Altocumulus means “high cumulus.” Floccus refers to tufts of wool. These mid-level clouds remind us of tufts of wool. They form when the mid-level of the atmosphere is conditionally unstable; meaning if clouds form the heat released by condensing water vapor create clouds with towers - altocumulus. The unstable layer isn’t very deep. In this photo we can see the cloud towers only penetrate a shallow layer overhead. No rain fell and the clouds dissipated. Sometimes these clouds grow large enough to develop into thunderstorms - if the air mass has a deep unstable layer. Photo copyright by Craig Johnson taken in Cedar Falls, Iowa looking west.
The Weather Briefing Blog
The right combination of sunlight and cloud created this beautiful rendition of altocumulus cloud over Cedar Falls today.
Cirrostratus over Cedar Falls on April 23, 2019
A sheet of ice in the form of cirrostratus covered part of the sky. A sharp edge to the cloud is a common sight with this cloud type. Cirrostratus always allow the disk of the Sun or Moon to be visible (although not in this photo because the Sun was out of position). If a cloud looks similar to this but blocks the disk of the Sun it is classified as altostratus - a mid-level cloud found between 6,000 and 18,000 feet above the ground.
It was winter in April on the 27th. This photo shows steady light to moderate snow collecting on grassy surfaces. Normally we are done with snow by the middle of April but not this year! A major low pressure center spread snow across northern Iowa. In Cedar Falls the equivalent of 4 inches of snow fell in a few hours, but with never more than an inch on the ground. Thankfully road conditions were only wet here while rural areas to the north reported snow and ice covered roads.
A large winter storm formed over the Texas Panhandle and moved northeast through Kansas and crossed eastern Iowa on its way to the upper Great Lakes. The low center exited Iowa near Dubuque after midnight on February 24, 2019. The pressure decreased as the storm approached and began rising after the storm center moved away.
A barograph traces changes in air pressure. The graph above shows the pressure beginning to fall at Noon on Friday, February 22nd. It reached its low point at Cedar Falls, Iowa around 11 p.m. CST on the 23rd. As the pressure began to fall cirrus clouds began increasing from the southwest. The cloud bases lowered throughout the day and night as cloud types changed from cirrus to altostratus. At the same time, temperatures warmed into the low to mid 30s.
As the pressured reached its minimum, the low center was passing southeast of Cedar Falls. With its passage colder air began drifting in from the north and snow began to fall. Winds also increased from the northwest. During the night winds increased and colder air lowered temperatures into the teens. Blowing and falling snow made travel hazardous with travel not advised and some roads in central and north central Iowa became impassible.
Air pressure is determined by many factors. The factors include the total mass of air above our heads, temperature, the amount of water vapor in the air, and whether air is rising or sinking. Those are topics for another occasion. In the meantime, a barograph, like the one in the photo above, is a useful tool for understanding how pressure changes with time how those changes are related to changes in our weather. Barometers were initially used to forecast the approach of storms. Falling pressure meant that a storm was approaching. The rate of fall and how far it fell was related to the intensity and speed of the storm. We have more reliable ways of forecasting the weather today but barometric pressure is still used to monitor storms. The barograph shown above is very useful for anticipating changes in the weather.
This view is looking southeast at a dramatic looking patch of altocumulus clouds. The clouds were distinct, partly because the light from the Sun, which was shining on the base of the cloud deck. This reveals individual cloud elements that look like pillows. The “pillows” are cells of upward motion where moisture is condensing as water droplets, even though temperatures are below freezing. The droplets are called supercooled.
Look what happened later. The clouds still exhibit cumulus characteristics, which are puffy cloud elements, but the elements are not as distinct as the clouds begin to look more like stratus (layered) clouds. The Sun has climbed high enough in the sky that it is shining on the cloud tops. The upward motion has been weakening so the clouds are turning into a more consistent layer than one with individual cloud elements. While the clouds in the upper photo are called altocumulus, which means high cumulus the cloud type below is called altocumulus stratiformis. The cloud is becoming increasingly more like a stratus cloud.
There are advantages to cold weather. We had two examples today; a sun pillar and sun dogs. They appeared this morning as the sun inched upward in the southeastern sky. The temperature was near zero as the pillar shown brightly, piercing the snow covered Iowa prairie. Sun pillars occur when sunlight reflects off ice crystals. The crystals are shaped like hexagonal plates and are slowly falling like leaves through the atmosphere. The result was this picturesque pillar.
Traditionally maximum and minimum temperatures were measured using the set-up shown in the photo above. Now many weather stations are equipped with electronic instruments. However, there are still many cooperative weather stations using “mercury-in-glass” thermometers. These thermometers, like the lower thermometer above, are mounted on a Townsend Support which places each thermometer in the proper alignment to measure the high (maximum) and low (minimum) temperatures.
The minimum thermometer, on top, uses red colored alcohol as the measuring fluid. Alcohol has a freezing point of -173 degrees F which is much lower than mercury’s -37.9 degrees F. The Townsend Support holds the thermometer tilted down slightly to the left. Inside the tube is a black index which always marks the lowest reading since it was last reset. The index allows alcohol to move past when the temperature warms rises. When the temperature cools the surface tension of the alcohol drags the black index down. Once the temperature reaches its lowest point and begins to warm the alcohol moves up the scale again allowing the marker to remain in place, marking the lowest reading. To reset the thermometer the observer tilts it down to the right and the black index moves down the tube stopping at the current temperature.
The maximum thermometer works like a fluid in glass thermometer used to take your temperature. There is a constriction just above the bulb which allows expanding mercury to move through when temperatures warms but stays in place when readings cool. When the mercury expands (warming) it is forced out of the bulb but when it contracts (cooling) it cannot go back into the bulb. As a result, the mercury stays at the highest point until it is reset by the observer. To reset the maximum thermometer the observer spins it to force the mercury down through the constriction.
The photo above was taken inside a medium size Cotton Region Shelter. The maximum-minimum thermometers are mounted on a cross bar (visible in the photo). The shelter keeps the thermometers in the shade to measure the air temperature, not the temperature of the sun shining on the thermometers, which is what would happened if they were exposed in the open. Sun shining on the thermometers would read too warm. The shelter also keeps the thermometers dry. Wet thermometers would tend to read too cool as water evaporates off them. In the background on the left is a mercury-in-glass thermometer that reads the current temperature. The minimum thermometer also reads the current temperature. The maximum thermometer does not.
Hartman Reserve, Cedar Falls, Iowa
Not every Christmas is white. Even in northern Iowa the odds for a white Christmas are about 6 years out of every 10. It seems like the odds should be higher. So far, this year has provided many opportunities to enjoy the great outdoors without snow and ice. Of course, skiers, snowmobilers, and snow enthusiasts in general have been disappointed - at least in our part of Iowa. However the south through east central and the northwest have had heavy snow already. Some spots in southern Iowa endured up to 17 inches in one storm. We have had 1 inch. But that is life in the Upper Midwest. At some point it will snow. It’s all part of how nature works in the middle latitudes. It’s a bit chaotic but part of the fun of watching the weather is the endless variety we experience. Take time to enjoy watching your weather. Notice the clouds and if you own a rain gauge, thermometer, or barometer read them regularly. Use the links on this website to learn more. It is truly an interesting hobby and there is always something new to learn.
The above photo was taken on Sunday, December 16th. It was a nice day for a walk in Hartman Reserve, Cedar Falls. The only snow and ice was on the frozen creek.
A barograph records air pressure on a rotating drum by amplifying pressure change through aneroid capsules. The capsules, which have a vacuum inside, expand and contract with changes in pressure. Multiple capsules amplify the pressure changes as does the arm extending from the capsules which traces the pressure by using an ink-filled nib on the end of the arm. The ink is drawn to the paper by capillary action - much like ink flows from a quill pen.
The barograph below was manufactured by Taylor Instruments. It is a Weather-Hawk Stormoscope Barometer No. 6450. The year this instrument was manufactured is unknown (so far) but they were available at least in the late 1960s and 1970s. There was also a thermograph version that measured temperature.
Barographs have been superseded by computer displays able to trace pressure change without using ink and paper. Computer systems can also use the pressure data in calculations. Barographs require an observer to read the trace to determine the date and time of the reading. Despite the drawbacks, barographs are still manufactured today and are mostly used for display purposes. Many come in exquisite wood cases and are displayed in cut glass panes. The first barograph was apparently made in the 1760s.
A barograph is a prized possession. Even though there are better ways to record pressure change there is nothing like a high quality barograph to grace a display case. They have the added benefit of letting us see pressure changes as they happen. If you like to do a little forecasting you can also use pressure change with other information, like clouds and wind to make a simple forecast. There is nothing like first-hand learning to encourage someone to get engaged with the world around them.
Graupel is precipitation that looks like pith balls. It is caused by snowflakes falling through moist air, such as water-droplet fog that is below freezing or super-cooled* larger drops of water. The water collects on the flakes creating a coating of rime ice. Graupel forms in convection, a condition that occurs when air is unstable. Convection is rapidly rising and sinking air currents. In the warm season convection is what occurs in showers and thunderstorms.
While it might not seem possible, convection occurs cold winter air under the right conditions. It has to do with vertical temperature differences that place heavier air next to lighter air. The heavier air must sink while the lighter air rises, much like a hot air balloon. In summer, temperatures are above freezing at altitudes extending at least several thousand feet above the surface, and often to 10,000 to 14,000 feet high. Precipitation falls as rain. In winter the entire air column is often below freezing so the tiny balls do not melt. The key is not the actual temperature. Convection occurs due to the difference in temperatures. The result is graupel instead of snowflakes.
The balls, about the size of BBs, are easily crushed by squeezing your fingers around them. Pith, is a spongy white material found inside the skin of an orange or lemon and also certain plant stems. While they look like pith balls, graupel is their name. According to Merriam-Webster, the term graupel was first used in an 1889 weather report. The term is Germanic in origin and is the diminutive of Graupe, meaning “pearl barley.” It may look like pith but it is really a form of snow.
Take a look at the photos below showing graupel that fell in Cedar Falls, Iowa on December 4th.
* Supercooled water is water that remains liquid even though temperatures are below freezing. Clouds that are below freezing usually contain liquid water droplets or a mixture of ice crystals and water. The percentage of ice to water changes with temperature. At around -40 degrees clouds are usually all ice crystals.
Cirrostratus photo by Craig Johnson, Copyright 10-24-2018
Appearing as a smooth whitish veil, this cloud is a distinct example of classic cirrostratus. Milky and smooth, this cloud type may produce a halo, either partial or complete. Often cirrostratus is fibrous with thinner and thicker regions of cloud but true cirrostratus never completely blocks the Sun. The solar disk is always visible, either distinctly if the cloud is very thin or as a very diffuse disk.
Cirrostratus contain ice crystals and are often high enough where temperatures are below zero, even in the summer.
Below you will find a different type of cirrostratus. Taken on the same day about 15 minutes before the photo above this cirrostratus is laced with cirrus fibers around the edges as well as fibers within the cirrostratus itself. Sometimes it is hard to recognize where the cirrus ends and the cirrostratus begins. Identifying clouds can be a tricky process because clouds cannot always be put in nice neat boxes.
Autumn is a great time of year. The humid air of summer gradually retreats from the Upper Midwest. The dry air masses create days of clear skies. This begs the question, “Why is the sky blue?” If you stand on the Moon the sky is black. Here on Earth it is blue. Why? Earth has an atmosphere and the Moon does not.
Another cold front passed by yesterday giving us a brief encounter with mild temperatures. It lasted about 2 hours when our high temperature touched 62 degrees. As the front approached scattered clouds formed. The photo below shows strips of altostratus with a few rag tag fractured cloud tags. At this point winds were breezy from the west and southwest.
This close-up in the photo below of an expanding area of altostratus was taken near the leading edge of the cold front.
As the front passed this patch of altocumulus shown below moved over head.
The next photo shows a close-up of the altocumulus. Notice the cell structure of this cloud time. The cloud layer is divided into cloud cells with lines of nearly clear sky around each cell. It should be pointed out that the areas of clear or nearly clear skies in the altocumulus are caused by sinking air. The clouds are found where condensation occurs in areas of rising motion. By contrast, the altostratus clouds are in a mostly continuous layer or sheet. Altostratus also occur in rising motion but rising motion with stratus type clouds is slower than in cumulus type clouds.
The first snow arrived early this year. October 14th is early. While a few flakes sometimes fly in October it is not common to have measurable snow this early. The day started with rain but by afternoon, as colder air moved in, it changed to what you see in this photo - large flakes collecting on grassy surfaces.
Temperatures were relatively warm - in the 30s. When temperatures are just above freezing the flakes are usually large due to moisture that condenses on the falling flakes making them “sticky.” Flakes stick together and condensation continues to grow the existing ice crystals creating the large flakes. The large flakes are easily visible in the photo. In the end we measured .3 (three-tenths) inches of snow, which melted almost immediately. We did manage to have scattered snow still on the ground the next morning but it melted quickly. It is all a reminder that winter cannot be far behind, although it is still anyone’s guess how soon a “serious” snowfall will occur.
Compare this photo to the previous post. The photos were taken about 30 minutes apart. Evaporation is taking place leaving clear patches in between the thicker cumulus clouds. So instead of a nearly continuous cloud layer we now have holes developing as drier air above the cloud area sinks through thinner portions of the cloud. Instead of stratocumulus the cloud type is now mainly cumulus. The next stage was the weakening of the upward motion that was making the cumulus clouds. After sunset the cumulus flattened into thin patches of stratocumulus.
What kind of cloud is this? It looks heaped which suggests it is a type of cumulus. On the other hand it also has some stratus characteristics - layered and areas that are flat. Meteorologists combine the two types into one - stratocumulus. This type has both cumulus and stratus elements. In this case the stratocumulus formed in seasonably cold air flowing into a departing low pressure area. The air above this cloud is sinking. Sinking dry air warms at 5.5 degrees F for each 1,000 feet it descends due to the higher air pressure at lower levels. The warming creates a layer of relatively warm air (but not necessarily warm) above the cool air below. The layer is stable - the lower air cannot rise through the upper layer. The result is a nearly continuous cloud with a flat layer expanse.
At the same time the cumulus shape forms when moist air near the surface rises during the heat of the day. The rising air cools and water vapor condenses into puffy cumulus cloud elements. In the end the combination of rising and sinking motion create a cloud that looks both cumulus and stratus at the same time - stratocumulus. By late afternoon the surface heating weakens as the Sun sinks lower in the sky. The cumulus elements disappear leaving a dissipating flat cloud that evaporates leaving a clear chilly night.
Autumn storms provide longer advance warning of their arrival than summer storms. The reason is the change in what causes storms during each season. Summer storms are smaller in horizontal extent than winter storms. They are also have much shorter life-spans. Winter storms are not as tall as summer storms. Summer storms develop quickly, winter storms take many hours or days to develop after they are identified.
NOTE: There is rarely a situation where storms are completely a “summer” or a “winter” storm. I am using that comparison to help readers understand that there are usually major differences in what causes storms to form and develop and how long they live during summer and winter.
Summer storms form and live off large amounts of warm air and moisture and flow patterns. Winter storms form and live off changes in temperature wind velocity with height.
The heat and moisture available in summer are the main driver of summer thunderstorms. Flow patterns determine the type of thunderstorm and severity. Winter storms depend on air flow vertically through the atmosphere. Differences in temperature, moisture, and flow patterns shape the intensity and type of storms. Those differences cause summer storms to be relatively small in size when compared to winter storms but summer storms are usually taller than winter storms. Winter storms sprawl over several states at once, sometimes influencing weather in one-third to one-half of the nation. Commercial airliners go around thunderstorms but can usually fly over winter storms.
Today was a good example of how the sky looks in winter compared to summer. On the prairie we can see storms coming for many miles so it is easy to change in cloud types when looking from horizon to horizon. The flat clouds of winter were on display. Below are several pictures of the sky this afternoon. From the top picture to the bottom the evolution of clouds indicated more of a winter than summer type storm was approaching. It is too bad the Sun went down because there would have been more to see.
Last evening a cold front moved across Iowa preceded by a narrow band of thunderstorms. Nearly a quarter-inch of rain fell in about 15 minutes. That is a rainfall rate of 1 inch/hour. How about temperatures? The high temperature ahead of the front was 85 degrees. Behind the front the low this morning was 37. The warm moist air ahead of the front was lifted by the cold front along a narrow band of instability.