Thursday, July 10, 2014

Looking Up: What are those clouds?

Kathleen Sayce

With an ocean to the west and mountains to the east, we live in an excellent region to see a variety of clouds. Basic cirrus, nimbus, stratus and cumulus clouds are on view regularly, sometimes all at the same time, along with mountain-capping lenticular clouds, ground clouds, fog, linear cumulus clouds along the spine of the peninsula, the occasional anvil cloud that brings a thunderstorm, and dramatic mackerel skies when a storm approaches. 

As storms approach, the arriving nimbus (rain-bearing) clouds may be preceded by regularly spaced cloudlets, called a mackerel sky, for the appearance of fish scales in the cloud pattern. 

Every now and then something less common appears. Late last fall, December 11, 2013, a striking shadow cast across clouds to the north of Naselle Ridge as the sun rose. It lasted only a few minutes, as the planet rotated east. For those few minutes, sunlight streamed under the clouds, and a shadow from the mountain appeared on clouds north of the ridge as a dark wedge across the sky. 

Clouds regularly cast shadows on clouds. Less common is the mountain shadow on clouds, at least on the coast. This shadow was seen in early December 2013, when a shadow from Naselle Ridge was cast on clouds at sunrise. 

In late spring, coming across the 101 bridge from Astoria at sundown, we saw unusual clouds over Cape Disappointment, and stopped at Chinook County Park to look at them as the sun set. They looked like sets of waves, curling over in a long row. These are Kelvin-Helmholtz  Instability waves, which appear at the interface between a calm layer and a turbulent layer, in this case in the atmosphere. What was even more unusual about this sighting was that the turbulent layer was underneath the calm layer, so the distinctive curling ‘waves’ were upside down. 

When a turbulent fluid layer interacts with a smooth layer, regular waves form. at the contact zone, called a Kelvin-Helmholtz Instability.  This happens when winds blow over water, and also when air layers interact. In this photo, the turbulent layer is beneath the calm layer, so the cloud waves are forming underneath a calmer upper layer. 

Summer is the time when marine fogs are most common. When it’s hot inland and cold upwelled water pushes onshore with onshore winds, dense fog forms over the water and flows over the land like a cotton blanket, thin or thick. Depending on how hard the marine air or the over-land air is pushing, this fog either stays just along the beach, or reaches across the bay and up the Columbia River for several miles. On an afternoon flood tide when there’s cold upwelled water onshore, look for fog right above the cold water in the channel as the tide comes in.  Cumulus clouds also form inland of the ocean under these conditions, and grow over the hills to the east as moisture keeps building. 
One of our commonest cloud types is the low to mid level cumulus cloud. in this image, the cumulus cloud is growing vertically, and if the air is very warm, could turn into a thunderstorm. 

Complex cloud layers make very striking dawn and dusk skies. It’s not uncommon to see high cirrus clouds (thin wispy clouds at very high altitudes), with nimbus or stratus layers beneath (thick dense clouds in horizontal layers), and beneath these, puffy scattered cumulus clouds. The evening sky showed a combination like this over Willapa Hills just the other day as the sun set. 

Cirrus clouds on top, with two jet contrails coming in from the left. Down below are stratus clouds in the far distance over the Willapa Hills, and in front, puffy cumulus clouds over Willapa Bay.  Layered, complex skies like these are common, and give brilliant skies at dusk and dawn. 
Keep a camera handy and watch the sky, dawn, day and dusk. You never know what striking new cloud formation or combination will be visible. As Jack Horkheimer, host of Star Gazer, says, ‘Keep looking up!”

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