The first thing that hit me when I stepped out for my daily mandated exercise today was how warm and humid it was. I already knew that it was +10°c outside thanks to my trusty new thermometer, but I wasn't prepared for the humidity. It had drizzled for most of the morning so everything was damp and the air felt thick and moist. According to the gauging station, the floodwaters had dropped by nearly 30cm since the peak, and were at a comfortable 4.03m so I thought I'd try my luck on Lower Load Lane to stay out of the fields with the wet grass. Unfortunately luck wasn't on my side today and the lane was still too flooded to pass without wellies. I took a detour over the playing fields behind the leisure centre and headed for the floodplains through the short grass of the golf course, which was my next best bet for dryness. Despite the waterlogged patches, I eventually got to the Severn's floodplains with only marginally wet feet.
Stepping out of the woods, I was greeted with an impressive sight. The floodplains were shrouded in a dense layer of mist, around 2 metres thick on average. I could clearly see the tops of trees far into the distance, but the bottoms of the trees were invisible just 30 metres away. My phone camera managed to get more detail than my eyes through the fog, so I wonder whether it might have a polarizing filter or something similar on the lens.
This fog is likely to be advection fog. This is a fog caused by advection (horizontal movement) of moist, warm air over a very cold surface. To begin with, I thought it was evaporation fog, where warm water evaporates into the cold air, which can't take in any more water so suspends the water droplets as fog. However, I determined that this was not the case as the air was so warm (although this could presumably still happen if the warm air was already at 100% saturation). It then occurred to me that the water in the floodplains must be very cold. After all, I'd seen a solid sheet of ice on it just a few days before. Plus, snowmelt must have contributed to a lot of it over the last few days. So, resisting the temptation to put my hand in and test the temperature, I concluded that advection fog must be the answer.
The first principle to understand is that cold air can't hold anywhere near as much water as warm air. This is why dew and frost form when the air cools down in the night. In the case of advection fog, when the moist warm air travels over the cold water, it cools down to the dewpoint (the temperature where the air reaches 100% saturation) and cannot hold any more water. As the air cools down even further, it has no choice but to expel the water it cannot hold, which condenses into water droplets suspended in the air as fog.
It's remarkable that the water can cool the air so much, presumably it can only occur if the air moves very slowly or not at all, much like today. Also, the air was very highly saturated to begin with, which means the dewpoint temperature is nearer.
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