Why has the rain been moving backwards?

I've been asked several times over the past week why our showers and thunderstorms have been moving "the wrong way." Isn't it more typical in the summer for us to be watching skies to the south or southwest than to the east? While sometimes we just call them "Crazy Ivans" (you remember this scene from "The Hunt for Red October," right?), there is a real explanation for this phenomena.

A Typical Summertime Weather Pattern

To answer the second question first - yes. Our weather usually originates from the southwest or south in the summer. To understand the answer to the first question, we need to know that the wind at the surface does not determine the direction clouds or storms move. The wind at the mid-levels of the atmosphere direct most weather patterns. Specifically, the wind between about 5,000-10,000 feet up (or 850-700 mb if you are a meteorologist using pressure values rather than feet).

Typically, much of the summer in the southeast U.S. is controlled by the "Bermuda High," a high pressure system that takes hold in the western Atlantic centered near Bermuda. Remember that wind flows clockwise around high pressure. That means that the air around the western side of that high, which is where we are located relative to it, flows from the south tor southwest. Thus our weather  usually follows that same pattern.

Bermuda High pressure extending into the southeast U.S. Clockwise air flow means south to southwest wind for Memphis when the Bermuda High controls our weather. (Original graphic credit: NASA Goddard Space Flight Center)

But sometimes we get Crazy Ivans...

In the past week or so, the Bermuda High has not had control over our weather. More influential highs have been located to our north or northeast. You undoubtedly heard about the record heat from the Midwest to the Northeast last week? That was because of a massive, hot high pressure center over that region. Even this weekend, despite it weakening some, high pressure at that crucial 5,000-10,000 -foot level has been centered over the Ohio Valley.

Satellite imagery from July 4 show the clockwise motion of clouds around high pressure over Kentucky. That results in weather moving from east to west across the Memphis area.

Let's look a little closer at Sunday afternoon, when our scattered afternoon showers were moving from southeast to northwest...The national picture at 10,000 feet shows high pressure over southwest OH and a strong low off the Carolina coast (that is Tropical Storm Chris). The green arrows show the flow of the wind around those features, while the colors show the wind speed (less than about 25 mph is white). Note that over our area the wind is light from the southeast.

Zoomed in a bit, you can see how the high over southwest OH results in clockwise wind flow around it, or from the southeast in the Memphis area.

Finally, if we look just at the metro area, the wind barbs are angled from southeast to northwest. The little flags at the tail of the wind barbs tell us how fast the wind is blowing. The longer flags are 10 knots (mph) and the short flags are 5 knots (mph). So the wind over the metro yesterday afternoon was blowing from the southeast at roughly 15 knots (mph).

And guess which way yesterday's showers were moving? To the northwest at 15-20 mph! Now you know why sometimes a radar loop shows rain moving in a different direction than you would expect! It's all in the (movement of the) air up there! Or maybe, we need to watch out for those Crazy Ivans!

Erik Proseus
MWN Meteorologist

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Updated look at the holiday weekend & "Dr. Dewpoint" strikes again!

It's been an interesting start to the week in Mid-South weather. We started off on Monday with an outflow boundary, or gust front, and thunderstorms (as predicted) that turned out to bring some short-lived flooding and sporadic wind damage and hail (that we didn't predict). Fortunately, the damage was all fairly low end, the water ran off quickly, and the hail melted just as fast.

Storm is rolling in downtown. #timelapse @memphisweather1 @joeysulipeck @RonChilders @NWSMemphis pic.twitter.com/OpxfT5VLXG

— Jonathan Louis May (@jmay11) June 27, 2016

The front that triggered the storms pushed through the area, bringing slightly less humid air (see a more accurate description below) on Tuesday. However, also with less notice than we like to provide, another strong gust front moved through the Memphis metro on Tuesday evening. It brought the customary strong wind but also spawned a couple of picturesque thunderstorms that dropped localized heavy rain. The storms were followed by a gorgeous (double) rainbow visible to many in the city, then a terrific sunset as the clouds cleared.

The prettiest of rainbows after todays rain...@memphisweather1 #memweather pic.twitter.com/OkSZwbly0Y

— Annabella Charles (@annabella232) June 29, 2016

.@memphisweather1 Quite the sight after the storm. pic.twitter.com/AG9YyNO4RD

— Memphis_Tom (@memphis_tom) June 29, 2016

Stunning! All your pics tonight are marvelous. Thanks for sharing! pic.twitter.com/us51wbgkst

— MemphisWeather.net (@memphisweather1) June 29, 2016

Looking ahead

In our weekend blog we promised cooler temperatures and lower humidity heading towards the holiday weekend. We're experiencing that now with highs near 90°, but more importantly, heat indices that barely register above the temperature due to reduced humidity in the wake of a reinforcing cold front yesterday. Unfortunately perhaps, the cooler and drier airmass will warm back up - and moisten up - as we head into the weekend. The long-range prediction for below average temperatures for the first week or more of July has wilted under the late June sun...

We now are looking at more mid 90s temperatures and typical summertime humidity/dewpoints for the Independence Day weekend, along with daily low thunderstorm chances as a frontal system sits just to our north. For now, storm chances are at 20-30% each day from Friday through the 4th, so it's darn near impossible to know whether your favorite fireworks event (and there are many) will go on as scheduled. The good news is that this type of pattern usually favors the highest storm chances in the heat of the day into early evening, so hopefully by dark, any scattered storms from that day will yield to the setting sun and allow the show(s) to go on! Stay tuned to the MWN Forecast on the web or our mobile apps for the latest forecast info, and follow us on social media (links below) for regular updates.

Dr. Dewpoint's lesson on "humidity"

We've touched on this topic before, but we have lots of new followers, plus humidity as it relates to your comfort level is best discussed when it matters - in the summer! So I want to take a moment to (re-)educate you on the topic of "humidity," specifically as it relates to summertime and heat index. So why did I place "humidity" in quotes? Glad you asked...

Humidity is a word that is used to describe how much moisture is in the air or how muggy it is, but in most cases, it's not actually the best word to define the "mugginess" of the air. The word humidity is usually used to mean relative humidity, the value given when you look at the current conditions for a location. But if you pay attention, you'll notice that the relative humidity is highest in the morning hours and lowest in the afternoon. This is true even if the amount of moisture in the air actually increases! Because it is relative, the humidity varies according to the temperature. As the temperature warms, even if the amount of moisture in the air remains the same, the relative humidity value actually goes down. As the air cools, the relative humidity rises, given the same amount of actual moisture in the air.

What we actually mean when we say "the humidity is rising" is best described using the term dewpoint. The dewpoint is a measure of the actual moisture content of the air and is not dependent on temperature. The dewpoint is the temperature that air must be cooled to in order to achieve saturation, or 100% relative humidity. It's an absolute measure of humidity rather than a relative one. So, if the dewpoint is 40° and the temperature is 80°, relative humidity (or just "humidity") is 24%.  Give the same amount of moisture in the air (a dewpoint of 40°), if the air temperature drops to 50° overnight, the relative humidity rises to 69%. But the amount of water molecules in the air has not changed!

Therefore, since humidity changes based on the temperature throughout the day, dewpoint is the best measure of how "muggy" it feels. Here in the south in the summertime, it is not unusual at all to have dewpoints of 70°. That is a muggy day and is pretty uncomfortable to most people! When we get dewpoints to fall closer to 60° (which we approached today), the air feels more comfortable, even though for most people living up north, that would still be a muggy day (it's all relative - ha!).

This chart provides an adequate description of how humid it feels in certain dewpoint ranges fin our part of the country. Graphic courtesy The Washington Post.

A week or so ago when we had Heat Advisories and heat indices of 110°+, dewpoints were well into the mid and even upper 70s. That's pretty oppressive! Sweat just doesn't evaporate that well with that much moisture already in the air, which means your body's cooling process doesn't work as well. That is why we recommend taking it easy with frequent breaks, proper attire, and plenty of water when Heat Advisories or Warnings are issued. The body simply can't cool itself well in those conditions.

So the next time you are wondering how muggy it will be - it's best to check the forecast dewpoint, rather than the humidity. It's a better measure of just how muggy it will be! Sometimes it really isn't the heat - it's the humidity dewpoint!

Erik Proseus
MWN Meteorologist

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How a mulch fire and atmospheric inversion create a smoky morning commute

As we drove into town last evening from a few (well-deserved I might say) days at the beach with family, a huge plume of smoke emanated from the area around I-40 and Sycamore View near Bartlett. I didn't know what was on fire at the time, nor did I realize (since I had been away from detailed atmospheric analysis since late last week) what the ramifications would be this morning.

However, an early glance at Twitter this morning and then a drive through Bartlett on my way to work quickly made clear the situation. A combination of fire and smoke continuing through the overnight hours from a huge mulch fire and a surface-based "thermal (atmospheric) inversion" had created a very smoky commute through central Shelby County!

Photos taken about 8:30am Wednesday. This one was several miles northeast of the mulch fire, but smoke was still present in the air, creating a beautiful set of "crepuscular rays" through the trees.

Approaching I-40 southbound on Sycamore View, the smoke got thick and acrid with visibility below 1/2 mile (note you can't see the I-40 overpass from Summer Avenue, a distance of  0.6 mile).

On I-40 east of Sycamore View, the dense low-level smoke is trapped below an inversion and is thickest in the area of the overnight fire that was still smoldering this morning

The Atmosphere, Inverted!

A surface-based inversion is created when the lowest level of the atmosphere (typically a couple thousand feet above the ground) cools more quickly than the layer above it. This is not uncommon and, in fact, occurs more often than not when the wind is light to calm. When an inversion is in place, the temperature actually warms with height, instead of cools (which is the typical thermal profile during the daytime).

The typical setup during a surface-based inversion and after it dissipates, courtesy UCSI University.

An aircraft sounding from Memphis International shortly before 7am Wednesday shows the temperature (in red) rising from the low 50s at the ground to the mid 60s at 1000 feet before falling slowly above that height. The pronounced temperature increase is the " thermal inversion" which effectively "capped" the polluted surface-based air from escaping.

The warming air creates a sort of "lid" on the lower atmosphere, trapping it near the ground and disallowing smoke, fog, smog, etc. from dissipating upwards. It's not unlike the "cap" we talk about relative to summertime thunderstorm development, only this inversion occurs close to the ground, not in the middle layers of the atmosphere.

About this morning's inversion

The "strength" of the cap, or inversion, is determined by the difference in the temperature between the cool air below and warmer air above. Sometimes it is only a few degrees, sometimes much more. The stronger the inversion (or bigger the temperature difference), the stronger the "lid" is between the low levels and the air above. This morning, the inversion was very strong! In fact, outside the city, temperatures were in the mid 40s (lower 50s in the city) and the warm layer above the ground was in the mid to upper 60s. That's a 15-20°F inversion!

The other interesting feature about this morning's inversion was that it was based very close to the Earth's surface. That warm layer in the mid 60s was only about 1000 feet above the ground. That means if you were to go up 1000 feet, it would be 20° warmer than at the ground. It also means that all that smoke was trapped in air that was only 1000 feet deep.  If it were 2000-3000 feet thick, the smoke would have had much more air (by volume) to disperse through. Instead, it spread out at the surface rather than escaping up.

Air quality alerts

As we know, smoke is not something you want to be breathing, as it is harmful to the lungs due to particulate matter that is an ingredient in the smoke. This morning, a Shelby County Health Department air monitoring station at Shelby Farms actually went "code red" for a while due to the harmful particulates in the air. (It's a good thing there wasn't a station at the Summer 4 Drive-In!) The Health Department issued a statement indicating that those with breathing issues or other illnesses stay indoors and others not exert too much energy in the affected areas.

An air quality monitor operated by us at Shelby Farms recorded levels of particulate matter at a concentration considered unhealthy.

— Shelby County Health (@ShelbyTNHealth) October 14, 2015

At the levels being recorded at this one monitor, everyone may begin to experience health effects.

— Shelby County Health (@ShelbyTNHealth) October 14, 2015

So how did the majority of the low-level smoke (outside of the source of it at the fire site) dissipate? The inversion disappeared as the low level temperatures warmed. As soon as the lowest 1000 feet of the atmosphere rose into the mid 60s (which occurred by mid-morning), the inversion was gone and air from the very low levels could mix with the air above, allowing the smoke to rise and disperse into the atmosphere above the morning inversion. However, the Health Department indicates that a Code Orange air quality alert remains in effect through midnight tonight, which means the air continues to be "Unhealthy for Sensitive Groups."

We have issued a “CODE ORANGE” air quality advisory for fine particulate matter due to an expected monitored exceedance.

— Shelby County Health (@ShelbyTNHealth) October 14, 2015

Sensitive groups include the very old and young and anyone with respiratory disorders or cardiovascular problems.

— Shelby County Health (@ShelbyTNHealth) October 14, 2015

So, the takeaway is that while an inversion is not rare, it was simply stronger and lower than usual this morning and we had a source of pollution to gauge just how it works. You can "breathe easier" now that you got your weather 101 for the day! How bad was the smoke where you live or work?

Erik Proseus
MWN Meteorologist

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Weather Mythbusters: Disproving urban myths related to today's "disappearing" snow

Many of you were diligently watching radar this afternoon as snow moved towards the area from the west. You were excited by it's approach. You were disappointed by it "disappearing" from the radar loop as it got into Crittenden County, to our west. Of course, this visual didn't help to dispel the urban myths of the powers of the Pyramid and Bluff.

So let me take a moment to explain what you were actually seeing using the example loops taken from this afternoon, a radar schematic, and a cross-section of the radar returns.

First, here's one example of what you were likely seeing (click play and watch it a few times). Whoa! The snow coming in from Arkansas vanished as it got closer to Memphis! And if you looks close, it re-appears to our east! Must be the bluffs.... or the Pyramid... or the snow dome... or, or... maybe it just needs scientific explanation!

Here's a loop taken just a bit earlier, as seen from StormView Radar, which many of you diligently use (thank you!). It seems there's a wall or giant snow-eater on the western side of Crittenden County.

So what was really going on?  Let me explain.

Below is a similar scenario from a cross-sectional point of view (pretend the rain drops are snowflakes - and this is not exactly to scale). Precipitation fell into a very dry layer of air beneath the clouds. As it did, the precipitation evaporated before reaching the ground. It's very common when dry air is in place in the lowest few thousand feet of the atmosphere. The falling precip that evaporates is called virga (with a hard G).

The radar scans upward slightly so the radar beam samples dry air (the red dashed line) out to about 20 miles in our scenario. When it gets past 20 miles, it "sees" the falling precipitation (blue dashed line) that had evaporated by the time it got to 3000 feet and so it paints precipitation on the radar display 20+ miles away from the radar. HOWEVER, as it turns out, that precipitation we see on radar is not actually reaching the ground!

In the loop from StormView Radar above, the "snow" west of Memphis (say at Wynne) was actually well above ground, but not making it to the ground, a.k.a. virga. As you get closer to the radar (in Millington), the precip appears to vanish because the radar beam is closer to the ground and the precipitation has evaporated at that lower level. Thus, it appears the precipitation is dissipating as it moves closer to Memphis when really none of the precip to the west was reaching the ground in the first place!

So how do we know it's not reaching the ground? A couple of ways - 1) your reports, or 2) cross-sections of the radar returns through the atmosphere (see below).  #1 is why we VALUE your reports! Except right near the radar, it doesn't actually see what's going on at the ground level. We need your eyes and reports to let us know (especially when we know there is a lot of dry low-level air) if the precip is reaching the ground and what type it is.  Note: it's also why we need reports of tornadoes, because the radar is typically looking above where where tornadoes form.  They can't "see" tornadoes, though they can infer them.

As for #2, let's take a look at a cross-section of virga from this evening. The first image below shows an area of precipitation east of Memphis at 7:30. There were no reports of precipitation in this case, but the radar sure looks like it's doing something!

The red dashed line above represents the line along which the cross-section was taken below.  In the image below, we are looking sideways along that dashed line with the ground at the bottom and the top of the image at 20,000'.  The 5,000', 7,500', and 10,000' lines are also marked.

Precipitation appears from about 5,000' to about 15,000' (on average). Based on nearby observations, the base of the clouds was at about 7,500' (red horizontal line). So, the precipitation being detected is in the clouds from 7,500'-15,000'. The precipitation was falling from the clouds down to about 5,000' before dissipating due to the dry air it was falling into. So, while the top image shows precipitation over Hardeman, Chester, and McNairy Counties, a closer look actually shows this precipitation wasn't reaching the ground.

Bottom line: The bluffs, Pyramid, snow dome, and snow-eaters had NOTHING to do with the precipitation dissipating as it moved into Memphis. It was snow aloft that was evaporating before reaching the ground and the radar happened to "see" that precipitation where it was occurring - in the clouds and just below. When you get closer to the radar, the radar beam was below the evaporating precipitation and not seeing it. The fact that it showed back up on the east side of Memphis in the first loop above means that the same thing was happening over the radar as well - it just didn't see it due to the angle of the radar beam.

(By the way, if the radar were in Wynne and you lived there, you'd see nothing over Wynne and precipitation over Memphis and think that the snow was going around you there too! There aren't any bluffs in Wynne...)

Thanks for reading, and let me know if this helped you better understand what you were seeing on radar today!

Weather 102 - A Step Further: The radar product being used above and on most web and app radars only shows what the lowest angle of the radar sees. Radar actually scan at many different "elevations" above ground. There are radar products that take data from all these levels and paint the radar image based on all of them. They are called Composite Reflectivity or Vertically Integrated Liquid Water (VIL). 

Some radar programs or web displays show these products. They don't necessarily show the "donut hole" around the radar site like the above loops do, but they do show all of the virga and tend to overestimate the actual intensity of the precipitation at the ground. That's a topic for another day!

Erik Proseus
MWN Meteorologist

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Microbursts produce wind damage in Shelby County on Monday night

Severe thunderstorms rolled through the Memphis metro on Monday late afternoon and evening ahead of a strong cold front of Canadian origin. A pre-frontal trough plowed into a very unstable airmass featuring temperatures in the mid 90s, high dewpoints, and high total atmospheric moisture that contributed to torrential rain in some storms, even though it generally didn't last very long.

One result of the early storms that formed in the late afternoon heat were strong downbursts of wind, called microbursts, that resulted in straight-line wind damage in some areas, including trees and branches down and even some power infrastructure damage. Microbursts, or downbursts, occur when a mature thunderstorm collapses (part of the normal life cycle of a storm). When this happens, the air that is forced up in thunderstorms (updrafts) quickly falls to the ground, hits the earth, and spreads out in all directions, like ripples in a pond spreading out when a rock is tossed in. Microbursts have been known to produce a great deal of damage due to wind exceeding 100 mph! The wind that spreads out creates outflow boundaries, or gust fronts. Often if the wind is strong enough (58 mph or higher), a Severe Thunderstorm Warning will be issued for these phenomena and it's best to seek shelter when the tell-tale outflow boundary, or shelf cloud, is approaching!

Schematic of a micrburst, in which a thunderstorm downdraft hits the ground and the wind spreads out in all directions.

It is important to know that Doppler radar only senses wind moving towards or away from the radar, not side to side. So while the wind near the surface spreads out in all directions when a microburst occurs, the radar only "sees" the components of the wind moving along the radar beam - towards or away from the radar.

Two radar images from late Monday afternoon show the microbursts as they were occurring. The first image below shows the Doppler data (wind direction and speed) for a microburst between Germantown and Collierville. The NEXRAD is northwest (up and left) of this location, so the green colors are showing wind blowing towards the radar and the red colors are showing wind blowing away from the radar. In between is where the downdraft wind hit the ground, then spread out. Doppler estimates are about 45 mph in the green area and 35 mph in the red area. Therefore, the wind that was collapsing down from the thunderstorm was traveling much faster than either of those values, resulting in the most damage directly under the microburst.

Doppler velocity data indicates the presence of a microburst near Collierville on July 14 with strong NW wind over Collierville and stronger SE wind on the east side of Germantown.

The second microburst image came from the Bartlett area about 20 minutes later. Once again, the NEXRAD is northwest (up and left) of this location, so the green colors are showing wind blowing towards the radar and the red colors are showing wind blowing away from the radar. This microburst (at least at this time) was not as strong as the Collierville one, but nevertheless produced very gusty wind over the east side of Bartlett in particular.

Doppler velocity data indicates another microburst near I-40 and Sycamore View with strong southeast wind over Bartlett and NW wind heading into Cordova.

When microbursts hit the ground, wind can seemingly come from all directions or "swirl" as the air moves chaotically in that localized area. Some people report this swirling wind as a tornado, but the microburst is a very different wind phenomena from a tornado. Typically, those surveying storm damage will see a wind pattern that spreads out from a central point in the case of microbursts, which make it fairly obvious that a straight-line wind event occurred, rather than a tornado. Your tree or the power pole really doesn't care what hit it when it's laying on the ground though! It just knows it got hit by a wind it couldn't stand up to!

Do you have any questions about microbursts or other wind phenomena? Leave them in the comments and we'll be sure to answer them!

Erik Proseus
MWN Meteorologist

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This rain won't clean your car off!

We've received several reports from social media this morning of sprinkles on cars and other outdoor objects that leave behind a residue, or dust, after evaporating. In fact, I had noticed the same thing on my windshield yesterday and assumed pollen, but didn't look closely. The pictures received though we definitely NOT pollen. It was dirt.

Morning sprinkles evaporated, leaving these dust spots on Carissa H.'s car (@rissa424)

A couple of thoughts immediately came to mind. First (and this one seemed less plausible than the second) was dust/dirt from Arkansas that was sucked up by tornadoes on Sunday and deposited downstream. I had heard of such a thing happening, but the timing (2 days later) and wind patterns didn't quite make sense. The second option was dust from the plains carried east on upper level winds. Knowing there was a massive trough over the middle of the country and a strong jet stream blowing from west to east across our area, I knew that option was more plausible.

Sure enough, after a little digging and checking in with the NWS to get their opinion, I'm fairly confident that it is indeed dust from the plains. For the past few days, strong and gusty north wind has prompted dust storms across the central and southern plains. In fact, the NWS watch/warning map below shows Wind Advisories and High Wind/Dust Storm Warnings in effect right now across the plains.

Then, I got a text from a family member who happens to be on the interstate in the TX panhandle describing vicious wind and "raining mud!" Sure enough, dust storms were in progress and light rain was dropping the dirt back to earth! Dust storms are known to eject fine dust particles thousands of feet into the air where they are carried by the currents downstream.  Below is the current upper level (about 35,000') jet stream courtesy of an awesome animated web tool called the "Wind Map."  The large L is the low pressure trough that the jetstream wind is flowing around. Key points are labelled.

So you can see that as the dust is picked up and ejected into the upper levels of the atmosphere, it gets carried downstream and deposited or rained out of the atmosphere! The light rain falling this afternoon is not going to be good at washing pollen off your car, but instead could result in a trip to the car wash thanks to strong wind and plains dust that is transported all the way to Tennessee!

Erik Proseus
MWN Meteorologist

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Watch for falling virga!

Some interesting cloud formations floated over the Memphis metro this morning as an upper-level disturbance moved through the area. Below is a great picture taken by Angela High along the north loop of I-40.

Virga descending from clouds over north Memphis on Wednesday morning. Photo credit: Angela High.

With the disturbance passing by, these clouds had enough moisture to generate precipitation.  However, due to very dry air in the layer beneath the clouds and above the ground, the precipitation evaporated as it fell.  This evaporating precipitation, which looks like rain falling but never reaches the ground is called virga.  Due to the evaporation process, some very interesting cloud formations can result - streaks that reach towards the ground, "balls" of cloud that appear to be descending, wispy clouds, etc.

Below is a "sounding" of the atmosphere over Memphis at 9am this morning.  Temperature is the red line, dewpoint is the green line and the top of the image is at about 26,000' while the earth's surface is at the bottom.  When the green and red lines are close together, the humidity is high; when they are far apart, humidity is very low, allowing evaporation to occur.

This morning, the base of the clouds were at about 15,000' and the precipitation was falling into a very dry area from a few thousand feet off the ground up to 10,000'. This is where the evaporation took place and virga formed.

Atmospheric sounding over Memphis Wednesday morning, showing how virga formed.

Did you see the virga, and did you know what it was when you saw it?  From now on, you will!

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Fog envelopes downtown Memphis on a frosty morning

Downtown Memphis was treated to a somewhat rare sight early this morning, especially if one had a vantage point from above.  A thick cloud of fog formed over the Mississippi River and drifted over downtown streets!

Photos taken by Jonathan May (Twitter: @jmay11) from the 29th floor balcony of One Commerce Square downtown (shown below) captured the essence of the surreal sight. Note that even the Hernando-DeSoto Bridge carrying I-40 traffic is nearly completely enveloped in the fog.  The bottom photo is taken looking southeast, away from the river, and shows that a "branch" of the fog had separated from the river and drifted east of the FedExForum towards the eastern portions of downtown.

Process of steam fog creation, courtesy USA Today.

What caused the fog?  On calm, chilly fall mornings, "steam fog" or "evaporation fog" is a common occurrence over relatively warmer bodies of water.  The necessary ingredients are the warm water below and cold, somewhat dry, air above, which are found in combination most frequently in the fall.  If you've ever driven by a pond or other small body of water, especially in an outlying or rural area on a cold fall morning, it's not unusual to see low-lying fog over the water.  This is called steam fog and it forms when cold air moves over warm water. As the warm water evaporates, it rises into the cooler air just above, cooling the rising air.  The cooling process changes the water vapor created by evaporation into visible fog.  In this case, the warm water of the Mississippi River produced steam fog on a grand scale!

Air temperatures over the water were in the mid 30s, while the temperature of the water was likely 20 degrees warmer. Fortunately, temperatures stayed just above freezing downtown, but had they been a few degrees colder, steam fog can quickly produce "freezing fog," which leads to black ice, particularly on bridges.  A few degrees cooler and this spectacle could have become a nightmare for commuters!

How can you create fog in the same manner steam fog is formed?  Exhale outside on a cold morning! See your breath?  You just mixed warm moist air into cold dry air.  You made steam fog!

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