Wavy Air

Stratocumulus Cloud on 9-25-2019. Photo by Craig Johnson

When we hear the word “waves,” we think of waves on lakes, ocean, or in rivers. We may not think of waves being in the air. But they are there and they are an important part of our weather and climate system.

When I was young I made waves in the bathtub by vibrating one hand on the surface of the water. The waves would look similar to what is visible in the photo above. When I used two hands, I created waves that came from two different directions. When the waves met, the patterns changed. Sometimes I would make the waves meet head-on; sometimes one set of waves would cross perpendicular to the other waves. The patterns would resemble forms that appear in clouds. It dawned on me that maybe the atmosphere had similar waves. Years later when I was studying physics we were able to use a wave generator that allowed us to make waves from a single source or multiple sources. Many different patterns formed. Many of those patterns are visible in our sky.

The photo below from NASA shows clouds associated with gravity waves. Imagine waves in water and then look at the curved cloud bands in the photo. The crest of the waves is where the clouds are found. That is where the air is rising. The clear banding in between the clouds occurs where the air is sinking.

Photo courtesy of NASA.

Gravity waves form where air is stable. Something puts the air in motion and waves oscillate between the crest and trough (top and bottom of the wave) while trying to return to equilibrium. When waves return to equilibrium the air flow becomes smooth again. Stability is important to the process. Without it the initial up and down motion may grow without stabilizing. An interesting fact:

What causes the waves in the first place? A thunderstorm billowing skyward pushes air out of the way. The air vibrates, moving up and down violently sending gravity waves out from the storm. Air moving over mountains causes gravity waves as the air flow ripples. So does wind shear (a change in wind direction and speed with height). The change creates a force that causes air to move up and down as it travels downstream. Eventually the waves will smooth out but along the way the air ripples. If clouds form the waves become visible. If there are no clouds the waves are invisible.

Gravity waves are important. They help dissipate the energy that builds in the atmosphere as the Sun heats the earth. The waves are part of the Earth’s energy balance. After-all, that’s what the atmosphere does. Day and night, 24-7, it works to balance the imbalances set up by the heating of the earth. They also may causes changes when they encounter thunderstorms and tornadoes - other other atmospheric phenomenon. They may even help start thunderstorms. Who knew that playing with waves in the bath tub could lead to this?

Other planets have gravity waves too. Check out Jupiter by clicking here.

Clear Cold Evening in Iowa

We are definitely headed for winter. I went out to the instrument shelter to take a late evening observation at 10:20 p.m. Overhead were a blanket of stars. It was a crystal clear night with no wind. Upon opening the door on the shelter I was greeted with a reading on the mercury-in-glass thermometer of 32 degrees F. This is the coldest so far this season at this time of the day. The coldest 24 hour temperature was 29 on the mornings of the 12th and 14th of October. It looks like we are about to break those readings.

Chart plotted by Digital Atmosphere available at www. weathergraphics.com.

Looking at the weather map as of the same time we see high pressure from the intermountain west over the Central Rockies of Colorado extending to Nebraska and Kansas to Iowa. Precipitation is located over Texas and Oklahoma. The station model plot indicates it is rain. We could use dry air here as harvest is well underway and farmers need dry weather with low relative humidity.

The high pressure area is a region of descending air. As the air sinks it is warmed by compression - the air is squeezed by increasing pressure at lower altitudes. This raises its temperature at a rate of 5.5 degrees F for each 1,000 feet it descends. The boxes on the map are the location of select weather stations. Skies are overcast in Texas and Oklahoma as indicated by the filled in boxes. Skies are clear under the high pressure area is indicated by the clear boxes. Note that Omaha, Nebraska is reporting a ceiling of 25,000 feet. The code for 25,000 is 250 because the code leaves off the final 2 trailing zeros.

The plots on this map are special versions of the station model plot. It is called a weather depiction plot because is shows sky cover, precipitation, and ceilings. I have added the isobars (lines of constant pressure) to show the main weather systems affecting surface weather.

Be sure to check out the station model plot section of this website to learn more about how weather observations are plotted on maps. I will share more in the future.

Ice Blowing in the Wind

Cirrus over Cedar Falls, Photo by Craig Johnson

Cirrus are blown into some of the most dramatic cloud formations. These patches of dense cirrus were flying high over Cedar Falls on October 3, 2019. Estimated at an altitude of around 18,000 feet, shafts of ice crystals are visible streaming below the thickest clouds. The cloud in the upper right of the photo should be classified as altocumulus and near the bottom of the photo are a few scattered cumulus. Strong winds aloft are blowing the clouds into long bands and stretching the ice crystals into long thin ribbons below the thick cloud formations.

Pressure Gradient - What Makes the Wind Blow

Map drawn by Digital Atmosphere. Weather Graphics.com

The solid thin lines on this map are isobars - lines of constant pressure. If you were to walk or drive along one of these lines your barometer reading (pressure) would not change (assuming you could do it instantly.). The map is a snapshot in time. As the weather systems move so will the isobars. The closer isobars are together the stronger the winds. Full disclosure: This is generally a true statement but there are factors that make it not so. That will be the subject of another post.

So on this map the isobars are closer together around the low center just north of Lake Superior. The spacing is wider to the southwest and the separation is greatest in the area of high pressure extending from off the Pacific Northwest coast to the northern Gulf of Mexico. If you look closely at the station model plots within the high pressure area, the wind speeds are light - less than 10 knots (9 mph). Around the low pressure center near Lake Superior wind speeds are 10 to 20 knots (9 to 18 mph). If you are unfamiliar with the station model plot format go to the station model plotting section on this web site to learn more.

The pressure gradient is the change in pressure over a specified distance. If the isobars (pressure lines) are close together the gradient is greater than if the lines are far apart.

Stratocumulus - One Sign of Winter

Surface Map, 1200Z (7:00 a.m. CDT) October 11, 2019, Map plotted by Digital Atmosphere, Weathergraphics.com; The solid lines are isobars (lines with the same atmospheric pressure. The low center is near lake Superior. The narrow spacing between the isobars indicates a tight pressure gradient over Iowa which causes strong winds. It was a windy day with stratocumulus.

Cold air sweeping in behind a low pressure center often forms stratocumulus clouds over Iowa. The clouds form as the colder air converges into the center of low pressure to the northeast. As low level air converges it rises to the top of the cold air where it meets sinking air from above. The sinking air is warming as it descends and creates a boundary when it meets the rising air below. Along the boundary is where the stratocumulus form. There is just enough instability and moisture in the colder air to create rising air and clouds as you see in the photo below. The clouds have the look of cumulus with rounded bottoms and tops but at the same time the entire layer looks like a sheet of stratus. The name is stratocumulus. Strato for the sheet or layered structure of the entire cloud sheet and cumulus for the rounded form of the individual cloud elements.

Stratocumulus over Adair County, Iowa. Photo by Craig Johnson 10-11-2019

Iowa is located close enough to the Great Lakes that storm systems moving northeast out of the state move over the waters of the lakes - usually Lake Superior and northern Lake Michigan. When the air mass behind the storm is colder than the lake water temperature the low pressure center may intensify. Intensification causes the circulation around the low to increase and contributes to low level moisture circling around the storm. The result? Stratocumulus.

Of course this does not always mean winter is coming because this phenomenon is common around the Great Lakes in other seasons - namely spring and fall. But this time of year stratocumulus with strong northwest winds after a storm us usually a sign of a change of seasons. Iowa is just close enough to get in on the action. Eastern Iowa feels this effect more often than the southwestern counties but in this case the clouds formed across the entire state, including Adair County.

2 Hours in the Life of the Prairie Sky on Friday, October 18, 2019

Heading to winter means big changes overhead. Sometimes they happen quickly. Let’s look at a 2 hour “snapshot” of the sky over northeast Iowa last Friday. The following images are photos taken Friday afternoon, October 18, 2019. The photos show the variety of cloud formations that appeared in a matter of two hours. The sky came alive as clouds changed shape and texture almost minute-by-minute.

Most of the United States resides under the Westerlies. The Westerlies is a west to east river of air found between 30 and 60 degrees north latitude. (For those living in the Southern Hemisphere you have your own westerlies between 30 and 60 degrees south latitude.). During the summer the core of the Westerlies is found near the United States - Canada border. It extends around the entire hemisphere at roughly the same latitude.). The Westerlies are weak during the summer with wind speeds at about 30,000 feet ranging between 20 and 70 mph. During the winter the Westerlies drop south, often flowing across the southwest United States to the region from the Carolinas to northern Florida. The speeds in the core reach often exceed 100 mph and can top 150 mph.

Why does this matter? The westerlies carry storms and fair weather around the hemisphere. As the winds aloft become more energetic during fall, clouds change dramatically! The photos below show some unusual shapes and patterns that formed and dissapated in a few tens of minutes. Check them out and if you live under the westerlies look for rapid weather changes as winter approaches. Temperature will rise and fall during autumn but the over-all trend will be down.

Cirrus and altocumulus, Cedar Falls, Iowa. Photo by Craig Johnson

Looking at the same clouds A few minutes later. Same The clouds have moved to the right. Photo by Craig Johnson

More altocumlus. Photo by Craig Johnson

Cirrus streamers with different textures. Photo by Craig Johnson

Cirrocumjlus, Cirrus fibratus, and altocumulus, Photo by craig Johnson. Notice the cloud patterns.

Cirrus again. Notice the banding. This formation has both Cirrus fibrates and Cirrus uncinus. Photo by Craig Johnson.

Altocumulus cumulogenitus

Photo by Craig Johnson, Copyright 9-28-2019, Fort Dodge, Iowa

Flying high over the Des Moines River Valley were two patches of Altocumulus cumulogenitus. Virga trails can be seen falling and streaming behind the clouds. These clouds are high based convection with drier air below. The precipitation is not reaching the ground. The cloud tops have reached a stable layer which has stopped their growth.

The photo is taken at the remains of the Hydroelectric Dam on the Des Moines River in Fort Dodge. Once source says the dam was built in 1916 while another places completion in 1922. In any case the dam provided electricity for downtown street lights until 1971. There were five gates with a spillway that extended left in the photo to the other side of the river. The dam height was 16 or 17 feet with a length of 366 feet. Only part of the skeletal structure of the building remains with one gate chute in place. The remaining gate does not function.

Cumulus in Layers

P:hoto by Craig Johnson, 9-28-2019, Near Williams, Iowa

This photo was taken on the Iowa prairie near Williams. High overhead are altocumulus clouds. Below are newly forming cumulus. While both layers feature cumulus clouds the processes creating the clouds are quite different.

The higher clouds are formed by the advection (horizontal movement) of moist air between 8,000 - 10,000 feet. The moist air begins to rise in cells, which form clumps of cloud. If you look closely you will also see parallel lines of cloud waves in the layer. Where the motion is upward, cloud forms. Where air is clear the air is sinking.

Down below we see evidence of a different process. The rising Sun is heating the ground, which in turn is heating the air. As the air warms it rises, forming cloud cells that look much like cotton balls. Air is rising in columns creating the puffy clouds. Where it is not rising the air remains clear.

The clouds are all in the cumulus family. Cumulus means “heaped” in Latin. Heaped clouds that form in the low levels are called cumulus. If they form at higher levels they are altocumulus, meaning high cumulus. The highest cumulus are called cirrocumulus because they form at the cirrus level.

Cirrocumulus lacunosis Clouds

Cirrus lacunosis, photo by Craig Johnson at Cedar Falls, Iowa

This is not a common everyday run-of-the-mill cloud. On the other hand they appear often enough that you should be on the look-out for them. Look past the lower cumulus clouds and you will see cirrocumulus lacunosis. The definition from the World Meteorological Organization states, “Cloud patches, sheets or layers, usually rather thin, marked by more or less regularly distributed round holes, many of them with fringed edges. Cloud elements and clear spaces are often arranged in a manner suggesting a net or a honeycomb.”

This cloud type is most often found at the cirrus level but can also be seen as a type of altocumulus and rarely a stratocumulus. The lacunosis in this photo are at the cirrus level. They have irregular edges with holes in the middle. The lower clouds are cumulus. Lacunosis often have a honeycomb shape. This cloud type forms when a layer of cool air mixes with a higher warmer layer in the atmosphere. When at the cirrus level the layers of air are only relatively warm and cold. Temperatures, even in the summer are below freezing.

Photo Copyright by Craig Johnson, Cedar Falls , Iowa 9-2-2019

Crepuscular rays are attention-getters. The shafts of light are created when sunlight passing through the air is scattered by dust and other small particles. The darker bands are simply shadows caused by light encountering clouds. The cloud shadows provide contrast between light scattered by particles in the air and the darker shadow bands.

Weird, huh?

Photo by Craig Johnson, September 2, 2019, Cedar Falls, Iowa

Weird, huh? Maybe not as weird as it appears. To begin, these are altocumulus. Altocumulus are a mid-level cloud found between 6,000 feet and 18,000 feet with a cumulus (puffy) structure. These altocumulus are also a type of wave cloud. There are waves moving from the bottom to the tip of the photo and from left to right. The crest of each wave (upward motion) forms a cloud while valleys (downward motion) create blue sky.

Weather involves processes that occur on a range of scales from large to small. Looking at this photo, notice 3 main bands of waves extending from left to right while moving from the bottom to the top. At the same time, within each wave, smaller cloud bands are lined up from left to right. It looks like there are large waves moving from the bottom to the top of this photo and smaller waves moving from left to right across each large cloud band. For example, the middle band has left to right oriented waves on the left and smaller waves on the right. The smaller waves feather out into the blue sky.

Most people glance at the sky. Try spending time looking closer. Watch how the clouds move and notice how their shapes change. See if you can identify the basic cloud types: puffy (cumulus), layered (stratus), and hair-like (cirrus).

Internal Lighting

Photo by Craig Johnson - September 2, 2019, Cedar Falls, Iowa

This is an example of the setting Sun creating special effects. We were out sitting on our deck enjoying dinner and watching clouds drift slowly overhead. The clouds were cumulus mediocris (medium size cumulus). The coloration was due to the changing color of the light as the Sun dropped lower in the sky. As the clouds approached they were non-descript - just white with a few shadows. But just as they began to pass, this view appeared, lasting only a couple of minutes. It’s a good example of how the right kind of lighting will enhance an ordinary cloud formation, if only for a few minutes. You need camera in hand because unlike portrait photograph, under controlled lighting, this cloud “posed” briefly before moving on. The back lighting lasted only briefly.

Bathed in Red

Photo by Craig Johnson - August 2019, Cedar Falls, Iowa

A hint of yellow, plenty of red, and a layer of cirrostratus set up a spectacular sunrise. Rays from the rising Sun were reflected off the cloud bottom to create a brilliant reddish hue. Sunlight contains the full spectrum of visible light but when the Sun is near the horizon most of the shorter wavelengths of light are backscattered by ice crystals in cirrus clouds. Found above 16,000 feet, blue, green and everything in between are filtered out leaving yellow and red light. As the Sun climbed above the cloud layer the red and yellow disappeared leaving a distinct white layer of cirrostratus.

Altocumulus and altostratus

Photo by Craig Johnson - August 2019, Cedar Falls Iowa

Another day and another sunrise looking the same direction as the photo above. This time sunlight was shining on a midlevel cloud at about 12,000 feet seen above darker lower clouds. The lower clouds were also in the mid-layer at around 6,000 feet above the ground. The mid-layer is defined as clouds between 6,000 feet and 18,000 feet, which is over 2 miles thick - easily enough room to have multiple cloud layers. The darker clouds are altostratus and altocumulus.

Cumulus under altocumulus

Photo by Craig Johnson - August 2019

There are two cloud layers in this photo. The lowest contains cumulus clouds which are below 6,000 feet. The upper layer is altocumulus - a middle layer cloud found above 6,000 feet but below 18,000 feet. Both cloud types form individual cells. the cells are made up of a core of rising air where water vapor has become visible due to condensation. These clouds are entirely separate from each other. The air around these cumulus is sinking as the air in the cloud is rising. The mid-level clouds are also cells of rising air surrounded by sinking air but the cells are closer together. The mid-level clouds resemble more of a layer cloud while still featuring the cell structure of cumulus. Cumulus in the lower layer are called “cumulus” but when they form in the middle layer are referred to altocumulus (high cumulus).

Photo by Craig Johnson - August 2019

This is a view of a cumulus from the bottom. Cumulus usually have flat, or nearly flat, bases. That’s because as air rises it reaches the condensation level. That level is often very consistent so the condensation occurs at the same height. It is also possible for condensation to occur at different levels in the same cloud. In that case the clouds have a ragged bottom. This cumulus did not have a distinctly flat bottom. In general, it was flat but there were variations in the level of condensation. From this angle it is possible to see the ragged cloud edge and the varying thickness of the cloud caused parts of the base to be brighter or darker than other parts.

Cumulus Mediocris

Cumulus mediocris, Photo by Craig Johnson, Copyright July 2019

Cumulus mediocris - a medium size cumulus. This cloud, and several others of similar dimension, turned the sky into a three dimensional extravaganza. The dark base and subtle shading at its top caused the cloud to “pop” out of the deep blue sky. It’s almost as if we could touch the cloud or bounce up and down like on feather-soft pillow. This cumulus mediocris drifted across the sky and slowly evaporated as new cumulus formed.

Cumulus commonly form during the warmth of the day. They are most common in summer when heated air, warmer than surrounding air rises. To see this principle in action watch hot air balloons rise. As the balloon rises the air inside cools, eventually becoming cooler than the surrounding air. The balloon then begins to sink. The pilot must add more heat to cause the balloon to rise again or it will eventually sink to the ground.

A cumulus mediocris does not have the luxury of having a pilot add more heat. Instead, if the air in the cloud does not remain warmer than the surrounding air it sinks and the cloud evaporates.

So how do cumulus clouds keep rising? As the cloud rises water vapor in the air condenses, releasing heat into the cloud. As long as the heat released into the cloud keeps it warmer than the surrounding air the cloud grows. Sometimes a cumulus mediocris grows into a thunderstorm - called a cumulonimbus. The cloud in the picture above did not grow larger. It became cooler than the surround air and began to descend and evaporate.

It turns out that cloud formation, size, shape, and growth are determined by several factors. Something must lift the air. There must be enough water vapor in the air to condense into a cloud. The temperature difference between the growing cloud and its surroundings determines the height of the cloud. And the wind shapes the cloud. In the end all of those factors determine if rain, snow, or hail fall from the cloud.

On this day nothing fell from the cloud. It simply grew to the height you see in the photo and then evaporated - disappearing into thin air!

Looking Down the Line

Shelf Cloud Blow Through Cedar FAlls, Iowa, Photo by Craig Johnson, Copyright 2019

Shelf clouds are among the most impressive on the planet. This one looked worse than it was. Forming on the boundary between warm moist air ahead of a thunderstorms and cooler air rushing out of the advancing storm, shelf clouds look scary! This one passed harmlessly overhead. Scroll down for more images of this and other shelf clouds.

Painted Sky

Photo by Craig Johnson, Cedar Falls, Iowa, USA. Copyright August, 18, 2019.

Enjoy this photo of nature’s handy-work. While the sky was smeared with ice crystals temperatures on the ground were in the lower 80s. The clouds in this photo are called cirrus. The name comes from Latin meaning “hair-like.” The strands in this photo certainly look like flowing hair.

Ice crystals do not evaporate - they sublimate. That means the ice changes directly from ice into water vapor without first becoming a liquid. Evaporation occurs when water changes into water vapor. Sublimation is slower than evaporation so the cloud edges are easily blown into streamers by winds aloft. It is the slowly sublimating ice crystals that make this effect possible.

Altocumulus Sunrise

The morning of August 10, 2019 dawned with the eastern sky full of backlit altocumulus. The contrast between light and dark was dramatic. Notice the cumulus elements in the midlevel cloud layer. Some are very small while others are larger and more dense. The dark cloud bases show where clouds are thickest. Altocumulus indicate instability in the middle levels of the atmosphere.

The second photo below shows the eastern sky a few minutes later. Clouds had receded eastward, meaning the layer was now lower in the sky. A hint of blue near the top of the frame shows what was coming for the day. In this case the cloud layer was created by an upper level disturbance (region of cooler air aloft) that crossed Iowa. Upward motion with the system released the instability causing the altocumulus to form. Lower clouds did not form because the upward motion and moisture was only sufficient above 6000 feet for cloud formation.

Think about air motion in terms of up and down and sideways. The sideways motion (horizontal) is generally stronger than the up and down motion. We call horizontal motion “wind.” When up and down motion becomes strong we often end up with thunderstorms - cumulonimbus clouds. This was not a thunderstorm day for us.