Fifty Meters of Doom -- The Difficulty of Winter Weather Forecasts

by: weatherdude

Sat Jan 26, 2013 at 23:31:50 PM EST



Parts of the southeastern United States had a mild but nasty ice storm this past Friday. Ice storms in January in the southeastern United States are far from unheard of, but this one threw people for a spin because it was so cold that one would normally expect snow rather than rain.

"It's 22 degrees. Why is it raining instead of snowing?"

One of the easiest things to forget for people who usually don't track weather is that the atmosphere is alive and deep. In the United States, there's usually between 7 or 8 miles of troposphere above you -- the layer of the atmosphere in which the weather occurs. Subtle changes at any level within that layer means big differences in what kind of weather you'll encounter at the surface.

weatherdude :: Fifty Meters of Doom -- The Difficulty of Winter Weather Forecasts
 
I °C What You Did There

First of all, we (Americans) have to keep in mind that we're pretty much the black sheep of the world. Everyone on earth (save for Burma, one of the African nations, and stubborn oldies in Canada) uses the Metric system. They measure temperature in degrees Celsius, we measure it in degrees Fahrenheit.

For the purposes of weather forecasting, we have to use Celsius to keep things standardized. Just remember that...

-20°C = -4°F
-10°C = 14°F
0°C =  32°F (Freezing)
10°C = 50°F
20°C = 68°F

...and you'll be okay for the rest of this post.

SKEW-T Charts

As you probably know, weather balloons are released at the same time all around the world twice a day. These balloons have radiosondes attached to them. They're essentially little weather stations in a cardboard box that measure temperature, dew point, wind direction and speed, and sometimes a few other things between the surface and the time the balloon ruptures about a dozen or so miles above us.

This weather data is transmitted back to computers safely on the ground and plotted on something called a SKEW-T/LOG-P chart.

The name comes from the fact that the temperature (T) lines are skewed diagonally from bottom left to top right, and that the pressure (P) goes up logarithmically (log) on the chart.

Pressure isn't consistent through the atmosphere. We measure air pressure in millibars -- 1013.25 millibars is standard sea level pressure. Pressure goes down very quickly as you go higher in the atmosphere. Denver is at or around 850 millibars most of the time, for example, which is why you have such a hard time breathing if you're not used to high altitudes.

10,000 feet off the surface is usually 500 millibars. 25-30,000 feet is usually between 200 and 300 millibars, where the jet stream is located.

When you draw these lines of constant pressure on a chart, the lines aren't evenly spaced out. The higher you get in the atmosphere, the further apart the lines are spaced.

Since the pressure goes up logarithmically, the temperature lines get skewed at a 45° angle. I don't remember precisely why, but I know that it makes it very easy for meteorologists to get a good idea of what the atmosphere looks like in order to forecast for things like severe weather (especially tornadoes) and winter storms.

Huh?

Okay. Here's a SKEW-T chart from Bufkit:

This one is a little bit different. Bufkit is a program developed by the NWS in Buffalo NY in order to help them forecast lake effect snow events.

Its charts are produced from model forecast data -- NOT actual weather balloon data -- so it allows us to look at what the models predict the atmosphere will look like for a specific point in the near future.

The only difference between charts generated by Bufkit and actual SKEW-T charts is that this one shows altitude instead of pressure. In other words, when you see a horizontal line on the chart, it's telling you altitude in feet.  

It makes it easy. Trust me.

Look back up at the black Bufkit chart I posted. It's pretty easy to interpret once you know what you're looking at.

Altitude is denoted by the horizontal gray dashed lines. The altitude corresponds with the number next to it, in thousands of feet. Thus, the "5" line denotes 5000 feet, "10" for 10000 feet, yada yada yada.

Temperature is shown by the blue and teal diagonal lines. The temperature corresponding to each line is in the brown area below. For instance, -30 would be -30°C. Remember that 0 is the freezing line.

The thick red line shows the temperature of the atmosphere. Put your finger on the thick red line at 5,000 feet, and drag your finger parallel to the blue/teal lines until you reach the brown surface. If you did it right, you should wind up around 3-4°C. That's the temperature at 5,000 feet. The thick red line crosses the 0°C freezing line between 6000 and 7000 feet.

The thick green line is the dew point line. It works the same way as the thick red temperature line. The dew point, if you don't know, is the temperature at which the the water vapor in the air will condense. Really low dew points indicate dry air. Really high dew points indicate very moist air.

When the thick green and red lines are far apart, that part of the atmosphere is dry.

When the thick green and red lines are close together, that part of the atmosphere is moist.

There's a lot more I could discuss, but it's irrelevant to the task at hand.

(PS: The barbs on the far right show the wind at different altitudes. The barbs show the direction from which the wind is blowing -- a barb pointing to the left shows wind from the west -- and the speeds are in knots.)

How does this relate to winter weather?

Glad you asked! We worry about 4 main precipitation types during the winter -- rain, snow, sleet, and freezing rain.

Unless you live in the Atacama Desert, you know what rain and snow are.

Sleet is an ice pellet -- essentially a rain drop that freezes solid before it hits the surface. It makes a loud tinging sound when it falls, and it's a lot slipperier and harder to deal with than snow. It can also freeze solid after it falls, turning into a sort of glacial pack ice that's impossible to remove once it solidifies.

Freezing rain consists of supercooled raindrops (water that falls below freezing, but doesn't have any particles to latch onto to form ice) that freeze on contact with a sub-freezing surface. Yes, it literally freezes into a sheet of ice when it hits the surface.

The line between rain, snow, freezing rain, and sleet is razor thin, and is a huge pain in the ass for meteorologists to predict.

RAIN

Remember that black SKEW-T chart I posted earlier? No? Hmph. I'm insulted. Well, here's another one:

Look below 5,000 feet at the thick red temperature line. See how it's well to the right of the teal 0° line? That indicates that the lower 5,000 feet of the atmosphere are well above freezing. Look right at the surface -- it's almost 10°C (50°F). There's no way that anything frozen would be able to survive a warm layer like that.

That's pretty much the story for any type of rain event in the winter. If there's any deep layer well above freezing (4°C or higher for more than 1,000 feet), there will be complete melting and the precipitation will fall as rain.

SNOW

Snow is almost the opposite, with some caveats of course. Here's a SKEW-T chart showing a snowstorm:

The first stipulation is that there has to be moisture in the "snow growth region." You see how there are two other teal temperature lines, between -20 and -10? Those two teal lines are -18 and -12°C, respectively. This is the snow growth region. This range of temperatures provides the perfect environment for snow to form. If there's moisture in this region, snow can form.

The next hurdle is that the entire atmosphere below the snow needs to be at or below freezing for the snowflake to survive. There can be a very small layer of temperature just barely above freezing, but for the most part it has to be all below freezing.

As you can see in the above chart, the entire atmosphere is moist and below freezing, indicating snow.

SLEET

Sleet forms from partially melted snowflakes. When the snowflake falls through a shallow layer of temperatures above freezing, the snowflake partially melts. The result is a little ice crystal left inside of a raindrop. When this raindrop falls back into the freezing air, it's able to freeze around the ice crystal, creating a little ice pellet.

As you can see on the above SKEW-T chart, the temperature goes slightly above freezing for a shallow layer of the atmosphere, allowing the snowflake to melt a bit, then reenter sub-freezing air and freeze into an ice pellet.

FREEZING RAIN

Freezing rain occurs when snowflakes completely melt into liquid, leaving no ice crystals left in the raindrop.

Here's a freezing rain sounding:

You can see that the sounding stays above freezing for almost 5,000 feet, which is way longer than necessary for the snowflake to freeze. The raindrop is completely liquid by the time it reaches 500-1000 foot deep sub-freezing layer right above the surface. Once the droplet falls onto something on the surface, the sub-freezing temperatures freezes it to ice.

Forecasting

I showed you as close to a textbook version of each precip type as I could find. I had to search through 30 model soundings and scroll through 84 hours worth of forecasts for each to find a near-textbook example. This stuff isn't always clear-cut, and just a small shift in winds can mean the difference between a cold rain and a prolonged ice storm, or the difference between 6 inches of snow and 3 inches of sleet.

This stuff is hard to forecast. When you're under the gun for winter weather, please give your local meteorologists a break. All it takes is a few hundred feet to throw off a whole forecast and turn a winter wonderland into an icy deathmatch.

(Lovely note to end the post on, eh?)

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Somewhat Unneeded Tip Jar (2.00 / 46)


Refugee in the War Against Sanity™

You forgot graupel. (2.00 / 27)
I get graupel year round up here at high altitude, usually as the first form of precip that falls.

Pic: http://upload.wikimedia.org/wi...

"Do your little bit of good where you are; it is those little bits of good put together that overwhelm the world." ~ Desmond Tutu


I purposely left graupel out. (2.00 / 22)
It's not as common as the top 4 types, and it doesn't really have as significant of an impact as sleet vs. snow would. It reacts like snow when you step on it.  

Refugee in the War Against Sanity™

[ Parent ]
Glad to see you stuck to the weather n/t (2.00 / 26)


I'm a Democrat.  Yellow.  New.  Progressive.  Blue.  Liberal.  Centrist.  We need them all in our big tent.

Hah! :D (2.00 / 25)


Refugee in the War Against Sanity™

[ Parent ]
More useless trivia from the Rockies - virga! (2.00 / 24)
Rain or snow that evaporates before it hits the ground.

This Texan is fascinated by the weather in the mountains. I knew what virga is, from driving in the desert, but graupel was something I had never heard of.

Thanks for the diary, 'dude. This stuff fascinates me almost as much as it does you.

"Do your little bit of good where you are; it is those little bits of good put together that overwhelm the world." ~ Desmond Tutu


Remember sneet? On those very rare Houston occasions when we'd (2.00 / 21)
get snow and sleet all rolled into one.  One of the local TV weathermen (can't remember which one) just loved to say "it's sneeting."

"I base most of my fashion sense on whether or not it itches"  -- Gilda Radner

[ Parent ]
I call it all "oobleck" (2.00 / 11)


[ Parent ]
Einstein believed weather would forever be unpredictable, (2.00 / 21)
or at least said this:

One need only think of the weather, in which case the prediction even for a few days ahead is impossible.  

Asimov similarly forecast far futures where weather was still not completely predictable.

These are very complex systems, at the least. Being able to predict them in general given the miniscule amount of data we have today is pretty amazing. For all the weather balloons and doppler radars out there, the amount of hard data acquired as compared to actual information in the system is almost immeasurably small.

A number of  years ago a meteorologist friend was excited because he had gotten super computers that could run global atmospheric models with meter-cubed units in realtime. In other words, it took three days to model three days of activity. When we are talking about years and centuries of activity at scales of droplets and dust particles the amount of horsepower we need is boggling, then multiply that by thousands if we don't want to take a decade to model a decade.

Climate change denial is like evolution denial. The level of evidence demanded by the most vocal opponents is orders of magnitude greater than reasonable certainty.

John Askren - "Never get into a pissing match with a skunk."


Thank you! (2.00 / 21)
Granted, being a desert rat and living in San Francisco makes that stuff alien to me, but it's nice to know. Actually, I was living in GA when a snowstorm came through. It was freezing rain followed by snow. I had to drive in it to get home, but I managed...barely.




Come visit us at our NON political blog jellybeansofdoom.com


I'm still just laughing over the name of the storm... (2.00 / 21)
Khan


Hey, I figure this is a good diary to ask in...

I need help with a math problem (my math skills are lacking...to say the least).

Any Mathematically Minded Moose™ willing to lend me their brain?

Photobucket


No. (2.00 / 16)
:)

John Askren - "Never get into a pissing match with a skunk."

[ Parent ]
lol...you rat bastid /grin (2.00 / 16)
I honestly don't even know where to begin with it.

It is a question on a toll of $1.84...that has a built in increase of 3.5% a year for 58 years.

I wanna know what the toll is going to be in 58yrs.

In fact, the whole of it has me so confused that I am not even sure if I have made the problem clear in WORDS.

I am an ignoramoose.  :(


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[ Parent ]
$5.58 (2.00 / 16)
yw

John Askren - "Never get into a pissing match with a skunk."

[ Parent ]
Close, but no cigar (you got it below...thank you!) n/t (2.00 / 14)


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[ Parent ]
yikes......between your question (2.00 / 16)
and all of WD's graphs I'm deep breathing math anxiety away!

"ignoramoose", heh  

Love is the lasting legacy of our lives


[ Parent ]
Or $13.53, (2.00 / 14)
if it compounds annually

John Askren - "Never get into a pissing match with a skunk."

[ Parent ]
Yes, that...and that is where my brain asploded. /grin (2.00 / 14)
Thank you, Blasky...you can have your brain back now.  :)

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[ Parent ]
Yes, but look back the other way: (2.00 / 14)
What Things Cost in 1955:
Car: $1,950
Gasoline: 29 cents/gal
House: $17,500
Bread: 18 cents/loaf
Milk: 92 cents/gal
Postage Stamp: 3 cents
Stock Market: 488
Average Annual Salary: $5,000
Minimum Wage: 75 cents per hour


John Askren - "Never get into a pissing match with a skunk."

[ Parent ]
I meant that 'compounding' is where my brain asploded... (2.00 / 14)
that is math beyond my capabilities.  /grin

As far as the actual number, it is actually significantly lower than I thought it was going to end up being.

The only thing that came to mind was that chess board and rice grain legend. Though, I guess that was 100% increase compounded by (with?) each square.

Anyhooo...thank you for crunchin the numbers for me, it really is appreciated.   :)


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[ Parent ]
Not quite! (2.00 / 12)
See my response, below. There may well be rounding error.  

"Most people worry about their own bellies and other people's souls when we all ought to worry about our own souls and others' bellies" Israel Salanter

[ Parent ]
If you'd like to know the method (2.00 / 16)
$1.84 at 3.5% for 58 years.

Start with one year. An increase of 3.5% is a multiple of 1 + (3.5/100) = 1.035. So 1.84*1.035 = 1.9044. Then that gets multiplied by 1.035 and 1.9044*1.035 = 1.971054. After 58 years, that will be 1.84*(1.035)^58 = 1.84*7.35428 = $13.53.

But wait! There's more!

After one year, the toll won't really be 1.9044. Tolls aren't given in fractions of a cent. It will be 1.90.

So, the above is not quite correct. I wrote a little R program to figure it out:


x <- vector("numeric", 58)
x[1] <- 1.84
for (i in 2:58) x[i] <- round(x[i-1]*1.035,2)

The final amount was $13.02

"Most people worry about their own bellies and other people's souls when we all ought to worry about our own souls and others' bellies" Israel Salanter


[ Parent ]
I'm in awe of your genius. I really, really, really hate numbers, (2.00 / 3)
and am horrendously bad at them. Words and language are my preferred tools. ;-)

Wer kämpft, kann verlieren. Wer nicht kämpft, hat schon verloren.
                       - Bertolt Brecht


[ Parent ]
i'm too brain dead tonight to really (2.00 / 19)
appreciate this Weatherdude. i'll be back in the morning when my brain has reset and is functioning.  also to hand out mojo to my favorite and trusted Weatherdude.  thank you for this.  i always learn so much from your diaries.  


Does the morgue wagon come with the job?

Wow (2.00 / 19)
I feel like I've just grasped more about meteorology reading five minutes of you than years trying to understand through others

I don't know if you are - or intend to be - a teacher or writer on this, but if not it"s the world's loss  

The p***artist formerly known as 'Brit'


D'awe, shucks. Thanks! :D (2.00 / 12)
I actually plan on being a high school government teacher. I might be able to sneak in some meteorology in there.  

Refugee in the War Against Sanity™

[ Parent ]
Great explanation, and very timely even in Europe. (2.00 / 16)
We had sheets of ice on the roads last Sunday, and it crippled the Frankfurt airport - and a few others - for some time.
Report from last week:
http://www.dw.de/frankfurt-air...
Frankfurt's main airport has been hit with further delays and cancellations as it deals with new snowfall following freezing rain. Recent heavy winter weather conditions have halted traffic at airports across Europe.
Around 200 flights Monday at Frankfurt International Airport have been cancelled so far due to snow.
A spokesman for the airport's operator, Fraport, had said earlier that further delays would be expected.
The spokesman added that the freezing rain has stopped, but it continues to snow, meaning planes must be de-iced.


Wer kämpft, kann verlieren. Wer nicht kämpft, hat schon verloren.
                       - Bertolt Brecht


45 degrees (2.00 / 14)
Cool diary! Cold diary! Wet diary! :-)


"Most people worry about their own bellies and other people's souls when we all ought to worry about our own souls and others' bellies" Israel Salanter

Got posted too quickly (2.00 / 13)
Near the beginning you said there was something about the 45 degree line that you didn't remember exactly.... If I understood what that was, then it's that the relationship between temperature and altitude is linear. e.g. in the first Bufkit, look at the line that is at 20 C at 0 elevation; it is at about 10 at 10 and 0 at 20. Straight line linear relationship. But, in the different diagrams, those blue teal lines are not all at 45 degrees (although they are all straight), this indicates that the relationship between temperature and altitude is different in the different charts (I am not sure if they represent different places or the same place at different times).  

"Most people worry about their own bellies and other people's souls when we all ought to worry about our own souls and others' bellies" Israel Salanter

[ Parent ]
On a standard SKEW-T chart they're at a 45 degree (2.00 / 8)
angle to the surface. On the Bufkit charts (black ones) that I posted, the angle of the isotherms changes based on the zoom.

The teal lines denote -12 and -18 degrees Celsius (snow growth region) and the freezing line.  

Refugee in the War Against Sanity™


[ Parent ]
You said that (2.00 / 18)
The line between rain, snow, freezing rain, and sleet is razor thin, and is a huge pain in the ass for meteorologists to predict.

Is this why we frequently see the forecasts for "Wintry Mix", because they just aren't sure how it is going to fall?

I am for the individual over government, government over big business and the environment over all. -- William O. Douglas


Yes. That and the fact that (2.00 / 12)
the precipitation type can change rapidly over the course of one or two hours, and multiple types of precip can fall at once.

Mindblowing, right?  

Refugee in the War Against Sanity™


[ Parent ]
Great stuff, weatherdude. (2.00 / 16)
I am going to hotlist this to read it later.  

Words have meaning. Our words will reflect what is in our souls.


Regardless of the weather (2.00 / 17)
your presence here is a ray of sunshine, weatherdude.

I know, I know... I'm a scientist. I should be able to say something more insightful or erudite, but I'm just so pleased that you are sharing your knowledge and passion with us over here.  

I never travel without my diary. One should always have something sensational to read in the train.


-- Oscar Wilde


Great post weatherdude, thank you (2.00 / 18)
Old man rant ON:

Isn't fahrenheit a easier and more minutia based scale when it comes to temperature?

I live in Texas and in August telling someone that it is 43.3 degrees outside just doesn't seem to convey the absolute misery the way that the fahrenheit scale does.

Old man rant OFF.


King of the Hill -
HANK: Dale, you giblet-head, we live in Texas! It's already 110 in the summer, and if it gets one degree hotter, I'm going to kick your ass!



Hahaha. (2.00 / 12)
I do prefer using Fahrenheit because it's more detailed than Celsius. Celsius turns out to be easier to work with for meteorologists, but Fahrenheit is easier to the users.

A 65 degrees sunny and slightly breezy day sounds so much nicer than 18 and sunny.  

Refugee in the War Against Sanity™


[ Parent ]
When I was in elementary school (2.00 / 11)
[back in Mesozoic times], we were admonished that if we did not learn the Metric system, we would be left behind by civilization, unemployable, and probably even unlovable. Yeah, well.  

I never travel without my diary. One should always have something sensational to read in the train.


-- Oscar Wilde


[ Parent ]
Ohhhhhhhhhhhhhhhhhhhhhhhhhhhh!!!!! (2.00 / 15)
I love it when you talk weather.

A weatherish note: Yesterday there was a good bit of sunshine. (2.00 / 11)
Today there is grey sky from horizon to horizon. But today there are several bunches of green shoots from one of my daffodil beds on a south -facing slope , not there at all yesterday.They are highly appreciated , those things showing up today.  

"COUNTY OFFICIALS TO TALK RUBBISH"- from 'Anguished English'

Hey Weatherdude, here's a topic to consider: (2.00 / 8)
Cities affect temperatures for thousands of miles

"The world's most populated and energy-intensive metropolitan areas are along the east and west coasts of the North American and Eurasian continents, underneath the most prominent atmospheric circulation troughs and ridges," Cai says. "The release of this concentrated waste energy causes the noticeable interruption to the normal atmospheric circulation systems above, leading to remote surface temperature changes far away from the regions where waste heat is generated."



John Askren - "Never get into a pissing match with a skunk."

Hey, weatherdude, good to see you! (2.00 / 10)
Thanks for the diary.  My brain is incapable of appreciating it fully, but I like that you wrote it. I've rec'd your diary, but am still trying to figure out how to tip. Is tipping allowed in this establishment?

Welcome to the Moose, Diana! (2.00 / 8)
We don't really tip here...just has never been a tradition we took on.

Check out the Insider's Guide to Motley Moose for some of the unwritten rules and traditions of the Moose.

If you have any questions...just ask in any active thread and I am sure someone will answer quickly (or, if you would prefer, use the Contact the Moose link at the bottom of the page).



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[ Parent ]
Hi Diana, (2.00 / 7)
The Moose doesn't do "mojo" in a standard fashion. Credit is earned on a more amorphous social basis than traditional blogoprofiling - more like real life.

You may have noticed that the rating system includes "Fail, Meh and Fierce". While it has no prize-generating capabilities, ratings have real meaning as a sort of poling function. Since we can't read faces in blogtopia, ratings help us all get a feel for inflection.

Fail is pretty awful, short of the Short Lived Moose Troll (a rapidly self-endangering species) it is only used when folks are in very strong disagreement. Meh is often funny, but can mean a strong but civil contrary view. Fierce is, well, fierce.

Make yourself at home, ignore Kysen (he's a bad influence), and that weatherguy has shown a habit of mixing concoctions that leave people bleary for several days so indulge with foreknowledge.

-cheers!  

John Askren - "Never get into a pissing match with a skunk."


[ Parent ]
Good to see you here Weatherdude (2.00 / 4)


squee! (0.00 / 0)
now i feel comfy, since you're here, too.  ;)

Twitter Doesn't Make You Martin Luther King


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