Weather Primer for Jongs

[Huskier]

So, I can't be the only one with this problem: This time of year I find myself searching the skies. Looking for a sign. Something, anything to indicate that winter is coming. My search proves fruitless, and I take to the interwebs in search of data. But scrolling through the NWAC and NOAA websites, it's hard not to get overwhelmed. "WTF does METAR mean?" "Oh hey look, radar. Wait, do I want base reflectivity or differential velocity?". Beyond 5-day's, beyond resort websites' fudged numbers, there have to be some mags out there with meteorological experience to school the rest of us with. Tricks, rules of thumb, etc. Lay it on us. If you have a specific question or there's something you don't get, throw it out there. Hell, somebody might actually learn something.

Stuff I've never understood:

-Wind @ elevation: Any reliable way of knowing that it's going to be blowing 60 knots on that summit you're headed for?
-Cloud deck: How do you figure out the max height of a given weather system? How high you'd have to get to break through the clouds?
-Radar: Types? Modes? What does what?
-Satellite images: Infrared? Water Vapor? Others? What shows what?
-Weather charts: Worth looking at, or leaving to the pros?

[userA]

make yourself friendly at TAY, they got a scientist named amar that throws info around like a rapper does bitches. serious weather smack is also spewed by cliffton mass at UW.
B

[Adamantium]

Beyond 5-day's

It is still extremely difficult to make accurate, specific predictions more than about this far into the future.

Regardless of what you can learn about the near-term, just keep that in mind.

[klar]

Stuff I've never understood:

-Wind @ elevation: Any reliable way of knowing that it's going to be blowing 60 knots on that summit you're headed for?
-Cloud deck: How do you figure out the max height of a given weather system? How high you'd have to get to break through the clouds?
-Radar: Types? Modes? What does what?
-Satellite images: Infrared? Water Vapor? Others? What shows what?
-Weather charts: Worth looking at, or leaving to the pros?

- Hard to say, depends on what you have available. Obviously there are specific mountain weather forecasts in some areas, or public access near real time AWS data. If you don't have that, you can look at weather maps. GFS (NOAA) maps for example are free online in a bunch of places. They show various parameters at different pressure levels, so for higher altitudes you would want to look at the 850hpa maps, or as a general indication maybe even 500hpa. If such maps of the windfield are not available for your area, you can look at pressure gradients and the strength/position of the jetstream. In general, some understanding of weather systems and the effects of local topography helps a lot. If you know what the greater pressure situation is like, you can usually figure out if you want to be on that summit or not.

- Airtraffic forecasts are excellent for this. If you can't get those, radiosoundings are good to see atmospheric layering. This is a great resource: http://weather.uwyo.edu/upperair/sounding.html You sort of need to know how to read them though. Skew-T diagrams are easiest, imo. Generally you will only break through the clouds if you have an inversion, not so much if there is a front (unless you are on a plane). GFS also has some cloud maps. Could try using satellite imagery too.

- This is a really broad question. Can you be more specific? Groundbased precipitation radar sends out microwaves, which bounce back from water and ice crystals. From this, intensity and distance of the precipitation is calculated. It can be hard to differentiate between different types of precip and this obviously works best closest to the radar station. Try reading the wikipedia articles on weather radar or doppler radar.

- Mostly you see clouds. They can tell you a lot about the weather you are having, where the fronts are, how they may be moving etc. Infrared usually shows the altitude of the clouds based on their temperature. Water vapour is also an infrared image in a band that detects water vapour well. What site are you looking at for these images? Usually they tell you what you are seeing somewhere, like here http://weather.unisys.com/satellite/details.php. Again, can you be more specific? If not, I suggest reading a textbook.

- Sure, look at them if you want to be geeky. Figure out what makes the most sense for your application and make a habit out of checking that particular product. You will figure out basic patterns quickly.

Unfortunately I don't know much about resources like blogs and weather forums in North America but there are a ton of extremely helpful sites for Europe, so I imagine it's just a matter of doing some googling for wherever you are.

[Huskier]

Awesome, thanks so much for the start.

Re: Radar: I guess I sort of found some of the answers to my own questions. But in case anybody else was wondering, these were somewhat useful: http://www.wunderground.com/radar/he...e_reflectivity
http://forecast.weather.gov/jetstrea...r/baserefl.htm

Re: Radar: So I guess I'm specifically wondering about base reflectivity, and what the different inclinations tell you: Say you have different values at different tilt elevations. What does that mean in terms of precip potential, form?

[klar]

Base reflectivity is the reflectivity at the lowest inclination, i.e. the closest to the ground. Each inclination gives you the reflectivity in a particular plane, at different altitutdes. (The inclination angle is the angle that the radar is tilted above the horizon. Base is typically 0.5°.) Composite images stack these planes and show the highest reflectivity of all elevation scans, rather than just the base reflectivity. If you see higher values in the composite than the base, that implies that there is something higher up in the atmosphere that is not (yet) reaching the ground. This happens for example with strong thunderstorms, where updrafts keep a large amount of water in the air for a while. Once the updraft dies, the water reaches the ground. In this case high values at higher inclinations can indicate severe weather. With snow it's fairly common that the crystals evaporate before they reach the ground (virga). This also shows in the composite. Regarding the form of the precipitation, liquid water has higher reflectivities than ice, so rain and especially hail will be far brighter than snow. This can cause various weird effects in the image but isn't directly related to inclination angle.

This is a nice article on how radar works and how to interpret certain things. Link downloads a pdf.

[theshredder]

Geostrophic flow is, imo, the most fundamental concept in meteorology (it's the end result of the balance between the coriolis force and the pressure gradient force), and understanding how the surface interacts with the upper level winds is the next most important thing to know. Basically winds are always parallel to pressure contours (except near the ground due to friction, which I'll get into next), and the greater the pressure gradient, the greater the winds. Its a linear relationship. If you can understand geostrophic flow then weather charts are easy.

Winds as a function of elevation are tricky. They depend mainly on atmospheric stability (atmospheric stability example: gasoline floating on water is stable but gas underneath water is not, because gasoline has lower density than water) and the 'roughness' of the ground (amount of friction). Unstable conditions and higher friction surfaces allow strong upper level geostrophic winds to mix down to the surface or to valley bottoms easily. The friction the ground creates varies depending on what's on the ground. Obviously buildings and trees create more drag on the winds than grass, although buildings can also channel winds, like water over a weir, making surface winds stronger (terrain can do that as well). Mountains are the roughest surface and when winds are strong, they can force a lot of mixing of the lower atmosphere, which generally results in precipitation. (Most of the precipitation is, however, orographic in nature.)

Stormy conditions are caused by a more unstable atmosphere, which allows ridge top winds to be easily mixed down to valley bottoms in the form of strong gusts, or turbulence. However, sometimes with strong storms there is often a capping inversion near ridge top which keeps the stronger winds well above valley bottom.

Stable high pressure systems usually result in very calm winds excecpt very close to ridges or mountain tops. The stability of the atmosphere keeps the geostrophic winds at upper levels and air below, in the valley bottoms, sort of just stagnates mainly. One thing is that often with weak geostrophic forcing, when the atmosphere is very stable (perhaps during a strong nocturnal inversion) the valleys will channel the winds, making them parrallel to the valley at valley bottom and at right angles on ridge tops.

So if you can absorb all that you're basically an 'expert'. The next step is to understand the Navier-Stokes equations and punch them into a computer, then you're an 'uber-expert'.

Satellite and radar are pretty much self explanatory imo. They show you were clouds and precipitation are.

(You asked about winds but the other huge component of the equation is solar radiative forcing and thermal radiative forcing (i.e. the earth's radiation). Thermal radiation from the atmosphere has a huge effect on the snowpack. Clouds re-radiate thermal radiation back to earth, keeping the nights warm. Snow can also be baked, essentially, by warm air high above, even when surface temperatures are well below freezing. Watch for conditions which can result in inversions (warm air above cold), they screw up the meteorologists and they screw up the computer models even more.)

Edit to add on winds: This is from experience, not school, but in general its always pretty windy at ridge top unless you're right in the middle of a high pressure system, or a ridge. Otherwise its gusty in the vallies (more gusty if the atmosphere is unstable/stormy) and the winds are strong and relatively constant at ridge tops. Also should note even though it might be obvious to some, that storms bring stronger winds than high pressure systems.

[klar]

supercool big picture wind/flow tool: http://earth.nullschool.net/

[MakersTeleMark]

If you can't summon your inner Gratzo from this, and not pull some bs second guessing, and maybe, just maybe have a good coin flip, then, well, you are hopeless.

I step outside before I go to bed, pee, then do the same in the morning. I factor in what day of the week it is, what went down in the last week, and then, usually decide to ski.

http://www.tetongravity.com/forums/s...14#post1687214

[klar]

well, the OP asked some specific questions about sat/rad images and apparently can tell the difference between the two, so I would assume he knows of images like the ones in that thread and probably even has a pretty good idea what they show, unlike most people.

I overlooked the "what does METAR" mean question the first time round. METAR is an internationally standardized way of transmitting&exchanging weather data. You have values for various parameters and turn them into a string of numbers accoring to a specific system that people anywhere can understand and that can easily be automated.

[theshredder]

Every day is a little different but the weather last week convinced me I should add a case study, from observations, which contradicts some of what was in my last post. figures.... (specifically, light winds with high pressure systems.)

Moderate storm rolls in with light winds except at ridge top and about a foot of snow on days 1 and 2. Day 3 is cooler, drier, and somewhat unstable (with light graupel, a sign of convection and a relatively deep unstable layer in the lower atmosphere), but still with relatively light winds even to ridge top. Day 4 is cold with a decent amount of sun and strong gusty winds to valley bottom. This is despite the absence of a low pressure system and I believe has more to do with channelling of winds by valleys and a deep, dry neutral layer, or possibly a quasi-stationary cold front.

From an avi perspective, days 4 and 5 are the days to watch, imo, because the winds have just moved a lot of the new snow to lee slopes. Gusty winds at valley bottom mean even gustier up high, and the snow is gonna get moved and fast, which causes avalanches.

[HansJob]

Go read Woody's books. If you have the opportunity take his class. 5 days out is tough to make accurate predictions.

[theshredder]

^^ Yup. That true. I still say you can get a pretty good idea of whats gonna happen over about the next 10 days though. sometimes more, sometimes less.

But I was talking about a 5 day period that already happened. I'll leave the forecasting to people who get paid to forecast.

[montanaskier]

pm Rontele

[klar]

Every day is a little different but the weather last week convinced me I should add a case study, from observations, which contradicts some of what was in my last post. figures.... (specifically, light winds with high pressure systems.)

yes, well, understanding the concept of geostrophic flow and being able to "punch the Navier-Stokes equation in a computer" does not have much to do with actual, day-to-day forecasting, or answering the question "will it be windy on my skitour today?", which is all most people are interested in, in case you haven't noticed...

Gusty winds at valley bottom mean even gustier up high

not neccessarily. in winter i would agree that this is usually true, in summer it is often gusty in the valley and calm up high particularly in stable high pressure due to valley wind systems. happens in winter also, but winds often don't get as strong due to lacking sun. you seem to underestimate the effect of topography on near-ground wind somewhat, often you don't need complicated explanations like a quasi-stationary cold front, imo.

[theshredder]

yea thanks klar... you've been helpful. OP also asked about weather charts and geostrophic flow is a simple to understand balance of forces concept that makes technical-looking weather charts readable to the laman. Theres also this thing called google and wikipedia that people who have some understanding of physics/engineering (of which there are many on this forum) can use to look stuff up if they so desire. I'm just giving them a starting point.

and as for ROLLBRAINDAMAGEROLL, take your negativity and your head shots to the padded room where people who are actually funny can school you like the retard that you are.