Wunderground Meteorologist Shaun Tanner

Coldest Thanksgiving?

By: shauntanner, 11:14 PM GMT on November 23, 2009

When was the coldest Thanksgiving?

Thanksgiving is caught in between weather extremes. Being two months after Summer ends and one month before Winter begins, the holiday often sees a wide variety of weather from region to region and from year to year. Early forecasts for Thanksgiving 2009 are eyeing a cold and wet holiday for the northeastern portion of the country due to a deep trough of low pressure, a wet day for the Northwest as a Pacific front moves into the area, and pretty much dry elsewhere. With more and more families traveling by car to their respective festivities, keeping an eye on the weather becomes all that much more important. Just a quick glance at weather conditions and traffic information before you leave the house can save you time on your way to your destination.

Using Weather Underground's vast database of past Thanksgiving weather for the contiguous United States there are a couple different ways to find the coldest Thanksgiving since 1950. For instance, when averaging all of the maximum temperatures for every airport nationwide each Thanksgiving, the year with the lowest maximum average temperature was 1993 with an average of 40 degrees. Another way to define the coldest Thanksgiving since 1950 is to find the day with the lowest average minimum temperature. Amazingly, this day was also Thanksgiving 1993 with an average minimum temperature of 25 degrees. That means that the average city in the country on that day was 7 degrees below freezing! However, the coldest ever Thanksgiving temperature occurred in 1985 at International Falls, Minn. with a bone-chilling minimum temperature of -29 degrees.

In contrast, the highest maximum average occurred in 1998 with an average of 61 degrees. Interestingly, 1998 is also the second warmest year on record. The warmest ever temperature reading occurred way back in 1950 when Ontario, Calif. reported a maximum temperature of 96 degrees. That year, 28 cities reported temperatures of 80 degrees of greater, most of them in the Southwest.

So what will this Thanksgiving bring? Whether you're in Minnesota or California, it probably won't break any of the records listed above, but the turkey will be warm.

For the best weather and driving information available this Thanksgiving, check out Weather Underground's new travel tool - the Road Trip Planner.


Figure 1. Average Thanksgiving highs and lows from 1950 to 2008.

Updated: 5:42 PM GMT on November 24, 2009

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Not All Vegans Are Annoying

By: shauntanner, 5:55 PM GMT on November 19, 2009

Okay, so this blog entry will not be entirely about meteorology. But it will actually have a environmental twist in regard to my life.

When searching for a college to go to for my freshman year, my father and I somehow stumbled upon a college in northern Wisconsin called Northland College. The college is located in a small town of only a few thousand people called Ashland. It is located directly on Lake Superior, and while I only spent one year there due to its high tuition cost, my stay had a profound impact on my life.

Northland College is pretty unique. This self-pronounced "environmental college" only had about 850 students enrolled during my freshman year and about half of those were very hard-core environmentalists. The other half are very well-informed environmentalists who would know how to survive if left stranded somewhere. To look at them, you would think they are the normal annoying hippy-type that you see in many large cities preaching how your lifestyle sucks and whatever they are doing is correct. The most amazing stat about this student body was about 75% was either vegetarian or vegan and most of the remaining percentage was the men's soccer team. I was a part of the latter group. In fact, during one of my first visits to the cafeteria I mistakenly thought the large bowl of tofu was chicken and promptly put it on my plate. What college eatery serves tofu! Upon eating it, I quickly spat it out and let out an audible groan.

My point in bringing all of this up is to let the world know that not all vegans are annoying. The students at this college were not the preachy-type who praise the benefits of a vegan diet one week only to abandon lettuce the following week in favor of a juicy T-bone steak. I remember one specific conversation I had with one of my female friends. I asked her simply one day while she was putting together some colorful salad, "Why are you a vegan?" Then I stepped back and waited for the obligatory explanation of how the cattle industry was destroying the land as well as poisoning our internal organs. But, much to my dismay, she said in response, "I just didn't like the taste of meat."

These young people walked the walk. I lived in a dorm room throughout the frigid winter with a heater that didn't work. Many other students, however, abandoned their cinder block prisons once snow began to fall in favor of customized snow caves built in the lightly wooded areas on the outskirts of campus.

When I was there, they just completed a new dorm that was only to be used by students for living. The unique thing about this dorm was that it was completely off the grid. Now remember, this was before sustainable living was cool or necessary. This dorm is surrounded by solar panels and a wind mill that supplies 100% of the power needed for the dorm. It also has compost toilets (not as bad as you think, they act just like "normal" toilets) and another compost heap that students use regularly. It is the perfect example of how to live sustainably.

I still keep in contact with a couple of the people I met up there. All are still glorious people, and most are doing something incredibly worthwhile. One is petitioning the government to stop the use of genetically-engineered food, another is working at an orphanage in Russia, another spent a summer cleaning up oxygen tanks that have littered the slopes of Mt. Everest, and yet another is investigating invasive species of insects on a military base in Minnesota.

It is enough to keep me inspired, enough to let me know that there are a few people worried about the planet and even fewer who are willing to do something about it. I am glad I got to meet and spend a year with them, and I am better off for it.

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These Are A Few Of My Favorite Things

By: shauntanner, 6:36 PM GMT on November 17, 2009

I was reflecting on what to write in today's blog while on the train this morning when my mind went very briefly to The Sound Of Music. We recently let my almost-3-year-old daughter watch the movie and I found it interesting she latched onto the song that has the lyrics "I am 16 going on 17" rather than the poppier songs like the title to this blog or "Doe, a dear". I hope that doesn't forebode her teenage years.

Anyway, that brief respite gave me a good idea for today's blog. I've decided to give a brief list of my favorite things regarding the atmosphere. Sure, it is self-serving but you have read this far so why not finish the reading? Also, I invite you to post some of your favorite things so the entirety of the blog does not sound so self-serving to me.

1. The last rain of the season
By nature, I am not a person who is attracted to dry, hot weather. There are plenty of people here in the office who celebrate hot, dry, boring weather so I don't need to add to the pile. Rather, I am energized by sloppy, cold weather which is why it might be perplexing why I enjoy the last rain of the season.

You see, I live in the San Francisco Bay Area. In our climate, we get the last rain of the season in April or May, followed by extremely boring weather until October or November when it might rain again. In the meantime, the area sets in for a boring pattern of fog comes in at night...fog goes out in the morning...fog in...fog out...in...out...zzzzzzzzzz.

But, before this pattern sets in, it rains in mid-Spring and I step out to enjoy and say goodbye to the last bit of the wet stuff. Then, one morning in late May I may wake up to foreboding clouds and I think to myself, "could it be?" I wrestle myself out of bed, put on my smoking jacket and step out into the, just barely, chilly Spring air. Then, I feel it. The tingling coolness of wet raindrops hitting my skin. Ahhh, the last rain of the season. It has come once again to say, "hello", before its deep slumber through the Summer months.

2. The second rain of the season
Then, after the long hibernation the rain threatens to return and it finally does. But the first rain of the season is not the best. This rain is very much welcome but its usefulness is spent on washing the atmosphere and buildings of the dirt and soot that has accumulated throughout the Summer months. So, it often rains down with a tinge of dirt smell that is not always nice.

Now that the dirt is out of the atmosphere, the second rain of the season comes down as a splash of freshness, watering the thirsty land and making puddles. Ah, now that's better. Finally the pattern is broken and my favorite season is upon us.

3. Making Distinguished People Undistinguished
There is a good episode of The Simpsons where it is explained that a pie in the face of a distinguished person is far more funny than a pie in the face of a clown. Well, the atmosphere works the same way. A sudden downpour on the head of someone wearing a tie walking down the street talking on his/her cell phone is far more funny than a sudden downpour on a group of nuns. Just is.

A few years ago I went on a business trip to Chicago with our Director of Business Development. Now, this Director usually dresses nicely and walks with a briefcase, while I am someone who always wears shorts and never checks bags on an airplane. Now, you can guess who the distinguished one was. I am not a person who normally likes downtown areas, but I was enjoying walking down Michigan St. when a thunderstorm broke over our heads and opened up a tremendous downpour right on top of our heads. I imagined this was penalty for making me eat at an oyster bar earlier in the night, but whatever the reason, the Director began a furious sprint to the nearest overhang while holding the briefcase over his head. I slowly chuckled to myself as I watched this business man get tremendously drenched. Leave it to Mother Nature to take us all down to the same level.

4. The simple elegance of a rainbow
Ever since childhood, we are bombarded with iconic images of rainbows that can represent hope, new beginnings, or funny-looking Irish men huddled around a pot full of gold. But, even with all of these worn out images of this pillar of the atmosphere, I am still taken aback when I spot one in person. Perhaps it is the entire picture; the fresh crispness of falling rain, the ray of sunlight poking through broken clouds, a stream of rain falling far off in the distance. Forget the complexity of how a rainbow forms, the simplicity of the final product is stunning. Mother Nature has a way of taking complex angles and equations and displaying them in simple, beautiful outcomes. Rainbows are no different with their long arc you just want to run and swing from and array of colors that stimulate every corner of your eye.

I must say, however, I am partial to the full rainbow rather than a partial arc. I will sit and stare like a moth to a flame at a full rainbow until it disappears.

5. Freeze my brain
Stepping out of your home to a cold burst of air that penetrates your soul is more stimulating than stepping out of your home in the morning to a warm pool of humid air that feels like you are walking through someone's mouth. Plus, it is even more attractive when you come in from out of the cold, slip under a cozy sleeping bag and watch The Sound of Music I mentioned earlier. Okay, so I don't make a habit of watching The Sound of Music, but it made for a good return to the beginning this blog.

Updated: 6:37 PM GMT on November 17, 2009

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WunderCast Discussion

By: shauntanner, 5:41 PM GMT on November 12, 2009

There are a couple of things I want to discuss that came up in the last blog regarding the WunderCast Competition.

First, we decided to do the precipitation the way it is done for simplicity. Right now, participants put in a probability of precipitation from 0 to 100 for what they think will happen during that day. If it does rain, then the score is 100, if it doesn't then the score is a 0. The farther you were away from those numbers versus what you forecast, the more points you will accumulate.

As a scorer, I want people to hedge their bets at times when they are unsure whether it will rain or not. People who say 100 and it doesn't rain really get punished. I believe the points used to punish when you do get the precipitation incorrect is appropriate for now.

Also, someone said that the forecast duration should be less than two weeks per city. Let me tell you my stance on this. This competition was set up as a learning exercise. Those with no forecasting experience can learn from those with considerable forecasting experience. Thus, I chose to use the two week duration to get the maximum amount of variation in any given city without completely boring people. If the duration is cut down to one week, we may not see any precipitation, no cold spells, or anything interesting during that timeframe.

In addition, since the valid forecast days are only Tuesday through Friday, there are only 8 forecast days per city. Thus, it isn't exactly two whole weeks, but rather 4 days followed by three days off.

Anyway, my idea for this blog was to get some ideas on forecast cities. What cities would you like to see in the competition?

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WunderCast Suggestions

By: shauntanner, 5:39 PM GMT on November 11, 2009

So I am thinking about restarting the WunderCast Competition again in December of January. If you have never heard of WunderCast, then let me explain. The WunderCast Competition is a battle royale forecasting contest where Weather Underground members forecast for various cities throughout the country. There are no prizes and members of any skill level can join and learn about weather and how to forecast. Scores are kept, but everybody who participates will learn something.

Participants forecast for a scheduled city for two weeks, Tuesday through Friday. Forecasts can be entered well ahead of time in case family vacations get in the way, but if you do not enter a forecast then you are given climatology.

Among the other participants are the various models so you can see how well you are doing versus these "professional", computerized forecasters.

Sound like fun? Then join the conversation. What I need from you now is suggestions. I am thinking about four or five cities that give a good cross section of the country. So, one city in the Northeast, one in the Northern Plains/Upper Midwest, one in the Northwest, and one wild card somewhere.

Forecast locations are airports (ICAOs) that have official climate reports. We use these airports because they have clear cut, official records that cannot be complained about.

So, any suggestions? I would love to hear them. Please post because I will not know what you are thinking unless you tell me.

Updated: 6:17 PM GMT on November 11, 2009

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Lesson: Mirage

By: shauntanner, 6:14 PM GMT on November 10, 2009

We've all seen movies where some lost wandering is crawling on his hands and knees across a parched desert. With his last once of energy he raises his head to see something splendid. An oasis a mile ahead complete with a shimmering water, a lone palm tree, a reclining beach chair, and a margarita. Okay, so I made the chair and the drink up. Nonetheless, as the dehydrated wanderer runs to this oasis, he is mystified because he cannot seem to get any closer to it. And then, it disappears?

Believe it or not, this is an actual atmospheric phenomenon that we have all seen before, just not likely in the same scenario as above. In the past blog entries, we have taken a look at the various hostilities that sunlight encounters once it enters the atmosphere. We have looked at reflection, scattering, and now we will look at refraction.

Refraction can be summed up by one simple statement...light can bend. I use this simple example in my class:

Picture a car traveling down a rural highway. On the shoulder is a deep lane of sand. So you have two types of surfaces, the asphalt of the highway and the sand shoulder. So, you are driving down the asphalt at 65 mph (you never speed, right), and you start to fall asleep. Your car drifts to the right, with your two right side tires eventually moving onto the sand shoulder. What will happen? Well, specially, your two right side tires are now driving through a surface that has more friction than your two left side tires. Thus, the two left tires start to move faster than the other tires and the car begins to turn to the right. Eventually, all four tires will be in the sand shoulder, experiencing the same friction once again and the car will move in a straight line away from the highway. This example is an experiment in refraction of sorts.

If you translate the above example into the atmosphere, the car is a ray of light, the asphalt is a layer of air of one density, and the sand shoulder is a layer of air of a different density. Many instances the layers of different density are simply layers of different temperatures. This is because cold air is more dense than warm air. Thus, it always wants to be lower than warm air.

Water can do the same thing to light. When you are fishing in a stream with a spear (I know you do this often), you always have to aim lower than where the fish is because that is actually where the fish is. Keep that in mind, my Neanderthal friend.

Now, we have everything set for what a mirage is. Take the typical oasis setting we set up at the beginning of this explanation. When blue rays of light pour through the atmosphere, it may encounter an unusual layer of warm air beneath a slightly cooler layer. This warm layer is warmed by the surface below, be it asphalt or warm sand. As the light enters this warm layer, it is bent or refracted towards your eyes rather than hit the ground. So, you see this ray of light in a position that appears to be on the ground rather than what should be there (sand). So the oasis of water is instead a refracted area of sky. That is why it looks like water. These mirages often shimmer because of the heat waves that are moving upward from the ground. Refraction is the same reason stars shimmer at night.

Updated: 6:17 PM GMT on November 10, 2009

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Lesson: Twilight

By: shauntanner, 5:49 PM GMT on November 05, 2009

Today, I will once again tackle an atmospheric phenomena that nearly everybody sees everyday. It has been so ingrained in our psyche because we see it everyday that most of us do not even know what twilight is and how it comes about. Have no fear, let's walk through it.

I am not too interested in the technical definitions of twilight as scientifically defined. It is interesting to note, however, that some crimes carry stiffer penalties if they are committed at night so a technical definition of when nighttime actually occurs is necessary. Rather, I am just interested in how twilight comes about. As usual, pictures follow if you are just interested in pretty WunderPhotos.

Try this someday. Turn on a singular source of light in your home. Make sure the room where the light is has no windows or do this experiment in the dead of night. Now, turn off the light. Notice anything? Absent of any other sources of light in the room, the atmosphere of the room should be pitch black. Now, turn the light back on. For this next experiment, keep the light on but take the light source outside the room. Put it down the hall, in another room, just put it somewhere else outside the room but relatively close. Now go back into the room. Notice anything now? Is the room completely dark as it was before? Chances are that at least some amount of light has made its way back into the room. Why? Well, nearly all the surfaces in your home has some sort of reflective quality. These surfaces reflect the light from the source outside the place where it is at, and at least some of it finds its way into the experiment room.

Well, the atmosphere reacts the same way as the reflective surfaces in your home. Without an atmosphere, the surface and sky would become immediately dark once the top of the sun moves below the horizon. Conversely, the surface and sky would stay dark until the moment the top of the sun peaks above the horizon in the morning. Imagine, dark...dark...dark...dark...BRIGHT! It would be an interesting change to our lives.

Rather, the atmosphere acts to reflect and scatter sunlight back to the surface even once the sun has already set. For instance, when the sun sets, that means the surface no longer has a direct line of sight. But, the higher you move up in altitude, the more likely you are to have a direct line of slight to the sun. Thus, the higher altitudes of the atmosphere still receive solar radiation even when the surface does not. This is the same atmosphere that is capable of reflection sunlight (clouds) and scatter sunlight (Nitrogen and other gases) back down the surface. Thus, the surface still receives sunlight well after the sun actually sets. The reverse is true for morning twilight. The upper atmosphere will receive solar radiation first, reflecting and scattering it to the surface before the sun rises.

So, now you know that twilight is not just some tweeny movie series.

Updated: 5:50 PM GMT on November 05, 2009

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Lesson: Sun Pillars

By: shauntanner, 5:44 PM GMT on November 04, 2009

And so the teaching continues. Who knew that the internet would be somewhere you could learn more than just about someone's wild ramblings on YouTube?

Yesterday we learned about Crepuscular Rays. Someone asked in the comments if the Green Flash was related to Crepuscular Rays. No, it isn't. We can learn about the Green Flash some other day...it is a bit more complicated.

Today we are going to learn about Sun Pillars. Crepuscular Rays were as a result of scattering of light entering the atmosphere. Well, Sun Pillars are as a result of another process, one that we utilize everyday...reflection.

Once again, I have placed WunderPhotos showing some nice Sun Pillars so feel free to glance to them during this relatively short discussion.

Sun Pillars often occur near the end of the sun's traverse across the sky, but not necessarily when it is close to the horizon. Generally, you need a significant clearing of the sky, but some high altitude ice crystals are necessary.

Okay, this description is going to require a bit of an imagination. I need you to picture ice crystals high up in the atmosphere. You got them? Now, transform those crystals into flat plates that are aligned parallel and horizontal. These ice crystals can be of any shape; rectangular, hexagonal, whatever; but they have to be horizontal with their widest surface facing up.

Now you are set. All you need is a light source and for our purpose that, of course, is the sun. You can imagine that if a bright light were shone at an angle (near setting sun), you would get a reflection of the light beam just like if you were to stand at an oblique angle from a mirror. So, standing at a distance, the reflected light is diverted to the observer's eyes. This shows up as a bright column that would not be there if the ice crystals were absent.

It is important to note that Sun Pillars are normally pretty narrow in comparison to the rest of the sky. Usually, they are near the same size as the width of the sun. If you think about it, that makes sense. If you picture a plane of ice crystals all oriented horizontally, you would only see a reflection from those crystals that directly divert the light beam into your eyes. The rest of the crystals are reflecting light, but not in the same direction as where you are standing.

One last note. You only need a light source a Sun Pillar. While it most likely is to happened as a result of the sun, pillars can form due to moonlight or other artificial land-based light source.

Updated: 5:45 PM GMT on November 04, 2009

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Lesson: Crepuscular Rays

By: shauntanner, 6:03 PM GMT on November 03, 2009

The laws of nature and science are capable of providing stunning sights. Here, at Weather Underground, we have hundreds of well-equipped photographers who go out of their way to capture these stunning sights and upload the photographs for our pleasure. It is something that you may take for granted when looking at some of the WunderPhotos. It takes time, patience, and talent to snap a good photograph, yet we have thousands of them at our disposal.

But, some of these photographs may show things you have not seen before. Perhaps you saw something in a picture that you would like to know more about. Well, here is the place you are looking for. Today, we will be talking about crepuscular rays and how they form. I have attached some beautiful WunderPhotos below so feel free to reference them during the discussion.

Crepuscular rays are something you have probably seen before and written off quickly. They are like a warm hand of a higher power reaching down from the heavens to give you a friendly pat on the back. Most commonly, they are seen over oceans or large bodies water near sunrise or sunset, but they can be seen during any part of the day as long as the sun is up and clouds are in the appropriate locations.

There are a couple things that you need to get a good display of crepuscular rays. First, you need...well..the sun. Without the sun above the horizon, you don't get the scattering required (we'll talk about scattering in a moment). Second, you need a heavy dose of clouds with some separation between them. The separation is required to focus the rays toward to your eyes, but the clouds also serve another purpose. The clouds also form shadowy regions on the opposite side of them so there is a large contrast between the bright rays and the dark regions adjacent to the rays behind the clouds. This contrast is what makes crepuscular rays spectacular. The third thing you need is something in the air that is capable of scattering sunlight. This can be sea salt (another reason these rays appear over oceans a lot), dirt, or other aerosols.

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My high school physics teacher used the following analogy to describe scattering (scattering is also responsible for a blue sky):

Picture a boat. It can be any sized boat you want. Now picture three different sized sets of waves. The first set has a trough that is twice the size of the boat, the second set has a trough half the size of the boat, and the third set has a trough the exact same size of the boat. Now ask yourself this question...which one of these wave sets will throw the boat around most violently?

For the large waves, the boat will simply ride up and down the troughs and hills and not be violently thrown. For the small waves, the boat will simply ride on top of the hills and not dip down into the valleys too much. But for the middle waves (the one with the troughs the same size as the boat), the boat will be thrown violently up and down quickly. So Goldilocks was right...not too big...not too small...just right.

Well, waves of light react this way when they encounter an appropriately-sized molecule or object in the atmosphere. For instance, when white light smacks into a Nitrogen molecule (78% of the lower atmosphere by volume), the blue wavelength of the white light is scattered. That is, the boat (Nitrogen) fits perfectly within wave (blue light wavelength) and is thrown about in all directions wildly. This is why the sky is blue.

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So, when sunlight hits sea salt, for instance, the white light is scattered in all directions, one of these directions is directly into your eyes. So it appears like a brighter than normal light in that area of the sky than in the darker, shadowy areas behind the cloud that aren't receiving solar radiation.

You can imagine people way back when staring out across open water and seeing this spectacular sight. With this in mind, it is no wonder this phenomena is also called "Jacob's Ladder" and "The Hand of God."

But, the scientific name of Crepuscular Rays simply describes the time of day you are most likely to see it. Crepuscular hours are the hours near sunrise and sunset. These are also the hours when the contrast between light and dark are most pronounced.

An interesting thing associated with these rays are anticrepuscular rays. These are rays that emanate in the opposite direction of where the sun is. They seem to converge to a single point on the horizon, but in fact this is an optical illusion. They are not converging at all. The last picture below is an example of these types of rays.

Updated: 6:13 PM GMT on November 03, 2009

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About shauntanner

Shaun Tanner has been a meteorologist at Weather Underground since 2004.

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