and Hawaii. These states experience a lot of precipitation due to the moist air that comes off their watery surroundings. The ocean and warm air currents keep moisture levels high in coastal regions.

Nevada, Utah, and Arizona are the driest states. They are landlocked and far away from large bodies of water. The prevailing winds that blow across the region are also dry. They lose much of their moisture as they pass over the coastal mountain ranges. The relatively high elevation in these states also contributes to their dry climate, since air is able to hold less moisture at higher altitudes.

Average precipitation data source:
National Oceanic and Atmospheric Administration

States were categorized into the following average rainfall totals in inches per year:
8-13, 13-18, 18-23, 23-28, 28-33, 33-38, 38-43, 43-48, 48-53, 53-58, 58-63

Köppen climate classification

The Köppen climate classification system is widely used to identify different climates based on temperature and precipitation. According to the the classification, the United States has five main climate types: tropical, dry, temperate, continental, and polar.

Tropical climates are found in Hawaii and Southern Florida. Dry climates are commonly found in Western states such as Arizona and Nevada. Most of the Southeastern states are considered to have temperate climates. Continental climates are commonly found in the the Midwest and the Northeast regions. Polar climates are found in Alaska.

The Köppen climate data source:
University of Veterinary Medicine Vienna

States were grouped into the following categories as a percentage of their land area:
Af, Am, Aw,
Cf, Cs
Df, Ds, Dw

Trewartha climate classification

Trewartha is another widely used system of climate classification. It was created to address the fact that Köppen classification does not quite match the vegetation in certain regions. One of the main differences between the two systems is that the Trewartha system places greater emphasis on the impact of air masses and weather patterns on climate. It is considered to be the more comprehensive system.

While Trewartha’s climate classification system is similar to Köppen’s, there are notable differences. For example, Trewartha considers the Southeast region of the United States to be subtropical due to the influence of air masses on its climate, while Köppen considers it to have a temperate climate. Furthermore, the Trewartha classifies many areas of California that are considered Mediterranean under the Köppen system as steppe or desert.

The Trewartha climate data source:
Oregon State University

States were grouped into the following categories as a percentage of their land area:
Am, Aw
Df, Ds

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