A longtime friend sent me this photo of an unusual cloud he saw from his property. He wanted to know if I could explain its extremely thin structure. Note: What looks like the “top” of the cloud is actually its leading edge, with the thin cloud extending toward the horizon.
This is a special type of stratus cloud that can form over bodies of water, and then be advected by prevailing winds to drift inland. There are several lakes near my friend’s property, so I’m pretty sure this cloud was what is technically called a marine layer cloud (MLC) — a subset of the stratus category.
Stratus clouds typically extend 10s–100s of kilometers horizontally, but are shallow in height, with base-to-top distances of 0.5–2 kilometers. They can form at altitudes from 2 kilometers to ground level — in which case they’re called fog. It’s hard to tell from a single photo at one angle, but judging from the background and allowing for altitude (estimated at 600-700 meters), I’d guess this cloud was a mere 50-100 meters thick.
All stratus clouds require an inversion to form. Air temperature typically decreases with altitude, but if a high pressure system is present, descending air will compress and warm. This results in an increase of air temperature with altitude over a short range. And it’s the range of the inversion that determines the thickness of the stratus cloud. That range depends on several factors: relative humidity of the air above the water, water temperature, water body altitude, and air temperature.
The altitude at which the MLC forms is wherever the air temperature reaches the dew point. That altitude is known as the lifting condensation level (LCL). It determines where the cloud base forms. Within the inversion layer rising water droplets encounter higher temperatures, the droplets re-evaporate, and the cloud fades out. Both the cloud base and cloud top are relatively laminar.
Here’s a graphic that shows the meteorology of an MLC:
The formation of these clouds starts when wind blowing over water mixes moist surface air upward in a vortex. As the air gains altitude, it expands and cools adiabatically. Cooling causes the relative humidity to increase, and once the relative humidity reaches 100% condensation of water vapor into liquid water drops takes place. A cloud is born. The altitude range through which the air mixes is referred to as the mixing layer.
The thickness of the MLC is a function of local environmental parameters. In this case the cloud thickness was on the extreme range of typical values. It’s a beautiful example of the varied phenomena Nature presents if we just take the time to observe and enjoy.
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