Tuesday, December 8, 2009

Thoughts About Really Cold Air

Note: Kind of a rambling post today. What can I say, it’s a crazy time for me at work. But you’re gonna love the graphics.

The seasons are officially tuned lockstep to the solstices and equinoxes, but I don’t think deep down any of us really believes that’s when they start and finish. Here in Utah, by September 21 Fall has already been underway for at least a couple of weeks; the real end of summer is more like Labor Day, or maybe LOTOJA. And the 2 or 3 weeks before the summer solstice, with their almost endless daylight, are surely more Summer than Spring.

Shoreline View1 Every season is like that, but none more so than Winter. Does anyone really think Winter doesn’t start till December 21? No, by then Winter is full-on. Depending on the year, it usually seems to start within a week or two, one way or the other, of Thanksgiving. This year, I know exactly when it started: Saturday, December 5th, at 11AM.

Watcher Lights Oh, it had already snowed a couple of times, and the week leading up to it was frigid- in the teens the 3 mornings before. But the foothills were still mostly dry and brown, and I’d still been mtn biking almost daily in the pre-dawn darkness. Saturday morning, hearing a change was in the forecast, OCRick, Vicente and I met at my place, pedaled over to the zoo, and started riding out Shoreline. The trail was frozen hard, as it had been the previous 3 or 4 days, and we rolled over the rock-hard ruts left by previous bikers during the thaws of the previous weekend.

Tangent: I a previous post, I mused that when you really get down to it, there are truly only 2 mysteries in life: the Mystery of Existence, and the Mystery of Self. I feel now that I misspoke, and that there are in fact 3 great mysteries in life: Existence, Self, and Why The Hell OCRick’s Camebak Is So Freaking Huge.

OCR Expand-O I’ve been riding with the guy for over 13 years and still have yet to figure out what exactly is in his camelbak. My current theory is that it contains one or more of the following: a) a ton of extra clothing, b)a cheesesteak, c) chemistry textbooks, or d) (dismembered & mummified) first wife. Knowing OCRick, it could be any of them…

Our plan was to ride to City Creek, then up to the radio towers and back, but as we crested the final rise before the City Creek descent, this is what we saw- the wall of the front.

Storm Front If you think of Salt Lake Valley as a big room, the “window” of the room opens to the West/Northwest. In that direction you can see out past the mountains, across the lake, into the West Desert, even into Nevada on a clear day. And it’s from that direction that the weather comes. While we’d wound our way through the draws of the foothills, a dark gray front had rolled in through the window and into the valley. We stopped, stared West, and small fast-moving flakes started blowing around us. We turned around and started pedaling home.

As we sped back to the East, we could see the front roll into the valley. In this shot, looking South from 6,000 feet, you can actually see the front moving in, pushing out the week-old inversion ahead of it.

Met Act Graphic There was more wind than snow on the ride back, but what snow did fall didn’t melt on the well-frozen ground, and we left clear tracks through the dusting. The lead edge of the front was a patchwork of clouds and flurries, and we rode through repeated “sunshowers” of snow. Down, down, through Dry Creek, behind the hospital and back to the zoo we rolled buffeted along by Old Man Winter.

At home I jumped in the hot tub to warm up, accompanied by Bird Whisperer, and as we sat in the open air the flakes grew in size and carpeted the adjacent deck and lawn. Winter had arrived.

IMG_3789 Side Note: Ever wonder why sometimes snowflakes are huge? It’s because they’re (relatively) warm and wet. Warmer flakes have a film of liquid water on the edges, and when 2 wet flakes come into contact, they tend to stick together*. When it’s colder, the flake-edges are frozen, and they flakes tend not to clump.

*This is because of the surface tension of water, which I explained in this post and showed in this Extremely Helpful (Albeit Ridiculously Obvious) Educational Video.

OK, you probably knew that already. But think about this: you ever notice that storms in the Wasatch often start up with monster-sized flakes, but as the storm continues, even though the snowfall continues, the flake size gets back to “normal”? Since leading edge of a storm front is where cold and warm air meet, and so the air temps are slightly higher, I’m guessing that’s why the flakes are big at the beginning. Once the storm “settles in”, the cooler air doesn’t melt the flake edges to the same extent.

BTW, I covered snowflake formation in this post last year.

IMG_3777 The rest of the weekend was frigid, and we’re locked in for a while; temps won’t break freezing again till Friday at the earliest. Usually during cold snaps in Utah, you always have an “escape valve” in the back of your mind: St. George. You know that you could, just could, call in sick and drive 4 hours South and everything would be nice and Spring-like again. Even if you only actually do so once or twice a year, the knowledge that you could somehow makes you feel less “trapped.”

But this week we really are locked in; the polar air mass has covered the entire state. It may snow today in St. George, and it’s unlikely to break 40F! Up on Little Creek Mountain it probably won’t break 30F. This is it; Winter’s here, there’s no escaping, and I’ll just have to “man up.”

Side Note: The pay-off of winter-weather is of course skiing, but this week’s storm has arrived so far with fairly minimal snowfall and terrible winds. So far, it’s just the yucky part of winter.

When you think about it, the strangest thing about the seasons is how darn late they all are. Think about it. The Winter Solstice, the darkest day of the year, is only 2 weeks away, but it just got really, really cold. A month before the solstice, in late November, the weather was still, on the whole, rather nice. But a month after the solstice, it almost certainly won’t be “rather nice.” Nor will it be much warmer 2 months, or possibly even 3 months after the solstice, even though the days will be so much longer and the sun far higher in the sky.

The reason of course is the oceans. They take a long time to heat and a long time to cool. It’s easy to forget about the ocean so far from it, but they act like a giant thermo-regulator throughout the year. Places far from the sea experience greater seasonal extremes in temperature. Utah is rough enough, but some of the wildest swings are in Central Asia, in places like Mongolia, thousands of miles from an ocean.

But what makes a week like this so rough is that the weather changes so suddenly. It’s not like every day for say 2 or 3 months, it’s like a ½ a degree colder or anything. No, it’s like in the span of a few days, the temps drop 20, 30, even 40 degrees. What’s up with that?

My first winter in Colorado was, on the whole, quite mild. Compared to a New England winter it was positively balmy, with daily highs in the 40’s and 50’s. Then one week, there was a minor snowstorm, following which it didn’t warm up for a full week. For a week the temps never climbed above 10F. The weatherman told us that an “Arctic Air Mass” had arrived. And every year I was in Colorado, once or twice a year- sometimes as early as October- an “Arctic Air Mass” would arrive, super-cooling the state down for a good 4-7 days.

Here in Salt Lake we don’t experience ‘Arctic Air Masses’ of the same severity, but we still do get them. This week it won’t break 30F until Friday, and freezing until Saturday (if at all.) So what is an “Arctic Air Mass” anyway, and why does it act like this?

Tangent: To be fair, the winter weather- and weather overall- of the Colorado Front Range is, on the whole, better* than the weather of the Wasatch Front. Their winters are warmer, their summers cooler. But we don’t get- by and large- those whacky Arctic freezes whereby it will stay under 5F or so for a week. I’ll explain why in a moment.

*Except for those annoying, clockwork-like summer afternoon T-storms. Nope, we don’t get ‘em.

323a-airmasses An arctic air mass is just one of several types of air masses (graphic right, not mine) that form over a particular part of the planet, and then create significant weather changes when they change location, often as a result of a shift in jet stream. These air masses are huge, often something like 1,000 miles across. There are maritime air masses and continental air masses, and polar and tropical air masses. What we call an Arctic Air Mass is a Continental-Polar air mass, designated “cP” in meteorology-speak.

Way, way up North, in Eastern Alaska and Northwest Canada is the birthplace of the cP air masses that affect us down in the lower 48. Over long, stable periods of high pressure, the troposphere- the part of the atmosphere below 30,000 feet (the part that we can breathe in) becomes very cold. The reasons for this may seem obvious, and some of them are, namely the low sun angle and super-short daylight hours. But there are other compounding factors at work. The low sun angle means greater tropospheric length, which is the distance a beam of solar radiation must travel between the tropopause (boundary with the stratosphere) and the surface. The longer the distance, the more of that radiation is scattered. (This is why sunsets/sunrises are orange- the tropospheric length is huge.)

IMG_3795There’s also much less tropospheric water vapor. Water vapor is actually the most important greenhouse gas in most of the world because of its quantity. (CO2 is a much more powerful greenhouse gas, but there’s way less of it.) But the cold polar regions evaporate little water into the air, the air retains less radiation, the air gets colder, the surface gets colder, and so on. And then there’s the high albedo of the arctic land-surface. Snow and ice are wonderful reflectors, and good amount of the radiation that does reach the surface is reflected back into space.

So the air is cold up in Canada. Tough luck for them. But what makes it tough luck for us is when these super-cooled cP air masses make their way South. The force that drives them South is the polar jet stream. Jet streams are powerful air currents up in the upper troposphere, just below the tropopause that move from West to East*. They’re caused by solar heating of the atmosphere and rotation of the planet**.

*Usually. Lesser, easterly jet streams can form in the tropics.

**Jet streams also occur in the atmosphere of Jupiter, where the planet’s internal heat is also a causative factor. For more on Jupiter, and its Way Cool Moons, see this post.

jet stream1 The thing about jet streams (diagram right, not mine), and the polar jet stream in particular is that they don’t just go West to East. It’s a Westerly air current, but follows a meandering path across the continent. This path is called a Rossby wave. Rossby waves are caused by shear in rotating fluids. In the case of the Earth’s atmosphere that fluid is the atmosphere, and the shear is caused by the Coriolis effect with latitude.

Tangent: Rossby is for Carl-Gustaf Rossby, a Swedish-American meteorologist who organized the training of military meteorologists in WWII. He’s probably the closest thing we have to a National Meteorological Hero, but of course you never heard of him because, well, we, uh… don’t have National Meteorological Heroes.

I think part of the reason is that meteorology- specifically weather-forecasting- has a bad rap among the public. We all love to go on about how inaccurate weather forecasts are, which is funny, because if you compare weather forecasts to, say economic forecasts, which arguably affect most of us much more, weather forecasts are far more accurate. Really, weather forecasts- like automobile tires, fuel injection, and LED headlamps- are one of those things that have consistently and quietly gotten better and better just within our lifetimes. In fact, the average 3-day forecast today is more accurate than the average 1-day forecast was in 1980.

All About The Coriolis Effect

The Coriolis effect is all about conservation of angular momentum. Let’s pretend you wanted to kill your Arch Enemy, who lived in Tempe, Arizona. And fortunately, you had a Wicked Powerful Rifle (WPR), capable of shooting a bullet 1,000 miles in a straight line. So you stood in the middle of Salt Lake Valley and fired your WPR exactly due South. Guess what? You’d miss him. As your perfectly-aimed bullet sped South, it would appear to veer right. And if he shot back at you with a similar WPR, his bullet would also veer right.

Cor Graphic 1 The reason is that Tempe is moving faster than Salt Lake City. Both spin around the same axis, but Salt Lake, sitting about 800 miles higher up on the globe, is spinning more slowly. When both bullets are sitting- unfired- in their respective WPRs, the Mesa bullet already has significantly higher angular momentum than the SLC bullet, and as it travels North, across more slowly-rotating earth surface, that angular momentum is expressed by its rotating faster around the Earth’s axis than the land over which it is traveling, which manifests itself as moving more rapidly to the East (the direction is which the Earth spins) and to the right.

Cor Graphic2 Conversely, the SLC bullet as it travels South, with its lower angular momentum, falls behind the land over which is passes with respect to the Earth’s rotation, and veers to the West, which is its right.

Tangent: I picked this example because I thought guns would hold your attention, not because I have an arch-enemy. In fact, as I was thinking about this example, there’s really no one in the world I’d want to shoot, even if I could get away with it. Oh, sure I might crack off a shot at Osama bin Laden or Kim Jong Il if the circumstances presented themselves, but I mean someone I actually know. Really, there’s just no one I have strong enough negative personal feelings toward that I’d want to hurt them.

Now, having said that, there are people I Really Don’t Want To Run Into. You know what I’m talking about- maybe a past co-worker you didn’t care for, or an annoying former neighbor, or an ex-girlfriend/boyfriend where you maybe handled the break-up poorly, or your wife’s friend-of-a-friend who manages to work every conversation into a pitch for why you need to start seeing her psychic*- that kind of thing. iphone_large5 And I think about this because it gives me an idea for an awesome smart-phone application: The Proximity-Annoyance Alarm. I got the idea from some iPhone commercial I saw where a couple of unbearably-hip yuppies are trying to meet up for sushi or facial wraps or some such and they’re sending each other directions, and monitoring each other’s locations on- what else- their iPhones. Who cares? I don’t need to see where my friends are- if I really care I’ll call ‘em up and ask them. But I would like to know where the people I Really Don’t Want To Run Into are, and more importantly, whether I’m in imminent danger of running into them. So the Proximity Annoyance Alarm would allow you to input the mobile #’s of those people and then an alarm would go off when one was nearby, and the app would give you directions to avoid them. Now that would be a useful iPhone app.

*You probably thought I made that one up. I didn’t. I swear, I have the weirdest friends-of-friends.

South of the equator, the effect is in the opposite direction, and North-South trajectories veer to the left. These forces work not just on make-pretend bullets, but on real-world air currents, and the North-right/South-left distinction is why cyclones always rotate counterclockwise in the Northern hemisphere and clockwise in the Southern*.

*It’s also the reason why cyclones rarely form near the equator; there’s little Coriolis effect.

Anyway, this effect causes shear that alters Rossby Waves, and so from time to time the Polar Jet Stream bobs way South, driving a thousand-mile wide cP air mass down into the heart of the Lower 48. But as it does so, the brunt of the mass is deflected by the Rocky Mountains, and this is why Denver’s cP air masses are so much more icy than ours. The cP air is frigid across the upper plains, but as it slowly makes its way East across the Mid-West and the Appalachians the mass is gradually warmed and broken up, and this is why I never even heard of “Arctic Air Masses” growing up in New England.

So that explains what an “Arctic Air Mass” is, why they really are worse in Denver than in Salt Lake, and why I never heard of them in New England.

isarnv_ Tangent: But what it doesn’t explain is why Central Nevada is so much freaking colder than the Wasatch Front in Winter. Surely, any Wasatch Front resident who’s looked at a wintertime weather map has noticed this: Central Nevada is an icebox. The lows in places like Elko and Ely are usually 10-15F lower than in Salt Lake. Why is that? The latitude is the same, the altitude more or less the same, and they’re far from the Polar Jet Stream path. Why is Nevada so damn cold?

The first real week of Winter always seems to come on a bit heavy; it’s dark and cold and Spring seems years away. But solstice is just around the corner, and then every day is just a little bit lighter, hinting at the Living Year to come.


Phil O. said...

Great post.

"So the air is cold up in Canada. Tough luck for them."

And me, going to Calgary soon -- and it's -8F there right now.

Ski Bike Junkie said...

Once again, excellent graphics.

Another interesting tidbit about snowflakes and snowpack: when they come in warm and big like that and gradually get smaller, it makes for what the avalanche forecasters call a right-side-up snowpack. The warm, big flakes don't just bond with each other, they bond with the snow they land on. Then as the drier, colder snow falls on top, you get a stable base with light, dry powder on top that's fun to ski and isn't going to all come crashing down on top of you.

When the storm comes in cold and gets warmer, however, you get an upside-down snowpack, where the dry, cold snow falls first and doesn't bond with the layer beneath it, then it gets heavier snow on top, and it's all quite a bit more likely to slide, either naturally, or less-desirably, when disturbed by a skier.

maggie said...

I love this time of year - especially the part where I get to laugh at all my friends in the states who are already getting snow while I haven't even unpacked my sweaters here in Toronto :D

Lucy said...

Great post reminding me of all that cool meteorology stuff I learned in high school but then promptly forgot but that I get reminded of every time I watch the weather forecast and think "why does it do that again?".

I hope you or someone can answer the Nevada-is-colder question for sure. My guess is that it is because the valley floors, where the cities being reported on are located, are significantly higher in elevation. The lowest elevation in Snake Valley (border of Utah and Nevada) is like 5200 feet, Elko is at 5000, Ely is at 6400, while Salt Lake International Airport is at 4221.

Watcher said...

Phil O.- Rough time for a Calgary trip. But on the bright side all those Canadians are really nice.

SBJ- Great snowpack explanation- thanks!

maggie- Shh! You're not supposed to say anything, lest you inundate your fair country with emigrating rude Americans!

Jube- You're definitely right about the elevation, but I don't think that alone can explain it. Ely and Elko both seem to routinely have winter lows lower than Park City/Kimball Jctn, which is higher than either. This morning at 7AM it was +3F in PC, -18F in Ely and -20F(!) in Elko. I think I'm going to call one of the local TV stations and see if I can reach an actual meteorologist...

KanyonKris said...

You need to stage a flat and everyone forgot their pump so you can find out what's in OCRick's pack.

While your explanation of the Coriolis effect is good, I still don't really get it. Angular momentum is one of those unintuitive things for me.

Are the Elko and Reno valleys like big sinks (like Peter Sink in Cache County)? Seems unlikely as the weather usually blows the cold air out regularly. Inversions?

Chris said...

Firstly, action graphics = awesome.
and the photo of the inversion getting pushed out, cool.

But I'm still not getting why Denver is impacted more as you say here. "But as it does so, the brunt of the mass is deflected by the Rocky Mountains, and this is why Denver’s cP air masses are so much more icy than ours."

Two things confuse me.
1. The jet stream is moving from west to east, so I would think the cP air mass would hit SLC first.
2. If my geography is correct, the mountains are on the east side of SLC, so wouldn't the cP air mass roll into SLC unobstructed? and the mountains are on the west side of Denver, so I'd think it would disrupt the flow of the cP air mass?

What did I miss?
You are far from the sea but you are close to a really big lake, does that have some regulating affect on temperature?

As for those of us in Austin, it's been in the 40's and rainy... not so good for biking (I'm a wimp), though at my folks house the storm this week dumped ~ 2 feet on the ski hill, that'll make Christmas more fun!

Colin said...

The two lakes on the Wasatch front probably have a moderating effect on the temperature felt in SLC while central Nevada (combined with its higher elevation) feels the brunt of the cold Canadian air. Too bad the universal health care idea doesn't travel unimpeded across the border.

Watcher said...

cmsparks- Check out this graphic and this graphic again from the post, as well as this graphic. The polar jet stream doesn’t go straight across North America, but dips down to the South as it passes through the middle of the continent, in a path determined by the Rossby wave. So the path of the center of a cP air mass from Eastern Alaska/Yukon is Southeast as it crosses the US border, which puts it on the East side of the Rockies (which run further to the East as you go South.)

A maritime polar (Pacific) mass (mP) would likely hit UT before CO, as you can see here. And you and Colin are on to something with the lakes, as we’ll see tomorrow.

KKris- and you are on to something with the inversions- check back tomorrow!

KB said...

Ideal timing for this post as it's been below zero for days in the Front Range.

I think that I understood most of it... But am I right that your explanation didn't explain the snake-like bends in the jet stream? You say that it's a Rossby wave that causes it but that doesn't really explain it (I thought that it was being attracted and repelled by low and high pressure systems). That's not a criticism - just checking that I do indeed 'get it'.

So, it's warmer where you are. This week, I might've moved there in a heartbeat. However, Dec is often our toughest month so it should turn around.

The graphics are awesome. You should be a teacher. I used to spend hours making graphics like that to teach about the mechanics of animals!

Watcher said...

KB- Yes, you’re right- the jet stream is where different temp air masses meet. But the Rossby Wave determines where they meet and how that path changes. Wikipedia has formulas associated with them (well beyond the kiddie-math I take on in this blog), and a somewhat helpful graphic of a meander evolving to send a cold air mass South.