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Saturday, March 29, 2008 - 9:11PM 
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Watching 850mb (and a primer on atmospheric layers)
 So what is all this talk on this blog about 850mb? 700mb? 500mb? I'll try to explain without being too verbose (hard for me, as you know if you've read this blog for very long). The atmosphere is made up of distinct layers, varying in thickness and number from day to day and season to season. Each layer has its own properties - wind speed, direction, humidity and temperature. If a layer is saturated (at or near 100% humidity), condensation occurs - and viola, clouds form. If you've ever taken a flight in a jetliner, you see these layers pretty clearly. At takeoff, you'll start out in clear air - the first layer (or layers). Then as you climb, you'll pass through clouds - another layer. Then another clear layer, then another cloud layer - and so on. Sometimes you'll see three or more distinct cloud layers sandwiched between clear ones - some thick, some very thin. Now, we live down in the layer at the surface (called the boundary layer, since fronts are usually confined to it), so it's the one that we most associate with our weather. However, what goes on in the layers aloft also play roles in determining what type of weather we see at the surface. As a result, meteorologists need to analyze all of the layers in the atmosphere to make their forecasts.
In meteorology, the atmospheric layers are measured by and referred to by their barometric pressure readings (in millibars, abbreviated mb) and not their actual distance above the ground. That can seem a little confusing at first, especially since the millibar levels are almost never at the same physical altitudes from day to day. This is what you'll see me talking about when I refer to the 500mb layer, the 850mb layer, etc. The higher in the atmosphere you go, the lower the pressure reading. 850mb refers to the layer above the surface boundary layer, usually a few thousand feet high. 700mb is another few thousand feet above that, 500mb is the 'midlevel', and the 300mb-200mb layers are the topmost in the troposphere. The surface layer can range from 1000mb-950mb or less depending on terrain elevation. In mountainous areas, sometimes the ground is even above the 850mb layer! In the winter, the layers are more 'compressed', not as thick - while in the summer, the thicknesses are greater.
Soundings: measuring upper air layers
At the surface, there are hundreds of weather stations in the USA reporting dewpoint, temperature, wind speed & direction and barometric pressure (you can see these on the METAR display). But that data only tells us what is happening at the surface - how do we measure the upper air layers? This is where weather balloons come in. Twice a day, weather balloons are launched from over 1000 sites around the world (90 of which are in the USA). The balloons rise up through all of the layers in the atmosphere, just like you do when you fly on a jet. The balloon carries an instrument pack that measures the temperature, dewpoint, barometric pressure and wind speed/direction as it rises. This measurement is called a sounding, and the data is plotted on a graph called a skew-t:
Skew-T for a Pittsburgh, PA sounding
Soundings and skew-ts are a complex subject in themselves, so I won't go into too much detail here. However, just looking at the graphic above you can see the millibar levels in blue on the left, the temperature plot in red, the dewpoint plot in green, and the wind speed and direction readings on the right. The circular graph at the top is the hodograph, which is used to determine wind shear. Where the green and red lines get close together (where the temp and dewpoint values are close), this marks layers that are at or near saturation - and therefore where clouds are most likely to be. This skew-t shows that there are probably clouds over Pittsburgh around the 800mb level. Just looking at this graphic, you can see how much that the conditions vary as you go upward in the atmosphere. The wind speed and direction, temperature and dewpoints vary greatly from what we see and feel down here at the surface.
850mb in forecasting winter weather
So what's the deal with 850mb? In the winter, you have to look at the layers from the surface up to around 700mb to determine the type of precip you will get at the surface - the most important of these usually being the 850mb layer. Above 500-700mb, the temperatures are typically below freezing year-round. If 850mb and higher are below freezing, snow can develop and begin falling through the boundary layer. If the surface (boundary layer) is above freezing and 850mb is below freezing, snow will usually melt before it reaches the ground (as rain), or if it makes it to the ground will melt quickly. If the boundary layer is also below freezing, the snow can make it to the ground and stick. In many cases, falling precip will bring some of the 850mb temps down to the boundary layer, causing it to cool. So even if the surface is above freezing, heavy snow falling from a very cold 850mb layer can cool the boundary layer below freezing in a relatively short time. It's almost like dropping trillions of little ice cubes down into a huge cooler.
Conversely, if 850mb is above freezing and the boundary layer is below freezing, liquid raindrops falling into the boundary layer (or partially melted snowflakes) will either freeze into ice pellets before reaching the ground (sleet) or will freeze on contact with the ground (freezing rain).
Upcoming 850mb temps
So why did I just go though explaining all of that? Well, as you've read recently I've been trying to get a handle on exactly when we can say that winter weather is truly over for the season. The main thing I've been using to try and accomplish that is the 850mb temperature forecasts on the various models. It's pretty safe to say that below-freezing air coming down from Canada in the boundary layer (surface) is very unlikely by this time of year. This is because the spring sun angle tends to heat the boundary layer air before it can make it very far south. However, sub-32°F air can and often does make it down here at the 850mb level and higher, sometimes as late as early May! Solar heating doesn't have as much influence at 850mb and above, so cold air at those levels can travel pretty far south even in mid-spring.
So, if 850mb temps over us are below freezing in April and May, does that mean it's going to snow? Maybe, maybe not - it depends. The influence of solar surface heating (called insolation) is increasing dramatically through the springtime, which usually keeps the boundary layer warm enough to prevent temps from dropping much in response to precip or mixing from a cold 850mb layer. However, it is possible for snow to reach the ground and even stick if the right factors come together. If the cold air at 850mb is persistent for several days, and if sunlight is limited by overcast skies, then the cold can mix down from 850mb - particularly if precip is falling. Also, if skies clear out at night and allow radiational cooling to cause temps at the surface to plummet, a cold 850mb layer moving in immediately afterward will have an easier time getting snow to the ground. This type of thing gets more rare the farther we get into spring - but it can still happen.
In the mountains and at higher elevations, the ground is closer to the 850mb level, so any frozen precip doesn't have as far to fall before reaching the ground. Some of the high ridgetops in West Virginia - such as at Snowshoe - can sometimes actually be in the 850mb layer! The boundary layer is also not as thick in the higher terrain, meaning a cold 850mb layer above it doesn't have as much work to do to get it to cool. As a result, snow is more common in the mountains during the winter, and is more likely during early to mid spring than at the lower elevations.
So that said, let's look at some model output for the 850mb layer in the next week. The GFS model carries a deep trough over us by the weekend, dropping 850mb temps well below freezing over us for several days (the models show 850mb temps in Celsius, where 0°C is freezing). The output for Saturday morning (April 5) shows a saturated (near 100% humidity) 850mb level over the mountains, suggesting some upslope precip (possibly snow) in the higher elevations:
GFS 850mb temp/humidity forecast for April 5
At the surface, temps are chilly but not in much danger of hitting the freezing mark outside of the mountains:
GFS surface temp forecast for April 5
However, unexpectedly high precip rates could start cooling the surface temps to the point that snow could briefly make it to the ground. Whether or not this is possible will become more apparent as the event gets closer.
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