Waving at Waves

2015-03-13 20:40:55.000 – Nate Iannuccillo, Summit Intern


Last Tuesday, fair weather allowed me to enjoy what proved to be a splendid day of hiking in the alpine zone. While I was thrilled by the good conditions, there was something else that got me really… really…
Ok, what the heck was I so excited about?
Lenticular Clouds!
Alright, so what are lenticular clouds?
In short, lenticular just means that they form in the shape of a lens, so they look really cool, as you will see.
But first, how do clouds form this shape?
Lenticular clouds are commonly associated with the lifting effect mountainous terrain has on airflow.
When air comes into contact with a mountain, it is forced to rise, and in most conditions, cools.
Water condenses with greater ease at colder temperatures, so as air is forced upward, it usually cools and will more readily form a cloud.
Conversely, as air moves down the leeward side (the side pointing away from the wind) of the mountain, the reverse effect tends to happen. Air warms as it moves back down the mountain, and water evaporates more easily, terminating cloud growth.
So depending on temperature profiles and the amount of water vapor in the air, the upward forcing provided by mountains can lead to a wide variety of different clouds initiated by this disturbance.
Sometimes, we can see what it is called a Cap Cloud that appears to sit on the top of the mountain.
And in fact, I got to observe one of these over the summit early Tuesday morning from Mt. Monroe.
Cap Cloud over Mount Washington NH as seen from Mount Monroe 

While these clouds tend to remain stagnant and may give the impression that there is very little wind, remember that this is not the case!
As air moves up over the mountain, “new” cloud is constantly forming as air rises, cools, and condenses, but as the air moves down the leeward side, cloud is continuously evaporating as air descends and warms.
I was delighted to get to see this interesting type of cloud on Tuesday, but this cap cloud was not the only lenticular cloud that I could see.
First, let’s think about the effect that an obstacle has on airflow.
Think of a large rock in the middle of a river. There is bound to be some notable disturbances as the water flows around the rock. There will likely be some ripples in the water downstream from the rock, and probably some turbulent eddies behind the rock as well.
Mountains have similar effects on airflow. This is fluid dynamics, folks!
The initial disturbance provided by the mountain often causes air to flow in a wavelike manner on the leeward side of the mountain. These waves can often cause clouds to form in a similar vain to the cap clouds that I described earlier. They will still have the same lenticular shape, but they won’t necessarily be situated over any specific mountain. These are called Lee Wave Clouds.
Mountain wave formation diagramSource: Dr. Robert Houze, University of Washington

So I bet you’re wondering if I saw any lee wave clouds on Tuesday…
You bet I did!
Check out the lee wave cloud over Boott Spur.
Lee wave cloud over Boott Spur 
So, this is a great example of how waves propagate in the horizontal direction, but do they still propagate in the vertical direction?
Of course they do, and this means that you can also get lenticular clouds forming well above the actual height of the mountains.


 Check out the lenticular cloud action over the northern presidentials…
Lenticulars over the Northern Presidentials 
Terrain can have some really cool effects on the cloud activity, so be sure to keep an eye out for sweet lenticular clouds, especially if you’re in the mountains!


Nate Iannuccillo, Summit Intern

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