The Weather Briefing Blog
I’d rather have questions that cannot be answered than answers that cannot be questioned.
— Richard Feynman, PhD
Photo Copyright 2019 by Weather Briefing, LLC
This is not a bad place to be - for the moment. The bottom of a thunderstorm is located in the distance near the horizon. The base of the storm is over the horizon. Coming straight at us is the storm’s anvil - a long flat cloud that is spreading from the distant storm top over our heads. The anvil is the outflow at the top of a thunderstorm being blown downstream by winds aloft.
How high is the anvil? This one was no more than 20,000 to 25,000 feet above the ground. Many anvils range from 30,000 feet and higher. Some are higher than 50,000 feet.
As you look at the photo try to think in three dimensions. The storm’s vertical column rises from near the horizon almost straight up to the storm top where winds aloft spread the cloud into a long layer - called the anvil. It gets that name because from a distance the cloud looks much like a blacksmith’s anvil. Look for thunderstorms with anvils during the warm season when thunderstorms are common in many parts of the country. This photo offers an unusual perspective as we look toward the storm from under the anvil.
By the way, can you locate two birds in this photo? Despite the storm in the distance it was rather quiet under the anvil. For the birds it was probably a pleasant evening for flying.
If you stand in front of an approaching thunderstorm you feel “it.” “It” is cool air that sweeps out of a thunderstorm. The cool air is a relief on a hot summer day. It is rain cooled, and it comes from thousands of feet above the ground. It splashes, twists, and turns as it reaches the surface.
The gust front is the leading edge of the onslaught. Gust fronts are sometimes visible on National Weather Service Doppler radars, as it is in this example. The radar picks up boundaries between warm and cool air. It also is able to detect insects. These radars operate in different modes which allow meteorologists to make the radar more or less sensitive, matching the radar to atmospheric conditions to help them observe what is happening.
The radar image above is from 9:31 am CDT on June 28, 2019. It shows a gust front from northwest to southeast through Ames . The line marks the leading edge of cooler air flowing southwestward out of thunderstorms to the northeast.
Wind speeds along a gust front may be quite strong, sometimes more than 50 mph. In extreme cases speeds winds may exceed 100 mph. Cooler air is heavier than warm air so it flows down and out of a thunderstorm. Depending on the storm and its environment, the outflow spreads in different directions. Most often it moves in advance of a thunderstorm but in this case the outflow was also spreading west and southwest away storms that were moving to the southeast. The radar clearly shows the leading edge of the advance.
Below I have added a very short video from Sunday, June 30, 2019 at 7:02 pm CDT showing cooler air moving toward the southwest. The boundary became visible on radar as it approached the radar location near Johnston, Iowa. Low level winds, like gust fronts, are more likely to be detected when they are close to the radar. Why: The radar beam is higher off the ground at greater distances from the radar making the beam too high to see the leading edge of the cool air. Once the outflow gets close enough to the radar, and conditions are right, it becomes visible on radar. The video was taken with an iPhone XR using the RadarScope Pro App.
Arcus is a low dense horizontal cloud that forms along the leading edge of some thunderstorms. Arcus come in two distinct forms; a shelf cloud or a horizontal roll cloud. When an entire arcus is observed it is often curved or a partial ring shape is visible - like an arc or segment of a circle. They form where the leading edge of cool air descending from the thunderstorm interacts with warm moist air streaming into the storm.
Arcus usually look very menacing. They are associated with strong straight-line winds rushing out of an approaching thunderstorm. Wind speeds can be too weak to cause damage in some situations or sometimes more than 100 mph in extreme cases. Winds of 45 to 70 mph are most common.
Arcus do not produce tornadoes but turbulence can create very chaotic conditions as winds rapidly change direction and speed. Circular motion is usually visible from under the cloud, as you can see in the video above. However the rotation is not caused by a tornado. Tornadoes are attached to the parent thunderstorm not an arcus cloud. However, it is possible for vertical rotation to occur along an arcus, and it could cause damage, but it is not a tornado. It is a good idea to be in shelter as the storm approaches.
The video above was recorded from underneath an arcus. It shows the turbulent flow. This cloud passed over with strong winds that littered the ground with small twigs and branches. It did not produce structural damage. This is typical of arcus. However, strong storms may produce downdrafts that do cause damage and for that reason it is a good idea to seek shelter when arcus approaches. The turbulence is very evident in this video.
In this video you will hear the wind blowing through the trees and into the microphone and you will also hear the call of a cardinal in the background.
Even if you have never flown you have probably been inside a cloud. Fog is a cloud that forms near the ground. Walking in fog means you have walked in a cloud. The short (3 second) video above was recorded as we flew while climbing through a cloud layer. It was recorded over south central Iowa as we were climbing toward 25,000 feet. I am not sure how high we were at this point. This cloud was made up of many tiny cloud droplets. At this altitude temperatures were above freezing so there were no ice crystals present.
The smallest rain drops are about .02 inches in diameter and the largest possible drop in a thunderstorms may be about .2 inches. The average cloud droplet is about 100 times smaller than the smallest raindrop. Cloud droplets can be suspended with much smaller vertical velocities than a raindrop. The terminal fall velocity of a small rain drop is about 4.6 miles per hour while the terminal (fastest) velocity of the largest drops is about 20.2 miles per hour. See table here:
Upward air motion of about 4.6 miles per hour would be needed to suspend a small raindrop in the air. In other words, it would not fall. If the upward flow is greater the raindrop would move upward inside the cloud. Larger drops need stronger upward motion to remain suspended. That is why storms with stronger upward motion and large moisture content often have larger rain drops.
Flying at 25,000 feet provides more than a birds-eye view of clouds. Click on the video for a 22 second clip showing altocumulus clouds on a trip from Des Moines to Dallas.
The overall scene features long cloud bands from left to right lined up in rows from the top to the bottom of frame. A closer look reveals much more detail. Individual cumulus cells are separated by clear lines perpendicular to the cloud bands. These breaks separate the clouds into cells. Along the bottom of the frame is a cloud band with more of a stratus (layered) structure.
Cumulus clouds indicate instability. Instability can be seen when a cloud forms and its vertical size is the same or greater than its horizontal size. The upward motion causes air to cool and if moisture is sufficient condensation into clouds occurs. Downward motion warms the air and evaporates clouds. Stratus clouds form in a stable atmosphere. The upward motion is much weaker so the rising motion is much weaker. Stratus clouds have a layered appearance because their horizontal size is much greater than the vertical extent.
We see both cloud types here. The left side of the cloud band at the bottom has more of a stratus (layer) shape but there are hints of a cumulus structure too, especially nearer the right half of the band. This show how clouds are not always distinctly separate types. Complex clouds may contain both cumulus and stratus shapes.
This is the setting Sun on June 2, 2019, visible from north of Ames, Iowa. The solar disk can be seen through a layer of smoke that had been hanging over much of Iowa and the northern United States east of the Rockies. The smoke was coming from fires in Canada.
The red/yellow hue is caused by sunlight passing through the smoke layer. The blue end of the light spectrum is being filtered out leaving red and yellow. While the sky looks cloudy, minus the smoke it would be clear. The smoke creates a smooth eerie look and feel to the sky because no cloud bases are visible - just an endless veil of smoke.
Two cloud layers dominate this scene; low and high. The low clouds are stratocumulus seen in the lower half of the photo. Stratocumulus have both stratus (layered) and cumulus (heaped) characteristics. The high layer is cirrus, which is found in the upper 3/4ths of the photo. Cirrus are mostly ice crystal clouds while stratocumulus are made of water droplets. The lower cloud layer is warmer than freezing while the high layer is below freezing. Low clouds are below 6,000 feet, by definition, and the high clouds are above 16,500 feet.
This lonesome patch of altocumulus appeared in the southeastern sky after several cumulus congestus exited the area. Clouds tell us about the processes going on in the atmosphere. The cumulus congestus indicate stronger vertical motion and greater instability than these altocumulus. A more stable air mass was moving in behind the exiting cumulus congestus leaving much weaker upward motion.
This is a great example of what happens to barometric pressure during the passage of thunderstorms. During the evening and night of Thursday, May 23rd into Friday morning May 24th several thunderstorms passed our location. Notice the unsteady pressure trace on the barograph beginning around 9:00 p.m. on the 23rd through 8:00 a.m. on the 24th. The pressure falls abruptly before a thunderstorm and rises quickly as the cooler down rush of air reaches and passes the barograph.
Typically the greater the fall-rise couplet the stronger the thunderstorm. Prior to and after the thunderstorms the pressure trace represented the pressure changes associated with the larger scale weather system affecting our area. The smaller scale changes are more dramatic and are superimposed on the larger scale changes.
Gulf moisture surged northward across Iowa on Saturday May 18th setting off showers and thunderstorms. At Cedar Falls 1.17 inches of rain fell between 5:55 p.m. and 9:45 p.m. This photo shows thickening clouds. Darker clouds are seen in the southern sky to the right. They were moving to the north (right to left).
The chaotic sky included two cloud layers; cumulus, cumulus mediocris, cumulus congestus and a second overcast layer of various mid-level clouds. A third layer of cirrus was visible on satellite imagery but not to a ground observer because of the mid-level cloud layer. Rapid vertical development is seen across the lower portion of this photo. The clouds were tilting to the left and upward due to increasing winds with height blowing south to north.
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 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.