The climate in the United States of America

The United States of America, with its size of almost 10 million km² and a wide variety of landscapes, extends over numerous climate zones. Northern Alaska has an arctic climate with temperatures down to 30°C below zero, while most of the land mass of the USA is in a continental temperate climate zone. A completely different climate prevails in the southern sun states. While the Californian Pacific coast is relatively mild and Mediterranean with an annual average temperature of 17 °C, the Californian deserts and mountains have significantly higher temperatures. All in all, it is warm all year round in all southern states.

On the opposite side of the USA on the Atlantic coast of Florida, the temperatures are noticeably higher again, but the rainy days are also more numerous. Here the climate is determined by the Atlantic currents and it is raining in the summer months. While spring is the driest time in Florida, autumn can be quite unpleasant with its hurricanes. It has a tropical climate.

In the interior, the up to 4,400 meters high Rocky Mountains share a large part of the country. The mountain range running in north-south direction also separates climate regions from each other. To the west of the Rocky Mountains, rain-rich low pressure areas are formed, which bring cool air masses from the Pacific and cause frequent rainfalls. On the eastern side, on the other hand, there is often drought at higher temperatures and lower humidity.

In contrast to Europe, there is no east-west mountain range in the USA. The Rocky Mountains separate the more western parts of the country from Pacific weather influences. Cold currents from the north and warm air masses from the south can move almost unhindered through the country. Thus, larger cold air infiltrations are felt up to the southernmost regions.

Last but not least, the USA has four large desert areas far away from the coasts (Chihuahua, Mojave, Sonora Desert and Great Basin Desert), which rarely consist of Saharan-like sand landscapes, but nevertheless have their individual mesoclimatic characteristics and vegetation. Famous is the Death Valley in the Mojave Desert, where the temperature reaches up to 50°C during the day.

Temperature records of the last 74 years

The hottest temperature measured from 1949 to January 2023 was reported by the Montgomery, AL weather station. In June 2007, the record temperature of 54.4 °C was reported here. The hottest summer from July to September, based on all 309 weather stations in the United States below 2,300 meters altitude, was recorded in 2022 with an average temperature of 23.2 °C. This average temperature will normally be measured every four to six hours, thus also including the nights. Normally, this value is 21.8 degrees Celsius. The average maximum daily temperature at that time was 29.3 °C.

The coldest day in these 74 years was reported by the Mc Grath, AK weather station. Here the temperature dropped to -51.9 °C in February 1999. Mc Grath, AK lies at an altitude of 103 meters above sea level. The coldest winter (January to March) was in 1971 with an average temperature of 0.1 °C. In the United States, it is usual to have about 3.2 degrees more at 3.3 °C for this three-month period.

The most precipitation fell in August 2017. With 44.8 mm per day, the Port Arthur, TX weather station recorded the highest monthly average of the last 74 years. Incidentally, the region with the most rainfall for the whole year is around Yakutat, AK. The driest region is near Las Vegas, NV.

Long-term development of temperatures from 1979 - 2022

In contrast to single record values, long-term development cannot simply be brought about by all weather stations in the country. Both the number and the locations are constantly changing. An average value would give a distorted result. If several measuring stations in particularly cold mountain or coastal regions are added in one year, the average would decrease as a result of this alone. If a station fails during the summer or winter months, it does not provide any values and distorts the average again. The subsequent long-term development was therefore reduced to only 25 measuring points in order to have comparable data over as long a period as possible.

In the years 1979 to 2022, there were only these 25 weather stations in the whole country, which reported continuous temperature values. From these weather reports, we have created a long-term development that shows the monthly average temperatures. The hottest month in this entire period was July 2022 at 25.2 °C. January 1979 was the coldest month with an average temperature of -1.0 °C.

The average annual temperature was about 12.8 °C in the years after 1979 and about 13.9 °C in the last years before 2022. So in less than 44 years, it has increased by about 1.1 °C. This trend only applies to the selected 25 weather stations in the United States of America. A considerably more comprehensive evaluation of global warming has been provided separately.

Data basis and methodology

The data from the individual measuring stations are based on the archives of the German Weather Service, individual values averaged and supplemented by own elements. In order to determine a representative national average, average values were first calculated for each part of the country, which were then summarized at the national level. Thus, if a disproportionate number of weather stations are located in a small area, their number does not affect the national average. There are 310 stations in the United States of America itself. In 13 cases, neighboring but nearby weather stations were also used to obtain more accurate values.