Occult precipitation

Occult precipitation refers to the deposition of atmospheric moisture onto surfaces through mechanisms other than direct rainfall. It includes fog interception by vegetation, dew formation, and direct cloud water deposition. Unlike conventional Precipitation, occult precipitation is not captured by standard rain gauges and requires specialized measurement techniques.

Mechanisms

Three main processes contribute to occult precipitation:

1. Fog interception: Wind-driven fog droplets impact vegetation surfaces and coalesce into larger drops that drip to the ground. This process is most effective on fine-structured vegetation such as epiphytes, mosses, and conifer needles.

2. Cloud immersion: At elevations within the cloud base, cloud droplets deposit directly on all exposed surfaces. This is the dominant mechanism in tropical montane cloud forests and on the highlands of the Galapagos Islands during the Garua season.

3. Dew formation: Radiative cooling of surfaces below the dew point causes condensation from ambient air. While volumetrically smaller than fog interception, dew can be ecologically significant in arid environments.

Significance in the Galapagos

In the Galapagos, occult precipitation is hypothesized to deliver a significant fraction of the total water input to highland ecosystems, particularly during the Garua season when conventional rainfall is minimal. The DARWIN project tested this hypothesis using a combination of field measurements and dynamical downscaling (see DARWIN publication thread).

In public-facing form, this research is represented through generalized highland reference sites and validation data rather than detailed station deployments.

Measurement approaches

• Fog collectors: Standardized mesh screens that intercept wind-driven fog droplets. The collected water is measured gravimetrically. Results depend strongly on mesh geometry and wind exposure.
• Eddy covariance: Can estimate total water flux including fog deposition, but requires careful partitioning of turbulent and gravitational contributions.
• Throughfall measurements: Compare precipitation below the canopy with above-canopy rain gauge measurements. The excess throughfall indicates fog interception by the canopy.
• Visibility sensors: Provide a proxy for fog presence and liquid water content, which can be combined with wind data and vegetation models to estimate deposition rates.

Global context

Occult precipitation is a major water source in many montane and coastal fog ecosystems worldwide, including Atacama fog oases, coastal redwood regions, and tropical montane cloud forests. In some systems, fog water can represent a substantial fraction of annual ecosystem water input.

See also: Garua, Precipitation, Water balance, Galapagos refined analysis validation data