High humidity levels in tropical climates are primarily a result of elevated temperatures and substantial rainfall.

Tropical climates are characterized by consistently high temperatures throughout the year, typically exceeding $18 \, \text{°C}$. These elevated temperatures enhance the rate of evaporation from various water bodies, including oceans, rivers, and lakes, as well as from the land and vegetation. The evaporation process introduces water vapor into the atmosphere, which significantly contributes to high humidity levels.

In addition to high temperatures, tropical regions experience considerable rainfall. This is largely due to the presence of the Intertropical Convergence Zone (ITCZ), a region near the equator where the trade winds from the Northern and Southern Hemispheres converge. This convergence causes the air to rise, resulting in the formation of clouds and precipitation. The frequent and intense rainfall in tropical areas further elevates the moisture content in the atmosphere, thereby increasing humidity.

Furthermore, the dense vegetation found in tropical regions plays a crucial role in sustaining high humidity levels. Through a process known as transpiration, plants release water vapor from their leaves into the atmosphere. In tropical rainforests, where vegetation is particularly abundant, this process can significantly enhance overall humidity.

Finally, the minimal seasonal variation in tropical climates contributes to the consistently high humidity levels. Unlike temperate climates, where humidity can fluctuate with the changing seasons, tropical climates maintain relatively stable temperatures and precipitation patterns year-round. This stability allows for a continuous supply of moisture in the atmosphere, resulting in persistently high humidity levels.