Humidity is caused primarily by the presence of water vapor in the air, which comes from the natural water cycle involving evaporation and condensation. When the sun heats water bodies like oceans, lakes, and rivers, water evaporates and rises into the atmosphere as vapor. This vapor increases humidity when it remains in the air rather than condensing back into liquid form like clouds or precipitation. The amount of moisture air can hold depends on the temperature—warmer air holds more water vapor, leading to higher humidity levels. Humidity is also influenced indoors by human activities such as cooking, showering, and breathing, all of which release moisture. Poor ventilation can trap this moisture inside, increasing indoor humidity. Additionally, water leaks, insufficient insulation, and the outdoor climate contribute to indoor humidity levels.

In summary, humidity is caused by water vapor from natural evaporation driven by solar energy and human activities that add moisture to the air, with temperature playing a key role in how much moisture the air can retain.