|HOW ARE TROPOSPHERE LAYERS DIVIDED FOR WX ANALYSIS?
METEOROLOGIST JEFF HABY
The troposphere is the portion of the atmosphere closest to the Earth’s surface where the typical weather takes place. It extends from the
Earth’s surface to the tropopause. The troposphere is divided up in a couple of common ways when weather analysis is done.
One way to divide the troposphere is to divide it into the lower levels and upper levels. The lower levels are from the ground surface to
the 500 mb level. The upper levels are from 500 mbs to 100 mbs. 500 mbs is taken to be the transition zone between these two levels but
it practice the 500 mb chart is considered an upper level chart. A similar way the troposphere is divided is from the ground surface to
550 mb and from 550 mb to 100 mb. This is due to 550 mb being near the level of non-divergence. Basically what this means is that
convergence in the low levels will become divergence at the upper levels or divergence in the low levels will become convergence
in the upper levels when there is a circulation of air throughout the troposphere such as from a deep low pressure or high pressure
system. This makes the 500 mb chart a transition chart between the low and upper levels since it is near the level of non-divergence. There
is not a commonly used 550 chart, thus the 500 chart is used for the chart that is just above the level of non-divergence where upper
level analysis processes and vorticity are analyzed. Being near the level of non-divergence is why vorticity advection is good to
analyze on this chart since vorticity analysis is more distorted moving farther away from the level of non-divergence. The surface
chart, 850 mb chart, and 700 mb chart are low level charts while the 500 mb, 300 mb are considered commonly used upper level charts.
Another way the troposphere is commonly divided up is to divide it into 3 layers. The surface to 700 mb is the low levels, 700 mb
to 500 mb is the middle levels and 500 mb and above is the upper levels. In practice, the 700 mb chart is typically classified
as middle levels while the 500 mb chart is classified as upper levels. Breaking the low levels into low and middle levels is
helpful since several tropospheric processes are much different between the surface and 700 mb than they are from 700 mb to 500 mb. The
surface layer and 850 mb are influenced more by the ground surface and air flow is more turbulent. Above 700 mb, the air flow
is typically smoother with less influence from the ground surface. This of course can change when storms occur. Using the 3 layer
approach, the surface chart and 850 chart are used for low level analysis, the 700 mb chart is used for middle level analysis and
the 500 mb and 300 mb charts are used for upper level analysis. In high elevation regions where the 700 mb level is closer to
the surface, then 700 mb is used more or less as another low level chart.