Hartbeespoort residents woke up during the last few weeks to the whole valley covered in dense mist. But what causes this weather condition?

Mist, a low-level cloud, often forms in valleys due to a combination of factors like change in temperature with cold air settling above the Hartbeespoort Dam. These conditions create an environment where water vapor in the air can condense into tiny water droplets, forming mist.

(Drone footage by Hein Nortje)

Here’s a more detailed explanation:

• Temperature Inversions:

During the night, the ground cools down more rapidly than the air above, creating a temperature inversion where warmer air layers are positioned above cooler air near the ground. This stable stratification of air can trap moisture near the surface and lead to fog formation.

• Cold Air Drainage:

Cold air is denser than warm air and will tend to settle into lower areas like valleys. This process, known as cold air drainage, can result in a pool of cooler air at the bottom of the valley.

• Higher Humidity:

Valleys often have higher humidity levels due to the presence of water bodies like rivers and streams, or because the valley floor is closer to moisture sources. The higher the moisture content in the air, the more likely it is to condense into fog when the air cools.

• Radiational Cooling:

Under clear, calm conditions, the ground radiates heat into the atmosphere, causing the air near the surface to cool down. This cooling process can lead to the air reaching its dew point and causing moisture to condense, resulting in fog or mist formation.

In summary, the combination of a stable temperature inversion, cold air drainage, higher humidity, and radiational cooling makes valleys prone to mist formation.