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The presence of high ice clouds in the sky is always an indicator of patterns in the atmosphere that will most likely shift in a couple of days. The reason for this is because the high ice clouds, known to meteorologists as cirrus clouds, are the advanced warning of distant storm conditions. Storms are most often the product of rising warm air. As the air over a given area of land or water heats up it rises into the upper layers of the atmosphere carrying water vapor with it. The rising water vapor forms clouds. The clouds build higher and higher until they turn to ice at about 30,000 feet. Most clouds at that level are made of ice. You can tell if they are made of ice if the sun looks prismatic through them or there are sun- dogs in the sky. If there are no prismatic effects then the clouds are most likely made of water vapor and not ice crystals. High ice clouds most often are showing the characteristics of the warm air that is rising from the center of the distant low. The sequences of high ice clouds can tell a keen observer a great deal about the forces of the rising air patterns from three to six hundred miles distant.
The first and most benign manifestation of high ice is known as buttermilk sky. The cloud forms resemble buttermilk spilled on a glass surface. The sky is showing signs of curdling. In England these clouds are called clabbered sky from clabbered milk (milk like thin yoghurt). Whatever the term used, the buttermilk sky is showing that the warm winds that are lifting up from a distant source are rising up to high levels in the atmosphere without much turbulence. It is as if the whole sky is lifting as one mass like a loaf of bread rising. The warm air lifts to a high altitude and the curdled clouds form these very characteristic patterns. This cloud can also be a fair weather cloud as large masses of warm air lift out of large valleys in the morning as the first heating pulse of the day unfolds. This is often seen at about 10:00 AM as the Sun heats the valley floor and large masses of warm air, lift together to a high level. Either that or buttermilk sky can tell of a large- scale change that is going to take place in a couple of days. Then these lyrical clouds tell of an impending change. An old cowboy song called buttermilk sky compares the cowboy's girlfriend to this harbinger of things that will change or maybe not. If buttermilk shows up at any other time than in the middle of the morning then it is time to start watching clouds for the subtle changes that can predict future storms.
In this image we see a buttermilk sky that has just begun to show the first signature that a wind aloft is creating ripples in the clouds. As a storm becomes more organized the winds that accompany the rapid rise of air in a low begin to form a stream or plume that rises up in a sloping pattern towards the east in the temperate zones with their prevailing westerly winds. This image shows the first wrinkle or rib that begins to perturb the gently rising air of the buttermilk condition. This development is speaking of a more organized air mass and a possible storm front to the west. However, for it to be a storm signal the ribs need to become much more organized.
In this image we see a set of ribs that have developed like a frothy wave on the crest of the current of rising warm air. The air in this cloud is moving from left to right. The ripples are showing the resistance of the cooler, denser air as the plume of warmer air pushes through it. Where the air rises in a ripple, a rib develops since the rising air in the ripple carries moisture upwards with it. When it gets to a cooler area the moisture in the rising current precipitates out as ice crystals. This forms the rib cloud form. The cool air is dense compared to the warm air. When it forms a cloud and cools it then sinks. The sinking cool air is represented by the gaps between the clouds in this image. As the cool air sinks it warms through increased pressure. The ice crystals in the sinking and warming air melt and turn back into vapor. The vapor phase is the blank area between each ripple. The blank areas can be seen in the image at the upper and lower edges of the cloud. In essence, where there is clear sky the cool air is sinking and warming and where the clouds are forming the warm air is rising and cooling. The alternating rising and sinking of the moist air forms the rippled clouds. These clouds tell that the rising air in the distant low is becoming much more organized and has now formed a rising current.
When the faint ribs and ripples become highly organized the formation of cloud streets, or what is known as mackerel sky, is the result. In this image we can see that the rippling effect has taken over the whole mass of clouds. A wide stream of air is forming these ice clouds and that means that the low in the west is moving closer to our position and is more organized because of the great extent of the influence seen in these broad masses of rippled clouds. The old rhyme was mackerel sky, mackerel sky, dry turns wet and wet turns dry.
In this image a winter type mackerel sky is depicted where the winds that are creating it are blowing strongly up the slope. Comparing the rippled effect in this image to the more gentle forms of the last image, we can see that the chiseled look of these clouds gives the impression that the winds that are creating them are blowing upslope much more vigorously. These types of ribs are called storm ribs and they speak of an enhanced velocity to the upslope wind at the storms source.
In this image the jet stream has entered the picture and the rippling has spread to large portions of the sky. This is a picture of the cloud forms typical of a plume of the jet stream. The jet stream is a high velocity river of air that steers storms across the continent. Even though this mass of clouds is telling that there is a strong pattern behind the flow we still don't know for sure if there is going to be a storm where we are. But, from these jet stream clouds we can guess that whatever is out to the west is going to bring some weather somewhere.
These clouds are showing that the ripples of the previous evolution of clouds are producing a thicker more puffy mass of clouds. Generally this means that the ice in the cloud is beginning to turn to water again and that means that the cloud is getting heavier and is starting to lower from a very high altitude. Each ripple now is producing a cumulus type of cloudlet that is a good indicator that the front line where the precipitation can be found is approaching the observer. If the wind is shifting to the southeast and this type of cloud has evolved from an early morning buttermilk sky, then there is a good chance of something developing in the position of the observer.
This picture shows a large bank of mackerel sky clouds that has formed in the lee of a coastal mountain range. The winds that are forming these clouds are moving from right to left. The ocean is to the right behind the mountains. Just beyond the mountains in the center of the picture where the clouds are massing up, is the Golden Gate of San Francisco bay. The Golden Gate,is a break in the coastal mountains where weather systems come into the bay from the ocean. The gathering of clouds lowering and massing up in the lower center of the picture is a lowering sky. The banks of clouds in the far background are masses of clouds that have lowered and are starting to form a front line. It is at the front line that higher and middle types of clouds turn into rain clouds.
The ice clouds in the top of the picture in the foreground are higher types that are under the influence of the updraft. On this side of the mountains the updraft from the low off of the coast is moving up along the mountain range off of the ocean and so it carries a lot of water helping to form the ice clouds. However, even these high clouds have started to lower and mass up. From these clouds we could read that the front was not yet in our locality but the sky was capable of lowering into masses of clouds that could produce rain. If the wind was out of the south and the barometer was falling or the flies were biting then these clouds would be a good indicator of impending wetness in our position.