Introduction to the SkewT Diagram
The SkewT Diagram, more formally as the SkewT-LogP Diagram,
is a very useful meteorological thermodynamic chart on which pressure,
density, temperature, and water vapor are plotted for a point on the Earth up
through the atmosphere. Every SkewT diagram consists of both a static set of
atmospheric calculations, which apply equally to all locations, and a dynamic
set of air temperature and dew point temperature calculations obtained by the
lofting of a small radiosonde instrument package, commonly referred to as a
weather balloon. NASA's Atmospheric Infrared Sounder (AIRS), with the help of
its sister instrument, the Advanced Microwave Sounding Unit (AMSU), have been
deriving these temperature and pressure profiles remotely from space since
shortly after their May 4, 2002 launch aboard NASA's Earth-orbiting Aqua
satellite.
The following sections represent a "mini-course" in meteorology as it relates to the general characteristics of the Earth's atmosphere. No formal mathematics or science training is assumed or required. After reading this material you will have a good basic understanding of the role of moisture in the atmopshere and gain a new appreciation for all those puffy white or ominous gray clouds you see when there is a change in the weather.
SkewT Diagram Layout
To begin to appreciate this very complex figure (see below for an "empty" SkewT Diagram), let's
first discuss why this is called a SkewT Diagram. Note along the left vertical
axis that the air pressure is plotted on a descending scale, with the highest
pressure (1050 millibars) at the bottom (which would be sea level) and 100 millibars at the top. This axis
uses a logarithmic scale (base 10) to accommodate the rapid changes in pressure
with increasing altitude. Therefore, if we extended the vertical axis all the way up to
10 millibars of pressure, the distance between 1000 and 100 millibars would be the same
as between 100 millibars and 10 millibars. If you are interested in more information
about logrithmic scales, check out this link
Nonetheless, the range of sea level to 100 millibars
covers the troposphere and a bit beyond, which is where most of Earth's weather occurs.
Along the right vertical axis, the altitudes are indicated
that correspond (for a standard atmospheric model) to the
pressure levels along the left vertical axis. Commercial airplanes have a cruising
altitude that corresponds to about 250 millibars.
Now, along the horizontal axis observe the increments of temperature in degrees Celsius. Out from each of these tic marks is a straight, slanting dark red line that runs across the figure from lower left to upper right. These are parallel lines of a constant temperature that have been purposely titled or "skewed" 45 degrees clockwise off of the customary vertical direction. This orientation has also been chosen to accommodate an important characteristic of the atmosphere. While most people have experienced that air temperature drops noticeably with an increase in altitude, this is in fact not true in the upper atmosphere. Specifically, as one moves up through the troposphere, passes the tropopause and enters the stratosphere, the air temperature (i.e., average motion) of air molecules actually increases rapidly across a wide range of upper altitudes before returning to lower temperatures. Therefore, if the temperature scale were just a set of parallel vertical lines, a plot of air temperature versus altitude would very quickly run off the right edge of the chart, just at the altitudes where interesting weather phenomena are developing. Skewing the temperature axis as shown allows these profiles to race towards the upper right corner of the figure rather than run off the page.
Finally, the curved and solid green lines and the dashed purple lines represent the behavior of parcels of air that rise and sink vertically through the atmosphere based on their temperature and how much water vapor they contain per unit volume. Stored water vapor represents trapped heat which makes an air parcel warmer and thus more bouyant than it surroundings. A warm, moist air parcel than passes into a region of cold dense air will shoot upwards like a cork submerged in water!
The dashed (purple) straight, parallel lines, running steeply from lower left to upper right, indicate for a given temperature and pressure, the "mixing ratio" of water with dry air that can be present in a parcel of air. For example, on a warm day of 25 deg C (77 deg F), at sea level a parcel of air could hold as much as 20 grams of water for every kilogram of air. Water vapor can contain a great deal of deal. For example, if you even have put your hand over a steaming kettle pot, it will get burnt as soon as the vapor condenses on your skin. When an air parcel contains the greatest possible amount of water vapor (i.e., saturated), we call it a "moist adiabat", which refers to the way in which it can naturally rise of sink in the atmopshere based on the difference between its temperature and surrounding air. This cooling occurs because the decreasing air pressure draws the water droplets out of the air parcel. The dashed green lines in the figure predict how a moist adiabat will cool off as it rises through the atmopshere, from various initial temperatures and water content on the ground. Any air parcel that is less than fully saturated is called a "dry adiabat". Its cooling behavaior is shown by the solid green lines. As you can see, the solid (dry adiabat) green lines are almost parallel to each other, regardless of the starting initial surface temperature. The dashed (moist adiabat) lines are very similar to the solid green lines at very low surface temperatures but are very different at higher surface temperatures. This is due to the fact that a warm air parcel can retain a great deal more water vapor than a cold air parcel. Therefore, the moist adaibat will cool off much more slowly than a dry adiabat as it rises.
Sample SkewT Diagram
Atmospheric
Soundings
The atmospheric soundings on a SkewT diagram consist
of an air temperature profile (solid thick red line) and the dew point
temperature (solid thick blue) line, each plotted against pressure and
temperature.
One important feature of the SkewT diagram is that where these
lines come close together (less than 10 degrees Celsius) there will be clouds, whereas widely separated lines
indicate fair weather and good visibility for aircraft pilots (See this article
Sample SkewT Diagram for Calm, Clear
Weather
When these atmopsheric profiles (soundings) are obtained from a
radiosonde
they occur over a few hours as the instrument package rises up and drifts
downwind of the release site. Therefore, although the data are accurate, they
represent the path traversed by the radiosonde, which is generally not a
straight vertical column of air. The unique benefit of a radiosone, therefore, is
valuable information on wind speed and direction, which AIRS cannot measure from space.
On the other hand, given that that an entire AIRS west-east swath of 45 adjacent
columns of air (also called footprints) can be scanned and measured from Earth
surface up to space, all in about 8 seconds, these data can more reasonably be
associated with a precise time and location. Thanks to a data sharing
arrangement with the National Oceanic and Atmospheric Administration (NOAA) and the NASA Goddard Space Flight Center,
data from scanning passes as recently as 4 hours after an Aqua/AIRS overpass are now available to produce
Skew T Diagrams for any location on the
globe.
As will be discussed in the next section, a great deal of valuable information
about an atmospheric profile can be obtained from a SkewT diagram simply by
reading the intersections of various lines on the chart. In addition, many
SkewT diagrams also include the trajectory of a theoretical parcel of air that
starts at the same position as the measured temperature profile but then rises
under very special thermodynamic assumptions which are different than the
actual atmospheric conditions.
Thermodynamic Background of the SkewT Diagram
In this section you will learn how thunderstorms are created!
Whenever a warm parcel is
less dense than the surrounding air it will naturally rise up through the
atmosphere. The surrounding air may or may not cool off at the same rate, due
to factors such as local wind patterns. It is therefore helpful to visualize a
theoretical parcel of air under very specialized assumptions and then compare
it during its ascent to the actual atmospheric conditions as mesured by instruments such as a
radiosonde or AIRS.
The notion of a rising air parcel (adiabat) is that it starts out unsaturated (dry) and rises up naturally to the point where the water vapor turns to water droplets and it becomes a ssturated (moist adiabat) air mass. This lifting mechanism is convection or sometimes a warm air parcel on the ground gets pushed up the side of a mountain by the wind. This is called an adiabatic process because, as noted earlier, no extra heat energy is being added to the parcel as it rises. This swicth from a dry to moist adiabatic is a straightforward calculation based on the initial surface temperature and water mixing ratio. It is called the "Lifting Condensation Level" (LCL) and is expressed in terms of pressure (i.e., altitude). This is shown as one of the many SkewT parameters along the right margin of the figure. On the SkewT diagram, then we can state that the air parcel follows a (solid green) dry adiabat line up to the calculated LCL and then switches to the (dashed green) moist adiabat line. It will only continue to rise, however, if it remains warmer and more buoyant than it surroundings. In most cases this is not true. If you look out an airplaneto observe a weather system which has a very low, flat and thick cloud deck you are seeing an LCL close to the ground. The air below the cloud deck is generally warm and moist and the air above the cloud deck is clear and cool.
First, let's consider an example of very stable atmospheric conditions
Return again to the previous SkewT plot for a calm weather day. You can see that the atmopsheric air temperature (red line) just happens to follow an dry adiabat line that starts at around 22 deg C. This is just a coincidence; the air tmeprature is what it is based on the actual measurements. You can see that the Lifting Condensation level is calculated to be at 854 millibars. This also is a farily low altitude (5,000 ft), yet there is a very large separation between the dew point (blue) and the air (red) temperature lines which suggests that at the LCL the air massess are not even close together. Therefore, this is cloud-free day at least up to 250 millibars (35,000 ft). The key is to look at the shape of the air temperateure cruve and how it makes a bow shape towards to the right (warmer temprature) as the altitude increases. If you look closely, this shape is significantly more curved than any nearly moist adiabat dashed green lines. In other words, one an air parcel reaches the LCL, even though it is saturated, it will immediately encounter sourrounding air that is warmer than it. Since a cooler air mass sink, the rising air parcel can rise no further. There is moisture in the air because we see indications of clouds at 250 millibars (millibars), but these are thin, wispy cirrus clouds, probably blown in from a separate, distant weather system. iAnother useful parameter along the right margin is Lifting Index(LI), a measure of the atmospheric instability. A negative value indicates unstable storm-like conditions. However, this SkewT Diagram has a LI of +7.6, which indicates extermely stable conditions.
Now, let's consider an example of very unstable atmospheric conditions
Take a look at the following figure that
represents a severe weather system. This is what happens when the rising (moist adiabat) parcel of air
reaches the LCL and find itself still WARMER than the surrounding air - it
has reached the "Level of Free Convection" (LFC), which is also expressed in the
figure as a pressure value along the right margin of the SkewT plot. Also, we have added the actual calculated
trajectory of the air parcel to this figure. If the
surrounding air continues to cool off at a rate slower than that for the moist
adiabat, the air parcel will always be warmer than the surrounding air and the
parcel is free to rise unimpeded up through the atmosphere. This is known as
the Zone of Free Convection and the amount of time a parcel spends in this
region is a good indicator of the likelihood of severe weather (e.g.,
thunderstorms and tornados). Notice how in this figure the air temperature
curve above the LCL is bowing to the left (cooler temperatures), as if making way for
the air parcel to rise into cooler denser air.
Eventually, the temperature profile of the
surrounding air does change dramatically and catches up with the slowly-cooling
adiabat. At the tropopause (the transition zone between the lower troposphere and upper stratosphere)
, air temperature starts to increase with altitude, such
that the adiabat temperature matches the surrounding air at the "Equilibrium
Level" (EL). Above this point, the surrounding air is warmer than the
adiabat and acts as a brake on its ascent. The highest altitude reached by the
air parcel before it stops rising and begins to sink down, is called the
"Maximum Parcel Level " (MPL). This point is
defined as the pint in which the kinetic energy acquired by the parcel in the
Zone of Free Convection (between LFC and EL) is balanced out by the potential
energy lost in rising above the EL.
Sample SkewT Diagram for Calm, Clear
Weather
If we look at the date, time and location for this SkewT plot, we find that it was taken at 2:43 PM on July 24, 2007 in nothern Georgia just east of the Appalachian Mountains. Most meteorologists would not be surprised to see thuderstorms at that location on a warm midsummer afternoon.In addition ,the Lifted Index for thei SkewT plot also has a very unstable value of -5.5. The Zone of Free Convection shown here as a hashed region is also translated into a helpful SkewT parameter known as the Convective Available Potential Energy (CAPE). The previous SkewT plot for a calm day has a CAPE value of 0.0. Therefore, the SkewT application will only include the temperature profile fo the adiabat air parcel when the CAPE value is greatre than zero (i.e. there is movement above the LCL).
In this example you can clearly follow the "wild rise" of the adiabat air parcel. It rises from the ground as a dry adiabat, up to the Lifiting Condesation level, and then jumps to a moist adiabat line all the way into the upper atmosphere. Note that this combination of storm conditions is fairly rare. In addition, the rising air parcel will loe both its heat and moisture content during the ascent. After reaching the MPL, unless it reabsorbs moisture from the surrounding air it will simply descend back down into the lower atmopsher along a dry adiabat line and start the process over again.
A comprehensive description of
the standard SkewT parameters can be found at the following
URL: http:www.theweatherprediction.com/thermo/parameters
Additional
References
Congratulations on completing your "mini-course" using the SkewT Diagram!.
The following web site provides additional information
about the SkewT diagram, including resources
on near-real-time SkewT measurements collected at many local airports.
http://www.skew-t.com/skew-t.htm