particles similarly increase cloud albedo by reducing precipitation and prolonging the lifetime of a cloud. This subsequently increases cloud albedo as solar radiation is reflected over a longer period of time. The Albrecht Effect is the related concept of increased cloud lifetime from cloud nuclei. Zenith Angle Cloud albedo increases with the total water content or depth of the cloud and the solar zenith angle. The variation of albedo with zenith angle is most rapid when the sun is near the horizon, and least when the sun is overhead. Absorption of solar radiation by plane-parallel clouds decreases with increasing zenith angle because radiation that is reflected to space at the higher zenith angles penetrates less deeply into the cloud and is therefore less likely to be absorbed. Influence on global climate Cloud albedo indirectly affects global climate through solar radiation scattering and absorption in Earth's radiation budget. Variations in cloud albedo cause atmospheric instability that influences the hydrological cycle, weather patterns, and atmospheric circulation. These effects are parameterized by cloud radiative forcing, a measure of short-wave and long-wave radiation in relation to cloud cover. The Earth Radiation Budget Experiment demonstrated that small variations in cloud coverage, structure, altitude, droplet size, and phase have significant effects on the climate. A five percent increase in short-wave reflection from clouds would counteract the greenhouse effect of the past two-hundred years. Cloud Albedo-Climate Feedback Loops There are a variety of positive and negative cloud albedo-climate feedback loops in cloud and climate models. An exampled of a negative cloud-climate feedback loop is that as a planet warms, cloudiness increases, which increases a planet's albedo. An increase in albedo reduces absorbed solar radiation and leads to cooling. A counteracting positive feedback loop considers the rising of the high cloud layer, reduction in the vertical distribution of cloudiness, and decreased albedo. Air pollution can result in variation in cloud condensation nuclei, creating a feedback loop that influences atmospheric temperature, relative humility, and cloud formation depending on cloud and regional characteristics. For example, increased sulfate aerosols can reduce precipitation efficiency, resulting in a positive feedback loop in which decreased precipitation efficiency increases aerosol atmospheric longevity. On the other hand, a negative feedback loop can be established in mixed-phase clouds in which black carbon aerosol can increase ice phase precipitation formation and reduce aerosol concentrations. References Atmospheric radiation Clouds Satellite meteorology