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Why do humid subtropical climates usually have hot and humid summers?

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Köppen climate classification map — Köppen climate classification The major climatic types are based on patterns of average precipitation, average temperature, and natural vegetation. This map depicts the world distribution of climate types based on the classification originally invented by Wladimir Köppen in 1900.

In the Köppen climate classification, humid subtropical climates experience hot and humid summers primarily due to their geographic location on the eastern sides of continents, where they are influenced by moist, maritime airflow.1

During the summer, these regions are dominated by the western side of subtropical anticyclonic cells located over low-latitude ocean waters. This circulation pattern transports warm, moisture-laden air from the oceans onto the land. As this air moves over the heated continental surface, it becomes unstable, leading to frequent convectional thunderstorm activity, which is a primary source of summer rainfall. In some regions, this moisture-rich environment is further enhanced by tropical cyclones.123

The combination of high temperatures—often averaging 27 °C (81 °F) with daily maxima reaching up to 38 °C (100 °F)—and the persistent influx of maritime moisture creates the characteristic hot and oppressive conditions associated with these summers.1