What causes the variations in Earth's cloud coverage during the course of a year and from year to year at a given location?

Clouds formation takes place when the air get warm and the moist or water droplet gets cool and expand. Clouds are nothing but small droplets of water evaoprated from the Earth surface.

Cloud coverage is defined as the fraction of the sky occupied by clouds as observed at a location.

The cloud coverage depends on the duration of sunshine as the location with more cloud covergae have as least exposure of sun and the least cloudy locales are the sunniest ones.

Other parameters affecting clod covergae are topography, temperature and humidity. Location with high mountains have large cloud coverage as it allows clouds to rain, Cool places have more water coverage as more water vaporise and expand in teh atmosphere, and humidity also support cloud coverage as more is the humidty more tiny droplets will evaporate to form clouds.

Changes in these factors causes variation in cloud coverage from year to year as environmental condition due to global warming is affected.