Clouds are visible masses of condensed water vapor or ice crystals suspended in the Earth's atmosphere. Their formation occurs when the air cools to its dew point, causing the water vapor to reach saturation and condense around microscopic airborne particles, known as condensation nuclei. Clouds are classified based on their structure and altitude, ranging from high, wispy formations composed of ice crystals to lower, sheet-like layers or large, towering vertical masses. These atmospheric features play a fundamental role in the global hydrological cycle by facilitating precipitation and are critical components in the Earth's energy balance, influencing the amount of solar radiation reaching the surface through both reflection and absorption.