Last time I went into detail about
sfc weather maps and how to read them, I also mentioned there were two types of
maps: sfc maps and upper air maps. Now it’s time to do that second one.
When meteorologists talk about upper
air, they’re referring to pretty much everything that is not at the sfc (or
very close to it). To identify how high some upper air phenomena is pressure
(in units of millibars, mb) is often used instead of elevation, such as feet or
meters above ground. What this all means is that the height of any given item
is expressed as the air pressure at the level of the item, for reference, the
average pressure at sea level is 1013mb. Pressure itself decreases with height,
so the lower the pressure, the higher the item is. However, pressure does not
decrease linearly with height as the figure below illustrates, instead is a
curve. Furthermore, the height of any given pressure level will vary from place
to place due to weather systems. For example, the altitude that the 850mb level
can be found at is in the neighborhood of 1460m above sea level, but in the
middle of a low pressure system 850mb might be observed at less than 1200m or
in a high pressure system it may be found at over 1600m. Therefore, if one were
to drape a giant blanket over the Earth such that the blanket was always at the
altitude where some given pressure was found, it would appear as a bumpy surface
with depressions, mounds, and wrinkles.
To plot upper air maps, this pressure
concept is basically turned inside out; the map lies on a single pressure
level, and the contours on it reflect the altitude at which the pressure value
is occurring. These lines of altitude (often referred to as height contours)
are plotted in the same way isobars were plotted on the sfc maps. Why is this
somewhat confusing convention used? Many important and fundamental equations
and concepts in atmospheric science work out much better is height units are
expressed with pressure. Another reason is that it helps visualize how air is
moving, since low pressure systems will have lower height values; one can imagine
objects sliding down into them like water does in a funnel. The same goes for
high pressure centers, because they have the highest height values in a given
area, it is as if air flows down them like water flows down a hill. Overall,
the height contours on an upper level map will have a very similar pattern to
the isobars on a sfc map.
When looking at an upper air map,
perhaps the most noticeable difference from sfc maps is that they have far
fewer features. In fact, fronts are not generally plotted on upper air maps at
all, because the sharp temperature gradient that defines a front pretty much disappears
by around the 700mb level. Below are the features that do appear on upper air
maps:
-Height
Contours (Isohypse)
These are the contours discussed
above. At lower levels, these will look much like the sfc isobars, at higher
levels height contours tend to become smoother with far fewer closed contours.
-Temperature
(Isotherms)
Isotherms are lines of constant
temperature, plotted in much the same way isobars or height contours are
plotted. They are generally depicted as dashed lines and are labeled in units
of degrees Celsius.
-Observations
Upper air observations are obtained
from weather balloon data, also called a radiosonde. On the maps, this data is
displayed on a station model similar to the ones found on sfc maps. However,
these are much simpler and typically include just wind data, temperature, dew
point, and height. There are also far fewer observations since very few
stations launch radiosondes compared to the total number of weather stations,
and they are launched just twice a day (at 00Z and 12Z).
-Wind Speed
(Isotachs)
For the higher upper air maps, such
as the 300mb and 200mb level maps which occur near the level of the jet stream,
lines of constant wind speed are plotted. These are plotted in the same manner
as isobars and isotherms and are typically labeled in units of knots. Regions
of particularly high wind speeds may be shaded, helping to identify the
location of the strongest parts of the jet stream, called jet streaks.
Below are a series
of images from the same time as the images in the post on sfc maps. Several
levels are shown, and some are overlaid on satellite imagery. Finally, I
included one of the sfc maps from the sfc map post in order to compare it to
the upper level maps.
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