Tools Of The Trade
Upper Air Observations
Measuring upper air information - Radiosondes
One of the most important tools a meteorologist uses to diagnose the atmosphere is an upper air observation. Like surface observations, an upper air observation can provide the meteorologist with a wealth of information regarding temperature, moisture, wind direction and speed in the atmosphere aloft. Displayed on a thermodynamic diagram, called a Skew-T/Log-P diagram, this information provides valuable clues as to the stability of the atmosphere which forecasters study to determine the potential for severe weather. Like surface observations, this data can also be plotted on a constant pressure chart to give a visual representation of the structure of waves in the upper atmosphere at various levels. Forecasters can then look at these "slices" to see when the next upper air disturbance will affect our area. Another useful format to look at upper air data is to plot the data on isentropic charts. Some examples of these types of charts will follow but first let's look at how the upper air observation is made.
Twice a day at designated reporting stations around the country upper air balloons are released carrying an instrument called a radiosonde. The radiosonde is very small and light and contains sensors which measure temperature, moisture, and pressure as the balloon rises. At the surface meteorologists track the speed and heading of the radiosonde using a dish antenna which locks onto a radio signal that is emitted from the radiosonde. Since the balloon drifts with the wind the wind speed and direction of winds aloft are determined by tracking the radiosonde. This information is then transmitted to a ground-based receiver where a computer encodes the information to be relayed across the country. Because pressure decreases with height, the pressure inside the balloon will eventually be greater than the surrounding atmosphere as it rises. The balloon stretches to its breakpoint and bursts. A small parachute attached to the radiosonde opens and the radiosonde drifts back to earth. If you see a small, white Styrofoam box lying in the fields with sensors in it, it is most likely a radiosonde. Look inside for a return address - it can be reused by outfitting it with new sensors.
Displaying upper air information
Skew-T/Log-P chart
In the past a team of meteorologists used to decode this information and plot it on charts by hand. Today we use computer software to quickly plot and display this information on two types of charts. The Skew-T/Log-p diagram, also known as a sounding, is a graph showing the vertical distribution of the temperature, dewpoint, wind direction, and wind speed of the radiosonde as it rises through the atmosphere near the site where the balloon was launched. Depending on the strength of the winds aloft though this information is not always representative right over the launch site because strong winds aloft blow the balloon quite a distance away downstream. Since pressure decreases with height logarithmically, the vertical axis of the graph shows higher pressure at the bottom of the chart decreasing to lower pressure at the top. The horizontal axis shows temperature in Celsius increasing from left to right but the temperature lines are "skewed" from lower left to upper right. This orientation makes it easier for forecasters to determine the stability of the atmosphere.
The two lines represent the temperature and dewpoint trace of the radiosonde. The temperature is always plotted to the right of the dewpoint because air temperature is always greater than the dewpoint temperature. Wind barbs representing wind speed and direction are always plotted on the right side margin. Like surface winds, wind direction aloft is always plotted with the barb pointing into the direction from which the winds blow and the wind speed is indicated by the length of the flags.
With this diagram meteorologists can display a wealth of information to diagnose the upper atmosphere. They can deduce the stability of the atmosphere over a "point" or points upstream to determine when unstable conditions or different moisture distributions will move into the area. For example before the severe weather outbreak of August 6, 1996 in southeastern South Dakota the airmass was comprised of a southerly flow of very warm and moist air. Thus the closest upper air sounding that was most representative of this airmass was located at Omaha, Nebraska.The sounding that morning showed extremely unstable conditions and when modified for maximum daytime heating indicated that the potential for very strong thunderstorms was significant. This is shown as the red shaded area which represents the Convective Available Potential Energy (CAPE) of the airmass. The data on the right side of the diagram contains various stability indices meteorologists use to further diagnose the potential for severe thunderstorms. Numerically, the CAPE for this airmass was indicated by the B+ index or 4,056 Joules/kg. Thus forecasters can quantify the stability of the airmass and measure the likelihood for severe weather. They can also get forecast soundings from computer models to anticipate when these unstable conditions might move into the area. They can use these soundings to generate cross sections along a line to show a vertical slice of the atmosphere across the country. They can also see rapid changes in wind direction or speed in the vertical, to measure wind shear, by plotting the wind information from a sounding on a hodograph.
Upper air constant pressure charts
Another way to display upper air information is on an upper air constant pressure chart. This information is plotted in much the same way as surface observations showing a standard plot over the reporting station. However instead of plotting just one level, as in a surface chart, the upper air charts show plotted information on horizontal slices at specific levels in the atmosphere. Since meteorologists work under pressure coordinates instead of height, these charts represent conditions at specific pressure levels, namely, 850, 700, 500, 300, and 200 millibars (mb). You can roughly equate these levels, respectively, as 5,000, 10,000, 25,000, 35,000, and 45,000 feet high in the atmosphere since pressure decreases with height. Click here for an example of the 500 mb chart used at the Sioux Falls National Weather Service Office. For example at Aberdeen, South Dakota, the air temperature is plotted to the upper left of the station (-11 C) and dewpoint depression (the difference in degrees between the temperature and the dewpoint) to the lower left (14 C). In the upper right portion the height (in meters) is plotted with the trailing zero omitted (in this case 5,770 meters). In the lower right is the change in height (in decameters) over the past 12 hours (+00 or no change in height).
Once the observation is plotted on a chart meteorologists can analyze various fields such as temperature, moisture, heights of the pressure levels, wind speed, and wind direction to determine your weather forecast. This is an example of a hand analysis of the same 500 mb chart showing temperature in red and wind flow in blue. In fact these upper air charts are crucial in determining where upper atmospheric waves or disturbances are located, how fast they are moving, and where they are moving to since they have a great affect on your forecast. They are also used to initialize all of our computer forecast models from which forecasts are generated showing changes in the upper air pattern.
Upper air isentropic charts
A different and more advantageous way to display upper air information is to plot them on an isentropic chart. For decades meteorologists have been trained to look at the upper atmosphere from a constant pressure framework. Recently, with the advent of computer power, we have begun to display upper air information on an isentropic framework. Instead of plotting information on a constant pressure level (say 850 mb) we can now plot information on a constant temperature level to see how constant pressure levels vary along that temperature surface . For isentropic analysis we use degrees Kelvin instead of Celsius, for example, the 300K surface. Here is a graphical representation of how pressure varies on an isentropic surface. In this example cold air is located to the north and acts as a dome. Higher pressure (which is lower in the atmosphere) is located to the south and lower pressure (which is higher in the atmosphere) is to the north. Air flowing from south to north then rides up this dome of cold air to higher regions of the upper atmosphere. We call this isentropic upglide motion and, given available moisture, the air cools and condenses to form clouds and precipitation.
There are many advantages to looking at the upper atmosphere on an isentropic surface the most important being that, making a few assumptions, air parcels will tend to follow an isentropic surface and remain on that undulating surface rising up "hills" and dropping into "valleys". Thus if the isentrope slopes upward there will generally be upward motion as the air parcel climbs the "hill" and vice-versa. This has great forecast implications since rising air, or lift, generally produces weather (rain, thunderstorms, snow) and sinking air generally produces dry conditions (fair weather). Thus forecasters can look at computer forecasts of various isentropic surfaces to actually see how much lift (or descent) will occur at given points in time. An example of the 310K isentropic surface shows that air flowing from left to right and following the solid lines would be crossing from 700 mb down to 850 mb (isentropic descent) until it crosses the solid line marked N-S. Then the air would be flowing up the isentropic surface as it went from 850 mb back up to 700 mb. On a constant pressure chart you cannot see the slope of the pressure surface (by definition it is constant) so there is no way to graphically see whether there is rising or sinking motion.
Isentropic information can also be plotted in a vertical cross section along a line to display the slope of the isentropic surfaces in another format. In this way one can see the undulating isentropes on which air parcels tend to follow. Forecasters can also use this type of format to analyze fronts, the amount of stability in the atmosphere, and temperature advection solely based on the slopes and vertical spacing of isentropic surfaces along the cross section. You can also see a more uniform moisture distribution through the atmosphere along an isentropic cross section as opposed to an atmospheric sounding.
Meteorologists taking upper air observations are as basic to diagnosing the atmosphere as a doctor taking your blood pressure to diagnose the condition of your heart. With these simple measurements we can analyze many structures in the upper atmosphere and understand the dynamics of weather systems aloft. Without these observations we would be as blind to the structure of disturbances aloft as not having satellite pictures. Your forecast depends on an adequate and reliable source of upper air observations!
