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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A hot mess - and why dewpoint is more important than humidity

An Excessive Heat Warning has been issued for the majority of the metro (Fayette County is technically under a Heat Advisory, but I'm not quibbling when the heat index has reached the danger level). The Excessive Heat Warning is in effect until Sunday at 8pm, but I have high confidence that we'll see it extended, possibly through Wednesday, before a front finally starts to move in our direction by week's end.

Excessive Heat Warning through Sunday in pink, Heat Advisory in orange. Graphic courtesy NWS-Memphis.

After a short break from the mid and upper 90s (but not the high humidity) earlier this week, plus a few rounds of strong thunderstorms, upper level ridging builds anew and can be thanked (or cursed) for pushing temps back into the mid 90s to near 100 for the next several days.  Rain chances are minimal to nil through Wednesday as well. These are the dog days... woof.

If it's any consolation, we are nearing the end of the hottest part of the year, climatologically. When eliminating rounding of temperatures to the nearest degree, the period from July 11-27 has the highest average daily temperature at 82.8°. Starting next week we slip a tenth of a degree or so! OK, that didn't help you any.  Maybe this will...

Perhaps this will make you feel a little better Memphis!
Posted by MemphisWeather.net on Saturday, July 25, 2015

So, besides hot temperatures, why are we seeing heat index values near or above 110°? Maybe you've heard this before: "It's not the heat, it's the humidity." (Well, in this case, it's the heat too. Let's not kid ourselves!) But there is truth to that statement. Let's look a little closer at humidity.

Relative humidity vs. Dewpoint

When humidity values are reported, it's actually "relative humidity" that we're talking about. It's relative to the temperature. For instance, if you track humidity during a normal day, you'll see that it peaks around sunrise and bottoms out in the late afternoon. However, even though the air is closer to saturation when relative humidity is highest, relative humidity doesn't actually tell you how much water content is in the air. Besides temperature, water content is the most important factor in determining how "uncomfortable" it feels.

Consider a calm, clear spring morning with temperatures in the mid 50s. The relative humidity is 90%. Now consider early this morning, with temperatures near 80 and relative humidity of 80%. In our example, the humidity was 10% lower this morning, so it wasn't as sticky as our spring example morning right? Wrong! Behold, I give you the more appropriate measure of water content (and uncomfortability - yes I made that up): DEWPOINT.  Dewpoint is the temperature to which air must be cooled to be saturated (or achieve 100% relative humidity).

Let's look at yesterday's observations from Memphis International Airport (below). Notice the humidity (yellow highlight column) falls from 88% in the morning when the temperature was 76° to 49% in the afternoon when the temperature was 95° (orange highlighted rows). That humidity value is "relative" to the temperature. Did the amount of water content in the air change from one time to another? Barely. We know this by looking at dewpoint (green highlight column). It was 72° when the temperature was 76° and 73° when the temperature was 95°. It inched up 1°, thus the amount of water content of the air changed very little. You'll also notice that the dewpoint is generally is more consistent (in the 72-76° range all day) than relative humidity, which varied from about 50-90%.

So, you say "50% humidity doesn't sound all that high!" Did you go outside yesterday afternoon? The air felt very humid! 50% in the morning is low humidity, but 50% in the afternoon is high! Dewpoint helps straighten all this out, since it measures the amount of water vapor in the air no matter the temperature!

Dewpoints above 65° usually cause people to start to notice the "humidity" in the air. Dewpoints above about 72° are very uncomfortable and when a dewpoint reaches the upper 70s to near 80° it's best to just avoid the outdoors altogether, especially if you have health issues! The amount of water vapor in the air at a dewpoint of 80° is so high that it can cause heat illness fairly quickly for those who aren't taking every possible precaution.

It's rare to see dewpoints this high at properly sited professional equipment that is well-maintained and in open areas, such as at large airports. However, in the MWN backyard, the grass is green and thick, moisture is constantly evaporating due to the heat, and there is less mixing of the air due to being sited in a neighborhood with obstructions. Even with well-maintained semi-professional equipment, the dewpoint has been reading 80° recently. I can assure you, having to mow that green thick grass that I can feel every degree of that ridiculous dewpoint! The sweat does little evaporating with that much moisture already in the air.

MWN's Bartlett reporting station, surrounded by grass and slightly sheltered by fences. These factors contribute to a higher dewpoint in this locale than at the airport.

So when someone comments on the humidity from now on, you can take a mental note that it's actually the dewpoint that determines how uncomfortable it feels!

Erik Proseus
MWN Meteorologist

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How humid is it? Why dew point is a superior measure of humidity

Humidity can often be a misunderstood concept due to the different ways in which it is measured. The most familiar of these measurements is probably relative humidity. Although it is most familiar, that does not mean it is the most understood. In fact, it’s probably the least understood.

Relative humidity is measured as a percentage and its name is appropriate because it truly is a relative measurement. Saying the humidity is 75% outside tells you nothing about how humid it actually feels. The percentage is relative to the current temperature. For example, a cold winter morning could have 100% relative humidity and the temperature could be 35 degrees. This just means the temperature of the air has dropped to the point of saturation. Now, on a warm summer night the humidity can also be 100% and actually feel incredibly humid. Let’s say the temperature in this situation is 75 degrees. In both situations, the relative humidity is 100%, but without knowing the temperature you have no idea what this humidity actually feels like. Clearly the cold winter night feels much less humid than the warm summer night, although both measurements of humidity are the same.

This brings us to the superior measurement of moisture in the air, known as dew point. Dew point will tell you exactly how humid the air is regardless of the temperature. It is a measure of "absolute humidity," or the actual moisture content in the air.  In our previous example, since the relative humidity was 100%, the dew points were equal to the temperatures, meaning the air was saturated in both cases. But a dew point of 35 degrees is very different than a dew point of 75 degrees! Just like higher temperatures equal hotter conditions, the higher your dew point is the more humid it feels, because the amount of moisture in the air is higher. A dew point in the 70’s, which is very uncomfortable to most people, can be common on a southern summer day. The dew point may not change much during the day, remaining in the 70’s. Our example demonstrated exactly how useless relative humidity can be when you want to know how moist the air actually is. This is because relative humidity gets close to 100% just about every night, while it gets lower during the day as the temperature rises - even if the dewpoint does not change! In other words, relative humidity is always changing.

As you can see relative humidity and temperature are inversely related. Dew point is an actual measurement of the current moisture content of the air, not relative to the current temperature of the air.

So the next time someone says "the humidity is awful this afternoon," you will know that they are likely referring to the dew point even if they don't know it! Humidity is relative - dew point is not!

--William Churchill, MWN Intern

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A few unsettled days then a taste of early autumn, plus a note on the Perseids

For well over a week, NOAA's Climate Prediction Center, which issues medium range forecasts that go out up to 14 days, has been advertising a cool spell for mid August.  I tried to imagine that as I sweat through my clothes Friday morning while mowing the yard in upper 70s dewpoints and 80% humidity! Turns out, it looks like they were right... but more on that in a minute.

For the first half of this week, we get to deal with a rainy, humid pattern that results from a frontal system that has taken up residence over the region.  When the front was north of us, it was the railroad track that allowed training storm systems to flood southern Missouri and portions of northern AR. The past day or so, it slipped south and brought much of the metro a couple of rounds of thunderstorms (nearly 2" fell today at MWN headquarters in Bartlett).  As of this evening, it has again retreated just to our north, but will remain in the area until it gets a good shove from a stronger system and change in the upper level wind pattern.

So, for the next couple of days (at least into Wednesday morning), the front and tropical moisture from the Gulf will mean continued humid conditions and chances of t'storms. In fact, as the front draws closer again on Tuesday (this time pushed by another cold front that will finally "clear the air" of all this humidity and rain chances), thunderstorm chances will increase - with a good chance everyone sees decent rainfall by Wednesday morning. Any storms will be capable of very heavy rain thanks to the moisture-laden atmosphere overhead, as well as copious lightning and a few strong wind gusts, especially from any afternoon storms in near-90 degree air.

On Wednesday, everything changes.

Early taste of autumn

A trough (basically a "valley" in the upper-level weather pattern which typically hosts cooler air) will set up over the northern half of the eastern U.S.  This trough will finally push a humidity-busting cold front through the Mid-South. With cooler high pressure at the surface building in over the Ohio Valley, we'll finally see an end (even if temporarily) to the oppressive humidity and rainy pattern. In addition, temperatures will be below normal to end the week and start the weekend! Recall that it will still be August though, which means summer is not over, but hopefully everyone will appreciate the reprieve and early taste of autumn!  A couple of graphics below demonstrate the change in airmass and it's results for Mid-Southerners.

(By the way, many have asked, and there is no correlation between a mild summer - one in which temperatures have not threatened 100 degrees even once locally - and the following winter.)

On Friday morning, the GFS model shows a significant trough across the Mississippi Valley at the 30,000' (jetstream) level. Upper-level troughs indicate an intrusion of cooler air into the region under the trough.

GFS model temperature (red), dewpoint (blue) and relative humidity (green) for this week.
Note in particular the dewpoint drop beginning Wednesday the 14th , with some 50s early Friday and Saturday mornings. Also, note temps that peak in the lower 80s late this week and fall to near 60 a couple of mornings. This is an early guess and is likely overstating cool air, but it's a hint of early fall weather that is likely.

The Climate Prediction Center gives Memphis a 65% chance of below normal temperatures for the period August 17-21 and a 32% chance of near normal temperatures (which basically means almost no chance of above normal temps!)

Perseid Meteor Shower

Last note: tonight and tomorrow night are the peak viewing nights for the annual Perseid meteor shower. It's a shame that the weather pattern is promoting clouds across the area, because this meteor shower is typically one of the best of the year. Perseids can be visible dozens of times per hour given the right conditions.  If you want to try and see a few, the best viewing is done well away from city and artificial lights, looking straight up, from midnight until just before dawn.  If you just want to learn more, here's a great article from Space.com: Perseid Meteor Shower is Peaking Now: How to Watch and here's a site where you can watch the meteors live online!

Image credit Space.com

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Weather 102: Basics on the wet-bulb process and why you should care

Warning: this post is a little technical and lengthy! I've tried to simplify it for the average reader.  Let me know how I've done!

In developing winter weather scenarios, meteorologists (often not in front of you, lest they forever be known as weather geeks!) will often refer to the "wet-bulb temperature," or the wet-bulb process.  In it's simplest form, the wet-bulb temperature is a combination of the air temperature (or what is actually called the dry-bulb temperature) and the amount of water vapor in the air, typically measured by the dewpoint temperature.  Temperature and dewpoint are terms you have probably heard before.  Dewpoint is a measure of the amount of moisture in the air, which, if the air were cooled to that temperature, there would be 100% relative humidity.  So, if the temperature is 40 degrees and the humidity were 55%, the dewpoint would be 25.  In other words, cooling the air down to 25 degrees would produce 100% humidity.

The wet-bulb, on the other hand, falls between the air temperature and the dewpoint temperature.  It is defined as the lowest temperature that can be reached by evaporation only.  As rain/snow falls from clouds, evaporation takes place as the precipitation falls into drier air.  As the evaporation occurs, the amount of moisture in the air goes up, the dewpoint increases, and the temperature decreases, bringing them closer together.

"Wet-bulbbing" is important in winter weather scenarios (and actually all precipitation onset situations) because as the initial precipitation falls from clouds, it first saturates the atmosphere it is falling into through evaporation.  This process (called "evaporative cooling") lowers the temperature and increases the dewpoint.  If the temperature lowers to a point below freezing (when the wet-bulb temperature is below freezing), a temperature in the upper 30s could end up falling into the lower 30s and producing winter weather!

An example of the wet-bulb process can be seen as precipitation began on January 9, 2011 in the graph of temperature and dewpoint in Bartlett below.  Notice the temperature (white) fall and dewpoint (yellow) quickly rise between 6-8pm.  The 30 degree temperature ended up in the mid 20s and the dewpoint rose from the single digits to the lower 20s.  Precipitation falling shortly after 6pm did not reach the ground, instead evaporating and causing the wet-bulb process to occur. By 8pm, moderate snow was reaching the ground.  The difference between the temperature and dewpoint until 6pm also delayed the initial onset of the precipitation, likely resulting in virga (precipitation seen falling from the clouds but evaporating before reaching the surface).

The wet-bulb process occurred between 6-8pm on January 9, 2011.

Why bring this up now? Well, the process is occurring right now in the Memphis area!  Very light precipitation is falling from the clouds and evaporating. The difference between temperature and dewpoint is about 10 degrees (38 and 28). Through the evening, those two numbers will approach each other (nearing the wet-bulb temperature) and light rain will change to light snow due to the surface temperature falling!  Watch for it on the WXLIVE! graphical page, noting the temp/dewpoint graph.

You can get an idea of what the wet-bulb temperature is by moving air over wet skin. Are you cold when you first get out of the shower? In essence, you're experiencing the wet-bulb temperature of your bathroom!  The air feels cooler when exposed to a wet surface - you!

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Weather Thoughts for August 4

There were a couple of weather events worth noting today...

First, the NWS cancelled our Heat Advisory mid-afternoon today as heat indices weren't reaching the critical 105 degree level. The reason? Dewpoints. See my blog from July 11 on The Importance of Dewpoint. Even though the temperature climbed to near 100 today, the absolute amount of moisture in the air (dewpoint) fell this afternoon and heat indices didn't reach the necessary 105 degrees to constitute a heat advisory. This tells you just how difficult it can be to forecast something like the dewpoint. Yesterday, we had no trouble reaching 105-110 heat index as dewpoints stayed in the mid 70s. Today, they dropped into the 60s. The NWS does expect dewpoints to be back up tomorrow (as do I) so the Heat Advisory has been re-issued for tomorrow afternoon. We'll watch the dewpoints to see!

Also, Tropical Storm Edouard (or just Eddie) is churning in the northern Gulf just offshore Louisiana. Eddie's moving west towards the Houston/Galveston area with a projected landfall around Galveston Bay mid-morning Tuesday. Max wind, though just 45 mph now, are expected to increase steadily overnight, reaching just less than hurricane strength by landfall (65-70 mph). The inland track takes him into central TX with no impact to the Mid-South (unless it pulls a Dolly!). Fortunately he isn't near the storm Rita was a few years back when the entire city of Houston tried to evacuate - leaving motorists stranded on interstates for HOURS and HOURS as the storm side-swiped the city.

Finally, since it's my blog, I guess I can brag just a little... The MWN Forecast accuracy stats for July are in and my second period temperature forecasts averaged an error of just 0.85 degrees. That means of forecasts made for the second period (47 morning forecasts of that night's low and afternoon forecasts of the next day's high), I was off by less than a degree. For all MWN temperature forecasts made (235 total), my margin of error was 1.46 degrees - my best showing since July 2003. For more accuracy stats, visit the MWN Forecast Accuracy page.

The importance of dewpoint

In an effort to educate on this blog, I bring you today's topic: the importance of dewpoint.

Dewpoint, as defined by the National Weather Service, is "the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant)" (ref: weather.gov). Or, more simply, it is a measure of the amount of atmospheric moisture. Why is it important? Well, the amount of moisture in the air helps determine things like whether clouds or precipitation will form and how "humid" it feels. Frequently in these parts, you'll hear someone say, "the humidity sure is high today." What they are talking about (maybe without knowing it) is how humid or sticky the air feels, as measured by relative humidity. Relative humidity is calculated using the temperature and dewpoint. Based on the definition above, if the temperature is 85 and the dewpoint is 65, that means the 85 degree air must be cooled to 65 for it to be saturated (or to reach 100% relative humidity). Therefore, the relative humidity can be calculated (using a formula) as 51%.

It is very important when forecasting temperatures and humidity to know what the dewpoint will be as well. In fact, it is so important, that it is one of the parameters I keep accuracy statistics on when making my forecasts. When I write a forecast, I always also forecast dewpoint, even though it does not appear on my actual forecast page. It is very helpful in knowing what a morning low temperature will be (the temperature can never fall below the dewpoint, otherwise humidity would be above 100%!), what the heat index will be (temperature and humidity are used to calculate heat index), and how much moisture is in place to feed potential thunderstorm development. It's also important to know the temperature and dewpoint spread above the surface (if temperature and dewpoint converge, the humidity nears 100% and clouds form at that level).

So, dewpoint is one of the parameters that must be taken into account when producing an accurate forecast. And next time you hear someone say "the humidity sure is high" - correct them by saying "perhaps it's the dewpoint." They'll probably look at you funny, and that's OK. ;-)