ÌÇÐÄÊÓƵ

A daily mismatch between temperature and humidity, observed in certain mountain and waterside regions, helps regulate atmospheric dryness. According to a new published in Science Advances, this protective effect may weaken under global warming.

Most people assume that temperature and relative humidity always move in opposite directions throughout the day: when the air warms up, it becomes drier, and vice versa. This common pattern makes the air driest in the afternoon, around the time of daily maximum temperature.

However, researchers at the Department of Atmospheric Sciences and the Department of Geography at National Taiwan University, studying the cloud forest in Chi-Lan, Taiwan, discovered something unusual. In this forest, the driest time of day occurs before the temperature reaches its peak.

This means temperature and humidity are mismatched, a pattern the researchers call "diurnal T/RH hysteresis." This daily time lag between temperature and humidity alters the typical course of atmospheric dryness, which can affect local water cycles and reduce stress on plants.

Inspired by the findings in Taiwan, they expanded the analysis to weather data around the world. They found similar mismatches in several waterside and mountainous regions, especially in the Andes, the Himalayas, southwestern Arabia, and New Guinea.

Many of these areas are cloud forests or fog-prone environments. The hysteresis appears to be influenced by local valley or sea breezes that transport moisture, along with surface evaporation, creating a mismatch between temperature and humidity.

The researchers found that this hysteresis pattern reduces peak atmospheric dryness during the day, potentially protecting plants from moisture stress. However, as the climate continues to warm, this natural buffering effect appears to be weakening. Ecosystems that once benefited from it may now face increased risks of dryness, heat stress, and extreme climate.

"Regions exhibiting diurnal mismatch between temperature and air humidity may serve as important indicators for early detection of climate-related risks and deserve sustained attention and monitoring," say Ph.D. student Ching-Hung Shih and Prof. Min-Hui Lo.