Last Night’s Thunderstorms & an Explanation of Lenticular Clouds

2016-07-23 12:20:47.000 – Tim Greene, Summit Intern

                We often see lenticular clouds at the Observatory, but yesterday evening we got to see a less common variety; a convective altocumulus lenticularis (try saying that ten times fast). Prior to the onset of a thunderstorm band that rolled over the summit last night, there was a spectacular 360 degree view of convective cloud formations. Unsurprisingly, the sky was dominated by cumulus towers bubbling up, the most impressive of which were located over southern Quebec. Bear in mind that if we are experiencing 60 statute miles of visibility, as we were yesterday evening, we can see significantly further distances in the atmosphere; this is due to the curvature of the earth and the fact that statute miles are measured along the ground. Similarly, if you were to watch a ship sail out into the open ocean, you would be able to see the tip of its mast (the tallest point of the ship) relatively long after the rest of the ship disappeared beneath the horizon; in fact, in the late 15^th century―a period when even the most educated scholars still believed the Earth was flat― Christopher Columbus suggested the Earth must be spherical because of this very phenomenon.

Thunderstorms building in the distant sky

                More locally, there was a massive lenticular cloud building overtop of the northern Presidential Range. In hindsight, I should have set up my camera for a time-lapse on the deck like I did when I captured a smaller one forming over Mount Jefferson last week. Lenticular clouds form when turbulent, moisture-laden air rises and cools to the dew point and condenses into water droplets that form a cloud. The striations visible on the cloud are a product of eddies in the air flow, which are in turn a function of the wind speed and the source of turbulence (in this case, air being forced up and over a mountain). Pilots are taught to avoid lenticular clouds because they are inherently indicative of turbulent air that makes for a bumpy, if not dangerous flight. Actually, there is one exception to this rule; glider pilots steer right into these areas because they denote regions of rising air that gliders use as a source of lift to propel them to higher altitudes and thus longer flights. 

What was so interesting about this lenticular cloud was its sheer size, yes, but also is convective, vertically-building nature. There was ample instability to provide the buoyancy needed to drive the cloud upward, which yielded a cloud that resembled a stack of pancakes.

                Another neat discernable feature in the above photo is the faint anti-crepuscular rays that can be seen underneath the cloud base. Crepuscular rays, which are much more common, emanate from the sun and are frequently seen piercing through gaps in the clouds on overcast days. Anti-crepuscular rays, predictably, are the inverse; they converge on an “anti-solar point” that is exactly opposite of the sun from the vantage point of an observer. Being farther from the sun, anti-crepuscular rays are much more difficult to see than their counterpart, so they made a nice addition to the already rare and impressive view!

Tim Greene, Summit Intern