The physics of a cloud tablecloth
The "tablecloth" that frequently drapes over Cape Town's Table Mountain is an example of an orographic cloud. This meteorological event happens when moist, stable air is forced to rise as it encounters a mountain range or other topographical barrier. In Cape Town, this process is driven by a strong, persistent southeasterly wind that blows during the summer months, from October to March. This wind, known locally as the "Cape Doctor," originates over the warmer waters of False Bay, collecting significant amounts of moisture before hitting the slopes of the Cape Peninsula.
As the Cape Doctor pushes this humid air up the eastern flanks of Table Mountain—which stands at 1,086 meters—the air parcel expands and cools due to the decrease in atmospheric pressure at higher altitudes. This process is called adiabatic cooling. The air cools at a predictable rate until it reaches its dew point, the temperature at which the water vapor within it condenses into tiny liquid water droplets, forming a visible cloud. The flat, 3-kilometer-wide plateau of the summit is where this cloud to form a dense, uniform layer. As the wind continues to push the cloud across the summit, it spills over the northern and western edges, facing the city. On this leeward side, the air begins to descend. As it sinks, it is compressed by increasing atmospheric pressure, causing it to warm adiabatically. This warming raises the air temperature above the dew point, and the cloud droplets evaporate back into invisible water vapor, creating the sharp, fabric-like edge of the tablecloth as it dissipates into the clear air over the city bowl.