Understanding the Nighttime Increase in Humidity in Tropical Regions

In tropical regions, the sudden increase in humidity that occurs just after sundown can be attributed to several factors. These factors create a noticeable change in the environment, which plays a significant role in local weather patterns and can have implications for both natural ecosystems and human activities. This article aims to explore these factors and provide a detailed explanation of why the relative humidity increases after sunset.

Cooling of the Atmosphere

After sunset, the ground and the lower atmosphere experience a rapid cooling. As temperatures drop, the air can hold less moisture. This process is known as condensation, where the air becomes saturated more quickly as it cools, leading to an increase in relative humidity. This cooling effect is a key factor in the sudden increase in humidity in tropical regions.

Radiational Cooling

In tropical regions, the clear skies often lead to significant radiational cooling at night. This process involves the cooling of the Earth's surface and the air directly above it. As the surface radiates energy to space and the air cools, it becomes more saturated, leading to an increase in humidity as the air temperature approaches the dew point. This phenomenon is particularly pronounced in areas with clear skies, where the cooling effect is more visible.

Decreased Evaporation

During the day, higher temperatures lead to increased evaporation from bodies of water and moist soil. After sunset, evaporation rates decrease significantly. This reduction in evaporation means that less moisture is being added to the air, but it also contributes to the increase in relative humidity. The air that was previously diluted with evaporated water now becomes more saturated, leading to noticeable changes in local humidity levels.

Calm Winds

Often at night, winds tend to calm down. This reduction in wind speed reduces the mixing of air layers, leading to a higher concentration of moisture near the surface. Without the turbulence that wind provides, moisture remains in a more concentrated area, contributing to the increased relative humidity.

Local Weather Patterns

In some areas, nighttime cooling can lead to the development of fog or dew, which further contributes to higher humidity levels. These local weather patterns are often a result of the cooling process and the subsequent condensation of moisture in the air.

The Science Behind Relative Humidity

In tropical jungle areas, the humidity is always high. However, the correct term is relative humidity, which means the moisture content of the air relative to the maximum saturation point of air at the current temperature. When the sun sets, the air will cool a bit. A quick drop in temperature, even a few degrees, will drive the relative humidity closer to 100%.

After sundown, the Earth radiates energy to space. This radiative cooling is more pronounced at night because there is no incoming radiation from the sun. The Earth and the air immediately above it cool due to their contact with the ground. This cooling causes the air to shrink slightly, so one cubic meter of air before sunset becomes a smaller volume but still holds the same amount of water vapor. The concentration of moisture per cubic meter of air increases, leading to higher relative humidity.

Relative humidity is defined as the partial water vapor pressure divided by the water vapor pressure there would be if the air was saturated at that air temperature. This means we need to consider the partial vapor pressure and the saturation vapor pressure.

The partial vapor pressure is the mole fraction of water vapor times the total air pressure. The mole fraction remains the same, and the total pressure is also unchanged. Therefore, the partial vapor pressure remains the same. The amount of water vapor that the air could hold at that lower temperature is now less because cooler air can hold less water vapor—the saturation vapor pressure is less. So, you have the same number (partial vapor pressure) divided by a smaller number (saturation vapor pressure), which gives a bigger number—the relative humidity is higher. This assumption holds true as long as air pressure at night is the same as during the day, and no moisture enters the system.

In conclusion, the sudden increase in humidity in tropical regions after sundown is a result of a complex interplay of atmospheric cooling, radiational cooling, decreased evaporation, calm winds, and local weather patterns. Understanding these factors is crucial for both scientific research and practical applications, such as agriculture, meteorology, and human comfort.