Watch the sky change in 6 hours

Here is a sequence of photos that began in mid-morning and ended a couple of hours before sunset. The overcast at the end of the day was a layer of Altostratus clouds ahead of a weak upper level system aloft approaching from the northwest. The progression of lowering cloud ceilings (cloud bases) is caused by increasing moisture in the middle levels being lifted by the upward motion ahead of the weak storm system. The lifting raises the moisture to higher levels where the air pressure is lower. The moisture condenses into clouds which lower as the moist layer deepens and rising motion intensifies the condensation coverage.

Several clouds in the Cirrus family are seen in the top half of this photo. In the lower half a lower layer of altostratus is evident with small altocumulus.

Wispy Cirrus fibratus, Cirrostratus and high thicker Altostratus are seen in this photo. Fibratus means the cloud appears as filaments while cirrostratus are a thin smooth layer. The prominent cloud in the lower center has Cirrostratus and Cirrus fibratus characteristics. While we like to apply specific names to clouds nature forms clouds that may appear as combinations of cloud types. We normally look for what is most dominant or include all of the types in our descriptions if they are significant. Either way, the different types occur because different cloud processes and motions are creating the different shapes, sizes and cloud textures.

Compare this photo to the one below. The clouds look like waves washing up on a beach after the top of the wave broke. linear Thicker clouds alternate with thinner bands of cloud and blue sky. In The photo below small drifts resemble the wavy clouds. The waves are in bands determined by variations in wind speed that align perpendicular to the camera. Turbulent wind is crossing the snow in waves which deposit the ripples on the snow surface.

Drifted snow mimics wave patterns that are seen in clouds.

Here is another view of the wave forms in the clouds above. Notice the numerous CONTRAILS (Condensation Trails) as water vapor in the engine exhaust of aircraft condenses in the cloud streaks. In the upper left there is a shadow cast by the sun shining on a CONTRAIL. These clouds look like Cirrostratus and Cirrus fibratus. There is a patch of Cirrocumulus in the lower center and a band of thicker cirrostratus which look suspiciously like it could have began as a CONTRAIL.

This is altostratus with an indistinct cloud base. Look at the cloud base in the lower part of this photo and you will see the cloud base hanging as a thinning cloud bottom that gives way to a whiter sky near the horizon.

This is an Altostratus cloud that is transparent to the sun. There are two layers. The sunlight is diffused by the higher layer and the lower layer appears to be another altostratus layer (above 6,500 ft) but may have invaded the lower layer (below 6,500 ft) which would make it a stratus cloud. As the layer of upward vertical motion and moisture deepens with the approaching storm the cloud depth thickens. Eventually the upper and lower cloud layers merge as precipitation forms and the cloud name changes to Nimbostratus. Nimbostratus is a precipitating cloud. When precipitation stops the cloud with either be labelled stratus (a low cloud below 6,500 ft) or altostratus if its base is above 6,500 feet.