Skew­T Diagram Informational and Current Data Website

By Chris Edlin

GE 406 Introduction to Meteorology

Professor Dr. Bill Rose

Michigan Technological University

I. Introduction

Radiosondes, as the balloon­borne instruments shown in the above photograph are called, are carried by special helium­filled meteorological balloons made of high­quality neoprene rubber. Over 100 stations in the United States and more than 1000 worldwide release, twice daily (00Z and 12Z), these balloons which are employed for elevating the radiosonde to very high altitudes of around 30,000 meters (100,000 feet); maximum altitude for balloon­borne radiosondes is about 50,000 meters. Observations of temperature and relative humidity at various pressures are radioed back to the station from which the balloons are released as they ascend at a predetermined rate. The balloons also are tracked by radar in order to ascertain winds from their drift. (Britannica Online, 1998)

Two basic systems are used for collecting data and coding it for transmittal. In the mechanical type, the thermometer is usually a bimetallic strip or a stretched wire, the humidity element a hair or goldbeater's skin hygrometer, and the pressure system an aneroid type of capsule. In the electrical change type system, a mechanical bellows, responsive to changes in atmospheric pressure, operates a switching arrangement to alternately transmit measurements of temperature and humidity; the temperature is indicated by a temperature­sensitive resistor and the humidity by a humidity­sensitive arrangement consisting of a strip of polystyrene coated with lithium chloride. Wind velocity can be determined by tracking the radiosonde with a theodolite or with an automatic tracking antenna. (Britannica Online, 1998) This information is transmitted to the surface and then on to a central site in Washington, DC at the National Weather Service where a Cray supercomputer uses the data to model the atmosphere up to 10 days in advance. This is a very powerful forecasting tool used by meteorologists.

When the balloon reaches higher altitudes the pressure inside the balloon, which is now much greater than that of the surrounding air, bursts the balloon. A small parachute then brings the radiosonde safely back to the ground. If you see asmall, white Styrofoam box with sensors in it look for a return address and send it back. (NOAA, 1998) These instruments can be reused by outfitting them with new sensors. Here is a good link that USA Today Weather page has on the f light and fall of the weather balloon.

Here is a very informative page on everything you need to know about radiosondes from the Department of Atmospheric and Oceanic Sciences at the University of Wisconsin­Madison. This site has a description, history, and detailed information on the various sensors and collection devices involved with the radiosonde.

II. Skew­T Diagrams

Skew­T diagrams are derived using the data that is collected by the radiosondes. 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. This thermodynamic diagram, or Skew­T/Log­P diagram, and 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. (NOAA, 1998)

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. (NOAA, 1998)

Measuring Stability

With this diagram meteorologists can display a wealth of information to diagnose the upper atmosphere. They can deduce thestability of the atmosphere over a "point" or points upstream to determine when unstable conditions or different moisture distributions will move into the area. They are very useful to meteorologists for determining the stability of the atmosphere very quickly, which is important in local and regional forecasting. (NOAA, 1998) Here is a good link from the University of Minnesota, Department of Soil, Water, and Climate that explains how to determine the stability in the atmosphere by using Skew­T plots.

The data on the right side of the Skew­T diagram contains various stability indices meteorologists use to further diagnose the potential for severe thunderstorms. Here is an excellent page describing Skew­T Plot abbreviations and symbols from the Purdue University Weather Page. 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 together 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. Like surface observations, this data can also be plotted on a constant pressure chart or map to give a visual representation of the structure of pressure waves in the upper atmosphere at various altitudes. (NOAA, 1998)

Effects of Moisture

Moisture levels play a major role in cloud formation, precipitation amounts, and severe storm development. Skew­T diagrams allow for the moisture levels to be tracked through the column of the atmosphere measured and are used in forecasting the above mentioned weather phenomenon. Here is a good site developed by the University of Minnesota, Department of Soil, Water, and Climate that explains why moisture is significant and discussion on how moisture levels are determined from the Skew­T diagram.

Inversions, Lapse Rates, and Adiabatic Lapse Rates

Temperature inversions are defined as the increase of air temperature with altitude. Such an increase represents a reversal of the normal temperature condition of the troposphere (the region of the atmosphere in contact with the Earth's surface), where temperature usually decreases with height. (Britannica Online, 1998)

Inversions play an important role in determining cloud forms, precipitation, and visibility. An inversion acts as a lid on the vertical movement of air in the layers below. As a consequence, convection produced by heating the air from below is limited to levels beneath the inversion. Diffusion of dust, smoke, and other air pollutants is likewise limited. In regions where a pronounced inversion is present at a low level, convective clouds cannot grow high enough to result in showers and, at the same time, visibility may be greatly reduced below the inversion, even in the absence of clouds, by the accumulation of dust and smoke particles. Because the air near the base of the inversion is cool, fog is frequently present there. (Britannica Online, 1998)

Another important influence of inversions is reflected in the diurnal range in temperature. The principal heating of the air during the day is produced by contact with the ground, the temperature of which is raised by the Sun's radiation to which the air is transparent. The solar radiation is absorbed by the ground, and the heat is communicated to the air by conduction and convection. Since the inversion base represents the upper limit to which heat is carried by convection, only a shallow layer of air will be heated if the inversion is low and large, and the rise in temperature will be great. (Britannica Online, 1998)

The lapse rate is the rate of temperature change observed in passing upward through the Earth's atmosphere. The lapse rate is considered positive when the temperature decreases with elevation, zero when the temperature is constant with elevation, and negative when the temperature increases with elevation (temperature inversion). The lapse rate of non­rising air­­commonly referred to as normal temperature, or actual, lapse rate­­is highly variable, being affected by radiation, convection, and condensation processes; it averages about 6.5 C per km (18.8 F per mile). It differs from adiabatic lapse rate, which involves temperature change due to the rising or sinking of an air parcel. Adiabatic lapse rate is usually differentiated as dry or moist. (Britannica Online, 1998)

The dry rate for air depends only on the specific heat of air at constant pressure and the acceleration caused by gravity. The dry adiabatic lapse rate for the Earth's atmosphere equals 9.8 C per kilometer (28.3 F per mile); thus, the temperature of an air parcel that ascends or descends 5 km (3 miles) would fall or rise 49 C (85 F), respectively. (Britannica Online, 1998)

When an air parcel that is saturated with water vapor rises, some of its moisture will condense, releasing heat and causing the parcel to cool more slowly than it would if it were not saturated. This moist adiabatic lapse rate varies considerably. The greater the amount of moisture contained in the air, the smaller the adiabatic lapse rate; as the air parcel rises, cools, and loses its moisture through condensation, its lapse rate increases and approaches the dry adiabatic. (Britannica Online,1998)

The comparison between the normal lapse rate in the atmosphere and the dry and moist adiabatic lapse rates determines the vertical stability of the atmosphere­­i.e., the tendency of an air particle to return to its original position or to accelerate away from its original position after being given a slight vertical displacement. For this reason, the lapse rate is of prime importance to meteorologists in forecasting certain types of cloud formations, the incidence of thunderstorms, and the intensity of atmospheric turbulence. (Britannica Online, 1998)

To wrap up this section on inversions, lapse rates, and adiabatic lapse rates here is are two links to Texas A&M; University's tutorials on Clouds and Precipitation and Precipitation and the Sounding Diagram. These tutorials are excellent. The first site will teach the effect of dew point temperature on the nighttime minimum temperature, the effect of fog on temperatures, the effect of clouds on temperatures, the effect of soil type on temperatures, and show how to apply these things to weather forecasting. The second site and will take you through an understanding of dry and moist adiabatic lapse rates, reading and interpreting a sounding diagram, determining the change in temperature as air parcels rise or sink, and using sounding diagrams to assess the possibility of severe weather, such as thunderstorms. These tutorials are the basis for understanding how to read the Skew­T diagrams.

III. My Skew­T Diagram List

Skew-T Diagrams

IV. Other Skew­T Diagram Data Links of Interest

  • Purdue University Skew­T Page. An excellent page containing both plots and raw data from radiosonde measurements. Also contains information on symbols and abbreviations that is helpful when reading the diagrams.
  • University of Wyoming Atmospheric Soundings Page.
    Soundings from stations located in the United States, Canada, South America, Mexico and the Carribean, Australia and New Zealand, and Antarctica.
  • Texas A&M; University Current Sounding Images.
    Includes present, 12 hours prior, and 24 hours prior.
  • Air Resources Laboratory
    Under the National Oceanic and Atmospheric Administration. Lots of options for developing the plot and step by step use.
  • University of Illinois Urbana­Champaign Weather Visualizer Upper Air Soundings.
    A Clickable map with text data given below each plot.
  • Florida State University's Skew­T Diagram List. Links for numerous stations throughout North America. Read the "Oreadme­skewt.1st" file for instruction.

    V. Acknowledgements

    Information taken from:

  • Britanica Online. 1998.
  • National Oceanic and Atmospheric Administration (NOAA) Central Region Headquarters. 1998.

    Brought to you by GE 406 Introduction to Meteorology
    Michigan Technological University (MTU), Houghton, Michigan

    **Author's Note**

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