Levels of the Atmosphere and Their Importance Relative to Weather Modeling

On the Weather Models 101 page, some of the mainstream weather models (GFS, NAM, Euro, Canadian) and their biases were discussed.  Now let’s look at some features that are shared among the various models.  Before launching into that discussion, it’s important to understand that weather forecasting is a “top-down” process.

When looking at any of the mainstream weather models, where do I start?

Why, at the beginning of course!  A long, long time ago, there was once a voluptuous…no, wait.  Sorry, wrong story.

At the risk of resurrecting dunce-cap wearing nightmares from third grade, we start at the sun.  The sun is the driving force behind our planet’s weather.  Uneven heating from the sun, the shape and tilt of the earth, topography, moisture levels, and other factors come into play that drive the weather we experience at ground level.

Meteorologists look at what’s happening in the upper levels (“upper air”) of the atmosphere, up to seven miles high in some cases, to figure out what might happen at the surface where we live.  Knowing basic physical truths such as: cold air moves in to replace warm air and moist air is less dense that dry air, allows scientists to calculate and track atmospheric energy.  Atmospheric energy (“dynamics”) plays a huge role in determining precipitation amounts, winds, or temperature changes that occur across the globe.

When I look at weather model tables, I see charts like “200mb, 300mb, 500mb, etc.”  What do these numbers mean?  Can I win the lottery using these numbers?

You might win the weather lottery, but not sure what that might buy you.  Instead, leave it to the weather folks to make measuring height in the atmosphere somewhat convoluted, but the atmosphere is a complicated entity.  It really helps to think three dimensionally when visualizing what’s happening in the sky.  These numbers correspond to “pressure surfaces” at which important meteorological phenomena/elements take place/reside.

Pressure and height have an inverse relationship with each other.  The higher one goes in the atmosphere, the lower the pressure (generally). You can think of 200, 250, and 300 mb as height levels in the atmosphere; in reality, they correspond to varying height levels based on atmospheric pressure, measured in millibars (mb).  Listed below are the approximate pressure/height relationships, courtesy of the Weather Informer blog (as a value added service, the conversion from feet to meters has been done for you since the NWS uses the metric system; you’re welcome!):

200 mb:  ~12,000 meters AGL (typical cruising altitude of a transcontinental commercial jet liner)
250 mb:  ~10,500 meters AGL
300 mb:  ~ 9,000 meters AGL
500 mb:  ~ 5,500 meters AGL
700 mb:  ~ 3,000 meters AGL
850 mb:  ~ 1,500 meters AGL
925 mb:  ~   750 meters AGL
1000 millibars(mb):  ~364 feet above ground level (AGL)
Surface:  Ground level.

Ok, now that I know what these numbers represent, how does that help me understand the purpose of each chart?

Well, now you should be an expert forecaster!  Errr, no.  It gets a little more complicated, so hold onto your weather balloon.  Let’s look at the primary use for each chart in isolation.  For illustrative purposes, screen shots from the GFS are used (other models are similar):

200, 250, & 300 mb charts – Locates the all-important jet stream.  The jet stream is a fast-moving ribbon of air that drives the movement of weather systems.  In the image below, the blue shaded region is the area of fastest wind speed.  The darker the blue, the higher the wind speed.  Purple-shaded areas can depict the core of the jet stream.

Forecasting weather without knowing the location/orientation of the jet stream would be like driving a car while blind.  Come to think of it, most drivers are blind when they text and drive, but that’s a topic for a different website.  On these charts you will commonly find wind speed and direction and the atmospheric height of the chart’s specific pressure level.  The location and characteristics of features on these upper air charts directly affect the placement of ridges and troughs on the all-important 500 mb chart.

My blind date once mumbled something about the 500 mb chart causing a personality incompatibility.  Please explainI
Your probability of dating success is better based on mood rings than the 500 mb chart, but the 500 mb chart is very important in meteorology circles.
500 mb chart – This chart is a key forecasting tool used by meteorologists.  The systems at the 500 mb level directly drive the weather experienced at the surface.  Key measures on this chart include temperature, vorticity, wind speed and direction, and of course the height of the 500mb pressure surface.  This chart also displays important features such as ridges (humps), troughs (dips), and sometimes even closed circulations (circles) depicted by the black line height contours.  The larger the feature, the more important the feature tends to be.  Vorticity is represented by the yellow/brown areas.  The darker the area, the greater the “negative vorticity” (explained below).

Ok, TIMEOUT!!!  What in the gol-durn world is vorticity and why do I care?  I’m getting dizzy!
Glad you asked, because that means you are not sleeping (yet).   The reason you might be dizzy is that vorticity is simply the measure of spin in the atmosphere.  The more spin, the greater the vorticity.  The spin that we are talking about is on a relatively small scale.

But hey!  Let’s not stop there.  The atmosphere has both positive and negative vorticity.  Positive vorticity is cyclonic (counter-clockwise) and promotes upward motion of air in the atmosphere.  Negative vorticity is anti-cyclonic (clockwise) and results in sinking or downward motion in the atmosphere.
The significance of vorticity cannot be understated.  Rising air (+ vorticity) is unstable, and in the presence of moisture can cause clouds and precipitation.   Sinking air tends to dry out and warm up; therefore + vorticity can be associated with inclement and possibly severe weather, depending upon its magnitude and the existence of other important atmospheric elements.

Going one step further, advection is movement of something toward a location.  In meteorology, advection usually involves the movement of heat, cold, moisture, or dry air toward someplace.  Likewise, vorticity can also “advect”.  PVA = positive vorticity advection and means atmospheric energy is  leading to an increasing incidence of rising air, meaning the atmosphere is tending toward destabilization.  NVA is just the opposite.

Awesome!  I’m feeling like a vorticity expert now, or maybe even a political PR expert in the use of spin, but so far I haven’t seen any charts mentioning moisture.   This FAQ is all wet unless moisture is discussed.  Are atmospheric moisture levels something we just go outside and wildly guess?

Nay, nay!  Meteorologists never guess at anything unless it’s a weather forecast!  Instead, they use the 700 mb chart to figure out moisture levels.

700 mb chart – So far we’ve mentioned parameters such as wind speed, temperature, and pressure.  Moisture has not been discussed.  The 700 mb chart is one main weather tool used to depict moisture in the atmosphere.  It is at this level that the precipitation process has its underpinnings.  In the winter, the 700 mb level is the upper level of what’s colloquially termed the “crystal factory”, the place where snowflakes are born.

In the absence of moisture, positive vorticity doesn’t do much in terms of sensible weather at the surface.

We’re done with charts now, right?  Please???
Kind of need a chart for charts, eh?  Nope, not done yet.  Let’s look at the 850 mb chart next.

850 mb chart - The 850 level is the level at which meteorologists key in on temperature.  Knowing where the freezing line (0ยบ C) is important for forecasts involving frozen precipitation.  A typical 850 mb chart looks like this:

The red lines are temperatures above freezing; blue lines are temperatures below freezing; and the purple line, if you can see it, is the freezing line.  As in previous charts, the solid black lines are lines of equal height, and the barbs depict wind direction and speed.

To determine precipitation type at the surface, meteorologists take the 850 mb chart one step further and look at predetermined atmospheric thicknesses, such as 1000 mb to 500 mb, or 850 mb to 700 mb.

What is meant by “thickness”?  Does this describe a typical meteorologist’s skull?

No, just my skull from trying to explain it.

1000-500mb Thickness chart – Since the atmosphere is technically a fluid, we treat it like one mathematically.   Generally, warm fluids are less dense than colder fluids.  This principle has a direct impact on the distance between the heights of pressure surfaces in the atmosphere.

If we look at a thickness chart and find that the height surfaces are very closely packed together, we can infer that there is some very cold air involved.  The opposite is also true.  Critical thickness values exist which help meteorologists figure out whether precipitation will fall in liquid or solid form.  Looking at different thickness levels in the atmosphere helps develop a clearer picture of the atmosphere’s temperature profile with height.  A single chart like the 500 mb chart cannot do this effectively, so the levels are combined to come up with an overall “thickness” measured in decameters.

In most cases the 540 dm thickness level is a rule of thumb to determine whether precipitation will fall as frozen (areas < 540 – blue lines) or liquid (> 540 – red lines).

I knew all this already.  Is that all you got?

Yes!  Now go out into the world and propagate more accurate forecasts than your local TV weathercaster!  Or, feel free to visit some of these other websites that will precipitate even more knowledge into your cranial cavity: