The Skew-T Diagram

 

The Skew-T diagram gives a "snapshot" picture of temperature, dewpoint, air pressure, and winds in the atmosphere above a particular point on the Earth's surface. The data is measured by launching hydrogen or helium filled balloons carrying weather instrument packages called radiosondes. As the balloon rises, the measurements are transmitted to a ground receiver. Twice a day, at 0000 and 1200 UTC (Universal Coordinated Time), about 800 radiosondes are launched worldwide, including two from Tucson, which are launched from the roof of the Arizona ENR Building (corner of 6th and Park). (NOTE: 0000 UTC corresponds to 5 P.M. local Tucson time and 1200 UTC corresponds to 5 A.M. local Tucson time. Tucson local time is always 7 hours earlier than UTC.) The measured data is then plotted on a skew-T diagam. Skew-T diagrams are used by meteorologists to help determine atmospheric stability and to assess the possibility for the development of severe thunderstorms.

In this class we are going to use stripped-down, skew-T diagrams to visualize the vertical structure of the atmosphere. This page describes how to read some of the information contained in the diagram. The skew-T diagram shown below was generated for Tucson on August 24, 2006 at 1200 UTC (labeled as 12Z, often 0000 UTC is labeled as 00Z and 1200 UTC as 12Z). The bold black line is a plot of the vertical temperature above Tucson and the bold blue line is a plot of the vertical dewpoint temperature above Tucson. The vertical axis is the air pressure in millibars (mb), and the horizontal lines on the graph are lines of constant pressure. The numbers printed on top of these pressure lines indicate the height above sea level in meters (m) at each of the pressure levels. For example, on this day and time, when the radiosonde balloon reached 5890 m above sea level, the measured air pressure was 500 mb. As expected air pressure must decrease as you move upward in the atmosphere.

I realize that we have not yet covered dew point temperature and that you may not understand what it means. The dew point temperature is really a measure of the amount of water vapor in the atmosphere and not a temperature that is measured with a thermometer. We are going to cover water in the atmosphere next. For now, I just want you to be able to read the dew point temperature information from the skew-T diagrams. The dew point temperature will always be less than or equal to the air temperature. When they are equal the relative humidity is 100%. This is typically the case if a cloud is present.

The tricky part about reading the skew-T diagram is that the lines of constant temperature are not vertical as in most graphs, but "skewed" at an angle of 45° from vertical. These lines are spaced on the graph every 10° C. To help you see this, the constant temperature lines are labeled on both the bottom and right axes of the plot. For example, at 700 mb (3182 m above sea level), the air temperature above Tucson was 10° C, since the bold, black temperature plot lies right over the 10° C constant temperature line. Now follow the bold, black line upward from there. Notice that the atmospheric temperature drops to the freezing level (0° C) somewhere between 700 mb and 500 mb (3182 m and 5890 m). Continuing upward to where the air pressure is 500 mb, hopefully you can see that air temperature at this point in the atmosphere is approximate -7° C.

The final bit of information on the skew-T diagram is the wind information shown along the right side of the diagram. The "wind barbs" indicate both the wind speed and wind direction at the corresponding air pressure and altitude. The wind barb points in the direction from which the wind is coming, with respect to standard compass directions. For a wind from the north the barb sticks straight up, from the east the barb points right, from the south the barb points straight down, and from the west the barb points left. For example the circled wind barb indicates that the measured wind at 200 mb was from the northeast. Wind speed is indicated by the line segments and flags attached to the end of the barb. There are long and short line segments. Each long line segment represents 10 knots of wind (there can be up to 4 long line segments). A short line segments represents an additional 5 knots of wind. For example the circled wind barb indicates a wind speed of 20 knots, whereas the wind barb directly below the circled wind barb indicates a wind speed of 15 knots. Each Flag on a wind barb represents 50 knots of wind speed. In the example all of the winds were too light to need a flag. A legend for wind speed is shown below on the right. (By the way a knot is a nautical mile per hour. 1 knot = 1.15 miles per hour). (See Figure D for further explanation)

Two More Explained Examples

You may wish to open the linked diagrams below in separate browser windows as you read the discussion below:
Tucson Skew-T August 28, 2012 at 00Z
Tucson Skew-T August 28, 2012 at 12Z
North American 500 mb map for August 28, 2012 at 12Z

You should be able to tell me the local Tucson time corresponding to the skewt diagrams above. For example, the local Tucson time corrsponding to August 28, 2012 at 00Z is 5 PM on August 27. A comparison of the 00Z and 12Z skew-T diagrams for August 28 shows that the lower atmosphere above the ground surface was considerably colder at 5 AM than it was at 5 PM the day before, from about 40°C [104°F] to 28°C [82°F]. Higher up there was little change in temperature between morning and evening. Notice also that there is a hint of a temperature inversion just above the ground surface at 5 AM. Remember that it is common to have a temperature inversion layer above the ground in the early morning hours just prior to sunrise. Temperature inversions are much more pronounced during the winter months when the nights are longer.

Both skew-T diagrams indicate the position of the top of the troposphere and the lower part of the tropopause. This happens where the air temperature stops decreasing and becomes nearly constant as you move upward. On the 00Z skew-T this transition takes place just above the 150 mb pressure level, which is estimated to be around 14500 meters above sea level. On the 12Z skew-T this transition takes place near the 120 mb pressure level, which is estimatated to be around 15500 meters above sea level.

As the radiosonde balloon moves upward, it can pass through clouds. There was apparently a cloudly layer in the 00Z sounding between the 700 mb and 500 mb pressure surfaces, where the air temperture and the dew point temperature come close together. We have not covered relative humidity yet, but when the air temperture and the dew point temperature are the same, the relative humidity is 100%. The relative humidity in clouds is always near 100%.

Looking at the Tucson 12Z skew-T for August 28, the winds at the 500 mb pressure level are from the east at 20 knots and the 500 mb height is 5920 meters. You should be able to verify that this 500 mb wind direction and height over Tucson is consistent with the height pattern shown in the corresponding 500 mb map for 12Z. You should also be able to tell that the 500 mb height pattern (and winds) are favorable for monsoon season thunderstorm activity in southern Arizona. The 5760 meter closed low in the northern Gulf of Mexico is the location of Hurricane Isaac.

Links to more examples of skew-T diagrams

The following links will allow you to view the most recent 00Z and 12Z skew-T diagrams for Tucson. These diagrams include some additional lines that we will not use in this class. Please do not let these confuse you. You can still see the skewed constant temperature lines.
Latest 00Z Tucson Skew-T diagram
Latest 12Z Tucson Skew-T diagram

Use this link to view the Skew-T diagram for any station on Earth for any day since 1973. After you click the link below, you must specify a geographical region and date for the sounding you want. You should also select Skew-T for the type of plot. This will bring up a map with all sounding stations for the specifed region and time. Click on the station you want and the corresponding Skew-T diagram will be drawn. Again these diagrams will have more lines than the stripped down versions that you have as handouts, but don't let that confuse you. You should be able to find the skewed temperature lines and read the vertical profile of air temperature and dew point temperature.
University of Wyoming's Sounding Data