The heat of the Sun does not melt clouds because clouds are composed of tiny water droplets or ice crystals that are suspended in the atmosphere. These droplets and crystals are held together by the forces of condensation, which occurs when water vapor in the air cools and forms liquid water droplets or ice crystals.
While the Sun's rays do contain heat and energy, the temperature of the atmosphere at cloud level is generally much lower than the temperature required to melt or vaporize water. The temperature at cloud level is determined by a combination of factors, including the temperature profile of the atmosphere, humidity, and the cooling effect of the cloud formation process itself.
Clouds can reflect and scatter a portion of the Sun's incoming radiation, which helps to keep them from becoming excessively heated. Additionally, clouds also absorb and emit thermal radiation, including the heat energy they receive from the Sun. This process helps to maintain the overall balance of energy within the Earth's atmosphere.
It's worth noting that the Sun's heat can certainly have an impact on weather patterns and the formation of clouds. Sunlight provides the energy that drives the water cycle, evaporating water from the Earth's surface and eventually leading to cloud formation. However, the specific characteristics and properties of clouds, such as their composition, altitude, and thickness, are determined by a combination of factors beyond just the heat of the Sun.