The increase in humidity after sunset, while the temperature remains the same, is due to the cooling of the air, which causes the relative humidity to rise. As the temperature drops slightly or stays constant, the air can hold less moisture, so the relative humidity increases, even if the actual amount of moisture in the air (dew point) doesn't change significantly. This is a common occurrence during summer evenings, especially in warmer climates.

Increase in humidity inside your flat without a temperature drop indicates that moisture is being introduced from an internal source, such as cooking, showering, or a humidifier. This could be causing the dew point to rise, making the air feel more humid. You might want to check for any activities or appliances that could be adding moisture to the air inside your flat.

The increase in humidity after sunset, while the temperature remains the same, is due to the cooling of the air, which causes the relative humidity to rise. As the temperature drops slightly or stays constant, the air can hold less moisture, so the relative humidity increases, even if the actual amount of moisture in the air (dew point) doesn't change significantly. This is a common occurrence during summer evenings, especially in warmer climates.