850 mb – Just a Few Thousand Feet Above

If you’ve kept up with the atmospheric pressure level discussions I applaud you! Meteorology has so much information to be learned and the upper atmosphere only scratches the surface.

The 850 mb pressure level is close to the surface at about 5,200 feet (1,590 meters). If you’re looking at this map (below) the heights are plotted and the last zero is dropped. We still continue to plot the heights in meters. If you have a height of 1,320 meters then it’ll appear as 132. The heights also allow meteorologists to depict highs and lows or centers of circulation.

Height contours in meters.

Temperatures or isotherms are also plotted in Celsius. This is probably the biggest aspect of this pressure level as frontal boundaries can be found from rising or cooling air within a warm or a cold front.

The temperatures that are at this level are not changed by the sun or the daily and night and day cycles. This gives meteorologists a good idea of where the warm mass of air lies or cold air.

Just like in the 500 mb chart where the 540 dm line was important to distinguish between rain/snow… the 850 mb also has a general rule of thumb. If you’re looking at the 850 mb pressure map and have the zero degree isotherm (line of constant temperature) that is probably the rain/snow line.

If it’s colder than zero degrees then snow is likely or if it’s above then expect rain.

With both the heights and temperatures plotted we can get the temperature advection. Temperature advection is the transfer of warmer or cooler air into a specific location. If winds are mapped then that also helps.

Say you have your height contours, isotherms, and winds… if the isotherms and wind is perpendicular to the height contours then you have temperature advection. This is called warm air advection if you have warmer air moving into an area of cooler air. The opposite is called cold air advection…if cold air is moving into warmer air. And this would be a rapid change in advection.

If the isotherms and height contours are parallel then little to no advection is taking place. But if you add in the winds and they’re perpendicular then that may cause strong advection.

In the short run, the spacing between both contours also matter. The closer the contours are then the faster the advection and the stronger the front. If it’s widely spaced then the change may be minimal… especially if it’s over a wide area.

Black lines: Height contours
Red and Blue: Isotherms

Looking at the example from above….

You will notice that the height contours are plotted in black and the isotherms are in red and blue (blue for below 0 deg C and red for above 0 deg C). If we look in relation to near Canada, North Dakota and Montana you can see that there is advection taking place.

Along the border of Montana and North Dakota there is cold air advection. The isotherms are perpendicular to the height contours and are tightly compacted. This probably means a strong cold front is developing. Just the opposite is occurring in Canada… warm air advection. You’ll notice that the isotherms remain perpendicular to the height contours but this is warm air advection.

If you look along the east coast and mid-west there is essentially no advection taking place at all. There are hardly any contours of any type and in general remain parallel to another.

This discussion only scratches the surface for the 850 mb analysis. We can also plot precipitable water, humidity, and dew point depressions. We will have late discussions on this as we continue to move forward. Stay turned.

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