Thunderstorm Terminology

This is my first post in a new series of blogs, where I’m hoping to: (a) provide some basic information about confusing weather topics, and (b) have a new sense of motivation for writing blog posts on a regular basis.

What does it mean when a weather forecaster says, “Expect scattered thunderstorms this afternoon”? What about “isolated thunderstorms”, or “widespread storms”? I will try to explain these common terms used to describe the thunderstorms that are so common during the summer months. Disclaimer: The following is my opinion, and may or may not be standard practice for the weather forecasting industry. However, I would like to believe that it represents a typical classification of thunderstorms.

When I hear the phrase “isolated thunderstorms”, I think of a predominantly sunny region with one or two thunderstorms in the area. Nearly everyone doing outdoor activities will have no weather troubles. Nonetheless, there may be a couple of people who are unlucky enough to have a thunderstorm interruption. The weather radar picture often looks something like this:

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Isolated thunderstorms (one or two storms southwest of Miami).

How about “scattered thunderstorms”? To me, this means a greater coverage of storms. At this point, some people’s outdoor activities will be impacted, but not all. A good number of people will see sunny skies all afternoon and might wonder why the forecaster called for a “40% chance of thunderstorms”. (There was a good chance of storms, not a guarantee.) The radar picture for scattered storms might look something like this:

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Scattered thunderstorms.

Finally, what about “widespread” or “numerous” thunderstorms? I see this as a significant amount of rainy, stormy weather. Almost everyone will see some impact from an afternoon of numerous thunderstorms. In this case, it would be rare to find a person that was not impacted by the storms. Some radar examples of this include the following:

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Numerous thunderstorms (only in the area north of Tampa and Orlando, FL).

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More numerous thunderstorms (a close-up of the area in the previous picture).

Another example of “numerous thunderstorms” can happen if the storms travel in a continuous line. In cases where there is a line of storms, most people will experience storms, just not all at the same time. I would still consider this an example of “numerous thunderstorms”, with a timing caveat:

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A line of numerous thunderstorms moving to the south-southeast.

Each weather forecaster or meteorologist will use different words to describe thunderstorm activity, but I’ve just picked the most common ones to describe here. If you have any questions or suggestions, feel free to leave a comment below. Also, if you have an idea for a future topic, please leave a comment as well. My goal is to write a blog post like this every Wednesday, which I’ll call “Weather Wednesdays.” (Really, every day is weather day for me, but I’m always an advocate of alliteration.)

New Writing Topics

This blog is, once again, on shaky ground due to a quiet, dry summer with not much interesting, dynamic weather to write about. Also, the 3-month break from school has left me in a very lazy frame of mind. A bad combination like this leads to the writer’s block (laziness) I’m having.

But today, I have a new idea. What if I write occasional (weekly?) blog posts about confusing topics about weather and weather forecasting? From a “general public” point of view, the weather seems more confusing than ever before, with the rapid spread of information (mainly aided by social media) about weather and forecasting. I’m thinking about writing once a week about a single topic, weather phrase, or phenomenon (e.g. what causes a tornado outbreak, the meaning/difference between scattered vs. isolated thunderstorms, what is the polar vortex, etc.), hopefully explained well enough that anyone can understand it. I’ll tentatively label these blogs “Weather Wednesdays”. Obviously, these posts could range in length from a short paragraph to a 8-10 page thesis paper, depending on the topic. However, I’d like to keep the blog posts on the shortish side. Thoughts?

Next to Nothing

Well, the lack of interesting weather to write about has lasted so long that it is becoming interesting. How’s that for a contradictory sentence? It has been so dry here over the past month that my CoCoRaHS gauge has become one of the driest gauges in the country. With about 0.5 inches of rain, my gauge is the driest one on the whole East Coast and the second-driest gauge east of the Mississippi! Only a gauge in Toledo, OH is drier, with 0.47 inches of rain.

Rounds of thunderstorms have narrowly missed here, going to the northwest and southeast. Hurricane Arthur missed me to the east earlier in July. Some gauges not far away have seen 8-11 inches of rain, adding insult to injury. Here’s a graphic showing the rain hole in Central Virginia.

Courtesy of NWS Wakefield.

Courtesy of NWS Wakefield.

What is to blame for this dry stretch? A lot of it is just the nature of erratic summer rainfall. Thunderstorms are short-lived and highly variable. This summer has just been a particularly extreme example of thunderstorms forming over the same places, separating the “haves” from the “have-nots”. But the overall weather pattern can take some blame as well. With a ridge of high pressure over the West and a trough of low pressure over Quebec, dry northwest flow tends to set up in the Mid-Atlantic. This has been a common weather pattern this summer. In this pattern, air coming into the area comes from the central U.S., which is dry compared to air coming from the Gulf or the Atlantic. Also, as the flow goes over the Appalachians, it tends to sink and warm on the lee side. This tends to lower the relative humidity somewhat near the surface. Without a deep moisture source, thunderstorms are less common (but still do occur) and tend to drop less rainfall.

Summer Sadness

While the blinding sun cooked the lowly Piedmont with T/Td spreads of 97/77 and heat indices around 105, one weather enthusiast grew excited about the potential for afternoon storms to cool the parched earth. As CAPE values exceeded 3000, and as the aforementioned daytime heating steepened near-surface lapse rates to dry adiabatic levels, this intrepid, hopeful soul watched puffy cumulus clouds fill the afternoon sky. Storm Prediction Center watch boxes filled the news feeds and spirits were high as the lee trough sharpened. Just then, rumors started to spread about slight capping inversions in the mid levels, and, at once, the cumulus clouds withered and died just as the grass was doing in the summer heat. Bitterness and frustration began to seep out as beads of sweat do in a sauna, not unlike the weather conditions of the afternoon. Fists were shaken at the empty promises of far-away convective outflow boundaries over the mountain foothills, as the sun began to set in a now-sunny sky. The Second of July held such potential as magnificent RAP forecast soundings illustrated, but this weather lover will have to settle for the fleeting greeting of much-needed rainfall from the supposed rendezvous of a cold front and the distant Arthur making its way out to sea.

In other news, I have spent a month volunteering at my local National Weather Service office. I promise that those experiences will be compiled into written form and shared here in the near future.

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A promising forecast sounding (from a short-range weather model called the RAP) only results in sun-bathed convective heartbreak.

Thunderstorm Time Lapse

I am always interested in time-lapse photography, and I got an unexpected chance to capture some thunderstorms yesterday! I was just milling around at home, and decided to set up my camera when I saw some developing cumulus clouds to the east. Five hours later, I had gotten a really great series of transient thunderstorms!

 

The first 20 seconds of the video are pretty boring, but then the thunderstorms really start to get going. It was hard to guess where they would form, so I kept moving the camera in the middle of the video. Right at the end, there is a faint rainbow along the right side of the video (but it’s mostly off-screen to the right).

Technical weather discussion begins here (continue reading at your own risk):

What is really happening in this time lapse video? As long as the summertime airmass is sufficiently unstable, meaning that air forced upward will want to continue rising on its own (given several limiting conditions/factors, such as density differences), thunderstorms are likely to form in the mid-afternoon. These types of storms are very common, and are not usually severe (generally no tornadoes, hail, or damaging winds), although they can still be hazardous to people doing outdoor activities. Given a lot of sunshine and warm temperatures in the low 90s like we had yesterday, this was an ideal day for this to occur. During the first 20 seconds of the video, plumes of air heated by the early summer sun are rising, condensing into tiny liquid water droplets, and forming puffy cumulus clouds. Interestingly, all the air condenses at about the same level in the atmosphere. This is called the Lifting Condensation Level, or LCL, and is found at the flat base of the cumulus clouds (notice that this level remains constant through the video).

However, there is another important level in the atmosphere called the LFC, or Level of Free Convection. Why is this important? Any plumes of warm air that bubble up below the LFC tend to not have enough energy to keep rising and form thunderstorms. They sink back down. However, clouds that do rise above the LFC tend to keep rising on their own. All the cumulus clouds that form and decay during the first 20 seconds of the video don’t rise above the LFC. However, the cloud to the far right around the 22-second mark has enough energy and momentum to surpass the LFC and keep rising. It forms a thunderstorm. A number of cumulus clouds later in the video rise above the LFC as well, forming other thunderstorms.

But will the air parcels in the thunderstorm keep rising forever? No. At a certain level, air parcels in a thunderstorm will lose momentum and stop rising. Due to differences in density caused by the difference between the temperature of the rising air parcels compared with the temperature of the surrounding environment, thunderstorms will not grow above a certain height called the Equilibrium Level (EL). Here, air parcels have neither an inclination to rise or sink, and are content to stay at the top of the thunderstorm. They slowly spread out like a pancake, in an area called the anvil of a thunderstorm (looks like an actual anvil), which can be seen in the thunderstorm at right around 0:30.

Why do thunderstorms form in the first place? This is a matter of instability. When the sun is out, it is only able to warm the Earth’s surface (and, by extension, a small layer of atmosphere near the surface). This leaves air aloft, 1000 feet above the surface and higher, a good deal cooler than the warm surface. Surface air parcels want to rise up and cool off in this situation (general definition of instability). One of the most common ways this happens is through thunderstorm formation. Due to complicated issues of thermodynamics (the transfer of heat energy), I won’t elaborate too much. What I will say is that air parcels that rise up in a storm and form rain are able to take heat from the surface and redistribute it into the upper atmosphere, decreasing the temperature imbalance formed by the sun. Paradoxical. You can (and rightfully should) blame the sun for stormy afternoons. It is the culprit.

In summary, what you see in the time lapse video are plumes of air that are too warm to stay put at the surface. They rise and form thunderstorms, trying the bring the atmosphere back into balance. Weather is the resulting dance of the atmosphere, trying to regain balance from imbalances caused by the sun. Neat stuff!

 

Atmospheric Optics

This is a quick post, since I am in the midst of finals week here at school. Yesterday, I was walking back from lunch after getting out of a final exam. I saw this up in the sky, to the west. At first, I was wondering if a lack of sleep and exhaustion was finally taking a toll. But I think the fact that I was able to take a picture of this proves that it actually occurred.

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According to various online sources, this was a circumhorizontal arc (colloquially referred to as a fire rainbow). Sunlight refracts through ice crystals formed in high cirrus clouds, causing the familiar spread of colors. Unlike halos around the sun, which are commonly observed in the winter, this type is only observed when the sun is high overhead. Ice crystals in the clouds must be flat, hexagonal plates in order for this type of rainbow to occur. It was a really neat find! Hopefully it was a good omen for the rest of finals week.

Finally, A Snow Worth Talking About

Well, it feels like a distant memory in a way, but we got an awesome snowstorm here in Asheville two Wednesdays ago (Feb. 12-13). I am finally writing a long overdue post on a great snowstorm. We ended up with almost 7 inches, which was Asheville’s biggest snow since January 10th, 2011! It was also about as much snow as Asheville has seen in the past three winters since then, combined. I hadn’t seen that much snow since February 2010, in the Snowmaggedons that dumped several feet of snow on Virginia and Maryland.

Since then, I have been in Asheville for almost two years now. After more dustings and 1/2 inch snows than I could bear to see (see northwest flow) over the past two winters, it was a really big relief to finally get “the big one,” the big snow. While we didn’t catch some of the very large accumulations that hit north of Charlotte, NC (over 12 inches), and near Roanoke, VA (around 24 inches), it was still a very nice week of snow.

Here’s the breakdown of the snow, which arrived over several days. On Monday, Feb. 10th, we had some heavy snow showers with the passage of a slow frontal system, which drifted south and east of the area. By afternoon, the snow was heavy enough that it accumulated rapidly, even though temperatures were just above freezing. We got around 1-1.5 inches of slushy wet snow by Monday evening.

Tuesday was cool but cloudy. Most of the snow from Monday melted, even though temperatures barely warmed above freezing. There was a small chance of flurries, but this never materialized. I went to bed early on Tuesday night because I was on the “on call” team on Wednesday for SEMPE (Sounding-based Experiment on Mixed Precipitation Events), our research project that launches weather balloons in winter storms. We planned to launch weather balloons starting at 4 am Wednesday morning, so I set my alarm for 2:30 am.

After being too excited to get much sleep, I got up at 2:30. (This was the earliest I have ever gotten up in the morning.) I walked over to the building where we were launching balloons. My teammate, Kurt, and I started preparing the weather balloon for launch. This process began with the setup of the sonde, the instrument that records temperature, humidity, and pressure. A base station tracks the sonde remotely via GPS, and calculates the winds. After the sonde was ready, we went out to fill up the latex weather balloon with helium. When full, it was about 3-4 feet in diameter. Then, we attached a parachute (for when the balloon pops and falls back to earth), and the sonde. Finally, around 4 am, we took the balloon out in the parking lot and released it under cloudy skies in the predawn darkness.

A few extra notes: Inside the sonde, there is a battery, pressure sensor, and other hardware. The only thing sticking out of the cardboard box filled with styrofoam is a metal probe with temperature and humidity sensors on it. The whole device is smaller than a paper carton of orange juice, about half that size.

After launch, we went back inside and tracked the balloon with our software program. As it ascended, we were able to plot the data on a graph called a sounding. The balloons took about 1.5 hours to reach their maximum height, at which point they popped due to the extremely low pressure high in the atmosphere. Some balloons got as high as 65,000 feet, or 12 miles, and ascended to a pressure of about 50 millibars (surface pressure in Asheville was about 945 millibars). We launched balloons every three hours until 4 pm. Another team of two students launched balloons from 7 pm Wednesday to 7 am Thursday. In between launches, we looked at forecasts and other various weather data for the incoming storm.

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Here’s an image of a balloon launch. This was taken in November, for another class. I was a spectator. The balloon is at the top, followed by the orange parachute in the middle, with the sonde (a small, white cardboard box) at the bottom.

On Wednesday morning, the snow was slow to move into the Asheville area. We were expecting snow as early as dawn, but we didn’t even get flurries until 10 am. We had flurries through most of the midday hours, and a light dusting by the afternoon. However, the storm was underwhelming at that point, and we were starting to worry about the potential of a forecast bust. Most weather forecasts had called for more accumulating snow during the day on Wednesday.

By late afternoon, we carried on watching the weather and launching balloons. Around 4 pm, the snow finally started to increase in intensity. Temperatures were steady around 27 degrees. We finally got more than a dusting around sunset as heavier snow bands started to move in. Heavy snow developed around 7 pm and lasted until after midnight. After my shift ended at 4 pm, I stayed around to watch the later balloon launches and watch the snow fall. By 1 am, we had gotten over 6 inches of snow, most of which had fallen between 7 pm and 1 am!

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This was the 10 pm launch, the last balloon launch that I saw before I left for the night. We had about 4 inches of snow at this point.

I left the launch site around 12:15 am on Wednesday night, about 22 hours after I got there. It was quite a marathon of a day, and I was pretty exhausted. But walking around in the magical snow got me excited again, and I walked around campus for a while in the powdery snow. Here are a few pictures. They’re a little blurry since it was dark, but I really enjoy the soft glow of lights in a snowstorm so I’ll show them anyway. Plus, the texture of 7 inches of powdery snow underfoot was hard to beat, along with the whirling of snowflakes as the storm lingered through the night.

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The loop road around campus, near the bulldog statue (mid-center) and the Sherrill Center (out of the picture to the right).

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A very snowy tree.

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The back side of Ramsey Library, from the mini-quad.

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Brown Hall (dining hall) from the steps leading up to Rhodes-Robinson and the main Quad.

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Governor’s Hall from the loop road.

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South Ridge.

We had a few more snow showers on Thursday morning (as well as Friday morning), but no additional accumulation beyond a dusting. We ended up having three days off of school anyway, from Wednesday through Friday! The snow generally stuck around through the weekend, before a warm spell melted it away. It was a really fun week of snow! However, my sleep schedule got pretty far out of whack after staying awake for the majority of a 48 hour period from Tuesday through Wednesday night. It was worth it, though. Hopefully, we will see a good snowstorm again soon. After a few days in the 60s this past weekend, there’s a chance of snow showers on Wednesday, although I don’t think it will amount to more than a dusting at best.